Extensive polymorphism of Ra86 genes in field populations of Rhipicephalus appendiculatus from Kenya.

Commercial vaccines based on recombinant forms of the Bm86 tick gut antigen are used to control the southern cattle tick, Rhipicephalus microplus, a 1-host species, in Australia and Latin America. We describe herein sequence polymorphism in genes encoding Ra86 homologues of Bm86 in the brown ear tick, Rhipicephalus appendiculatus, isolated from four Kenyan field populations and one laboratory colony. Sequencing of 19 Ra86 sequences defined two alleles differentiated by indels, encoding 693 amino acids (aa) and 654 aa respectively, from the Muguga laboratory reference strain. Ra86 sequences were also determined from gut cDNA from four field populations of R. appendiculatus collected in different livestock production systems in Kenya. Analysis of approximately 20 Ra86 sequences from each of the four field sites in central and Western Kenya; Makuyu, Kiambu, Kakamega and Uasin Gishu, revealed three additional size types differentiated by 39-49 amino acid indels resulting in a total of 5 indel-defined genotypes. The 693 aa type 5 was isolated only from the laboratory tick stock; genotypes 1, 2 and 3 were identified in ticks from the four Kenyan field sites and appeared to be derivatives of the shorter RA86 genotype found in Muguga laboratory stock genotype 4. By contrast no large indels have yet been observed between R. microplus sequences from Australia, South America or Africa. Evidence that selection contributes to the observed sequence variation was provided by analysis of ratio of synonymous and non-synonymous substitutions and application of the selective neutrality and neutral evolution tests to the primary data. Phylogenetic analysis clustered sequences from all Ra86 size types and Bm86, into four major clades based on amino acid substitutions, but there was no evidence that these groupings correlated with geographical separation of R. appendiculatus populations.

Early biting of the Anopheles gambiae s.s. and its challenges to vector control using insecticide treated nets in western Kenya highlands.

Long term use of insecticides in malaria vector control has been shown to alter the behavior of vectors. Such behavioral shifts have the potential of undermining the effectiveness of insecticide-based control interventions. The effects of insecticide treated nets (ITNs) use on the composition, biting/feeding and sporozoite rates of Anopheles gambiae s.l. mosquitoes in Musilongo village, Vihiga County of western Kenya highlands were investigated. Adult mosquitoes were collected in selected sleeping spaces inside six randomly selected houses using miniature Centre for Disease Control and Prevention (CDC) light traps. Mosquito sampling in each house was conducted twice every week for 16 consecutive months (May 2010-August 2012). At each sampling a single trap was set in the selected space inside each house such that it collected mosquitoes alternatively from 18:00 to 21:00h and 21:00 to 06:00h every week. All collected mosquitoes were morphologically identified. Female Anopheles mosquitoes were classified according to their physiological status as unfed, fed, partially gravid and gravid, sorted and counted. Members of the A. gambiae complex were identified using a Polymerase chain reaction (PCR) method. Enzyme-linked-immunosorbent assay (ELISA) was used to determine blood meal sources and Plasmodium infection rates in A. gambiae s.l. mosquitoes. Blood meal tests were conducted on DNA extracted from gut contents of blood fed A. gambiae s.l. The head and thorax section of dried samples of A. gambiae s.l. were used in testing for the presence of Plasmodium falciparum (Pf) sporozoites. Overall, 735 adult female Anopheles comprising 708 [96.3%] A. gambiae s.l. and 27 [3.7%] Anopheles funestus mosquitoes were collected. A. gambiae s.l. population collected comprised, 615 [86.9%] unfed and 38 [5.4%] fed adult mosquitoes. The rest were either partially or fully gravid. The proportion of A. gambiae s.l. biting indoors within 18:00-21:00h was 15.8% (103/653) at a rate of 3.2bites per person per hour compared to 84.2% biting from 21:00-06:00h at a rate of 3.8 bites/per/h. An estimated 97.7% A. gambiae ss and 2.3% A. arabiensis constituted the indoor biting A. gambiae s.l. The population of An. gambiae s.l. biting from 18:00 to 21:00h had a Plasmodium faciparum (pf) sporozoite rate of 3.8% compared to 3.5% observed in populations biting within 21:00-06:00h. Human blood constituted 89% of An. gambiae s.l. blood meal sources. The risk of malaria transmission from 21:00 to 06:00h was approximately 5 fold the risk within 18:00-21:00h. Majority of the infective female A. gambiae s.l. adults were biting deep into the night than in the early hours of the night. Humans remain the preferred source of blood meal for A. gambiae s.s. the dominant malaria vector in the highlands. ITNs remain a fundamental control intervention against malaria transmission since female blood seekers were more during bed time than pre-bed time. Advocacy on enhanced net availability, integrity and usage in Kenyan highlands can reduce Pf transmission. Additional complementary interventions are required to control the biting and parasite transmission encountered before bed-time.

Differential transcription of two highly divergent gut-expressed Bm86 antigen gene homologues in the tick Rhipicephalus appendiculatus (Acari: Ixodida).

The transcriptional control of gene expression is not well documented in the Arthropoda. We describe transcriptional analysis of two exceptionally divergent homologues (Ra86) of the Bm86 gut antigen from Rhipicephalus appendiculatus. Bm86 forms the basis of a commercial vaccine for the control of Rhipicephalus (Boophilus) microplus. The R. appendiculatus Ra86 proteins contain 654 and 693 amino acids, with only 80% amino acid sequence identity. Reverse-transcription PCR of gut cDNA showed transcription of only one genotype in individual female ticks. PCR amplification of 3' untranslated sequences from genomic DNA indicated that both variants could be encoded within a single genome. When both variants were present, one of the two Ra86 genotypes was transcriptionally dominant.

Microgeographic variations in Burkitt's lymphoma incidence correlate with differences in malnutrition, malaria and Epstein-Barr virus.

BACKGROUND: Endemic Burkitt's lymphoma (eBL) has been associated with Epstein-Barr virus (EBV) and holoendemic Plasmodium falciparum malaria. But recent evidence suggests that other risk factors are involved. METHODS: We hypothesised that selenoprotein glutathione peroxidase (GPx), a surrogate of nutritional status, is an important biomarker for eBL risk. We measured plasma GPx, anthropometric markers of malnutrition, EBV viral loads and malaria parasitaemia in children aged 1-9 years (n=258) from two locations in Nyanza Province, Kenya, with higher-than-expected and lower-than-expected incidence of eBL. The study participants were malaria asymptomatic children from the community. RESULTS: Children from eBL high-incidence areas had significantly lower GPx levels, high EBV viral load and more evidence of chronic malnutrition than children from eBL low-incidence areas (all P<0.001). Additionally, GPx levels were significantly lower in children with the highest EBV viral load and for those with P. falciparum infections (P=0.035 and P=0.004, respectively). CONCLUSIONS: These results suggest that selenium deficiency may be a risk factor for eBL.

Discriminative feeding behaviour of Anopheles gambiae s.s. on endemic plants in western Kenya.

Anopheles gambiae Giles s.s. (Diptera: Culicidae) is known to feed on plant sugars, but this is the first experimental study to consider whether it discriminates between plant species. Thirteen perennial plant species were selected on the basis of their local availability within the vicinity of human dwellings and larval habitats of An. gambiae s.s. in western Kenya. Groups of 100 or 200 mosquitoes were released into cages either with a cutting of one plant type at a time (single-plant assay) or with cuttings of all 13 plants simultaneously (choice assay), respectively, and left overnight. In the choice assay, direct observations of the percentages of mosquitoes perching or feeding on each plant were recorded over four 1-h periods each night. For both types of assay, mosquitoes were recaptured and the percentage that had fed on plants was assessed by testing them individually for the presence of fructose. To identify which plants the choice-assay mosquitoes had fed on, gas chromatography (GC) profiles of samples of mosquito homogenates were compared with GC profiles of extracts from relevant parts of each plant. Four of the plants that were observed to have been fed on most frequently in the choice assay (Parthenium hysterophorus L., Tecoma stans L., Ricinus communis L., and Senna didymobotrya Fresen) were also shown to have been ingested most often by mosquitoes in both types of assay, suggesting that An. gambiae is differentially responsive to this range of plants, regardless of whether the plants were presented singly or mixed together. Significantly more females than males fed on plants, with the exception of P. hysterophorus L., one of the plants most frequently fed on. For most plant species (ten of 13), GC profiles indicated that An. gambiae obtained sugars primarily from flowers. The exceptions were P. hysterophorus L., Lantana camara L. and R. communis L., on which An. gambiae fed more often from leaves and stems than from flowers.

Plasmodium falciparum malaria disease manifestations in humans and transmission to Anopheles gambiae: a field study in Western Kenya.

Transmission of the malaria parasite Plasmodium is influenced by many different host, vector and parasite factors. Here we conducted a field study at Mbita, an area of endemic malaria in Western Kenya, to test whether parasite transmission to mosquitoes is influenced by the severity of malaria infection in its human host at the time when gametocytes, the transmission forms, are present in the peripheral blood. We examined the infectivity of 81 Plasmodium falciparum gametocyte carriers to mosquitoes. Of these, 21 were patients with fever and other malaria-related symptoms, and 60 were recruited among apparently healthy volunteers. Laboratory-reared Anopheles gambiae s.s. (local strain) were experimentally infected with blood from these gametocyte carriers by membrane-feeding. The severity of the clinical symptoms was greater in febrile patients. These symptomatic patients had higher asexual parasitaemia and lower gametocyte densities (P = 0.05) than healthy volunteers. Ookinete development occurred in only 6 out of the 21 symptomatic patients, of which only 33.3% successfully yielded oocysts. The oocyst prevalence was only 0.6% in the 546 mosquitoes that were fed on blood from this symptomatic group, with mean oocyst intensity of 0.2 (range 0-2) oocysts per mosquito. In contrast, a higher proportion (76.7%) of healthy gametocyte carriers yielded ookinetes, generating an oocyst rate of 12% in the 1332 mosquitoes that fed on them (mean intensity of 6.3, range: 1-105 oocysts per mosquito). Statistical analysis indicated that the increased infectivity of asymptomatic gametocyte carriers was not simply due to their greater gametocyte abundance, but also to the higher level of infectivity of their gametocytes, possibly due to lower parasite mortality within mosquitoes fed on blood from healthy hosts. These results suggest that blood factors and/or conditions correlated with illness reduce P. falciparum gametocyte infectivity.

Health status and health seeking behaviour of the elderly persons in Dagoretti division, Nairobi.

OBJECTIVE: To determine the health status and the health seeking behaviour of the elderly people aged 65 years and above. DESIGN: A descriptive cross sectional study of individuals interviewed through questionnaires and focus groups discussions. SETTING: Dagoretti Division, Nairobi Province, Kenya. SUBJECTS: Four hundred non-institutionalised elderly persons. RESULTS: Four hundred people including 276(69%) women and 124(31%) males were interviewed; 44% had no independent source of livelihood and 51% were widowed. The majority 376(92.5%) of the respondents had been sick within the last three months, preceding the study with 111(27.8%) being sick all the time. The prevalent diseases included musculoskeletal (80%), respiratory (68%), sight (44%) and dental conditions(40%). Three hundred and sixteen (79%) of the respondents were functionally independent in activities of daily living. One hundred and sixty one (40.3%) were satisfied with their current way of life while (63%) perceived themselves as healthy, 24.8% of the respondents lived alone. The reported social problems included:- economic dependency (96%), poor housing (76%), loneliness (60%) and feeling not needed (42%) of the respondents. Only 26% were on treatment, lack of money hindered health care access to 73% of the respondents (p<0.001). Sixty two per cent of the respondents were buying over-the-counter drugs. Walking was under taken by 217(67%) as a physical exercise, and (26%) of the respondents consumed addictive drugs. The focus groups criticised the lack of health facilities for the elderly in the community. CONCLUSION: The effects of ageing, low economic status and inadequate access to health care contributed to the elderly poor health status. The use of over-the-counter drugs was indicative of the inefficient health facilities in meeting the health needs of the elderly. The study points out the need to formulate policies that will target on the health needs of the elderly.

Infectivity of Plasmodium falciparum gametocytes in patients attending rural health centres in western Kenya.

BACKGROUND: Experimentally studying the transmission of the malaria parasite and its regulating factors requires availability of human blood donors carrying infectious gametocytes. The difficulty of identifying gametocyte carriers from the community is often limited due to financial and human resources constraints. The available alternative is rural health centres where malaria patients go for treatment. In this study, the potential of recruiting volunteers and acquiring infectious blood for experimental infections from rural health centers in malaria endemic area was examined through routine patient diagnosis. OBJECTIVE: To examine the patients presenting at rural health centers for the potential to carry sexual stage malaria parasite and test their infectivity to Anopheles gambiae mosquitoes. SETTING: Mbita Health Centre, Mbita Town Ship, Suba District, western Kenya. METHODOLOGY: Routine survey of all patients attending Mbita Health Centre with suspected malaria. Patients were examined for Plasmodium falciparum trophozoites and gametocytes. Gametocyte-positive volunteers were recruited for their potential to infect Anopheles mosquitoes via membrane feeding. RESULTS: Three thousand nine hundred and eighty seven patients were screened between May 2000 and April 2001. Plasmodium falciparum was the predominant parasite species and P. malariae being the only minor species, accounting for 0.9% of malaria cases. Clinical malaria varied with age and prevailed throughout the year with a slight seasonality. Gametocyte prevalence was low (0.9-6.6%), and gametocyte densities were generally very low with a geometric mean of 39 gametocytes per microl blood. Children aged > 5 years constituted 67% of all gametocyte carriers. Only 22 volunteers with mean gametocytes density of 39.62 per microl blood (range: 16-112) were recruited for study of parasite infectiousness to laboratory-reared mosquitoes. Only two patients infected 1% of 1099 mosquitoes with one or two oocysts. CONCLUSION: The low gametocyte densities or other possible host and vector related factors regulating infectivity of gametocyte carriers to mosquitoes may have caused the poor infections of mosquitoes. This study indicates that rural health centers in malaria-endemic areas may not be suitable for recruiting infectious gametocyte donors for studies of vector competence. They are suitable for passive clinical case surveillance and for evaluation of the effects of control measures.

Traditional use of mosquito-repellent plants in western Kenya and their evaluation in semi-field experimental huts against Anopheles gambiae: ethnobotanical studies and application by thermal expulsion and direct burning.

Ethnobotanical survey in 2 communities in western Kenya revealed that the most commonly known repellent plants were Ocimum americanum L. (64.1%), Lantana camara L. (17.9%), Tagetes minuta L. (11.3%) and Azadirachta indica A. Juss (8.7%) on Rusinga Island, and Hyptis suaveolens Poit. (49.2%), L. camara (30.9%) and O. basilicum L. (30.4%) in Rambira. Direct burning of plants is the most common method of application for O. americanum (68.8%), L. camara (100%) and O. basilicum (58.8%). Placing branches or whole plants inside houses is most common for H. suaveolens (33.3 and 57.8% for the respective locations), A. indica (66.7 and 100%), and T. minuta (54.8 and 56.0%). The repellency of plants suggested by the ethnobotanical survey and other empirical information was evaluated against the malaria vector Anopheles gambiae s.s. Giles in experimental huts within a screenwalled greenhouse. Thermal expulsion and direct burning were tested as alternative application methods for the selected plants O. americanum, O. kilimandscharicum Guerke, O. suave Willd., L. camara, A. indica, H. suaveolens, Lippia uckambensis Spreng and Corymbia citriodora Hook. When thermally expelled, only H. suaveolens failed to repel mosquitoes, whereas the leaves of C. citriodora (74.5%, P < 0.0001), leaves and seeds of O. suave (53.1%, P < 0.0001) and O. kilimandscharicum (52.0%, P < 0.0001) were the most effective. Leaves of C. citriodora also exhibited the highest repellency (51.3%, P < 0.0001) by direct burning, followed by leaves of L. uckambensis (33.4%, P = 0.0004) and leaves and seeds of O. suave (28.0%, P = 0.0255). The combination of O. kilimandscharicum with L. uckambensis repelled 54.8% of mosquitoes (P < 0.0001) by thermal expulsion. No combination of plants increased repellency by either method. The semi-field system described appears a promising alternative to full-field trials for screening large numbers of candidate repellents without risk of malaria exposure.

Utilisation of antenatal and maternity services by mothers seeking child welfare services in Mbeere District, Eastern Province, Kenya.

BACKGROUND: Utilisation of antenatal and maternity services is an important maternal health indicator. Increasing the proportion of mothers who are cared for in health facilities during pregnancy, childbirth and puerperium reduces the health risks to mothers and their children. OBJECTIVE: To determine the utilisation of antenatal and maternity services by mothers of Mbeere District bringing their children to the child welfare clinic. DESIGN: Cross-sectional, descriptive study. SETTING: Four rural health centres in Mbeere district. SUBJECTS: Two hundred mothers bringing their children aged one year and below to the child welfare clinic between September and December 2000. RESULTS: The proportion of mothers who utilised health facilities for antenatal and maternity services was 97.5% and 52%, respectively. Utilisation of health facilities for maternity services was significantly influenced by number of children and distance to health facility in that, as number of children increased, utilisation of maternity services reduced (chi2=8.99; p=0.027; df=1). Mothers living less than 5 km to a health facility utilised the services better than those living 5 km and beyond (chi2=7.57; p=0.0059; df=1). Among the reasons given by the mothers (individual respondents and through Focus Group Discussions) regarding dissatisfaction with the services offered included shortage of drugs and essential supplies, lack of commitment by staff, poor quality of food and lack of cleanliness in the health facilities. CONCLUSION: Coverage for antenatal services was high among mothers during their last pregnancy. However, only about half of the mothers interviewed utilised health facilities for maternity services (labour and delivery). The major constraints experienced by the mothers as they sought for the services (as reported by individual respondents and through Focus Group Discussions) included lack of transport, lack of money for transport and hospital fee and delay in admission to health facility once mothers report in labour. Lack of satisfaction with quality of care given could be the major demotivating factor in the use of health facilities for maternity services.

Childcare practices of commercial sex workers.

OBJECTIVE: To determine the childcare practices of commercial sex workers (CSWs). DESIGN: A descriptive cross-sectional survey was conducted between July and December 2000 during which a structured questionnaire was administered. SETTING: Kibera slum, Nairobi, Kenya. SUBJECTS: Three hundred eighty five CSWs and four focus group discussions (FGDs) held. Health cards from 126 under five years old children belonging to the respondents were reviewed for immunization status and regularity of growth monitoring. RESULTS: The mean age of the 385 CSWs surveyed was 32 +/- 7 years and mean duration of sex work was 6 +/- 4 years. The mean number of living children was 3.4 +/- 2 and 81.2% of the mothers lived with their children. Three quarters of the CSWs practised prostitution at home. The most common daily childcare activities by the mothers were food preparation (96.2%) and washing children's clothes (91.3%). Overall 96.8% of their under-five years old children were fully immunized and 80% of their under one year old children had their growth monitored monthly. About three quarters of the mothers with adolescent children educated them on HIV/STDs. Health seeking behaviour for the children was hampered by health care cost (71.4%) and consumption of alcohol by the mothers. Like other mothers, the CSWs encouraged their adolescent children to take up some adult roles such as maintaining a clean house (93.3%). However only 2.0% took time to converse or counsel the children. Focus group discussions (FGDs) with the CSWs showed that children were left unattended at night while the mothers went out in search of clients. Efforts to provide better education for the children were undermined by lack of funds (52.2%) and truancy (46.6%). One third of the study population had invested for the future maintenance of their children. CONCLUSION: There was more emphasis on physical, rather than psychological aspect of childcare. The practice of living with the children ensured that earnings from the sex trade were used for the immediate needs of the children such as food. However this practice had a negative influence on the children as the majority of the respondents conducted their sexual business at home with little or no privacy. Health seeking behaviour for the children was hampered by lack of funds and to some extent alcohol consumption by the mothers. Efforts to invest in the education of their children were undermined by lack of funds and truancy.

Vector-related case-control study of severe malaria in Kilifi District, Kenya.

A case-control study examined vector-related and environmental parameters associated with severe malaria in Kilifi District along the coast of Kenya. Over an 11-month period, 119 children identified with severe malaria infections at the Kilifi District Hospital were matched by age with control children who reported to the outpatient clinic with nonsevere infections. Intensive mosquito sampling was done in each of the case-control houses over a four-day period, beginning within a week of index case admission. A total of 109 environmental, demographic, behavioral, and animal husbandry variables were characterized for each household. Vector species (Anopheles gambiae s.l. and An. funestus) were detected in 40.1% and 36.1% of case and control houses, respectively. The relative abundance of vectors in individual houses was stable over the two-week resampling periods (r = 0.9). Both the overall abundance of anopheline mosquitoes (odds ratio [OR] = 1.5) and P. falciparum sporozoite rates (OR = 1.5) were not significantly different between case and control houses. In a matched analysis, 11 of 109 house variables associated significantly with severe malaria were also associated with vector abundance, as determined by chi-square linear trend analysis. Under conditions of year-round, low-level transmission on the coast of Kenya, the risk of severe disease in children is multifactorial and not governed strictly by transmission intensity or environmental heterogeneity affecting vector abundance and distributions. This suggests that current interventions that appear to be achievable only in areas where transmission is already low to moderate should be appropriate. However, such interventions should be monitored so that inappropriate and possibly disastrous control activities can be avoided in Africa.

Sporozoite loads of naturally infected Anopheles in Kilifi District, Kenya.

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.

Relationships between Plasmodium falciparum transmission by vector populations and the incidence of severe disease at nine sites on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at nine sites in the Kilifi District in Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s. l., An. funestus, An. coustani, An. squamosus, An. nili, and An. pharoensis. Anopheles gambiae s.l. was the predominant vector, comprising 98.4% of the total anophelines collected. Overall, 3.5% of 2,868 An. gambiae s.l. collected indoors and 0.8% of 261 collected outdoors contained P. falciparum sporozoites. Transmission was detected during 10 months, with peak periods from June to August and December to January. In eight of the nine sites, entomologic inoculation rates (EIRs) averaged only four infective bites per year (range 0-18); an annual EIR of 60 was measured for the site with the highest intensity of transmission. The incidence of severe malaria infections, ranging from 8.6 to 38.1 per 1,000 children (0-4 years), was not associated with EIRs. At these sites on the coast of Kenya, a high incidence of severe disease occurs under conditions of very low levels of transmission by vector populations. With respect to conventional approaches for vector control in Africa, decreases in transmission, even to levels barely detectable by standard approaches, may not yield corresponding long-term reductions in the incidence of severe disease.

Severe childhood malaria in two areas of markedly different falciparum transmission in east Africa.

Malaria remains a major public health challenge in sub-Saharan Africa, yet our knowledge of the epidemiology of malaria in terms of patterns of mortality and morbidity is limited. We have examined the presentation of severe, potentially life-threatening malaria to district hospitals in two very different transmission settings: Kilifi, Kenya with low seasonal transmission and Ifakara, Tanzania with high seasonal transmission. The minimum annual rates of severe disease in children below five years in both populations were similar (46 per 1000 children in Kilifi and 51 per 1000 children in Ifakara). However, there were important differences in the age and clinical patterns of severe disease; twice as many patients were under one year of age in Ifakara compared with Kilifi and there was a four fold higher rate of cerebral malaria and three fold lower rate of malaria anaemia among malaria patients at Kilifi compared with Ifakara. Reducing malaria transmission in Ifakara by 95%, for example with insecticide-treated bed nets, would result in a transmission setting comparable to that of Kilifi and although this reduction may yield early successes in reducing severe malaria morbidity and mortality in young, immunologically naive children, place these same children at increased risk at older ages of developing severe and potentially different manifestations of malaria infection hence producing no net cohort gain in survivorship from potentially fatal malaria.

Evaluation of light traps for sampling anopheline mosquitoes in Kilifi, Kenya.

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.

Low-level Plasmodium falciparum transmission and the incidence of severe malaria infections on the Kenyan coast.

The transmission of Plasmodium falciparum was studied in relation to the incidence of severe malaria infections at Sokoke and Kilifi town, Kilifi District, Kenya. Intensive mosquito sampling during a one-year period yielded Anopheles gambiae s.l., An. funestus, and An. coustani. Anopheles gambiae s.l. was the predominant vector, comprising 87.9% and 97.9% of the total anophelines collected in Sokoke and Kilifi town, respectively. The proportion of An. gambiae s.l. with P. falciparum sporozoite infections was 4.1% (20 of 491) in Sokoke and 2.2% (3 of 138) in Kilifi town; no infections were detected in An. funestus or in An. coustani. Entomologic inoculation rates indicated that residents were exposed to only 8.0 infective bites per year in Sokoke and 1.5 in Kilifi town. Transmission was detected during only six months in Sokoke and three months in Kilifi town despite low-level, year-round vector activity. The yearly incidence of severe P. falciparum infections in children, 1-4 years of age was 24.1 per 1,000 in Sokoke and 4.2 per 1,000 in Kilifi town. Monthly patterns of transmission corresponded closely with the incidence of severe infections. At these sites on the coast of Kenya, the spatial and temporal incidence of severe malaria infections is associated with low-level P. falciparum transmission by vector populations.

Bloodfeeding behavior of Anopheles gambiae s.l. and Anopheles funestus in Kilifi District, Kenya.

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.

Plasmodium falciparum infection rates in malaria vectors at two ecologically district sites in Kilifi Districts; Kenya

Transmission of Plasmodium falciparum was studied for one year at two ecologically different sites in Kilifi district. Anopheline mosquitoes were collected by three sampling methods and tested for P.falciparum sporozoites by ELISA. Anopheles gambiae s.l. was the predominant vector. Plasmodium falciparum sporozoite rates were 4.07 (20/491) and 2.17 (3/138) for An.gambiae s.l. at Sokoke and Kilifi town respectively. No sporozoite infections were detected in An.funestus or An.coustani. Transmission was year-round with highest levels occurring after the long rains (June to September). Entomological inoculation rates indicated taht each person could receive 8 infcetive bites per year at Sokoke; compared with 1.3 infective bites per year at Kilifi town. Our studies indicate taht the public health problem of malaria in Kilifi district is associated with extremely low levels of P.falciparum transmission by vector populations