Medical arid Vewitiary Entomology (1992) 6 , 16-22

Relations between Anopheles darlingi breeding habitats, rainfall, river level and malaria transmission rates in the rain forest of Suriname J . A . R O Z E N D A A L Medical Mission, Paramaribo, Suriname

Abstract. Irregular peaks of P. falciparum malaria incidence were monitored in Suriname during 1982-86. An attempt is made to correlate these fluctuations of malaria rates with rainfall and river level in relation to the periodic availability of breeding habitats for the malaria vector Anopheles darlingi Root.

Densities of adult female An. darlingi were recorded in the Upper-Marowijne region of Suriname, in a focus of malaria along the Marowijne River. Peaks in biting densities correlated well with periods of (i) high water level in the long rainy season, (ii) low water level in the long dry season, and (iii) abundant rainfall in the short rainy season. Critical levels of river-height and rainfall were defined which could explain most of the monthly fluctuations in malaria parasite incidence observed in this area.

Key words. Anopheles darlingi, malaria epidemiology, Plasmodium falcipurum, river level, rainfall, rainforest, Suriname.

Introduction

In the interior of Suriname, the main focus of malaria is situated near the border with French Guyana in the Upper- Marowijne Region (Figs 1 and 2). During 1983-85 more than 80% of Suriname’s Plasmodium fakiparum malaria cases were recorded from this area, which is inhabited by a Maroon tribe. Other receptive areas were often re- infected by semi-immune carriers originating from the Upper-Marowijne region, raising the need for effective control of malaria in the focus area (Rozendaal, 1991). Many years of residual house-spraying with D D T and some- times dieldrin for malaria vector control have not reduced the rates of malaria prevalence, although Rozendaal et al. (1989a) demonstrated the efficacy of this method against the principal vector Anopheles darlingi Root. The lack of success was probably due to the low spraying coverage (between 15% and 60% of the houses were sprayed per cycle) caused by the poor cooperation of the local population and by organizational problems (Barnes & Jenkins, 1972).

The efficacy of pyrethroid-impregnation of the local mosquito nets has been evaluated, but this method ap- peared to be incompatible with the local habit of washing bednets every week (Rozendaal et al . , 1989b). Vector

Correspondence: Dr J. A. Rozendaal, Division of Control of Tropical Discascs, World Health Organization, 1211 Geneva 27, Switzerland.

control with impregnated strips of wide-mesh netting on the open eaves of houses was suggested as a more appropriate method (Rozendaal, 1991). Detection and treatment of malaria cases therefore remain the mainstay of malaria control in the Upper-Marowijne region; but drug-resistant P. falciparum, frequent travelling of the population, logistical problems, poor cooperation by semi- immune infected people and other problems hamper the effective control of malaria.

Anticipation of malaria epidemics has been difficult because the fluctuations in malaria incidence were not understood. Early investigators (e.g. Flu, 1912; Bonne, 1924) found that most cases of malaria were detected during and shortly after periods of heavy rainfall, in the months July and August, although transmission seemed to be associated with tidal influence. Data collected by the Anti-Malaria Campaign (AMC) often revealed a peak in the number of positives during the long dry season, and a smaller peak during the long rainy season (e.g. Fleming, 1963; Hudson, 1984). In French Guyana, where most positive cases also originate from the Upper-Marowijne region, similar seasonal fluctuations have been observed by Juminer et al. (1981). However, a serious epidemic in 1984, which peaked at the end of the long rainy season, raised the need for better understanding of malaria epi- demiology in Suriname. Detailed ecological mapping of An. darlingi breeding habitats should improve the targeting of control activities, leading to more effectiveness and reduced costs (cf. Gabinaud, 1987).

Ih

Anopheles darlingi breedirzg habitats in Suriname 17

58OW 5 7O 56O 55O 54O

Fig. 1. Map of Suriname. Indicated are the limits of tidal influence, the study area in the Upper-Marowijnc region, inhabited by the Djuka tribe (shaded) and the area along the Upper-Surinamc River inhabited by the Saramaccan tribe (shaded).

A survey of the breeding-sites of An.darlingi in riverine habitats indicated that a relationship between the incidence of malaria and the river water-level and rainfall is likely (Rozendaal, 1990). Larvae of An. darlingi were recorded during the dry season, with low river water-level, from: (a) creeks: at sunlit places under the forest canopy, between roots, fallen trunks and other obstacles; (b) river edges: shaded from direct sunlight, in patches of floating vegetation or debris trapped in emergent or trailing vegetation along the banks, where water current is impeded (cf. Fleming, 1963; Panday, 1980; Hudson, 1984); (c) pools: when the river level dropped, during the dry season, pools formed in or near the river-bed; larvae were collected only in partly shaded pools, close to the forest edge.

During periods of high and unstable water level, these habitats become flushed out. Consequently, in the rainy season, An. darlingi larvae occur mainly between grass stems and debris floating in open sunny places in flooded forest areas and riverside swamps (Bonne & Bonne-Wepster,

1925; Van der Kuyp, 1950; Bruyning, 1052; Hudson, 1984; Rozendaal, 1987).

Fig. 3 depicts the breeding habitats of An.darlingi in relation to river water-level and rainfall. When the water height becomes sufficient (level w) during the rainy season, the river overflows and the wet season breeding-sites be- come flooded (Wl) . Depressions in the forest floor, remote from the river, are filled by the rain (W2). At the end of the rainy season, the river level drops and many of these breeding-sites of Amdarlingi dry up. At the beginning of the dry season, when the current slows down and the river water-level falls (level d) , breeding-sites become available between plants and obstacles along the river edge (D2) and in pools (DI). With a further retreat of the river level, these habitats mostly dry up, except for perennial swamps. Sunlit pools (D3) were found to be devoid of An. darlingi, but many An. nuizeztovari larvae were found in them (Rozendaal, 1990).

I8 .I. A. Rozeiiduul

Fig. 2. The study area, thc mcdical district ‘Stoclmanseiland’. in thc Upper-Marowijnc Rcgion. Indicated arc: (1) Abetredjocka, (2) Stoclmansciland, (3) Asclikamp, (4) location of several cer- cmonial villagcs.

It depends on the local geomorphology as to which types of breeding habitats are available. Fleming (1963) recorded larvae and the highest densities of An. darlirigi biting during the long dry season along the Upper-Suriname River (Fig. 1). Along the Lawa River (Figs 1 and 2), Hudson (1984) and Rozendaal (1987) found a pronounced peak in An.dar/ingi biting density during the long rainy season in several consecutive years, but almost no biting during the remainder of the year. In six other localities in the Upper-Marowijne region, Rozendaal(1987) also observed differences in seasonal biting densities of An.dar/ingi, with peaks during both the dry and the wet season.

This paper assesses the relations between the river water- level, rainfall, the availability of An. durlirigi breeding habitats and seasonal fluctuations in the incidence of ma- laria in the Upper-Marowijne region of Suriname.

Area of study and Methods

General. In Suriname, Atiopheles durlirigi and malaria transmission occur mainly in the tropical rain forest area, south and up-river of the tidal limit (Fig. 1) (Rozendaal, 1990, 1991). The river basin of the Upper-Marowijne and its tributaries is shallow with extensive floodplains. The river attains a width of up to 4 km in places with many islands. From Suriname’s total population, estimated to be 400,000 people, only about 33,000 live in the rain forest area. Of these, 31,000 are Maroons (calling them- selves bush negroes) and 2000 are Amerindians. The Maroons are organized in several tribes, each with their own territories.

The Djuka tribe comprises approximately 10,OOo Maroons inhabiting the Upper-Marowijne region shown in Fig. 2, with the Marowijne River and its tributaries the Lawa and Tapanahony. Other tribes of Maroons live mostly along the Suriname River and some on the Saramacca River (Fig. 1). The Djuka practice slash-and-burn cultivation and spend most of the year in very small settlements on islands in the river, close to their cultivations but far from their main villages.

Climate. The tropical rain forest climate has a major rainy season from March to mid-August, a major dry season from August to November, a minor rainy season from December to January and a minor dry season from February to March. Monthly total rainfall data were cal- culated from daily measurements by the Department of Meteorology at Stoelmanseiland airstrip (Fig. 2).

Malariology. As part of the primary health care (PHC) programme, a system of malaria case detection was in- troduced in 1982 in the interior of Suriname, whereby bloodsmears were collected monthly from approximately 10% of the population in the villages (Schaapveld, 1984). Plasmodium v i v a is scarce, with overall slide positivity rate (SPR) 0.4-0.9%, except among Amerindians in the south. P. falciparum is more widely prevalent: overall SPR 2.6-5.9% in 1982-85. Most P.falciparurn cases were detected among the Djuka in the Upper-Marowijne region (Table 1). In the Upper-Suriname River area,

Anopheles darlirigi breeding habitats in Suriname 19

Fig. 3. The rclation bctwccn the rivcr watcr-level, rainfall and thc availability of brccding habitats for An.dur/ingi. In thc wct season, with rivcr watcr-lcvel w, forest arcas arc Boodcd (W1) and pools arc fillcd by rainwatcr (W2). In thc dry season, with rivcr watcr-lcvcl d. pools (D1) and still watcrs near the riverbank (D2) arc suitable brccding habitats. Pools (D3) bccomc availablc with a furthcr rctrcat o f the rivcr. but thcsc placcs arc unshadcd and do not constitutc suitable brccding placcs for An.dur/ingi.

malaria incidence has been generally low in recent years, but an epidemic in 1979-80 involved several hundred cases.

The monthly malaria parasite incidence (PI) in Stoel- manseiland, comprising most of the Djuka tribal territory, was taken as an index of malaria transmission in the Upper- Marowijne region. Stoelmanseiland Medical District has a population of approximately 4525 people of the B i b sub-tribe, living in eleven ceremonial villages and more than 200 small settlements along more than 100 km of the river. Malariological data from January 1982 until August 1986 were used. This period commenced with reorganiz- ation of the malaria activities in the PHC programme (see above) and terminated due to political problems.

Hydrology. The Marowijne river level was recorded daily near Abetredjoeka (Fig. 2), downstream from the confluence of the Lawa and Tapanahony rivers, by the Department of Hydrology, and monthly averages were calculated for the period 1982-86.

Enfornology. Seasonal fluctuations in the biting density of An.darfingi were monitored over the period March 1983 to August 1986 at Abetredjoeka (Fig. 2 ) , as part of entomological studies reported elsewhere (Rozendaal,

1987, 1989, 1990; Rozendaal et a/ . , 1989a, b), using a series of different sampling routines.

(a) March 1983 to May 1984: outdoor (peridomestic) human bait collections, using an aspirator, were done in a village on 4 days per month for three periods per night: 18.30-20.30, 23.30-01.30 and 04.30-06.30 hours. In April, June and November 1983 and January and April 1984 no collections were made. The monthly totals o f An.darfingi collected were converted into an hourly average per month, from which an estimate of the probable biting rate indoors was derived by multiplying by 0.59, based on the observation by Rozendaal (1989) that simultaneous indoor and outdoor biting collections yielded only 59% as many An.darlingi indoors as outdoors.

(b) July 1984 to November 1984: indoor human bait collections were undertaken for 13h every night, in an experimental hut, from 18.30 to 07.30 hours.

(c) December 1984 to August 1986: exit trap collections were made from a man-baited experimental hut. Nightly totals of Arz.darlirigi were recorded and the monthly av- erage number of An.darlingi bites per man-hour in the hut was estimated.

Table 1. Blood smcar cxaminations and malaria dctcction ratcs by thc Medical Mission in thc intcrior 01 Suriname. SPR = slidc positivity rate.

No. positive (SPR) No. and pcrcentagc of P. fulcipururn from: No. slidcs

Year examined P. fakiparum P . vivu.u Uppcr-Marowijnc Uppcr-Surinamc Other arcas

1082 37,866 2171 (5.7%) 261 (0.69%) 1209 (55.7%) 85 (3.9%") 877 (40.4%,)

1984 42,355 2477 (5.9%) 164 (0.39%) 2281 (92.1%) 30 (1.2% 166 (6.7'%,) 1983 36.300 1012 (2.8%) 328 (0.YOY") 820 (81.0%) 42 (4.2%) 150 (14.8%))

1985 42,264 1090 (2.6%) 241 (0.57%) 949 (87.1%) 14 (1.3%) 127 (11.6%)

20 J . A . Rozentluul

300

200

Riverheight (cm) . - Rainfall (mm)

, D l + D 2 , J F M A M J J A S O N D J F

*-> month

Fig. 4. The thcorctical seasonal availability of An.dur/ingi brccding habitats i n relation to the averagc monthly watcr-lcvel near Abctrcd- joeka, 3982-85. Indicated arc thc water level w = 260crn abovc which wet season breeding is possible, and level d = 170cm bclow which dry season brccding is possible until thcsc habitats have dried up after an cstimatcd 1.S months. Also indicated is the influence of rainfall above 250 inndrnonth in the short rainy scason.

Duru correlation. This paper evaluates the relationship of monthly malaria parasite incidence (PI) to river-level (w and d, Fig. 3) during the week before each peak in Att.durlingi density, bearing in mind that the data on malaria incidence and river height reflect the situation throughout Stoelmansciland Medical District, whereas fluctuations in An.durlirzgi densities at Abetredjoeka reflect the local effects of water level on breeding-sites at this one small locality in the Marowijne Region.

Results

Rdutioti of An. durlirzgi density with river height arid ruirt,full

Months with Anopheles durlirzgi biting densities at least 50% more than in the month immediately preceding or following were: August-September 1983, July 1984,

November 1984, June 1985, October 1985 and July 1986 IFig. 5). Peaks in the months of June and July came within the long rainy season (three peaks), while the other three peaks coincided with the long dry season. The fact that ‘wet season peaks’ are higher than ‘dry season peaks’ suggests that, in the Abetredjoeka area, more suitable breeding habitats are available during the rainy season than during the dry season.

Fig. 6 shows the monthly average river-height near Abetredjoeka and total monthly rainfall at Stoelmansei- land. Arrows indicate the waterlevels 1 week before an increase (upward arrow) or a decrease (downward arrow) in An.durlingi densities. Suitable breeding habitats pre- sumably became available or were wiped out during these periods. River water-levels during the weeks preceding the three dry season peaks were 150, 130 and 170cm re- spectively. Comparing Figs 5 and 6, the critical river height, below which favourable breeding habitats become available in the long dry season (level d), is estimated to be 170cm.

Fig. 5. The monthly incan biting density of An.dur/ingi per hour. The number of observation nights for each month is indicatcd in par- entheses. The observation methods are also indicated and furthcr explained in the text. Fig. 6. Average nlonthly rivcr hcight (cm) near Abetredjoeka and total monthly rainfall (mm) on Stoclmanseiland. Indicated arc thc water lcvcls w and d and the pcriods during which An.durlingi breeding habitats are expected to be available in the wet season (W) and dry season (D) . Fig. 7. Monthly parasitc incidcnec (PI) for medical district ‘Stoelmanseiland’ (‘Too). Indicated are the periods during which an incrcase in PI is cxpcctcd following the availability of A.dur/ingi brccding habitats in the wet season (W’) and the dry season (D’). Fig. 8. Expcctcd (lower arrow) and observed (upper arrow) increase or decrcasc of malaria incidence in the medical district ‘Stoclmanseiland’ during thc periods following the availability or unavailability of suitable breeding habitats for An.durlingi.

AnopheIe.s darlirigi hreedirig hahitats iri Sirririume 2 1

The estimation of level w, above which wet season breeding habitats are formed, is prevented by the lack of data on An.darlitzgi densities for June 1984 and May 1985. During the wet seasons of 1983, 1984 and 1985, water levels of 260,280 and 300 cm gave rise to peak populations of An.darlingi which lasted for 1-2 months. Based on these figures, the level w is provisionally estimated at 260cm. Wet season peaks declined in August 1984, July 1985 and August 1986. The water levels 1 week before the end of these peaks were 220, 320 and 220cm respectively, followed by retreats of the river. Level w (260cm) drained the flooded forest areas (W1 in Fig. 3). No explanation can be suggested for the decline in July 1985.

In October 1983, December 1984 and 1985, 1 week before the end of the dry season peaks of An.darlirzgi, the water levels were 100, 130 and 120cm, respectively, well below the estimated level d of 170cm. Disappearance of the dry season breeding sites was presumably due either to drying out of the pools (DI in Fig. 3) and still waters nearthe riverbank (D2 in Fig. 3), as the river level dropped, or to an increase in the water level which flushed out these larval habitats,

The overall density of An.darlingi was much higher in 1984 than in other years, with the peak population in July, following unusually high rainfall during December 1983 and January 1984: cf. 327mm in 1982-83; 543mm in 1983-84; 398mm in 1984-85; 393mm in 1985-86. During the dry season in late 1983 (Fig. 6), the dry season pools (DI) and wet season pools (W2) dried up completely. The ensuing short rainy season in December 1983 to January 1984 was followed by a 3-month period with a stable river water level, just below level d, which provided both cat- egories of dry season habitats (D1 and D2) yielding rela- tively high An. darlingi densities during the long rainy season of 1984.

In contrast, during January- April 1985, the river level fluctuated around level d. The relatively low densities of An.darlingi during 1985 (Fig. 5) were attributed to insta- bility of the river level (Fig. 6).

Fig. 4 summarizes the above observations. A period of 1.5 months has been used for the duration of dry season habitats. Duration of wet season habitats is determined by the period of river height above level w = 260mm. I t is inferred that breeding habitats (W2 and D1) are favoured when the rainfall during December-January exceeds 250mm per month, since peaks of Ari.darlingi were not observed following less than this amount of rainfall.

Relationships between malaria incidence, river height and rainfall

Periods during the wet season (W) or dry season (D) when breeding habitats are most productive of An.darlitzgi in Stoelmanseiland can be defined by reference to Figs 4, 5 and 6. Thence Fig. 7 shows the relationship between observed malaria PI in Stoelmanseiland and the peaks predicted for wet (Wl) and dry (Dl) seasons, assuming an egg-adult period of An. durlingi of 8 days and a sporogonic

cycle for P . falcipariim of 12 days, with an incubation period of 10 days or more.

Fig. 7 shows associations between observed and predicted increases of malaria PI in 1982, 1984 and 1985 but not in 1983. Numbers of malaria cases detected in August and September 1983 were very low, which can be explained by an exceptionally poor rainy season. The exceptionally high number of malaria positives throughout most of 1984 corresponds with the long duration of the availability of breeding habitats during the first half of the year.

Arrows in Fig. 8 indicate the expected (lower) and ob- served (upper) increase or decrease in malaria incidence during 1982-86, derived from Fig. 7. I n fourteen o f the twenty periods, the arrows correspond upwards or down- wards, but for six periods the arrows point in opposite directions. Overall there was no significant correlation ( r z = 20; t = 8; P > 0.05).

Conclusions and Discussion

In the study area of Stoelmanseiland in Suriname, where An.darlitigi is the vector, the highest peak of malaria incidence occurs during the long dry season (August- November) and a smaller, secondary peak during the long rainy season (March-August). An epidemic of ma- laria is likely to occur when An.darlirigi breeding habitats become plentiful between these two peak periods, for example when the short rainy season has high rainfall giving a period of relatively stable river height below the maximum level for dry season breeding habitats.

In the coastal area of Suriname, where the rivers are tidal, An.darlitigi breeding conditions are unfavourable and limited to flooded areas in the wet season (Rozendaal, 1990). Isolated outbreaks of malaria in this area occur mainly during or shortly after the long wet season. In the coastal area of Guyana, to the west of Suriname, Giglioli (1951) observed peaks of malaria transmission only during the rainy season. This could be explained by the avail- ability of rain-flooded places near irrigation canals which provided the only suitable breeding habitats for Ari.darlirigi in that area.

In the Amazon region of Brazil, the situation is more complicated. Ferraroni & Hayes (1979) reported peaks in malaria transmission during the transitions between the wet and the dry seasons. Charlwood (1980) found peaks in An. darlirzgi biting density during approximately the same periods. It was suggested that heavy rains might flush out breeding sites while, in the dry season, the limited amount of standing water may cause the population of Aii.darlirrgi to decline, except possibly in the forests where breeding continues.

Because Arz.darlirzgi thrives under various hydrological conditions, fluctuations in malaria incidence cannot be generally predicted from rainfall patterns or river levels in these areas. Surveys of Aiz.darlirzgi breeding habitats and population dynamics need to be correlated with malaria epidemiology in each malarious situation, as reported here for Stoelmanseiland in eastern Suriname, to provide

22 J . A . Kozeridaal

a basis for planning improved malaria control activities in the context of rural development programmes.

Acknowledgments

I wish to thank the Department of Hydrology (BWKW) and the Department for Meteorology both in Paramaribo for making their data available to me. 1 am grateful for the cooperation and support offered to me by the staff and personnel of the Medical Mission and the Bureau of Public Health in Paramaribo, Suriname. Also 1 wish to thank Professor Dr J . J . Laarman, Royal Tropical Institute, Amsterdam; D r R. Hayes, London School of Hygiene and Tropical Medicine; Dr A. B. Knudsen, Dr L. Molineaux and D r J . A . Cattani, World Health Organization, Geneva, for their constructive comments on the manuscript.

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129- 142.

(2). 135-146.

Accepted 3 May 1991