diseases and pathogens of mustela spp., with special ...phthiraptera.info/publications/46969.pdf ·...

© Journal of The Royal Society of New Zealand

Volume 31 Number 4 December 200] pp721 744

Diseases and pathogens of Mustela spp., with specialreference to the biological control of introduced stoatMustela erminea populations in New Zealand

Robbie A. McDonald*, Serge Larivieref

Controlling populations of introduced stoats is a high priority for the conservation ofavian biodiversity in New Zealand Existing technology for stoat control is labourintensive and expensive, therefore new techniques and approaches, such as biologicalcontrol, are needed We reviewed the literature on the diseases and pathogens of stoats,and closely related mustelids, with a view to identifying potential biological controlagents Aleutian disease virus, mink enteritis virus, and canine distemper virus holdpromise as agents of lethal control, though the risks to non-target species posed by theseviruses are serious Host-specific ectoparasites such as Tnchodectes ermineae, nematodessuch as Skrjabingylus nasicola, and bacteria such as Hehcobacter mustelae andBartemella spp could have a role as vectors for the transmission of fertility controlagents We urge some caution in developing biological control technology without aparallel investigation of the potential effects of biological control on stoat populationsand the resulting survival of threatened birds

Keywords biocontrol conservation ferret mink Mustelidae pest management predator control wildlife disease

INTRODUCTION

The endemic fauna of New Zealand has evolved in the absence of mammalian predators andhas proven particularly vulnerable to some of the mammals introduced since human settlement(King 1984) Stoats Mustela erminea, weasels M nivahs, and ferrets M furo were introducedto New Zealand in the 1880s in an attempt to control rabbits Oryctolagus cuniculus (King1984) Stoats were almost immediately implicated in the declining abundance of native birds,a trend initiated by the earlier arrival of numerous other mammalian predators In the 21stcentury, stoats are still contributing to the decline of native fauna They now present a seriousthreat to the future existence of several endemic bird species (McLennan et al 1996,O'Donnell et al 1996, Wilson et al 1998) McDonald & Murphy (2000) provided a recentreview of the problems caused by stoats and of the steps taken so far to manage stoats in NewZealand

'School of Biological Sciences, University of Bristol, Bristol BS8 1UG, UKPresent address Game Conservancy Trust, The Gillet, Forest in Teesdale, Barnard Castle,DL12OHA, UK email robbie mcdonald@stoats comDepartment of Biology, University of Saskatchewan, 112 Science Place Saskatoon, SK S7N iE2

CanadaPresent address Delta Waterfowl Foundation R R #1, Box 1, Portage La Prairie, Manitoba R1N 3A1,Canada email slanviere@deltawaterfowl org

722 Journal of The Royal Society of New Zealand, Volume 31, 2001

Reducing predation by stoats is clearly essential for the survival of several endemicspecies on the mainland of New Zealand. The main effort so far has been in developing andemploying methods of lethal stoat control. Hence, current programmes rely to a greatextent on "traditional" kill-trapping using steel spring traps, which is very labour intensive.More recently, the use of stations baited with poisoned hen eggs has become widespread.These methods have proven useful in temporarily reducing stoat abundance and enhancingthe nesting success of certain birds. However, more cost-effective and sustainable approachesto controlling stoats are urgently needed.

The use of diseases as agents of biological control is often the most appealing of a rangeof options for the control of pest species (van Driesche & Bellows 1996; Lynch 1998;Courchamp & Sugihara 1999; Norbury 2000). The objective of our review was to provideecologists with an accessible account of the literature on stoat diseases, with particularreference to areas that may be pertinent to the biological control of stoats in New Zealand.A range of infectious diseases can affect small mustelids but they have not received asmuch attention as the diseases affecting larger carnivores (Murray et al. 1999). Consequently,the pathology and epidemiology of the numerous diseases that affect stoats are poorlyknown. Since certain aspects of the biology of stoats have not yet been described in detail,we expanded our review to take into account work on closely related species. Fortunately,the economic incentive for rearing ferrets, and mink Mustela vison, as well as conservationinterest in black-footed ferrets M. nigripes, has stimulated a good deal of research on thediseases that afflict them. Ferrets and mink are useful models for stoat disease because ofgood evidence that congeneric species are similarly vulnerable to many diseases. Forinstance, domestic ferrets, Siberian polecats M. eversmanni, and ferret-Siberian polecathybrids were extensively used as disease models in developing black-footed ferretconservation plans (Williams et al. 1991; Williams & Thorne 1996). However, it is notalways correct to assume comparable responses to infection between congeners (Williams& Thorne 1996). Where work on the biology of Mustela spp. was scarce, we have alsodrawn on literature about other mustelids.

Diseases of the Mustelidae in captivity have been reviewed previously (Williams &Thorne 1996). While they were shown to be particularly susceptible to a range of viraldiseases, their review did not consider bacterial, protozoan, or metazoan agents of diseasein detail. The reviews provided by Davis et al. (1981) and Addison et al. (1987) areinvaluable to a consideration of infectious diseases in wild mammals and furbearers. Thegeneral reviews of stoat biology provided by King & Moody (1982), King (1983, 1989),Fagerstone (1987), and McDonald & King (in press) have also proven useful. Norbury(2000) recently reviewed the potential options for biological control of stoats, though hefocused on fertility control rather than on diseases. Here, we have reviewed studies of allknown pathogens, including viruses, bacteria, and parasites, and have presented our findingsby types of disease agent. We have outlined the relevance of disease agents to the controland limitation of mustelid populations and have identified areas that we believe will bemost productive for future research. It is likely that animal welfare will be a major topic ofconsultation during the development of biological control agents for stoats. Unfortunately,the current level of understanding of the pathology of disease in stoats is such that adetailed consideration of animal welfare would not be supportable in this review. Therefore,there remains a clear need for further investigation of welfare aspects associated with usingany novel biological control agent.

McDonald & Lanviere—Diseases of stoats 723

VIRUSES

Morbilliviruses and parvoviruses

Morbi Hi viruses cause a range of wildlife and human diseases including measles, rinderpest,and canine distemper (Barrett 1999) Parvoviruses are apparently able to spread worldwide,because most viruses are not genetically distinct even when geographically separated(Parnsh 1995) Recent epidemics of morbilliviruses and parvoviruses in wildlife anddomestic animals suggest that these are highly variable pathogens capable of rapid adaptationto alternative hosts For instance, the incidence of feline panleukopema virus (FPV) incaptive large cats was suggestive of interspecific transmission from domestic dogs (Steinelet al 2000) Mustehds can also be host to non-specific parvoviruses For example, FPV,but not canine parvovirus (CPV), has been found in wild honey badgers Melhvora capensis(Steinel et al 2000)

Isolates of phocine distemper virus (PDV) from harbour seals Phoca vituhna collectedin Denmark, Norway, Greenland, and the United States were comparatively distinct fromreference strains of canine distemper virus (CDV) However, similarities between Danishand Norwegian isolates of PDV and morbillivirus isolates from Danish mink farms suggestthat epizootics among farmed mink may have arisen by transmission from diseased seals toterrestrial carnivores (Blixenkrone-Moller et al 1992)

Two parvoviruses that cause very different diseases have been described in mink indetail Aleutian disease virus (ADV), also referred to as Aleutian mink disease parvovirus(AMDV), is associated with persistent, low-level viral replication and chronic severeimmune dysregulation (Storgaard et al 1997) In contrast, infection with mink enteritisvirus (MEV) is associated with rapid, high-level viral replication and acute disease

Aleutian disease virus

Aleutian disease of mink is a naturally occurring persistent viral disease first described in1958 (Helmboldt & Jungherr 1958) It is caused by ADV and in adult mink results in achronic disease that can be broadly characterised as an immune disorder with a persistentinfection of lymphoid organs ADV is particularly lethal to the Aleutian strain of mink, butall strains are susceptible to some degree (Bloom et al 1994) The virus is transmissible toother mustehds, particularly Mustela spp including ferrets and stoats (Kenyon et al 1978,Alexandersen et al 1985) Symptoms similar to Aleutian disease have been described in anotter Lutra lutra, though while the pathology was consistent with infection by ADV, noabsolute diagnosis was provided (Wells et al 1989)

When stoats were inoculated with ADV isolated from farmed mink, antibodies to thevirus were detected by counter immuno electrophoresis However, the stoats did not showclinical signs of the disease, I e , abnormal accumulation of lymphocytes in kidney or livercells or hyperplasia of lymphoid organs (Kenyon et al 1978) In a sample of 446 domesticferrets in England, 8 5% were seropositive for ADV (Welchman et al 1993) implying thatthere could be significant reservoirs of this virus among species less affected by thedisease

Clinical signs of the chronic disease in adult mink include plasmacytosis, hyper-gammaglobulinaemia, high antiviral antibody titres, and immune complex disease (Bloomet al 1994) In severe cases, the structural organisation of the thymus gland is destroyedand T-cells are found throughout the organ, whereas they would normally be found in


greatest numbers in the inner medulla (Chen & Aasted 1998). Elevated levels of antigen-antibody complexes appear in the circulation and, when deposited in arterial walls, lead toarteritis (Kostro et al. 1999). The virus is present in faeces, saliva, and, intermittently, urea,about 15 days after infection (Kostro et al. 1999). Mink lymph nodes are suitable for theculture of ADV (Jensen et al. 2000).

The disease is transmissible vertically and horizontally (Kostro et al. 1999). Airbornetransmission is possible, but is less efficient than mechanical transmission (Jackson et al.1996). In farmed mink, ADV is transferred between infected mothers and their kits, but notbetween infected fathers and kits (Jackson et al. 1996), suggesting that ADV crosses theplacental barrier between infected female mink and embryos (Broil & Alexandersen 1996).The percentage of dead and resorbed foetuses was much higher in females infected withADV before mating than in those infected after the assumed date of implantation (Broil &Alexandersen 1996). In contrast to adult mink, infected newborn mink develop acuteinterstitial pneumonia that is fatal in most cases (Bloom et al. 1994). Inoculation of 449mink kits with ADV of various strains resulted in 48% mortality, though the severity ofeach strain was variable. In kits, high virulence strains included ADV-K, ADV-Utah I, andADV-DK and resulted in mortality rates of 90-100%. Low virulence groups ADV-GL andADV-Pullman resulted in 30-50% mortality. Kits that survived challenge by ADV developedchronic Aleutian disease as normally expressed in adult mink (Alexandersen et al. 1994).Certain strains of mink that are more susceptible to ADV are better at transmitting theinfection (Jackson et al. 1996).

The genome of ADV is highly variable, with at least three subgroups and manygenotypes. More than one genotype has been found at one farm (Olofsson et al. 1999).Structurally, ADV is similar to human parvovirus B19, CPV, FPV, and minute virus of mice(MVM). Specific patterns of tropism and pathogenicity of ADV are related to structuraldifferences between ADV and these related viruses (Parker & Parrish 1997; McKenna et al.1999). Differences between virus types, including their host range, can be due to only threeor four sequence differences in capsid protein genes (Parrish 1999).

The severity of disease varies between strains of ADV. The ADV-Utah strain causessevere Aleutian disease and death in mink within 6-8 weeks. The ADV-G strain does notreplicate in mink, but does in cats. The difference between these two strains amounts to aslittle as five amino acids (Bloom et al. 1998). Changes of single amino acids at particularlocations can cause changes in host-specific replication and can cause less acute, but notclassical, forms of Aleutian disease, where unmodified versions were benign (Fox et al.1999). This genetic variability presents both a strength and weakness for the use of ADVagainst stoats. While it appears likely that a virus variant that would affect stoats could beidentified, the viruses may be unstable to the degree that non-target species could also be atrisk.

Mink enteritis virus

Mink enteritis virus is part of the feline parvovirus subgroup and is closely related to FPVand CPV (Bittle 1981; Steinel et al. 2000). We are not aware of any attempts to infectstoats with MEV. Clinical signs of infection by MEV in mink include a rapid onset ofdepression, lethargy, and high temperature. MEV replicates very rapidly in Crandell'sfeline kidney cell cultures, at least 20 times faster than ADV. This may be because MEV ismade up of a higher proportion of structural proteins whereas ADV codes for a highproportion of non-structural proteins (Storgaard et al. 1997). Under normal outdoorconditions, MEV is very robust and survives well. Viruses contained in mink faeces


collected from infected individuals survived outdoors and were able to infect mink for upto 10 months after collection, which was 1 month after standard cell culture tests provednegative The virus was not tolerant of drying out but did survive well in soil and dampconditions (Uttenthal et al 1999) Protection against MEV in mink can be induced by asingle inoculation with vaccines developed for protection of dogs against canine parvovirus(Langeveld et al 1995)

Canine distemper

Canine distemper is caused by the canine distemper virus (CDV) which is classified as amorbillivirus and has been considered the most important viral infection of mustehds (Williamset al 1988) While all members of the Mustehdae have been reputed to suffer from distemperthis has not been confirmed in many of the 67 species and data on the fatality rate caused bythe disease are very scarce Confirmation that stoats are susceptible to distemper has beenobtained only once and is based on the death of one individual (Keymer & Epps 1969) Acaptive colony of 7 stoats, 4 weasels, and 3 least weasels experienced an outbreak of caninedistemper that was diagnosed by early clinical signs including muscular spasms, reducedactivity, photophobia, and discharges from their eyes (Keymer & Epps 1969) Two stoats, 4weasels, and 2 least weasels showed clinical signs of the disease, and, of these, 1 stoat, 3weasels, and the 2 least weasels died Distemper was confirmed by taking inoculates from thedead animals and inoculating distemper-immune and non-immune ferrets The immuneferrets survived while the others did not (Keymer & Epps 1969) The progression of thedisease was very slow and in certain individuals lasted as long as 12 weeks (Keymer & Epps1969) The slow progression of this disease in infected individuals may therefore lend itselfto widespread transmission in free-living animals

Black-footed ferrets are particularly susceptible to CDV (Williams et al 1988) and thevirus was responsible for the near elimination of remnant populations of this species Inblack-footed ferrets, clinical signs included pruritus, hyperkeratosis, and loss of body condition(Williams et al 1988) In a sample of 146 mustehds (132 stone martens Martes foina, 5badgers Meles meles, 5 polecats Mustela putonus, and 4 weasels) collected in Germany,CDV antigens were found in the brains of 54 (37%) including 1 of the 4 weasels, and weremainly found in the grey matter Histological brain lesions were detected in 45% of the CDVpositive animals The high prevalence and seasonal variation in prevalence of the antigensand lesions suggested that in Germany at that time there was an epizootic of CDV amongmustehds, particularly stone martens (van Moll et al 1995) Two of 10 stone martenssampled in Germany were seiopositive for CDV and a similar proportion (2 of 13) of aseparate sample of stone martens contained CDV RNA (Frohch et al 2000) None of 468badgers sampled in England were seropositive to canine distemper suggesting that, incontrast to continental Europe, the disease is not endemic among British badgers (Delahay &Frohch 2000)

Using immunocytochemical techniques, few differences could be detected in the structureof CDV found in domestic dogs and that found in martens, polecats, and weasels Thissuggested that the mustehd virus was not antigemcally distinct from the canine virus (Alldmgeret al 1993) Frohch et al (2000) have also confirmed the genetic similarity of most isolatesof CDV from mustehds and canids, and suggested that horizontal transmission between wildcarnivores and between domestic dogs and wild carnivores is commonplace However, thegenetic sequence of three CDV isolates taken from ferrets and stone martens were distinctfrom the main group of other mustehd and canid isolates This led to the suggestion of apossible second wild type of CDV (Frohch et al 2000)

726 Journal of The Royal Society of New Zealand Volume 31 2001

Inactivated canme distemper vaccines with adjuvant and a modified live virus were usedcomparatively successfully in vaccinating black-footed ferret x Siberian polecat hybrids All8 animals vaccinated with the modified live virus survived and resisted virulent CDVinoculation Of the 7 hybrids given the inactivated vaccine, 1 was euthanised following thedevelopment of clinical signs of distemper A further 2 developed clinical signs of the diseasebut survived (Williams et al 1996a) Multivalent avian-ongin vaccines tor CDV inducedtypical clinical signs of canine distemper in 4 inoculated European mink Mustela lutreolaAll 4 were producing CDV antigens, confirming clinical signs, and died of CDV 16-26 daysafter inoculation (Sutherland-Smith et al 1997)

Symptoms similar to distemper have been observed in farmed mink of particular strainsHowever, these can occasionally be related to tyrosinemia II, known as pseudodistemper,which is a rapidly fatal genetic disease prevalent in standard black ranched mink and iscaused by a deficiency of the enzyme hepatic tyrosine ammotransferase (Sanford 1988)

RabiesRabies is one of the best known and most widespread zoonotic diseases Rabies virus is amember of the family Rhabdovindae, genus Lyssavirus (Dietzschold et al 1996) In humansthe virus causes an acute, incurable encephalitis that results in 40 000-100 000 deaths peryear worldwide (Meslin et al 1994, Rupprecht et al 1995) In infected hosts, the virus isexcreted mostly in the saliva and is transmitted through bites from infected animals (Charlton1994) Rabid mammals exhibiting the furious form of the disease are characterised by wide-ranging and often erratic movements, as well as aggression towards other animals includinghumans (Kaplan 1985) Such behavioural modification is thought to enhance rabiestransmission and persistence in host populations For wildlife species, recovery of individualsfrom rabies seems possible and may have occurred in as many as 25% of infected animals(Carey & McLean 1983) Oral rabies vaccines have been successfully tested and used in thefield to protect foxes, raccoons Procyon lotor, and coyotes Cams latrans, but not skunksMephitis mephitis (Hanlon et al 1999)

Although many animals may contract rabies, only a few species act as reservoirs Thereare five rabies variants currently recognised in North America, raccoon rabies in the east,grey fox rabies in the south-west, skunk rabies in the west and north west, and arctic/red foxrabies in the north (Charlton et al 1988 Rupprecht & Smith 1994, Krebs et al 1999) Casesof rabid stoats have been reported (Ballantyne & O'Donoghue 1954, Plummer 1954, Rausch1958, Johnson 1959), but such cases are insignificant compared with rabies in other speciesMany stoats may die of rabies undetected, but even then the significance of the disease tostoat populations is probably minimal The role of stoats in rabies transmission remainsunclear (Johnson 1959) but is unlikely to be important Following inoculation with theraccoon variant of the virus, the incubation period of the disease in ferrets was 28 days, and33 days with the skunk variant Clinical signs were typical, including ataxia, hypothermia,tremors, and lethargy Following clinical signs, death ensued within 4-5 days Viral excretionranged from 1 day before onset of clinical signs to 6 days thereafter (Niezgoda et al 1997,1998) While ferrets are susceptible to rabies, wild Mustela spp appear to represent a deadend m the propagation of the rabies virus

Other viruses

Inoculation of mink with the herpesvirus that causes Aujeszky's disease resulted in salivation,vomiting, and coma after an incubation period of 72-96 h The clinical signs of Aujeszky'sdisease are similar to rabies, hence the disease is sometimes termed pseudorabies Lesionswere detected in the brain stem in the form of non suppurative encephalitis, and degeneration


of vessel walls was widespread The disease virus was detected in the central nervous system(Quiroga et al 1997) Contrary to previous thinking, replication of the related canmeherpesvirus-1 is supported in cells other than those of canine origin, e g , in foetal mink lungcells This property has been used to develop immunoassays for canine herpesvirus infectionin kennel dogs (Reading & Field 1999)

Coronavirus antisera have been isolated in mink This virus apparently occurs m up to100% of mink kept on farms in Denmark The putative mink coronavirus (MCV) is similar instructure to transmissible gastroenteritis virus (TGEV) and porcine epidemic diarrhoea virus(PEDV) Young mink infected with MCV develop acute enteritis that may be related to"sticky kit disease" (Have et al 1992) However, sticky kits do not have any greaterprevalence of coronavirus, rotavirus, or calicivirus than normal kits, nor any difference inEschenchia coh communities (Jorgensen et al 1996)

Mink are susceptible to avian influenza-A virus (subtype HI0), particularly strain mink/84(H10N4), which causes pneumonia, lower rates of weight gain, and higher rates of viralexpression (Englund & af Segerstad 1998) In fact, mink lung cells were more sensitive thanother more commonly used cells for rapid detection of influenza and other respiratory viruses(Huang & Turchek 2000) Ferrets are also susceptible to influenza A-virus (Buchman et al1995)

Infectious canine hepatitis (ICH) is caused by an adenovirus While canids are mostdirectly susceptible, mustelids including mink, skunks, and otters (Harris 1968, Addison etal 1987) can also contract the disease In foxes and skunks, progression of ICH is very rapidand clinical signs include convulsions and lethargy, rapidly progressing to coma after whichdeath follows in minutes to hours (Cabasso 1981) A screening exercise of 10 514 animalsfrom all over Russia for a viral haemorrhagic fever with renal syndrome, revealed that noneof four weasels was positive for the disease antigens (Tkachenko et al 1983)

BACTERIA

Stoats and, more commonly, ferrets are competent hosts for Mycobactenum bovis, whichcauses bovme tuberculosis (TB) in domestic stock, particularly cattle and red deer Cervuselaphus in New Zealand Clinical signs of TB m stoats have been recorded in the formerSoviet states (Lavrov 1944) but accurate diagnostic tools were not then available None of 33stoats examined in Britain between 1971 and 1986 tested positive forM bovis (MAFF 1987)In samples collected on farms during outbreaks of bovine TB in cattle in New Zealand, 1 of62 stoats (1 6%) and 98 of 548 ferrets (17 9%) exhibited tuberculous lesions (Ragg et al1995a) Ferrets infected with M bovis most commonly had lesions in the mesentenc (35%),retrophasyngeal (17%), and prescapular (16%) lymph nodes (Ragg et al 1995b)Mycobactenum avium paratuberculosis has been isolated in stoats in Scotland In cattle, thisorganism causes Johne's disease which is a chronic and frequently lethal enteritis In stoats,the effects are presently unknown (Beard et al 1999) but in ferrets M avium causesgranulomatous enteritis (Schultheiss & Dolginow 1994) In a sample of 44 stoats collected inBritain, 5 exhibited pulmonary granulomatous inflammation associated with bacterial infection(McDonald et al unpubl data)

Stoats can apparently be resistant to tularaemia, but the disease is thought to causesignificant mortality (Lavrov 1944) The susceptibility of stoats to the disease was demonstratedby experimental infection (Lavrov 1944), though it is found naturally in a range of othermustelids (Bell & Reilly 1981) It is caused by Francisella tularensis and can be transmittedby arthropod vectors, mainly rabbit fleas, or through water, hence it is also known as a"swamp fever" (Addison et al 1987) In carnivores, including weasels, the disease presumablyoriginates from eating infected prey, although the potential role of arthropod vectors is not


clear. It causes a plague-like acute febrile infection frequently leading to rapid mortality. Thedisease has not been recorded in Britain (Bell & Reilly 1981), hence is unlikely to be presentin New Zealand stoats. It is transmissible to humans and so may present a significant risk incertain areas, either by contaminating water supplies or by arthropod transmission fromrodents to humans.

A sample of 33 of 45 stoats was shown to contain the DNA of Bartonella spp. (McDonaldet al. 2000), a genus of bacteria that infect red blood cells (Breitschwerdt & Kordick 2000).There is potential for the use of a possible stoat-specific strain of Bartonella as a vectororganism for control of stoats in New Zealand and this is the subject of current investigation.At least one species, Bartonella henselae, has previously been found in domestic cats in NewZealand (Joseph et al. 1997). From the same sample of stoats collected in Britain, 10 of 45were shown to contain the DNA of Borrelia burgdorferi sensu lato, the organism that causesLyme disease in humans (McDonald et al. 2000). However, this organism is unlikely to beuseful because of its status as a significant human pathogen.

Pseudotuberculosis or yersiniosis, caused by Yersinia pseudotuberculosis, has similarclinical signs to tularaemia and plague. The disease has been reported in American martensMartes americana, mink, and otters but not stoats (Wetzler 1981). The disease is transmittedvia the oral-faecal route, although again arthropod vectors may have a role in transmission.While there have been acute and highly fatal outbreaks in laboratory animals, no epizooticsin wild animals have been reported (Wetzler 1981). Plague, caused by infections of Yersiniapestis, has been detected in captive colonies of black-footed ferrets. This disease may have arole in controlling wild black-footed ferret populations given the high prevalence of Yersiniaamong wild prairie dog populations (Williams et al. 1994; Dyer & Huffman 1999). However,earlier attempts to inoculate ferrets and Siberian polecats with Yersinia pestis failed to induceclinical signs of the disease (Williams et al. 1991). American badgers Taxidea taxus are alsosusceptible to bubonic plague (Dyer & Huffman 1999). In North Dakota, the black-tailedprairie dog Cynomys ludovicianus is the most likely primary reservoir of bubonic plague. Ahighly contagious plague-like disease was recorded in 1940 among stoats in Kazakhstan.Symptoms included the rapid onset of lethargy, loss of appetite, nasal discharges, convulsions,and paralysis, followed within 24 h by death (Lavrov 1956). Unfortunately, no diagnosis ofthis condition was made at the time, though it was thought to resemble symptoms of bubonicplague in sable Martes zibellina.

Erysipelothrix rhusiopathiae has been recorded in a number of mustelids, includingSiberian polecats, American mink, sable Martes zibellina, and kolinskys Mustela (sibirica)itatsi. American mink were apparently resistant to the affects of the bacterium. However, inother hosts it causes a septicaemia of varying severity. Transmission occurs by consuminginfected prey (Wood & Shuman 1981). Between 1 of 3 (Twigg et al. 1968) and 1 of 8 weasels(Michna & Campbell 1970) sampled in Britain were positive for Leptospira spp. whichcauses leptospirosis or Weil's disease in humans. However, none of 9 stoats, 9 ferrets, or 4weasels was serologically positive for Leptospira in New Zealand (Hathaway & Blackmore1981). Pasteurella multocida, which causes a haemorrhagic septicaemia in domestic stock,has been isolated from weasels Mustela spp. (Rosen 1981).

The prevalence of gut bacteria (anaerobes, aerobes, and staphylococci, but notenterobacteriaceae) increases with age in farmed mink, but the total incidence was much lowerthan for other mammals, perhaps because of the rapid passage of food through the gut.Campylobacler was rare and Salmonella and Shigella were not detected (Williams et al.1998). Specific strains of Staphylococcus interniedius have been identified in mink (Hesselbarth& Schwarz 1995). This bacterium has been identified as the cause of acute adenitis of thecervical apocrine glands in neonatal farmed mink and of vaginitis and mastitis in adult females


(Hunter & Prescott 1991, Schneider & Hunter 1993a) Infections of Staphylococcus may alsobe linked to urohthiasis (disease caused by kidney stones) (Zimmermann & Witte 1988)

Ferrets are often host to Hehcobacter mustelae and so they are a common model forstudies of H pylori infection in humans In humans H pylori causes stomach ulcers ofvarying seventy In ferrets, the infection is associated with gastric lymphoma in the pyloncantrum with characteristic epithelial lesions (Erdman et al 1997) and adenocarcinoma (Foxet al 1997) H mustelae has been isolated from wild and captive ferrets in New Zealand(Forester et al 2000) and so this organism holds potential as a vector for biological control offerrets, and possibly stoats, in New Zealand

A Siberian polecat in captivity failed to reproduce after mating The uterus was infectedwith heavy growth of Enterococcus faecahs causing reproductive failure by filling the uteruswith purulent material, a condition known as pyometra The strain of Enterococcus isolatedfrom this polecat was resistant to a wide range of standard antibiotics (Johnson et al 1999)

PROTOZOA

We are not aware of any specific investigations of protozoan infestations in stoats Pneumocystisspp can cause pneumonia in a range of hosts that may include stoats Pneumocystis is avariable organism that may live in several host-specific "special forms" (Wakefield 1998)There are pronounced genetic differences between these forms (Stringer & Cushion 1998)Two of 46 least weasels in Finland were host to Pneumocystis cannn (Laakkonen et al1998) High levels of mortality in farmed mink are caused by initial infections of Pneumocystiscannn and by secondary infection of CDV or other viruses (Dyer & Schamber 1999)

In a sample of 29 wild mink from Kansas and Missouri, 66% had antibodies to Toxoplasmagondii, a protozoan that causes abortion in sheep but rarely causes clinical disease in its mainhosts (Smith & Frenkel 1995) Similarly, in Ireland, 7 of 15 wild mink caught in Ireland hadpositive titres for Toxoplasma gondii, although no cysts were detected (O'Crowley & Wilson1991) Although the prevalence of Toxoplasma in farmed mink was low (3%), the largenumbers handled in a typical farm present a substantial risk to mink farmers and furriers(Hennksen et al 1994)

Two species of Eimena and other Coccidian oocysts were detected in black-footed ferretsthat had died from canine distemper (Williams et al 1988) Various life stages of Eimenaspp were located in the epithelium of the bile ducts and gallbladder of a ferret (Williams etal 1996b) Eimena ictidea and E furoms were located in the faeces and intestinal contents ofwild and captive black-footed ferrets (Jolley et al 1994) Giardia spp were also found inblack footed ferrets (Jolley et al 1994) Infection by the Coccidian Cryptospondium sppfrom goats caused several fatalities in a colony of captive ferrets (Gomez Villamandos et al1995) Captive stone martens shed Cryptospondium, probably C parvum, oocysts duringtemporary diarrhoea episodes (Rademacher et al 1999)

Mink are susceptible to infection by Sarcocystis causing muscular sarcocysts Infection intwo mink was associated with meningoencephahtis and meningomyehtis (Ramos Vara et al1997) Four of 42 (10%) American martens collected in Washington were host to Sarcocystisspp (Foreyt & Lagerquist 1993) In a sample of 70 wild-caught Japanese martens Martesmelampus, 67 (96%) were host to schizonts or gametocytes of Hepatozoon spp that causednodular lesions, most commonly in the heart (Yanai et al 1995) Antibodies to Encephalitozoontunicuh that causes abdominal distension, paralysis, and other symptoms similar to rabieswere found to be widespread in mammals, including mink, in Iceland (Hersteinsson et al1993) In farmed mink, Encephalitozoon causes cataracts and renal lesions (Zhou et al1992) A sample of mustehds including stoats, long-tailed weasels, and ferrets were foundnot to host Neospora caninum, which causes abortion in cattle (McAllister et al 1999)


FUNGIStoats are not thought to host dermatophytes, e.g., Trichophyton, which cause ringworm,though only small numbers of samples were tested (English 1969). In Finland, 21 of 46(46%) least weasels were infected by adiaspores of Chryosparium spp. This fungal infectioncaused granulomas around the adiaspores (Laakkonen et al. 1998). A case of fataladiaspiromycosis has been recorded in a wild British otter (Simpson & Gavier-Widen 2000)and the condition is relatively common among British badgers but is not thought clinicallysignificant (Gallagher & Nelson 1979).

HELMINTHSThe best known parasite of stoats is the nematode Skrjabingylus nasicola, which appears toinfect all species of Mustela (Dougherty & Hall 1955). The parasite is common throughoutthe Holarctic and elsewhere including New Zealand (King 1974). It causes skull deformityby eroding bones of nasal sinuses, presumably leading to pressure on the brain. Rates ofinfestation in stoats range from 17 to 31% in Britain (Lewis 1967; van Soest et al. 1972), upto 50% in Ireland (Sleeman 1988), but can be as much as 100% in North America (Dougherty& Hall 1955; Jennings et al. 1982). In New Zealand, there is an average overall prevalence of10% infestation (King & Moody 1982), ranging from 0 to 37%. The obligate intermediatehosts of Skrjabingylus are terrestrial snails and the paratenic hosts were once thought to beshrews (Hansson 1967), but shrews are rare in stoat diet and are absent from New Zealand.Invasive third-stage Skrjabingylus larvae have been found encapsulated in Apodemus, whichreadily eat molluscs both in the wild and in captivity, and these larvae can experimentallyinfect stoats (Weber & Mermod 1985). Heavy infestations of Skrjabingylus were believed toadversely affect skull size on Terschelling Island in the Netherlands (van Soest et al. 1972)and density, fertility, and body weight in Russia (Popov 1943; Lavrov 1944). However, nostunting of infested individuals was detected in a sample of 1492 stoats examined in NewZealand (King & Moody 1982). Skrjabingylus may induce fits or spasms and has beenassociated with "dancing" behaviour or playing dead under stress (King 1989). Skrjabingylusis not thought to cause significant mortality in stoat populations and so is not a likelycandidate for lethal control without modification in some respect. However, Skrjabingylusnasicola is sufficiently host-specific that it could be used as a vector for biocontrol agents,perhaps by genetic modification.

Of 22 stoats from Washington, 41% were infected by one or more of five helminthspecies: Taenia mustelae, Alaria mustelae, Molineus patens, M. mustelae, and Trichinellaspiralis (Hoberg et al. 1990). The trematode fluke Troglotrema acutum has been identified inSwedish stoats, but it appears to be rare (Hansson 1968). A survey of common helminths ina Russian sample of stoats included: nematodes, Capillaria putorii, Molineus patens, andStrongyloides martis; cestodes Taenia tenuicollis and Mesocestoides lineatus; and, rarely,Acanthocephala Acanthocephalus spp. (Lavrov 1944). Filaroides martis has been recordedin a single adult male stoat in New Zealand (McKenna et al. 1996). Guinea worm Dracunculushas also been recorded in stoats and other mustelids in Canada (Crichton & Beverly-Burton1974). Of 40 stoats screened in Canada, Taenia mustelae was identified in 8 (20%), Capillarialarvae in 16 (40%), and Aelurostrongylus pridhami in 5 (13%) (Jennings et al. 1982). In asample of 44 stoats collected in Britain, inflammatory responses associated with nematodeparasitism were detected in the intestines of 6 animals and in the lungs of 5 (McDonald et al.unpubl. data).

The cestode tapeworm Taenia mustelae has been recorded in weasels in Japan (Iwaki et al.1995) and in black-footed ferrets (Rockett et al. 1990). In weasels, ingested larvae of the


spiroroid nematode worm Gnathostoma nipponicum migrate from the stomach to musclewhere they enlarge and mature into adult worms The adults invade the oesophageal wall andcause tumours (Ando et al 1994) Another spiroroid nematode Physaloptera sp was found inblack-footed ferrets (Jolley et al 1994) In ferrets, infection by gut parasites such as thenematode Tnchinella spirahs can lead to changes in neuromuscular function, particularlymuscle contractabihty and gut neurotransmission, that persist after the symptoms of infectionhave cleared up (Venkova et al 1999)

Filaroides infections have been detected in two feral ferrets m New Zealand (McKenna etal 1996) Ferrets are susceptible to infection by Dirofdana (McCall 1998) and are acompetent host for Dracunculus insigms (Eberhard et al 1988) Wild polecats are host to thetrematode fluke Troglotrema acutum, which infects nasal sinuses and causes cranial lesionssimilar to Skrjabingylus (Artois et al 1982)

Of 259 stone martens in Germany screened for helminth infection, 87% were host toCapillana putoni, 27% to Taenia martis, and 10 4% to Mohneus europaeus Toxocara spplarvae were detected in only seven individuals None was infected by Skrjabingylus (Schoo etal 1994), though Skrjabingylus petrovi has been identified as the principal species ofSkrjabingylus infecting Martes spp in France (Gerard & Barrat 1986) and in Sweden(Hansson 1968) Japanese martens were host to Mesocestoides paucitesticulus (Sato et al1999a) and to Aonchotheca putoni in the stomach, Concinnum in the pancreatic duct, andMohneus and Euryhelmis costancensis in the small intestine Eucoleus aerophdus andSobohphyme batunni have also been identified in Japanese martens (Sato et al 1999b) Of42 American martens, 36 (86%) were host to Capillana putoni in their stomachs, 14 (33%)had Mesocestoides hneatus in the small intestine, and 2 (5%) had Tnchinella spirahs in thetongue Unusually, the prevalence of Mesocestoides hneatus was significantly higher injuveniles than in adults (Foreyt & Lagerquist 1993)

The cestode Dioctophyma renale has been lecorded in mink (Wren et al 1986) Recordednematodes include Bayhascans devosti, Capillana mucronata, Euparyphium melis, Filaroidesmartis, Spirometra ennacei (Sidorovich & Savcenko 1992, Dunstone 1993), and Skrjabingylusnasicola (Hansson 1967) In 50 mink from Illinois, the following helminths were recordedFilaroides martis (62%), Capillana putoni (34%), Paragommus kelhcotti (14%), Dirofilanaimmitis (2%), and Mohneus sp (2%) (Zabiega 1996) North American river otters hostnumerous endoparasites including cestodes (Greer 1955), nematodes, trematodes, andacanthocephalans (Hoover et al 1984, Hoberg et al 1997)

ECTOPARASITES

The specific louse Trichodectes (Stachiella) ermineae has been recorded on stoats in Canada(Jennings et al 1982), Ireland (Sleeman 1989), and New Zealand (King 1975, 1990) Thereis also a specific flea Nearctopsylla brooksi that has been recorded in arctic Canada andnorthern Scandinavia, but apparently not in Britain or in New Zealand (Holland 1964, King1976) Stoats are host to several ectoparasites associated with their prey species and with nestparasites from species that are not eaten European records list a total of 26 flea species(Debrot & Mermod 1982) Rhadinopsylla pentacantha, an uncommon flea specific to volenests, Megabothris rectangulatus specific to voles, Orchopeas howardi specific to squirrels,and Spilopsyllus cuniculus specific to rabbits have all been recorded in Britain (King 1976,Mardon & Moors 1977) Ctenophthalamus nobihs, Dasypsyllus galhnulae, Nosopsyllusfasciatus, and S cuniculus have been recorded on stoats in Ireland (Sleeman 1989) InCanada, Monopsyllus vison infested 8 of 40 stoats from Newfoundland (Jennings et al 1982)In New Zealand, rat fleas N fasciatus make up 97% of records, but Leptopsylla segms,


Ceratophyllus gallinae, and Parapsyllus nestoris have also been recorded (King & Moody1982). Amphipsylla kuznetzovi and Ctenopsyllus bidentatus have been collected from stoatsin Kazakhstan (Lavrov 1944).

In Ireland, stoats are host to ticks Ixodes canisuga, I. hexagonus, and /. ricinus, liceMysidea picae and Polyplax spinulosa, and the mite Neotrombicula autumnalis (Sleeman1989). The mites Demodex erminae (Nutting et al. 1975), Listrophorus mustelae (Sweatman1962), Gymnolaelaps annectans (Tenquist & Charleston 2001), Hypoaspis nidicorva, andEulaelaps stabularis (King 1975) and the tick Haemaphysalis longicornis (King 1990;Tenquist & Charleston 2001) have also been recorded on stoats in New Zealand. Canadianstoats are host to the mites Laelaps multispinosus and Androlaelaps fahrenholzi (Jennings etal. 1982). Symptoms of mange caused by the mites Sarcoptes spp. and Demodex spp. havealso been recorded in stoats (Lavrov 1956). In Canada, mink, martens, and weasels wererecently found to host a new species of tick Ixodes (Pholeoixodes) gregsoni sp. nov.(Lindquist et al. 1999).

Weasels are host to a specific louse Trichodectes mustelae, a specific itch mite Psorergatesmustelae, the follicle mite Demodex spp., the tick Haemaphysalis longicornis (Tenquist &Charleston 2001), and numerous flea species (King 1976; Mardon & Moors 1977). Of 1391mink sampled in England and Wales, Ixodes hexagonus and /. canisuga were found on 40%and 2.5%, respectively. Ixodes ricinus and /. acuminatus were also found. Infestation rateswere lower in summer and male mink had more nymphs than females (Page & Langton1996). Mink fleas include Ctenophthalmus, Megabothris, Malareus, Nosopsyllus, Paleopsylla,and Typhloceras (Fairley 1980; Chanin 1983). Domestic and feral ferrets are susceptible tosubcutaneous infection by Demodex spp. mites causing alopecia and pruritus, the classicalsymptoms of demodecosis (Noli et al. 1996; Tenquist & Charleston 2001). Sarcoptic mangecaused by Sarcoptes scabiei has also been recorded in domestic and feral ferrets (Phillips etal. 1987; Tenquist & Charleston 2001). The tick Haemaphysalis longicornis (R. A. McDonald& A. Heath pers. obs.), the fur mite Listrophorus mustelae, and the ear canker mite Otodectescynotis have been recorded in feral ferrets in New Zealand (Tenquist & Charleston 2001). O.cynotis was also recorded in wolverines Gulo gulo (Wilson & Zarnke 1985). Ectoparasites ofNorth American river otters include ticks (Eley 1977; Serfass et al. 1992), the louseLatagophthirus rauschi (Kim & Emerson 1974), and the flea Oropsylla arctomys (Serfass etal. 1992).

OTHER DISEASES

Neoplasia

There have been no screening studies for neoplasia (tumour growth) in stoats, though there isan extensive literature on neoplasia in laboratory and domestic ferrets. In a 30-year study,574 of 4774 (12%) ferrets had 639 tumours of various types distributed through all organsystems. The most common tumours were pancreatic islet cell (22%), adrenocortical cell(17%), and lymphoma (12%). Affected animals ranged in age from 1 month to 15 years, butincidence was highest in animals 4-7 years old. No sex bias was detected, though spayedanimals were more likely to have tumours (Li et al. 1998). Of 57 ferrets diagnosed withhaving pancreatic islet cell tumours, 34 had only pancreatic carcinoma, while 23 had bothcarcinoma and hyperplasia or adenoma. Despite treatment, long-term survival of ferrets withpancreatic carcinoma was low since the tumours were malignant (Caplan et al. 1996).

A single spayed female ferret was found to have epitheliotropic lymphoma leading to arange of skin disorders and other conditions, including renal disease, though this may havebeen the result of inappropriate treatment with corticosteroids (Rosenbaum et al. 1996). Two


5-year-old ferrets had malignant lymphoma resulting in a cranial mediastinal mass of thymiccells, leading to a diagnosis of thyoma (Taylor & Carpenter 1995) Synovial sarcoma hasbeen recorded in a laboratory ferret (Lloyd & Wood 1996) Domestic ferrets are occasionallysusceptible to prostatitis consisting of prostatic cysts and/or prostatic squamous metaplasiaThe disease was commonly associated with prohferative adrenal lesions and adrenal gland-associated endocnnopathy (Coleman et al 1998) Five of six ferrets with urogemtal cysts hadadrenocortical hyperplasia or neoplasm (Li et al 1996) A ferret was diagnosed with Hodgkinslike lymphoma involving lung, liver, kidneys, and lymph nodes As in humans, the diseasewas associated with eosinophihc granulomas caused by abnormal proliferation of T-lymphocytes (Blomme et al 1999)

Perhaps significantly from the perspective of biocontrol, inoculation of ferrets with non-cellular extracts from ferrets suffering from lymphoma caused the inoculated ferrets todevelop lymphoma Evidence of reverse transcnptase activity in inoculated animals suggestedretrovirus activity Therefore, viral agents may have a role in horizontal transmission ofinfectious lymphomas (Erdman et al 1995) This led to the idea that high incidences oflymphoma and leukaemia in cohabiting ferrets may be associated with infection by ADV orFPV However, the incidence of these viruses in 35 ferrets living in three groups with 21cases of lymphoma was not significantly different from the incidence in 52 ferrets living inthree groups with no lymphoma Therefore, these agents could not be shown to be involvedin the disease, though other viruses may be (Erdman et al 1996) Similar inconclusive resultsarose from another study of a cluster of cases of juvenile mediastinal lymphoma in ferrets(Batchelder et al 1996)

Nursing sickness

Nursing sickness is a major cause of mortality among breeding female mink It caused up to14% total mortality among 1774 lactating females in Denmark (Clausen et al 1992) In asample of 48 farms in Ontario that experienced mortality rates of 0 2-10 1% during thelactation period, nursing sickness was diagnosed in 56% of mortalities (Schneider & Hunter1993a)

Increasing risk of nursing sickness is related to increasing litter size and not to the age ofthe mother (Schneider et al 1992) Sick mothers have significantly larger litters (5 4 kits)than healthy mothers (5 0 kits) However, age, litter size, and female weight loss were allmajor determinants of the risk of nursing sickness In the last 2 weeks of lactation, healthyfemales lost about 14% of their body weight, whereas sick females lost about 31% Sickfemales exhibited signs of advanced dehydration, emaciation, and other indicators olprogressive catabohsm (Clausen et al 1992) In the advanced stage of the disease, coma anddeath appear to be the inevitable outcome of the strain of continuing milk production(Wambergetal 1992)

Nursing sickness appears to be alleviated by supplementary salt However, it is unknownwhether salt is actively involved in avoiding the disease or whether it acts as a dietarystimulant that can help prevent starvation (Clausen et al 1996) Recent studies indicate thatlow sodium is a symptom rather than a cause of the disorder (Hansen et al 1996) The variousclinical signs of nursing sickness are essentially metabolic indicators of energy stress Thedisease results in lethargy, emaciation, dehydration, and other symptoms including eventualmortality of lactating female mink However, there are few indications of the cause of thedisease Viral screening has proven negative for ADV Campylobacter was isolated fromaffected individuals, but controls were also positive The basic cause of the disease appears tobe the energy demands placed on the mother by lactation Even in normal cases, the energydemands during lactation in female mink are very high Despite increases in energy intake


during the early weeks of lactation, females lose body weight by using up their body reserves.This is especially true in the final part of the lactation period, when they have reached amaximum of energy intake (Hansen 1999). Nursing sickness appears particularly prevalentin farmed mink because of the large numbers of females observed and the susceptibility of aminority of individuals (Schneider & Hunter 1993b).

Pregnancy toxaemia is a common cause of mortality among pregnant female ferrets andtheir young. Clinical signs of the disease include anaemia, hypoproteinaemia, azotaemia,hypocalcaemia, hyperbilirubinaemia, and high liver enzyme activities. Hepatic lipidosis wasobserved during histological examination. Causes of the disease are unclear, though theymay be related to nutrition and stress (Batchelder et al. 1999).

Transmissible encephalopathy

Mink are susceptible to a rare transmissible mink encephalopathy (TME) that is similar toother conditions that cause progressive neurological disease, such as scrapie in sheep(McKenzie et al. 1996). Feeding mink with material infected with transmissible spongiformencephalopathy (TSE) agents was the cause of original infection (McKenzie et al. 1996). Themost likely source was feed derived from cattle infected with bovine spongiformencephalopathy (BSE) (Robinson et al. 1994) and not, as originally thought, sheep infectedwith scrapie (McKenzie et al. 1996). TME is transmitted comparatively easily to a range ofalternative hosts (McKenzie et al. 1996), but not to ferrets (Bartz et al. 1994). A range ofTME agents can also be used to re-infect cattle with more serious results than back-passageinfections of cattle with scrapie agents (Robinson et al. 1995).

Ferrets are also susceptible to chronic wasting disease (CWD), another form of TSE.Clinical signs include spongiform degeneration of brain tissue and reactive astrocytosis.Experimental inoculation of ferrets led to shortening of incubation periods after severalpassages. Unlike the original CWD isolated from deer, the ferret form was transmissible torodents (Bartz et al. 1998).

DISCUSSION

While the incidence of various pathogens in stoats is quite well described, information on theeffects of disease on stoat populations is very sparse. Although Lavrov (1941) asserted thatdeclines in ermine harvests were brought about by a combination of infectious and helminthdiseases, there is little contemporary evidence to support this view. Regrettably, the presentlack of an economic incentive to assist the fur industry by understanding stoat populationdynamics appears to have halted most relevant work in the former Soviet Union. Of themajor diseases reviewed here, several are clearly unacceptable as agents of biocontrol in NewZealand because of conflicts with the health of humans, domestic, and companion animals.These include rabies, Aujeszky's disease, Lyme disease, tuberculosis, plague, and leptospirosis.There is insufficient information about the epidemiology of nursing sickness, transmissibleencephalopathy, and fungal and protozoan parasites to advocate novel research directed atstoats. However, future work on mustelids in which there is still an economic or humanhealth interest may reveal possibilities for stoat control and this research should be closelymonitored.

Disease agents that are already enzootic in wild stoats in New Zealand, or in their ancestralpopulation in Great Britain, do not appear to limit stoat populations. Nonetheless, theirpotential role in a biological control programme should not be underestimated. If bacteria,viruses, or parasites are already widespread in wild stoats, they may be used as vectors for thetransmission of other forms of biological control, including fertility control agents. Therefore,we advocate a more detailed study of already widespread disease agents, particularly those


such as Bartonella spp and Hehcobacter mustelae, that tend to be host-specific Thetransmission and epidemiology of Skrjabingylus nasicola has already been studied in somedetail but existing knowledge is far from exhaustive The ecology of the louse Tnchodecteseimineae and the mites Demodex erminae and Listrophorus mustelae are also, to ourknowledge, largely unknown and warrant further investigation

Of the pathogens described here, viruses hold the greatest promise for the lethal control ofstoats Canine distemper is unlikely to be socially acceptable as an agent of lethal control ofstoats, since it is a serious pathogen of domestic dogs However, distemper can be induced incertain mustelids by exposure to vaccines produced for domestic animals (Williams et al1996a, Sutherland-Smith et al 1997) This suggests that vaccines routinely used in dogs inNew Zealand could potentially be used to induce the disease in wild stoats Althoughdescribed mamly in mink, ADV and MEV appear likely to be transmissible to stoats andearly trials of the pathogemcity of these viruses to stoats would be very useful The dangersof using these agents lie primarily in transmission to non-target species, either wild species ofconservation concern or domestic and companion animals Therefore, an extensive andinevitably costly programme of research into the stability and specificity of disease agents isunavoidable

Of these viral diseases, ADV perhaps holds greatest promise as an agent for biologicalcontrol of stoats In adult mustelids, ADV results in long-term chronic disease of the immunesystem and in breeding females reduces fertility by approximately 50% In contrast, ADV inyoung animals results in acute pneumonia with rapid mortality There is also evidence forvertical transmission in utero Under field conditions, the variable pattern of mortalityinduced by ADV might reasonably be expected to reduce adult stoat populations in themedium term, markedly reduce female fertility, and rapidly increase postnatal juvenilemortality Depending on the epidemiology of the disease, such a combination of effects couldsubstantially reduce stoat populations in the long term In contrast to ADV, MEV results inrapidly propagating acute enteritis causing rapid mortality of adults and young with similarsymptoms The rapid propagation of this disease suggests that in unstable populations ofstoats repeated inoculations may be required, since contact rates might be reduced so rapidlyby acute disease that the virus would not be self-sustaining in the wild MEV may be a short-term remedy to the marked irruption of stoat populations and the localised distribution ofbirds for which protection is needed Furthermore, MEV appears to survive well under fieldconditions, hence it could be dispensed easily and effectively in the long term, perhaps byremote equipment

It is unlikely that any single approach to the control of stoats will resolve the problemscaused by stoat predation in New Zealand The extreme rarity of certain important birds, suchas kiwi Apteryx spp , kokako Callaeas cinerea, takahe Porphyrw hochstetteri, and mohuaMohoua ochrocephalus, means that serious losses can be the result of occasional predationevents perpetrated by very small numbers of stoats Therefore, the combined approach oftraditional technology, namely trapping and poisoning, together with disease and fertilitycontrol is most likely to ensure the future status of New Zealand's birds Indeed, Norbury(2000) has emphasised that the cumulative effect of simultaneous deployment of fertilitycontrol and disease agents could enhance control efforts by more than the sum of theireffects

The cost of developing novel biotechnology means that it is essential to understand thelikely effects that new control agents would have, not just on stoat populations but, moreimportantly, on the survival of birds In the first instance, simulation modelling exercisesshould be undertaken to examine the effects of a range of control strategies on stoatpopulations and on the bird species they eat (Courchamp & Sugihara 1999) Unfortunately,


the primary data on stoat population biology and the relationships between stoat density andbird survival that are needed to inform such a modelling exercise are still fairly limited Thepeculiarities of certain New Zealand ecosystems, for example, fluctuating populations ofstoats and their prey resulting from southern beech Nothofagus spp mast cycles, will presenta particular challenge to modellers Valuable analyses of the high degree of temporalvariability in stoat fertility and mortality in beech forests have been provided by King &McMillan (1982), King & Moody (1982), and Powell & King (1997) and in podocarp forestby King et al (1996) However, these studies are unpaialleled in other New Zealandecological communities Therefore, there is a clear need for further research on the populationbiology of stoats and their prey These should focus on variation in fertility, mortality, thespatial ecology of stoats, and on the relationship between these factors and the survival ofthreatened birds

ACKNOWLEDGMENTS

This work formed part of Research Investigation 3358 commissioned by the New Zealand Departmentof Conservation R McDonald was supported by a Royal Society Postdoctoral Fellowship, hosted bythe Centre for Biodiversity and Ecology Research, University of Waikato Thanks to M Artois, KFrolich, M Jackson, and C King tor providing reprints, to M Day, R Delahay, and E Murphy foradvice, and to G Norbury and two anonymous referees for their valuable reviews

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R00026 Received 5 September 2000 accepted 1 March 2001

diseases and pathogens of mustela spp., with special ...phthiraptera.info/publications/46969.pdf ·...

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