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Distribution of mosquito larvae in inundated rice fields is poorly known despite its profound implications in implementation of vector control programs. Based on oviposition behavior of gravid females and biotic and abiotic conditions of the rice field, distribution of mosquito larvae within the paddy may vary greatly. As a guide to implementation of mosquito vector control program targeting the aquatic stages in the rice fields in Mwea, studies were conducted to determine the distribution of mosquito larvae within the paddy. Twenty-eight cages measuring 50 cm3 were distributed randomly within the paddy during the transplanting stage of the rice growth cycle, and were examined twice per week up to the flowering stage to determine mosquito oviposition pattern. A total of 17,218 mosquito larvae were collected at the periphery and a further 17,570 at the center of the paddy. These comprised 7,461 larvae from the genus Anopheles and 27,327 from genus Culex. The number of pupae collected at the periphery was 1,004 and 1.5 times greater than the number collected at the center. Significantly higher counts of Anopheles larvae were collected at the center (1.00 +/- 0.11) than at the periphery (0.55 +/- 0.05) of the paddy during transplanting stage, but the difference was not significant during the tillering stage. In contrast, significantly higher numbers of Culex larvae were collected from the periphery (3.09 +/- 0.39) than at the center (2.81 +/- 0.24) of the paddy. More pupae were also collected at the center than at the periphery of the paddy. These findings indicate the distribution of Anopheles and Culex larvae in rice fields to be nonrandom; however, for successful achievement of an integrated vector control program targeting the diverse mosquito fauna occurring in rice fields, there is need to target the whole paddy for larvicidal application.

Water quality of aquatic habitats is an important determinant of female mosquito oviposition and successful larval development. This study examined the influence of environmental covariates on Anopheles arabiensis mosquito abundance in the Mwea Irrigation Scheme, Central Province of Kenya, prior to implementation of a malaria vector control program. Experimental rice plots were used to examine the environmental covariates responsible for regulating abundance and diversity of the aquatic stages of malaria vectors. Mosquito larval sampling and water quality analysis were done weekly from the flooding stage to the rice maturation stage. Sampling for mosquito larvae was conducted using standard dipping technique. During each larval collection, environmental covariates such as pH, temperature, conductivity, salinity, dissolved oxygen, water depth, and rice stage were measured. Anopheles arabiensis larval density was highest between 1 wk before transplanting and 4 wk after transplanting with peaks at weeks 0, 3, and 8. The fluctuation in values of the various environmental covariates showed characteristic patterns in different rice growth phases depending on the changes taking place due to the agronomic practices. Using a backward linear regression model, the factors that were found to be associated with abundance of An. arabiensis larvae at any of the rice growing phases included the following: dissolved oxygen, pH, turbidity, water depth, rice height, number of rice tillers, salinity, conductivity, and temperature. The environmental covariates associated with abundance of An. arabiensis were associated with early vegetative stage of the rice growth. For effective control of developmental stages of mosquito larvae, the application of larvicides should be done at the vegetative stage and the larvicides should persist until the beginning of the reproductive stage of the rice.

The influence of body size of Anopheles gambiae s.s., Anopheles arabiensis, Anopheles merus, and Anopheles funestus on the transmission of Plasmodium falciparum was studied at 3 sites along the Kenyan coast. Adult mosquitoes were collected inside houses by pyrethrum spray collection (PSC) from April to September 2001. Wing length was measured microscopically to the nearest 0.01 mm as an index of mosquito body size. The P. falciparum circumsporozoite protein (CSP) was detected by enzyme-linked immunosorbent assay technique. A total of 1715 anopheline mosquitoes were collected, of which 785 (45.77%) were An. gambiae s.s., 54 (3.15%) An. arabiensis, An. merus 27 (1.57%), and 849 (49.5%) were An. funestus. The mean wing length for An. gambiae s.s. was 2.94 mm and that of An. funestus was 2.50 mm. There was no site-to-site variation in the body size of An. gambiae or An. funestus. There was no significant difference in wing length between sporozoite-infected and uninfected An. gambiae s.s., and the same was found for An. funestus. At the 3 rural coastal sites in Kenya, the efficiency of malaria parasite transmission does not appear to be influenced by variation in vector body size.

The number of salivary gland malaria sporozoites (sporozoite load) was determined by hemacytometer counts for 2,055 field-collected Anopheles mosquitoes from Kilifi District, Kenya. Of 48 gland-positive Anopheles gambiae s.l., sporozoite loads ranged from 125 to 79,875, with a geometric mean of 1,743 sporozoites per infected mosquito. About half of the infected mosquitoes had sporozoite loads < 1,000. Following hemacytometer examination of salivary gland samples, the same samples were subsequently tested for Plasmodium falciparum circumsporozoite (CS) protein by enzyme-linked immunosorbent assay (ELISA). The confirmation by ELISA of CS protein in 89.6% (43/48) of the salivary gland-positive samples compared to only 1.4% (28/2,007) of the dissection-negative mosquitoes indicated that dissection methods with hemacytometer counts of sporozoites were adequate for detecting even low numbers of sporozoites in field-collected mosquitoes. Detection of 17 or fewer sporozoites in blood meals of 7 freshly bloodfed An. gambiae s.l. provides a further indication that the actual number of sporozoites transmitted during bloodfeeding may be quite low.

Anopheline mosquitoes were sampled inside houses, where residents slept under untreated bednets, by CDC light traps and human-biting catches to evaluate light traps as a means for determining human exposure to malaria vectors in Kilifi District, Kenya. Mosquitoes were sampled during 2 all-night collections by light traps and one all-night biting catch in a series of 262 houses. Collections yielded 1,721 Anopheles gambiae s.l. and 46 An. funestus, and 60.3% of the houses were negative for anophelines. There was a significant correlation in numbers of An. gambiae s.l. captured by light traps and human-biting collections (r = 0.64), but light traps were biased and underestimated An. gambiae s.l. abundance. This bias increased with increasing mosquito abundance. In addition, the proportion of An. gambiae s.l. infected by Plasmodium falciparum was 2.3-fold higher in light traps than in human-biting collections. Along the coastal zone of Kenya where vector abundance is low, light traps do not provide an adequate estimate of man-vector contact when such information is required at the household level in epidemiological studies of malaria parasite transmission.

Blood meal samples were tested by ELISA for 534 Anopheles gambiae s.l. and 76 Anopheles funestus collected from 25 sites in Kilifi District, Kenya. Human IgG was detected in 94.4% of the An. gambiae s.l. and in 90.8% of the An. funestus. No samples were positive for cow and only a few were positive for goat. Both species fed predominantly on humans irrespective of host availability. At these sites on the Kenyan coast, the high degree of human-feeding by malaria vectors contributes to the efficiency of malaria parasite transmission and the high incidence of severe malaria.