Bancroftian filariasis in Kenya

Agricultural chemicals: life changer for mosquito vectors in agricultural landscapes?

BACKGROUND

Although many mosquito species develop within agricultural landscapes where they are potentially exposed to agricultural chemicals (fertilizers and pesticides), the effects of these chemicals on mosquito biology remain poorly understood. This study investigated the effects of sublethal concentrations of four agricultural chemicals on the life history traits of Anopheles arabiensis and Culex quinquefasciatus mosquitoes.

METHODS

Field and laboratory experiments were conducted to examine how sublethal concentrations of four agricultural chemicals: an insecticide (cypermethrin), a herbicide (glyphosate), and two nitrogenous fertilizers (ammonium sulfate and diammonium phosphate) alter oviposition site selection, emergence rates, development time, adult body size, and longevity of An. arabiensis and Cx. quinquefasciatus.

RESULTS

Both mosquito species had preference to oviposit in fertilizer treatments relative to pesticide treatments. Emergence rates for An. arabiensis were significantly higher in the control and ammonium sulfate treatments compared to cypermethrin treatment, while emergence rates for Cx. quinquefasciatus were significantly higher in the diammonium phosphate treatment compared to glyphosate and cypermethrin treatments. For both mosquito species, individuals from the ammonium sulfate and diammonium phosphate treatments took significantly longer time to develop compared to those from cypermethrin and glyphosate treatments. Although not always significant, males and females of both mosquito species tended to be smaller in the ammonium sulfate and diammonium phosphate treatments compared to cypermethrin and glyphosate treatments. There was no significant effect of the agrochemical treatments on the longevity of either mosquito species.

CONCLUSIONS

These results demonstrate that the widespread use of agricultural chemicals to enhance crop production can have unexpected effects on the spatial distribution and abundance of mosquito vectors of malaria and lymphatic filariasis.

Heterogeneous dynamics, robustness/fragility trade-offs, and the eradication of the macroparasitic disease, lymphatic filariasis

BACKGROUND

The current WHO-led initiative to eradicate the macroparasitic disease, lymphatic filariasis (LF), based on single-dose annual mass drug administration (MDA) represents one of the largest health programs devised to reduce the burden of tropical diseases. However, despite the advances made in instituting large-scale MDA programs in affected countries, a challenge to meeting the goal of global eradication is the heterogeneous transmission of LF across endemic regions, and the impact that such complexity may have on the effort required to interrupt transmission in all socioecological settings.

METHODS

Here, we apply a Bayesian computer simulation procedure to fit transmission models of LF to field data assembled from 18 sites across the major LF endemic regions of Africa, Asia and Papua New Guinea, reflecting different ecological and vector characteristics, to investigate the impacts and implications of transmission heterogeneity and complexity on filarial infection dynamics, system robustness and control.

RESULTS

We find firstly that LF elimination thresholds varied significantly between the 18 study communities owing to site variations in transmission and initial ecological parameters. We highlight how this variation in thresholds lead to the need for applying variable durations of interventions across endemic communities for achieving LF elimination; however, a major new result is the finding that filarial population responses to interventions ultimately reflect outcomes of interplays between dynamics and the biological architectures and processes that generate robustness/fragility trade-offs in parasite transmission. Intervention simulations carried out in this study further show how understanding these factors is also key to the design of options that would effectively eliminate LF from all settings. In this regard, we find how including vector control into MDA programs may not only offer a countermeasure that will reliably increase system fragility globally across all settings and hence provide a control option robust to differential locality-specific transmission dynamics, but by simultaneously reducing transmission regime variability also permit more reliable macroscopic predictions of intervention effects.

CONCLUSIONS

Our results imply that a new approach, combining adaptive modelling of parasite transmission with the use of biological robustness as a design principle, is required if we are to both enhance understanding of complex parasitic infections and delineate options to facilitate their elimination effectively.

Shrinking the Lymphatic Filariasis Map of Ethiopia: Reassessing the Population at Risk through Nationwide Mapping

BACKGROUND

Mapping of lymphatic filariasis (LF) is essential for the delineation of endemic implementation units and determining the population at risk that will be targeted for mass drug administration (MDA). Prior to the current study, only 116 of the 832 woredas (districts) in Ethiopia had been mapped for LF. The aim of this study was to perform a nationwide mapping exercise to determine the number of people that should be targeted for MDA in 2016 when national coverage was anticipated.

METHODOLOGY/PRINCIPAL FINDING

A two-stage cluster purposive sampling was used to conduct a community-based cross-sectional survey for an integrated mapping of LF and podoconiosis, in seven regional states and two city administrations. Two communities in each woreda were purposely selected using the World Health Organization (WHO) mapping strategy for LF based on sampling 100 individuals per community and two purposely selected communities per woreda. Overall, 130 166 people were examined in 1315 communities in 658 woredas. In total, 140 people were found to be positive for circulating LF antigen by immunochromatographic card test (ICT) in 89 communities. Based on WHO guidelines, 75 of the 658 woredas surveyed in the nine regions were found to be endemic for LF with a 2016 projected population of 9 267 410 residing in areas of active disease transmission. Combining these results with other data it is estimated that 11 580 010 people in 112 woredas will be exposed to infection in 2016.

CONCLUSIONS

We have conducted nationwide mapping of LF in Ethiopia and demonstrated that the number of people living in LF endemic areas is 60% lower than current estimates. We also showed that integrated mapping of multiple NTDs is feasible and cost effective and if properly planned, can be quickly achieved at national scale.

Bayesian calibration of simulation models for supporting management of the elimination of the macroparasitic disease, Lymphatic Filariasis

Background

Mathematical models of parasite transmission can help integrate a large body of information into a consistent framework, which can then be used for gaining mechanistic insights and making predictions. However, uncertainty, spatial variability and complexity, can hamper the use of such models for decision making in parasite management programs.

Methods

We have adapted a Bayesian melding framework for calibrating simulation models to address the need for robust modelling tools that can effectively support management of lymphatic filariasis (LF) elimination in diverse endemic settings. We applied this methodology to LF infection and vector biting data from sites across the major LF endemic regions in order to quantify model parameters, and generate reliable predictions of infection dynamics along with credible intervals for modelled output variables. We used the locally calibrated models to estimate breakpoint values for various indicators of parasite transmission, and simulate timelines to parasite extinction as a function of local variations in infection dynamics and breakpoints, and effects of various currently applied and proposed LF intervention strategies.

Results

We demonstrate that as a result of parameter constraining by local data, breakpoint values for all the major indicators of LF transmission varied significantly between the sites investigated. Intervention simulations using the fitted models showed that as a result of heterogeneity in local transmission and extinction dynamics, timelines to parasite elimination in response to the current Mass Drug Administration (MDA) and various proposed MDA with vector control strategies also varied significantly between the study sites. Including vector control, however, markedly reduced the duration of interventions required to achieve elimination as well as decreased the risk of recrudescence following stopping of MDA.

Conclusions

We have demonstrated how a Bayesian data-model assimilation framework can enhance the use of transmission models for supporting reliable decision making in the management of LF elimination. Extending this framework for delivering predictions in settings either lacking or with only sparse data to inform the modelling process, however, will require development of procedures to estimate and use spatio-temporal variations in model parameters and inputs directly, and forms the next stage of the work reported here.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-1132-7) contains supplementary material, which is available to authorized users.

Immuno–parasitological assessment of bancroftian filariasis in a highly endemic area along the River Sabaki, in Malindi district, Kenya

Lymphatic filariasis is endemic in the coastal areas of Kenya, with four major foci identified in the early 1970s. The prevalence and intensity of Wuchereria bancrofti infection, together with antifilarial antibody responses, were assessed in a historically highly endemic focus along the River Sabaki, in Malindi district. The prevalences of microfilaraemia and antigenaemia (detected by Og4C3 ELISA) were >20% and >40%, respectively, and both increased steadily with age. The high prevalences of antifilarial IgG1 (86%) and IgG4 (91%) responses indicate that most people living in this setting are exposed to W. bancrofti infection. The children investigated had higher levels of antifilarial IgG1 than the adults. The results of this study, based on a battery of currently available parasitological and immunological methods, provide an epidemiological update on lymphatic filariasis on the northern Kenyan coast. They show that the River Sabaki area is still an important focus for bancroftian filariasis and highlight the importance of implementing an elimination programme, to interrupt the transmission of W. bancrofti in all areas of endemicity in Kenya. The detailed baseline data collected in the River Sabaki area make the communities studied ideal as sentinel sites for epidemiological monitoring and the evaluation of the impact of mass drug administrations to eliminate lymphatic filariasis.

Bancroftian filariasis in 12 villages in Kwale district, Coast province, Kenya — variation in clinical and parasitological patterns

As part of a larger study on the effects of permethrin-impregnated bednets on the transmission of Wuchereria bancrofti, subjects from 12 villages in the Coastal province of Kenya, south of Mombasa, were investigated. The aims were to update the epidemiological data and elucidate the spatial distribution of W. bancrofti infection. Samples of night blood from all the villagers aged i 1 year were checked for the parasite, and all the adult villagers (aged >/= 15 years) were clinically examined for elephantiasis and, if male, for hydrocele. Overall, 16.0% of the 6531 villagers checked for microfilariae (mff) were found microfilaraemic, although the prevalence of microfilaraemia in each village varied from 8.1%-27.4%. The geometric mean intensity of infection among the microfilaraemic was 322 mff/ml blood. At village level, intensity of the microfilaraemia was positively correlated with prevalence, indicating that transmission has a major influence on the prevalence of microfilaraemia. Clinical examination of 2481 adults revealed that 2.9% had elephantiasis of the leg and that 19.9% of the adult men (10.8%-30.1% of the men investigated in each village) had hydrocele. Although the overall prevalence of microfilaraemia in the study villages had not changed much since earlier studies in the 1970s, both prevalence and intensity varied distinctly between the study villages. Such geographical variation over relatively short distances appears to be a common but seldom demonstrated feature in the epidemiology of bancroftian filariasis, and the focal nature of the geographical distribution should be carefully considered by those mapping the disease.

Mosquito species abundance and diversity in Malindi, Kenya and their potential implication in pathogen transmission

Mosquitoes (Diptera: Culicidae) are important vectors of human disease-causing pathogens. Mosquitoes are found both in rural and urban areas. Deteriorating infrastructure, poor access to health, water and sanitation services, increasing population density, and widespread poverty contribute to conditions that modify the environment, which directly influences the risk of disease within the urban and peri-urban ecosystem. The objective of this study was to evaluate the mosquito vector abundance and diversity in urban, peri-urban, and rural strata in Malindi along the Kenya coast. The study was conducted in the coastal district of Malindi between January and December 2005. Three strata were selected which were described as urban, peri-urban, and rural. Sampling was done during the wet and dry seasons. Sampling in the wet season was done in the months of April and June to cover the long rainy season and in November and December to cover the short rainy season, while the dry season was between January and March and September and October. Adult mosquito collection was done using Pyrethrum Spray Collection (PSC) and Centers for Disease Control and Prevention (CDC) light traps inside houses and specimens were identified morphologically. In the three strata (urban, peri-urban, and rural), 78.5% of the total mosquito (n = 7,775) were collected using PSC while 18.1% (n = 1,795) were collected using the CDC light traps. Using oviposition traps, mosquito eggs were collected and reared in the insectary which yielded 329 adults of which 83.8% (n = 276) were Aedes aegypti and 16.2% (n = 53) were Culex quinquefasciatus. The mosquito distribution in the three sites varied significantly in each collection site. Anopheles gambiae, Anopheles funestus and Anopheles coustani were predominant in the rural stratum while C. quinquefasciatus was mostly found in urban and peri-urban strata. However, using PSC and CDC light trap collection techniques, A. aegypti was only found in urban strata. In the three strata, mosquitoes were mainly found in high numbers during the wet season. Further, A. gambiae, C. quinquefasciatus, and A. aegypti mosquitoes were found occurring together inside the houses. This in turn exposes the inhabitants to an array of mosquito-borne diseases including malaria, bancroftian filariasis, and arboviruses (dengue fever, Yellow fever, Rift Valley fever, Chikungunya fever, and West Nile Virus). In conclusion, our findings provide useful information for the design of integrated mosquito and disease control programs in East African environments.

The role of monitoring mosquito infection in the Global Programme to Eliminate Lymphatic Filariasis

In addition to monitoring infection in the human host, there is also a need to assess larval infection in the vector mosquito population to evaluate the success of interventions for eliminating lymphatic filariasis transmission from endemic communities. Here, we review the current status of the available tools for quantifying vector infection and existing knowledge and evidence regarding potential infection thresholds for determining transmission interruption, to assess the potential for using vector infection monitoring as a tool for evaluating the success of filariasis treatment programmes.

LYMFASIM, a simulation model for predicting the impact of lymphatic filariasis control: quantification for African villages

LYMFASIM is a simulation model for lymphatic filariasis transmission and control. We quantified its parameters to simulate Wuchereria bancrofti transmission by Anopheles mosquitoes in African villages, using a wide variety of reported data. The developed model captures the general epidemiological patterns, but also the differences between communities. It was calibrated to represent the relationship between mosquito biting rate and the prevalence of microfilariae (mf) in the human population, the age-pattern in mf prevalence, and the relation between mf prevalence and geometric mean mf intensity. Explorative simulations suggest that the impact of mass treatment depends strongly on the mosquito biting rate and on the assumed coverage, compliance and efficacy. Our sensitivity analysis showed that some biological parameters strongly influence the predicted equilibrium pre-treatment mf prevalence (e.g. the lifespan of adult worms and mf). Other parameters primarily affect the post-treatment trends (e.g. severity of density dependence in the mosquito uptake of infection from the human blood, between-person variability in exposure to mosquito bites). The longitudinal data, which are being collected for evaluation of ongoing elimination programmes, can help to further validate the model. The model can help to assess when ongoing elimination activities in African populations can be stopped and to design surveillance schemes. It can be a valuable tool for decision making in the Global Programme to Eliminate Lymphatic Filariasis.

Evaluation of four sampling techniques for surveillance of Culex quinquefasciatus (Diptera: Culicidae) and other mosquitoes in African rice agroecosystems

Field studies were conducted in a rice, Oryza sativa L., agroecosystem in Mwea Kenya to compare the efficiency of CO2-baited Centers for Disease Control (CDC) light traps against nonbaited CDC light traps and gravid traps against oviposition traps in outdoor collection of Culex quinquefasciatus Say (Diptera: Culicidae) and other mosquitoes. Collectively, 21 mosquito species from the genera Culex, Anopheles, Mansonia, Ficalbia, and Aedes were captured during the 10-wk study period. Cx. quinquefasciatus was the predominant species in all trap types with proportions ranging from 57% in the nonbaited CDC light traps to 95% in the gravid traps. Significantly higher numbers of Cx. quinquefasciatus and Culex annulioris Theobald were collected in the CO2-baited CDC light traps than in the nonbaited CDC light traps, but the numbers of other mosquito species, including malaria vectors Anopheles arabiensis Patton and Anopheles funestus Giles did not differ significantly between the two trap types. More Cx. quinquefasciatus females were collected in grass infusion-baited gravid traps than egg rafts of this species in oviposition traps containing the same infusion. Although most mosquitoes captured in CO,-baited and nonbaited CDC light traps were unfed, most of those collected in gravid traps were gravid. From these findings, it is concluded that at least in the rice-growing area of Mwea Kenya, CO2-baited CDC light traps in conjunction with gravid traps can be used in monitoring of Cx. quinquefasciatus both for control and disease surveillance.

Bancroftian filariasis: patterns of vector abundance and transmission in two East African communities with different levels of endemicity

Intensive monitoring of Wuchereria bancrofti vector abundance and transmission intensity was carried out in two communities, one with high-level endemicity for bancroftian filariasis (Masaika, Tanzania) and the other with low-level (Kingwede, Kenya), on the East African coast. Mosquitoes were collected in light traps, from 50 randomly selected households in each community, once weekly for 1 year. They were identified, dissected and checked for parity and filarial larvae. Anopheles gambiae s. l., An. funestus and Culex quinquefasciatus transmitted W. bancrofti in the two communities but the importance of each of these taxa differed between the communities and by season. The overall vector densities and transmission intensities were significantly higher in Masaika than in Kingwede (the annual biting rate by 3.7 times and the annual transmission potential by 14.6 times), primarily because of differences in the available breeding sites for the vectors and in the vectorial capacity of the predominant vector species. A marked seasonal variation in vector abundance and transmission potential contributed to the complex transmission pattern in the communities. Generally, these indices were higher during and shortly after the rainy seasons than at other times of the year. Considerable differences in W. bancrofti transmission were thus observed between communities within a relatively small geographical area (mainly because of environmentally-determined differences in vector habitats), and these were reflected in the marked differences in infection level in the human populations. The variation in vector abundance, vector composition and transmission intensity in the two communities is discussed in respect to its cause, its effects, and its significance to those attempting to control bancroftian filariasis.

Lymphatic filariasis in Ghana: entomological investigation of transmission dynamics and intensity in communities served by irrigation systems in the Upper East Region of Ghana

We conducted an entomological study to document the effect of irrigation on the vectors and transmission dynamics of lymphatic filariasis in the Upper East Region of Ghana. Mosquitoes were collected by indoor spraying of houses in a cluster of communities located around irrigation projects (Tono and Vea) and others without reservoirs (Azoka). Anopheles gambiae s.s. was the dominant species and major vector, followed by An. funestus. Anopheles arabiensis constituted 9--14% of the An. gambiae complex but none were infective. Culex quinquefasciatus was also not infective in these communities. Chromosomal examinations showed that >60% (n=280--386) of the An. gambiae s.s. in irrigated communities were Mopti forms whilst 73% (n=224) in the non-irrigated area were Savannah forms. Infectivity rates (2.3--2.8 vs. 0.25), worm load (1.62--2.04 vs. 1.0), annual bites per person (6.50--8.83 vs. 0.47) and annual transmission potential (13.26--14.30 vs. 0.47) were significantly higher in irrigated communities.

Bancroftian filariasis on Pemba Island, Zanzibar, Tanzania: an update on the status in urban and semi-urban communities

Cross-sectional clinical, parasitological and entomological surveys for bancroftian filariasis were conducted in Konde, Chake Chake and Kengeja, three urban and semiurban communities on Pemba Island, and the results were compared with similar surveys done 15 years earlier. The overall prevalences of clinical manifestations among males aged 15 years or more (n = 614) was remarkably similar to those recorded 15 years earlier: elephantiasis 1.4% in 1975 and 1.1% in 1990; hydrocele, 22.4% and 21.8%, respectively. However, when the communities were compared individually, there was a reduction in the hydrocele prevalence in Konde from 22.4% to 11.5% and an increase in Kengeja from 27.0% to 35.5%. The overall microfilarial prevalence found during night blood surveys of all individuals aged 1 year or more (n = 2687) was 9.7%, compared to 14.2% recorded in 1975. The reduction was most pronounced in Konde. Of 1052 female mosquitoes caught with CDC light traps, 95% were Culex quinquefasciatus and 5% Anopheles gambiae s.l. Infective larvae of Wuchereria bancrofti were found only in the former. The filariasis situation in urban and semiurban communities on Pemba Island appears not to have changed considerably over the last 15 years.

Bancroftian filariasis in an irrigation project community in southern Ghana

An epidemiological study to document the endemicity and transmission characteristics of bancroftian filariasis was conducted in an irrigation project community in southern Ghana. In a 50% random sample of the population, the prevalence of microfilaraemia was 26.4% and the geometric mean microfilarial intensity among positives was 819 microfilariae/ml of blood. Hydrocoele was found in 13.8% of the males aged > or =18 years, and 1.4% of the residents examined, all females, had tymphoedema/elephantiasis. Detailed monitoring of the microfilarial intensity in 8 individuals over a 24-h period confirmed its nocturnal periodicity with a peak at approximately 0100 hours. The most important vector was Anopheles gambiae s.l., followed by An. funestus. The abundance of these mosquitoes and their relative importance as vectors varied considerably between the wet and the dry season. Opening of the irrigation canals late in the dry season resulted in a remarkable increase in the population of An. gambiae (8.3% of which carried infective filarial larvae) to levels comparable to those seen during the wet season, suggesting that the irrigation project is responsible for increased transmission of lymphatic filariasis in the community.

Herd immunity to filarial infection is a function of vector biting rate

Despite the existence of an impressive body of work on human immune responses against filarial infections, the occurrence of a protective response to infection remains unclear. Here, we use a combined modelling and comparative data analysis framework to address this issue for human infections with the filarial parasite, Wuchereria bancrofti. By analogy with previous work, the analysis involves the comparison of observed field patterns of infection with epidemiological patterns predicted by a mathematical model of parasite immunity. Unlike most other human helminths, which are transmitted by ingestion or dermal penetration, exposure to infection with lymphatic filariasis can be measured explicitly in terms of vector mosquito biting rates, thereby also allowing, probably for the first time, examination of the suggested role of exposure in generating herd immunity to macroparasites. Observed field patterns in this study were derived from 19 different published studies, which gave parallel estimates of community exposure rates and the corresponding age--prevalence patterns of infection, while predictions of the epidemiological impact of herd immunity were obtained using a catalytic model framework. The results provide the first conclusive evidence to date that variations in the observed age--prevalence patterns of infection in filariasis can be effectively explained by the occurrence of an exposure-driven acquisition of herd immunity. We discuss this result in terms of implications for the new World Health Organization-led initiative for the global control of this parasitic disease.

Bancroftian filariasis in Kwale District of Kenya. I. Clinical and parasitological survey in an endemic community

A cross-sectional parasitological and clinical survey for Wuchereria bancrofti infection was carried out in an endemic community of south-eastern Kenya, to obtain background epidemiological information for detailed serological studies on bancroftian filariasis in the same community. Quantitative examination of night blood samples (100 microliters) from 1129 individuals (76% of the population), using the counting chamber technique, revealed circulating microfilariae in 13.7% of the study group. Prevalence increased with age, and was higher in males (15.9%) than in females (11.6%). The geometric mean intensity among infected individuals was 223 microfilariae/ml blood (336 microfilariae/ml for males and 212 microfilariae/ml for females). Approximately 16.5% of the males aged > or = 15 years had hydrocele and 2.4% of the population had elephantiasis. The prevalence of these conditions increased with age; in those aged > or = 50 years, 23.8% of males had hydrocele and 6.1% of the total population had elephantiasis. Acute cases of epididymo-orchitis, adenolymphangitis and funiculitis were also seen. The microfilarial prevalence in males with hydrocele was not significantly different from that in asymptomatic males, but none of the elephantiasis cases had microfilaraemia. The striking difference in microfilaraemia pattern between hydrocele and elephantiasis patients may reflect different mechanisms underlying the development of these two chronic manifestations. The overall prevalences of microfilaraemia and clinical manifestations in this community were moderate when compared with those found in other studies carried out along the coast of eastern Africa.

The ability of Aedes aegypti mosquitoes to survive and transmit infective larvae of Brugia pahangi over successive blood meals

The mortality of Aedes aegypti mosquitoes increased; immediately following a blood meal containing microfilariae of Brugia pahangi, when infective larvae began to migrate out of the flight muscles and when infective larvae were lost from the mosquitoes during a blood meal. When infective mosquitoes took a second blood meal 86.2% of the infective larvae escaped from their bodies. However, only 50.3% escaped when mosquitoes fed through a thin layer of cotton. Infective larvae in the abdomen of the mosquitoes stood the least chance of escaping from the insects. When infective mosquitoes were offered a third blood meal four days later, the proportion of infective larvae in the head and labium had risen from 56.6% in the control group to 66.0% and 69.4% in the two test groups. At this third feed 54.7% and 75.7% of the infective larvae were lost from mosquitoes with a low and medium pre-feeding worm burden respectively. This suggests that the escape of infective larvae from mosquitoes with only a few worms is less efficient than from mosquitoes with a medium worm burden.

Vectors of Wuchereria bancrofti in the Sepik Provinces of Papua New Guinea

The vectors of Wuchereria bancrofti were investigated in two areas of the Sepik Provinces of Papua New Guinea. At the village of Yauatong, indoor-resting Anopheles punctulatus had an infection rate of 47.3% and an infective rate of 3.4%. No infections occurred in 382 Culex annulirostris obtained in night-landing catches. At the village of Yankok only An. koliensis contained infective larvae but An. punctulatus and Cx quinquefasciatus were infected. Probit values of the cumulative percentages of filariae-positive mosquitoes plotted against the logarithm of the corresponding filaria count are illustrated.

The effect of different types of skin surfaces on the transmission of Brugia pahangi infective larvae by the mosquito Aedes aegypti

The transmission of Brugia pahangi from Aedes aegypti into Meriones unguiculatus was compared for four different skin surfaces (HAIRY, CLOTH, CLEAN, GREASY). Unshaven jirds reduced the feeding avidity of infective mosquitoes. The loss of larvae from infective mosquitoes was greatest when these insects fed on "exposed" skin surfaces. Significantly fewer infective larvae penetrated the host when infective mosquitoes fed on a jird through a thin layer of cloth.

Bancroftian filariasis in Kenya. V. Mass treatment given by members of the local community

Diethylcarbamazine citrate was given for a total of 13 days to all people above one year of age in the following areas: (1) Mambrui, a town with a population of about 1300; (2) Jaribuni, a rural area with a population of about 1500; (3) The towns and villages on the islands off the north coast of Kenya with a population of over 20 000. The people were not weighed, but the daily dose was calculated according to age, being approximately 6 mg kg-1; drug taking was not supervised. The drug was mainly distributed by local people who also treated the side-effects; a doctor remained available in the background for consultation and advice. In Mambrui and Jaribuni, where baseline data had been collected, the number of infective larvae per 1000 vector-mosquitoes had decreased by more than 90% in the six to 12 months period after the campaign. Two years after the campaign, the microfilaria rate in Mambrui had dropped by 75%; in Jaribuni, a reduction of 75% was recorded in men, but in women it was only 48%. The overall rates after the mass treatment were 5.3 and 9.3% in Mambrui and Jaribuni, respectively. On the islands to the north, particularly good results were obtained in the village of Siyu, where the highest elephantiasis rate had been recorded; three years after the campaign, the microfilaria rate there had fallen to 3.6%, while none of 48 children under ten were found infected. The spectacular results in Lamu town may have been partly due to excellent mosquito control in preceding years. However, no control was achieved on the remote island of Ndau, probably because the treatment team left before the side-effects had disappeared, causing many people to stop the course of tablets. Thus, on the East African coast it is possible and cheap to control Bancroftian filariasis by mass treatment campaigns carried out by local, non-medical personnel.

Bancroftian filariasis in coastal North Trinidad, West Indies: intensity of transmission by Culex quinquefasciatus

The intensity of transmission of Wuchereria bancrofti was measured in a coastal community in north Trinidad where 15% of the residents were microfilaraemic. The only significant vector, Culex quinquefasciatus, was studied in detail to determine seasonal abundance, longevity, infection and infectivity rates. Only 2.1% of the indoor biting population and 7.0% of the indoor resting population were infected. The daily mortality rate of the vector increased with age from 27% at four days to 44 to 47% at 12 days and is considered to be an important factor contributing to the low infectivity rate (0.1%). The mean number of C. quinquefasciatus bites received by a resident of the village during the year of observation was estimated to be 17,948 of which 14 were from mosquitoes harbouring infective larvae of W. bancrofti. The results from similar studies elsewhere are outlined, but the need for standardization of procedure is emphasized before more reliable comparisons can be made.