Medical Entomoly

Medical Entomology

Medical Entomology A study of the arthropods of medical importance is known as medical entomology.

Arthropods: Arthropods are invertebrates with jointed legs.

Class:Insecta

Class:Arachnida

Class:Crustacea

1. Mosquitoes:AnoplelinesCuilicines2. Flies:HousefliesTsetsefliesSandfliesBlackflies3. Human Lice:Head and body lice,Crab lice4.Fleas:RatfleasSand fleas5.Reduvidbugs

1. TicksHard ticksSoft ticks2. Mites:Leptotrombiculidandtrombiculidmites,Itch mite

Cyclops

Arthropods of Medical Importance

Distinctive characters of arthropods of medical importance

Insecta

Arachnida

Crustacea

Body divisions

2. Legs

3. Antennae

4. Wings

5. Where found

Head, Thorax, Abdomen

Pairs

1pair

One or two pairs; Some are wingless

On land

Cephalothoraxand Abdomen

Pairs

None

None

On land

Cephalothoraxand Abdomen

Pairs

2 pairs

None

In water

Arthropod-borne diseases

Arthropod

Diseasestransmitted

1. Mosquito

Malaria,filaria, viral encephalitis, viral fevers (e.g. Dengue, West Nile), viral haemorrhagic fever (e.g.Yellow fever, dengue haemorrhagic fever)

2.Housefly

Typhoid and paratyphoid fever, diarrhoea, dysentery,cholera, gastro-enteritis,amoebiasis,helminthicinfestations, poliomyelitis, conjunctivitis, trachoma,antrax, yaws etc.

3.Sandfly

Kala-azar,oriental sore,sandflyfever,orayafever

4. Tsetse fly

Sleeping sickness

5.Louse

Epidemic typhus, relapsing fever, trench fever,pediculosis

Arthropod

DiseasesTrnsmitted

6. Rat flea

Bubonic plague, endemic typhus,chiggerosis.

7.Blackfly

Onchocerciasis

8.Reduvidbug

Chagasdisease

9. Hard tick

Tick typhus, viral encephalitis, viral fevers,viralhaemorragicfever,tularemia, tick paralysis.

10. Soft tick

Q fever, relapsing fever.

11. Itch mite

Scabies

12. Cyclops

Guinea-wormdisease, fish tape worm

13. Cockroaches

Enteric pathogens

Terminology

Vector- An arthropod or other invertebrate which transmits infection by inoculation into or through the skin or mucous membrane by biting, or by deposit of infective materials on the skin or on food or other objects.Definitive host- The host in which the sexual cycle of the agent occurs is called the definitive host. e.g. Mosquito is the definitive host in malaria.

Intermediate host- The host in which the asexual cycle of the agent occurs is called the intermediate host. e.g. Mosquito in filaria, cyclops in guinea-worm disease.Infestation- Infestation is meant the lodgement, develpoment and reproduction of arthropods on the surface of the body or in the clothing. e.g. Louse infestation.

Transmission of Arthropod-borne diseases: There are three types of transmission cycles which are involved in the spread of arthropod-borne diseases:

(1) Direct Contact- directly transferred from man to man through close contact. e.g. scabies and pediculosis.

(2) Mechanical Transmission-diarrhoea, dysentery, typhoid, food poisoning by housefly through their wings, legs.

(3) Biological Transmission- when the disease agent multiplies or undergoes some developmental change with or without multiplication in the arthropod host.

Biological transmission may be of three types: a) Propagative no cyclical changes of disease agent only multiplies in the body of the vector

Example- Plague bacilli rat fleas.

b) Cyclo-propagative disease agent undergoes cyclical changes multiplication occurs Example- malaria parasite in anopheline mosquito.

c) Cyclo-developmental disease agent undergoes cyclical changes but multiplication does not occur

Example- filarial parasite in culex mosquito and guineaworm embryo in cyclops.

Principles of arthropod control

The general principles of arthropod control are: Environmental control Chemical control Biological control Genetic control

Environmental control:elimination of breeding placesfilling and drainage operationcarefully planned water managementprovision of piped water supplyproper disposal of refuse and other wastescleanliness in and around houses

Chemical control: A wide range of insecticides belonging to the organochlorine, organophosphorus and carbamate groups of compounds are available for vector controlBiological control: The use of larvivorous fish (Gambusia) is well known in mosquito control.

Genetic control: Different techniques can help in genetic control of mosquito. Such as sterile male technique, cytoplasmic incompatibility and chromosomal translocations.Integrated approach: No single method of control is likely to provide a solution in all situations. Integrated approach for vector control combining two or more methods with a view to obtain maximum results with the minimum effort and to avoid the excessive use of any one method.

Mosquito

The body of a mosquito consists of three parts:a) Head: semi-globular in outline and bears the following structures- i. A pair of large compound eyes ii. A long needle-like structure, called proboscis iii. A pair of palpi, situated on either side of proboscis iv. A pair of antennae.

b) Thorax: the thorax is large and rounded and bears- i. A pair of wings dorsally. The buzzing noise which the mosquitoes produce is due to the beating of their wings, and not to singing. ii. Three pairs of leg ventrally

c) Abdomen: The abdomen is long and narrow and composed of 10 segments, the last two of which are modified to form the external genitalia.

There are four stages in the life history of mosquitoes:1) Egg: Eggs are laid on the surface of water, 100-250 at a time. Under favourable conditions, the egg stage of mosquitoes lasts for 1-2 days.

2) Larva: The larva is a free swimming creature with an elongated body divisible into head, thorax and abdomen.It feeds on algae, bacteria and vegetable matter. The larva stage occupies 5-7 days.

3) Pupa: The pupa is coma shaped in appearance, with a large rounded cephalothorax and a narrow abdomen. The pupa represents the resting stage in the life history of the mosquito which lasts for 1-2 days.

4) Adult: When the development is complete; the pupal skin splits along back and the adult mosquito or imago emerges. Under favourable conditions of temperature and food supply the life cycle from egg to adult is complete in 7-10 days. Normally the adult mosquito lives for about 2 weeks.

Differentiation between anophelini and culicini

Tribe Genus

AnopheliniAnopheles

CuliciniCulex,Aedes,Mansonia

Eggs

Laid singlyEggs are boat-shaped, and provided with lateral floats

Laid in clusters or rafts, each raft containing 100-250 eggs (except-Aedes)Eggs are oval-shaped, and not provided with lateral floats

Larva

1)Rest parallel to water surface2) No siphon tube3) Palmate hairs present onabdominal segments

Suspended with head downwards at an angle to water surface.Siphon tube presentNo palmate hairs

Pupae

Siphon tube is broad and short

Siphon tube is long and narrow

Adults

When at rest, inclined at an angle to surfaceWings spottedPalpilong in both sexes

When at rest, the body exhibits a hunch backWings unspottedPalpishort in female

Anopheles Mosquito: Anopheles mosquitoes are characterized by dark and pale scale blocks arranged on their wings. Wings are spotted. For breeding they prefer clean, stagnant water. Anopheles phillipinensis- Plain area Anopheles minimus- mountainous area (Mymensingh) Anopheles sundicus- coastal region (Coxs bazaar) Anopheles ballabacensis- Hilly and forest (Ctg. Hilltrack, Sylhet) Anopheles annularis- Narayanganj Anopheles vagus- Kishoregonj.

Culex Mosquito: They are known as nuisance mosquitoes. An important member of this group is Culex fatigans, the vector of Bancroftian filariasis. It breeds profusely in dirty water collections. Eg- stagnant drains, septic tanks, burrow pits.

Aedes Mosquito: They are easily distinguished by white stripes on a black body. For this sometimes they are reffered as tiger mosquitoes. Important members of this group are : Aedes aegypti, Aedes albopictus. They breeds in artificial accumulations of water in and around human dwellings, such as water found in discarded tins, broken bottles, flower pots, coconut shells, earthen pots, tree holes

Habits of mosquitoes:1) Feeding habits: The males never bite. They subsist on plant juices. The females are haematophagous. They require a blood meal for the development of eggs.

2) Time of biting: In general mosquitoes bite in the evening or in the early part of the night.

3) Resting habits: Mosquitoes obscure themselves during the day in dark and cool corners. Some rest indoors (endophilia), and some outdoors (exophilia).

4) Breeding habits: The anophelines prefer clean water; the cuilicines prefer dirty and polluted water; the aedes prefer artificial collections of water.5) Hibernation: Mosquitoes are known to hibernate in the adult stage when the environmental conditions are not favourable.6) Dispersal: Mosquitoes do not generally fly far from the place where they bread unless swept by currents of wind.7) Life span: The life of a mosquito is influenced by temperature and humidity. The normal life span varies from 8 to 34 days.

Type of mosquito

Disease

1. Anopheles

Malaria,Filaria

2.Culex

Bancroftianfilariasis,japaneseencephalitis, West Nile fever, Viral arthritis

3.Aedes

Yellow fever, Dengue, Dengue haemorrhagic fever,Chikungunyafever

4.Mansonoides

Malayanfilariasis,Chikungunyafever

Mosquito born diseases:

Mosquito control measures: 1. Anti-larval measures a) Environmental control- source reduction b) Chemical control-mineral oils, paris green c) Biological control

2. Anti-adult measures a) Residual sprays- DDT b) Space sprays c) Genetic control

3. Protection against mosquito bites a) Mosquito net b) Screening c) Repellents

HousefliesMusca domestica, M. Vicinia, M. Nebula, M. Sorbens. General characteristics1) Head: A pair of antennae, a pair of large compound eyes and a retractile proboscis, which is adapted for sucking liquid foods.2) Thorax: A pair of wings, three pairs of legs.3) Abdomen: Abdomen is segmented.

Life history: a) Egg b) Larva c) Pupa d) Adult

Habits:1) Breeding habits: fresh horse manure, human excreta, manure of other animals, garbage, decaying fruits and vegetables, rubbish dumps containing organic matter.2) Feeding habits: The housefly does not bite. It is attracted to food by its sense of smell. It vomits on solid food to make a solution of it and sucks in a liquid state.

3) Restlessness: The fly is a restless insect. It moves back and forth between food and filth and thus spread infection mechanically.4) Vomit drop: The fly vomit frequently and spread diseases.5) Defecation: The housefly has the habit of defecating constantly all the day. Thus it deposits countless bacteria on exposed food.

6) Resting habits: Rest on vertical surfaces and hanging objects.7) Dispersal: Normally they remain close to their breeding places, but disperse frequently up to 4 miles.

Transmission of disease: Diseases: Typhoid and paratyphoid fevers, diarrhoeas and dysenteries, cholera and gastroenteritis, amoebiasis, helminthic infestations, poliomyelitis, conjunctivitis, trachoma, anthrax, yaws.

3) Defecation

Ways of transmission:

1) Mechanical transmission: Through their feet and hairy legs.

2) Vomit drop

Fly control measures-1) Environmental control- Elimination of their breeding places by, a) storing garbage, kitchen wastes and other refuse in bins with tight lids.

b) efficient collection, removal and disposal of refuse by incineration, composting or sanitary landfill. c) provision of sanitary latrines d) stopping open air defecation e) sanitary disposal of animal excreta

2) Insecticidal control- a) Residual sprays: DDT (5%), methoxychlor (5%), lindane or chlordane b) Baits c) Cords and ribbons d) Space spray e) Larvicides 3) Fly papers 4) Protection against flies 5) Health education

SandfliesSandflies are small insects, light or dark-brown in colour. They are smaller than mosquitoes. Their bodies and wings are densely clothed with hair.

General characteristics: a) Head-The head bears a pair of long, slender and hairy antennae, palpi and proboscis. Only the female bite. b) Thorax-The thorax bear a pair of wings and three pairs of legs. c) Abdomen- The abdomen has 10 segments and is covered with hair.

Special characteristics- Sandflies hop about, and do not fly.Habits: Sandflies are troublesome nocturnal pests. Their bite is irritating and painful. They take shelter during day in holes and crevices in walls, holes in trees, dark rooms, stables and store rooms. Sandflies are incapable of flying over long distances, they merely hop about from one place to another. They are generally confined to within 50 yards of their breeding places.

Diseases transmitted: Kala-azar, Sandfly fever, Oriental sore

Control of sandflies: 1. Insecticides: A single application of 1 to 2 g/m2 of DDT or 0.25 g/m2 of lindane has been found effective in reducing sandflies.

2. Sanitation: Removal of refuse and vegetation within 50 yards of human dwellings, filling up cracks and crevices in walls and floors, and location of cattle sheds and poultry houses at a fair distance from habitations.

Itch Mite The itch mite (Sarcoptes scabiei) is an extremely small, globular arthropod. The female parasite burrows into the epidermis where it breeds and causes the condition known as scabies. Mode of spread: a) Close contact: Scabies is usually transmitted by close contact with an infested person. This is often due to sleeping in the same bed or children playing with each other or nursing an infested person.

b) Contaminated clothes: The disease may be acquired sometimes from contaminated cloths and bed linen. Site of lesion: The disease classically affects the hands and wrist, the extensor aspect of elbow. The axilla, buttocks, lower abdomen, feet and ankles, palms in infants.

Diagnosis: a) The patient complains of itching which is worse at night b) Examination reveals follicular lesions at the affected site c) Other members of the household are affected d) Confirmation of the diagnosis may be made by searching for the parasite in the skin debris under microscope.

Control of scabies: 1. Benzyl benzoate: Before commencing the treamtent the patient is given a good scrub with soap and hot water. Benzyl benzoate should be applied to every inch of the body below the chin including the soles of the feet and allowed to dry. In case of babies, the head must also be treated. The application should be repeated after 12 hrs and after a further 12 hrs a bath given and all cloths and sheets changed and washed.

2. HCH (hexachlorocyclo-hexane) 3. Tetmosol 4. Sulphur ointment

Insecticides Insecticides are substances which are used to kill insects. The word pesticides is a general term that includes insecticides, fungicides, rodenticides, herbicides, disinfectants, repellents etc.

Insecticides are classified into three groups- a) Contact poisons-are those which kills insects primarily by contact, e.g- pyrethrum, DDT,HCH, dieldrin. b) Stomach poisons-are those which when ingested cause the death of the insects.e.g. Paris green, sodium fluride.c) Fumigants-are those which give off vapours which have a lethal effect on the insects, e.g. Sulphur dioxide.

Another classification may be: Group i Organochlorine compounds: DDT, HCH, dieldrin, chlordane, methoxychlor etc. Group ii Organophosphorous compounds: malathion, fenthion, abate etc. Group iii Carbamates: Propoxur, carbaryl.

Natural

Synthetic

PyrethrumMineral oils

Paris greenSodiumFluoride

Hydrogen cyanideSulphur dioxideCarbon disulphate

Organo-ChlorineCompounds-DDTMethoxychlorHCHLindaneChlordaneDieldrinHeptachlor

Organo-PhosphorusCompounds-ChlorthionDiazinonMalathionFenthionParathionMethyl ParathionAbate

Carbamates-CarbarylPropoxur