Evaluation of potential role of Anopheles ziemanni in malaria transmission in western Kenya

Observations of considerable numbers of Anopheles ziemanni Grunberg resting inside human dwellings in Ahero, western Kenya, raised the question of its status in malaria transmission. To address this question, samples of this species were tested for Plasmodium falciparum infection rates and blood-feeding pattern, and data were compared with those of known vectors in the area. Although no An. ziemanni mosquitoes were infected with P. falciparum sporozoites, the proportions of this mosquito species among all anophelines captured and its human blood index compared well with those of other vectors, suggesting a possible role in the transmission of malaria. Additional studies, including both indoor- and outdoor-resting mosquito populations as well as incorporating laboratory-based parasite susceptibility tests should clarify the situation.     

The effects of climatic factors on the distribution and abundance of malaria vectors in Kenya

Since 1988 malaria epidemics have occurred in multiple sites in western Kenya highlands. Climatic variability has been associated with some of the recent epidemics. We examined influences of climatic factors on the distribution and abundance of three malaria vector species, Anopheles gambiae, Anopheles arabiensis, and Anopheles funestus in western Kenya and in the Great Rift Valley. Mosquito samples were collected from the lowland and highland areas with various climatic conditions. The three vector species were abundant in the lower part of western Kenya. An. arabiensis was not found in the areas above 1,400 m elevation in western Kenya Although An. gambiae and An. funestus were found in the sites above 1,700 m in western Kenya, their densities were < 1 per house. In the Great Rift Valley, An. gambiae was not recorded. An. funestus was more widely distributed than the other two species. A stepwise multiple regression analysis found that moisture index was the most important variable in shaping species composition of the An. gambiae complex. Relative abundance of An. gambiae was positively associated with moisture index, suggesting that An. gambiae is more adapted to moist climate. Seasonal differences in species composition were significant in western Kenya, and the proportion of An. funestus was higher in the dry season than the rainy season. Influence of temperature on vector density was significant for all three species. These results imply that climate changes alter the distribution and abundance of malaria vectors in future.     

Characterization of malaria transmission by Anopheles (Diptera: Culicidae) in western Kenya in preparation for malaria vaccine trials

Malaria transmission was studied for 33 mo in the villages of Kisian and Saradidi in western Kenya in preparation for field trials of malaria vaccines. Abundance estimates of Anopheles gambiae Giles sensu lato and Anopheles funestus Giles, which constituted over 99% of 26,645 anophelines collected, were compared for all-night biting collections inside houses, outdoors, and in tents. The overall numbers of Anopheles per man-night were 2.3 times greater in Kisian than in Saradidi. For the three types of collections, mean sporozoite rates by dissection ranged from 2.2 to 5.4% for 13,072 Anopheles in Kisian and from 9.9 to 13.6% for 7,058 Anopheles in Saradidi; greater than 90% of the infections were Plasmodium falciparum, either alone or mixed with P. malariae or P. ovale. Heaviest transmission from April to July coincided with the end of the long rainy season. Entomological inoculation rates (EIR) averaged 0.82 infective bites per man per night inside houses in Kisian and 0.65 in Saradidi. Outdoors, EIRs averaged 0.09 in Kisian and 0.52 in Saradidi. In tents, which were evaluated to identify methods for exposing nonindigenous volunteers during vaccine efficacy trials, EIRs were 3.3 and 2.5 times less than inside houses for Kisian (EIR = 0.25) and Saradidi (EIR = 0.26), respectively. Exposure in tents averaged one infective bite every 4.0 d in Kisian and every 3.8 d in Saradidi. The use of tents in vaccine efficacy trials should provide adequate exposure for nonindigenous volunteers. Malaria vaccine trials could be conducted efficiently in western Kenya, with timing dependent upon the intensity of transmission required by vaccine trial objectives.     

Survivorship of immature stages of Anopheles gambiae s.l. (Diptera: Culicidae) in natural habitats in western Kenya highlands

We examined the survivorship of Anopheles gambiae s.l. Giles (Diptera: Culicidae) larvae and habitat productivity in three major habitat types in the western Kenya highlands. The age-specific distribution was determined for larvae and pupae, and survivorship curves were constructed. Larval-to-pupal survivorship was 6.8% in drainage ditches, 4.3% in cow hoofprints, and 1.8% in disused goldmines, respectively. High mortality rates were observed in all developmental stages. The estimated daily survival rate was highest in drainage ditches (0.74), followed by cow hoofprints (0.71), and it was lowest in disused goldmines (0.62). Productivity of emerging An. gambiae adults was generally low in these larval habitats (1.35, 1.55, and 1.84 mosquitoes per m2 per wk in drainage ditches, disused goldmines, and cow hoofprints, respectively). In total, seven families of larval mosquito predators were identified from the larval habitats, including Hydrophilidae, Dytiscidae, Corixidae, Nepidae, Notonectidae, Belostomatidae, and Cordulidae. Predator density in disused goldmines was significantly higher than that of other habitat types. Determination of the relative importance of predation, habitat stability and food contents on natural mosquito habitat productivity would help to design cost-effective vector control methods specifically targeted at the productive habitats.     

Avian malaria infections in western European mosquitoes

In the complex life cycle of avian malaria parasites (Plasmodium sp.), we still have a poor understanding on the vector-parasite relationships. This study described the community of potential avian malaria vectors in four Portuguese reedbeds. We tested if their geographical distribution differed, and investigated on their Plasmodium infections. The mosquitoes' feeding preferences were evaluated using CO(2), mice, and birds as baits. The most abundant species were Culex pipiens, Culex theileri, and Ochlerotatus caspius (and, in one site, Coquillettidia richiardii). Plasmodium lineages SGS1 and SYAT05 were found in unengorged Cx. pipiens and Cx. theileri, respectively, suggesting that these mosquitoes were competent vectors of those lineages. The species' abundance was significantly different among sites, which may help to explain the observed differences in the prevalence of SGS1. At the study sites, SGS1 was detected in the most abundant mosquito species and reached a high prevalence in the most abundant passerine species. Probably, this parasite needs abundant hosts in all phases of its cycle to keep a good reservoir of infection in all its stages. Cq. richiardii showed an opportunistic feeding behavior, while Cx. pipiens appeared to be more mammophilic than previously described, perhaps because the used avian bait was not its preferential target. In one of the study sites, mosquitoes seem to be attracted to the Spotless Starling Sturnus unicolor, an abundant bird species that may be an important local reservoir of avian malaria infections. To our knowledge, this is the first report of detection of avian Plasmodium DNA from European mosquitoes.     

Measurement of antibody levels against region II of the erythrocyte-binding antigen 175 of Plasmodium falciparum in an area of malaria holoendemicity in western Kenya

Region II of the 175-kDa erythrocyte-binding antigen (EBA-175RII) of Plasmodium falciparum is functionally important in sialic acid-dependent erythrocyte invasion and is considered a prime target for an invasion-blocking vaccine. The objectives of this study were to (i) determine the prevalence of anti-EBA-175RII antibodies in a naturally exposed population, (ii) determine whether naturally acquired antibodies have a functional role by inhibiting binding of EBA-175RII to erythrocytes, and (iii) determine whether antibodies against EBA-175RII correlate with immunity to clinical malaria. We treated 301 lifelong residents of an area of malaria holoendemicity in western Kenya for malaria, monitored them during a high-transmission season, and identified 33 individuals who were asymptomatic despite parasitemia (clinically immune). We also identified 50 clinically susceptible individuals to serve as controls. These 83 individuals were treated and monitored again during the subsequent low-transmission season. Anti-EBA-175RII antibodies were present in 98.7% of the individuals studied. The antibody levels were relatively stable between the beginning and end of the high-transmission season and correlated with the plasma EBA-175RII erythrocyte-binding-inhibitory activity. There was no difference in anti-EBA-175RII levels or plasma EBA-175RII erythrocyte-binding-inhibitory activity between clinically immune and clinically susceptible groups. However, these parameters were higher in nonparasitemic than in parasitemic individuals at enrollment. These results suggest that although antibodies against EBA-175RII may be effective in suppressing some of the wild parasite strains, EBA-175RII is unlikely to be effective as a monovalent vaccine against malaria, perhaps due to allelic heterogeneity and/or presence of sialic acid-independent strains.     

Complexity of the malaria vectorial system in Cameroon: contribution of secondary vectors to malaria transmission

Malaria transmission in Africa is a dynamic and complex system that is so far superficially understood. Further knowledge is required to improve control of the disease. In the present report, we highlight the contribution of the so-called "secondary" malaria vectors to the overall parasite transmission intensity in several sites across Cameroon, through a retrospective analysis of surveys from the Organisation de Coordination pour la lutte Contre les Endémies en Afrique Centrale database. In total, 48,490 female anophelines belonging to 21 different species were collected between October 1998 and March 2003. Anopheles gambiae Giles, Anopheles arabiensis Patton, Anopheles funestus Giles, Anopheles nili (Theobald), and Anopheles moucheti Evans represented 89% of the total anopheline fauna. Beside these major vectors, malaria parasites or their circumsporozoite proteins were found in nine secondary malaria vectors: Anopheles ovengensis Awono-Ambene et al., Anopheles carnevalei Brunhes et al., Anopheles coustani Laveran, Anopheles hancocki Edwards, Anopheles marshallii (Theobald), Anopheles paludis Theobald, Anopheles pharoensis Theobald, Anopheles wellcomei Theobald, and Anopheles ziemanni Grtünberg. The mean infection rate of secondary vectors (1.36%) was significantly (P < 0.001) lower than that of major vectors (3.08%). An. pharoensis and An. ovengensis were repeatedly found infected by Plasmodium falciparum Welch and contributed substantially to the total malaria transmission intensity in some areas where they were abundant. Both species have strong exophilic and/or exophagic habits such that they might elude vector control directed against endophilic and endophagic malaria vectors.     

Effects of microclimatic changes caused by land use and land cover on duration of gonotrophic cycles of Anopheles gambiae (Diptera: Culicidae) in western Kenya highlands

Studies were carried out at a malaria epidemic-prone highland site in western Kenya to determine the effects of deforestation-caused microclimate change on the duration of the gonotrophic cycle of the malaria vector Anopheles gambiae Giles. Gonotrophic cycle duration was measured using field-collected F1 A. gambiae females. Average ambient temperature in the deforested area of Kakamega (elevation 1,430-1,580 m above sea level), western Kenya, was 0.5 degrees C higher than that of the forested area over a 10-mo period. During the dry season, deforested areas showed an increased mean indoor temperature of 1.8 degrees C, and a shortened duration of the mosquitoes' first and second gonotrophic cycles, by 1.7 d (59%) and 0.9 d (43%). During the rainy season, the average indoor temperature of houses located in the deforested area was 1.2 degrees C higher than in houses in the forested area. The duration of the first and second gonotrophic cycles was shortened by 1.5 d (17%) and 1.4 d (27%), respectively, in the deforested highland site. A shorter mosquito gonotrophic cycle implies increased daily biting frequency and thus increased vectorial capacity. Together with evidence that deforestation reduced A. gambiae larva-to-adult developmental time and increased larval and adult survivorship, this study suggests that deforestation can further enhance malaria transmission potential in the highlands through increased indoor temperature and shortened gonotrophic cycle durations of A. gambiae mosquitoes.     

Plasmodium falciparum rosetting is associated with malaria severity in Kenya.

Rosette formation in 154 fresh Plasmodium falciparum isolates from Kenyan children with mild (n = 54), moderate (n = 64), or severe (n = 36) malaria was studied to determine whether the ability to form rosettes in vitro is correlated with malaria severity. There was a wide distribution of rosette frequencies within each clinical category; however, a clear trend towards higher rosette frequency with increasing severity of disease was seen, with the median rosette frequency of the mild-malaria group (1%; range, 0 to 82%) being significantly lower than those of the moderate-malaria group (5%; range, 0 to 45%; Mann-Whitney U test, P < 0.02) and the severe-malaria group (7%; range, 0 to 97%; Mann-Whitney U test, P < 0.003). Within the severe-malaria category there was no difference in rosetting among isolates from cerebral malaria patients or those with other forms of severe malaria. We also examined the ABO blood groups of the patients from whom isolates were obtained and found that isolates from group O patients (median rosette frequency, 2%; range 0 to 45%) rosetted less well than those from group A (median, 7%; range 0 to 82%; Mann-Whitney U test, P < 0.01) or group AB (median, 11%; range 0 to 94%; Mann-Whitney U test, P < 0.03). We therefore confirm that rosetting is associated with severe malaria and provide further evidence that rosetting is influenced by ABO blood group type. Whether rosetting itself plays a direct role in the pathogenesis of severe malaria or is a marker for some other causal factor remains unknown.     

Phospholipases during membrane dynamics in malaria parasites

Plasmodium parasites, the causative agents of malaria, display a well-regulated lipid metabolism required to ensure their survival in the human host as well as in the mosquito vector. The fine-tuning of lipid metabolic pathways is particularly important for the parasites during the rapid erythrocytic infection cycles, and thus enzymes involved in lipid metabolic processes represent prime targets for malaria chemotherapeutics. While plasmodial enzymes involved in lipid synthesis and acquisition have been studied in the past, to date not much is known about the roles of phospholipases for proliferation and transmission of the malaria parasite. These phospholipid-hydrolyzing esterases are crucial for membrane dynamics during host cell infection and egress by the parasite as well as for replication and cell signaling, and thus they are considered important virulence factors. In this review, we provide a comprehensive bioinformatic analysis of plasmodial phospholipases identified to date. We further summarize previous findings on the lipid metabolism of Plasmodium, highlight the roles of phospholipases during parasite life-cycle progression, and discuss the plasmodial phospholipases as potential targets for malaria therapy.     

Population movement and malaria persistence in Rameswaram Island

During 1982-84, the Vector Control Research Center (VCRC) of the Indian Council for Medical Research (ICMR) at Pondicherry studied the role of population movement in the persistence of malaria transmission in Rameswaram Island, Tamil Nadu, between mainland India and Sri Lanka. While the island supports a population of 56,000, mostly fishermen, there is also a periodic, back and forth migration of fishermen between mainland villages and Rameswaram. This population movement greatly contributes to the high prevalence of malaria in both areas, since fishermen can be either donor or recipient of malaria in either place. The VCRC monitered and recorded the movement of fishermen in various seasonal camps by questioning them and by the VCRC staff accompanying them when possible. In 9 fishing camps 412 of 1098 families had migrated from mainland villages; 686 families had migrated from villages within Rameswaram Island. A mass blood survey found 138 of 4073 individuals examined positive for malaria; 107 of 680 fever cases examined were positive for malaria. Mosquito collections, the lack of permanent treatment facilities for the transient population, and ecological factors indicate a high receptivity for malaria on Rameswaram Island. With the island attracting between 1000-4000 tourists daily and over 200,000 travelers annually between India and Sri Lanka, evidence exists for considerable danger from the importation of chloroquinine resistant malaria strains into Rameswaram. Adequate attention to human ecology will be needed for malaria control in this area.     

Placental malaria diminishes development of antibody responses to Plasmodium falciparum epitopes in infants residing in an area of western Kenya where P. falciparum is endemic

To determine the effect of placental malaria (PM) infection on the development of antibody responses to malaria in infants, we measured immunoglobulin G levels to seven different Plasmodium falciparum epitopes by using plasma samples collected at monthly intervals from infants born to mothers with and without PM. Overall, PM was associated with diminished antibody levels to all of the epitopes tested, especially with infants aged >or=4 to 12 months, and the difference was statistically significant for four of the seven epitopes (P<0.0035). These findings suggest that PM can negatively influence the development of immune responses to malaria in infants.     

Anopheline mosquito survival strategies during the dry period in western Kenya

The dry season survival mechanism of Anopheles gambiae Giles is one of the most vexing deficiencies in our understanding of the biology of the major malaria vectors. In this study, we examined the dynamics of anopheline adult mosquitoes, their larval habitats, and egg survival potential during the dry season in the basin region of Lake Victoria, western Kenya. Through field surveys, we demonstrated two survival strategies of An. gambiae sensu stricto during the dry season: continuous reproduction throughout the year and embryo dormancy in moist soil for at least several days. We further demonstrated that An. gambiae shows a strong preference for moist soil as an oviposition substrate rather than dry soil substrate under the insectary conditions. The observation that anopheline eggs remain a dormant stage to resist desiccation clearly contrasts the conventional wisdom that anopheline eggs hatch shortly after they are laid. Our results from western Kenya are consistent with the suggestion that anopheline mosquitoes do not necessarily suffer a severe population bottleneck during the dry season and thus maintain a large effective population size.     

Antigenic similarity between the mosquito vectors of malaria and filariasis.

Rabbit antisera were raised to the malaria vector Anopheles tessellatus, head-thorax, abdomen, and midgut preparations. Reactivity of the antisera with An. tessellatus and the filariasis vector, Culex quinquefasciatus, were examined by enzyme-linked immunosorbent assay (ELISA) and Western blots. Although many antigens were shared between the two species of mosquitoes, tissue- and species-specific antigens also were identified.     

Effect of permethrin-impregnated nets on exiting behavior, blood feeding success, and time of feeding of malaria mosquitoes (Diptera: Culicidae) in western Kenya

The impact of permethrin-treated bednets on the feeding and house entering/exiting behavior of malaria vectors was assessed in two studies in western Kenya. In one study, matched pairs of houses were allocated randomly to receive bednets or no bednets. Exiting mosquitoes were collected in Colombian curtains hung around half of each house; indoor resting mosquitoes were collected by pyrethrum spray catches. The number of Anopheles gambiae Giles and An. arabiensis Patton estimated to have entered the houses was unaffected by the presence of bednets; Anopheles funestus Giles was less likely to enter a house if bednets were present. Anopheles gambiae and An. funestus were less likely to obtain a blood meal and significantly more likely to exit houses when bednets were present. No difference was detected in An. arabiensis rates of blood feeding and exiting. In a second experiment, hourly night biting collections were done on 13 nights during the rainy season to assess whether village-wide use of permethrin-treated bednets caused a shift in the time of biting of malaria vectors. A statistically significant shift was detected in the biting times of An. gambiae s.l., although the observed differences were small. No change was observed in the hourly distribution of An. funestus biting. Our study demonstrated that, at least in the short-term, bednets reduced human-vector contact and blood feeding success but did not lead to changes in the biting times of the malaria vectors in western Kenya.     

Tick infestations on Zebu cattle in western Kenya: individual host variation

Monthly collections were made of all ticks from 25 mature East African Zebu cattle and 5 yearlings for 13 mo on Rusinga Island, Lake Victoria, Kenya, from September 1986 to September 1987. The most common species were Rhipicephalus appendiculatus Neumann and Amblyomma variegatum F.; Boophilus decoloratus Koch and Rhipicephalus evertsi evertsi Neumann also were present in fewer numbers. Cattle were ranked according to their degree of resistance to each of the four species and to each stage in the life cycle of the three-host ticks. Ranking according to infestations of females and nymphs of R. appendiculatus, nymphs and larvae of A. variegatum, or all three stages of these two species when compared with the total burden gave statistically significant correlations. Sampling difficulties prevented the ranking of cattle for infestations of larvae of R. appendiculatus, although the numbers on cattle were high. On the other hand, failure of attempts to rank the cattle on females of A. variegatum, B. decoloratus, or R. e. evertsi counts was attributed to the small differences between tick numbers on cattle. Highly resistant cattle showed little or no seasonal fluctuations in tick numbers for most of the period compared with animals of low resistance, which showed an up to sevenfold increase in the magnitude of the tick burden when the tick challenge was high. Cattle with low resistance were responsible for much of the tick multiplication in the field, whereas highly resistant animals tended to limit the populations. There were no distinct differences between the numbers of each tick species on yearlings and mature cattle.