Bancroftian Filariasis in Tanganyika: A Quantitative Study of the Uptake, Fate and Development of Microfilariae ofWuchereria BancroftiinCulex Fatigans

Modelling co-infection with malaria and lymphatic filariasis

Malaria and lymphatic filariasis (LF) continue to cause a considerable public health burden globally and are co-endemic in many regions of sub-Saharan Africa. These infections are transmitted by the same mosquito species which raises important questions about optimal vector control strategies in co-endemic regions, as well as the effect of the presence of each infection on endemicity of the other; there is currently little consensus on the latter. The need for comprehensive modelling studies to address such questions is therefore significant, yet very few have been undertaken to date despite the recognised explanatory power of reliable dynamic mathematical models. Here, we develop a malaria-LF co-infection modelling framework that accounts for two key interactions between these infections, namely the increase in vector mortality as LF mosquito prevalence increases and the antagonistic Th1/Th2 immune response that occurs in co-infected hosts. We consider the crucial interplay between these interactions on the resulting endemic prevalence when introducing each infection in regions where the other is already endemic (e.g. due to regional environmental change), and the associated timescale for such changes, as well as effects on the basic reproduction number R₀ of each disease. We also highlight potential perverse effects of vector controls on human infection prevalence in co-endemic regions, noting that understanding such effects is critical in designing optimal integrated control programmes. Hence, as well as highlighting where better data are required to more reliably address such questions, we provide an important framework that will form the basis of future scenario analysis tools used to plan and inform policy decisions on intervention measures in different transmission settings.

Ecological meta-analysis of density-dependent processes in the transmission of lymphatic filariasis: survival of infected vectors

The survival rate of infected vectors represents one of the fundamental components that influence the transmission dynamics of mosquito-borne diseases. Despite the occurrence of a number of studies investigating mosquito survival after infection with filarial worms, there remains conflicting evidence from both laboratory and field experiments as to the existence and mechanism for parasite-induced mortality among filarial mosquitoes. Here, we used a mixed effects meta-analytical framework to combine the data from all available vector-human host blood feeding experiments to evaluate the evidence for the impact of parasite load on the mortality rates of the three major lymphatic filariasis transmitting mosquito genera, Culex, Aedes, and Anopheles mosquitoes, over the extrinsic incubation period of parasitic infection. The results show that, despite the application of this approach, or in the case of Anopheles using a convention fixed effects logistic regression analysis supplemented with additional survival analysis of longitudinal data, no strong association between mortality rate and microfilariae (mf) uptake for either of the three mosquito genera is apparent in the combined data. Instead, a key finding is that study effects played a more crucial role in determining the levels of mortality observed in these experimental studies. This was most revealing in the case of Culex, given that the largest single study in terms of both the number of data points and range of mf intensities, in contrast to smaller studies, showed a significant positive association between mf intensity and mortality, indicating that in this genus at least, the detrimental effect of infection may be manifested only at the highest mf intakes. Although no density dependence in vector mortality was also observed for Aedes, possibly because of the use of restricted human mf intensity range in previous studies, an intriguing finding was that a significantly higher overall mortality was observed for this genus over mfintake ranges that produced much less corresponding mortality in Culex and Anopheles. The results also indicate that currently very little can be said about the survival rate of Anopheles mosquitoes infected with filarial worms because of the striking paucity of data for this genus. Further studies, using standardized methods and covering an appropriate range of mf uptake intensities and using study frameworks that allow the design and comparison of data from both experimental and field experiments, are clearly indicated if we are to reliably quantify the likely effect of filarial infection on vector survival.

Transmission dynamics of lymphatic filariasis: vector-specific density dependence in the development of Wuchereria bancrofti infective larvae in mosquitoes

The principles of meta-analysis developed in a previous study were extended to investigate the process of Wuchereria bancrofti (Cobbold) (Filarioidea: Onchocercidae) infection in mosquito (Diptera: Culicidae) hosts, focusing specifically on the functional forms and strength of density dependence in the development of ingested microfilariae (mf) to infective (third instar) larvae (L3). Mathematical models describing observed mf-L3 functional responses for each of the major three parasite-transmitting vector genera, Aedes, Culex and Anopheles mosquitoes, were fitted to paired mf-L3 data collated from all available studies in the published literature. Model parameters were estimated and compared by deriving and applying a data synthetic framework, based on applying a non-linear weighted regression model for fitting mathematical models to multistudy data. The results confirm previous findings of the existence of significant between-genera differences in the mf-L3 development relationship, particularly with regard to the occurrence of limitation in Culex mosquitoes and facilitation in Aedes and Anopheles mosquitoes. New and unexpected findings regarding L3 development from ingested mf were discovered as follows: (1) for Culex, overcompensation in L3 development at higher intensities of mf (or a peaked mf-L3 functional response) was detected; (2) for Aedes mosquitoes, facilitation (with an apparent asymptotic constraint on L3 development at high mf densities) was shown to be the major process governing L3 development, and (3) for Anopheles, a stronger facilitation type of response with no apparent saturation in L3 development appears to govern L3 output from ingested mf. These results yield major new insights regarding filarial vector infection dynamics and their potential impacts on parasite control, and demonstrate the efficacy of employing a data synthetic approach to reveal and estimate parasitic infection processes in host populations.

Vector survival and parasite infection: the effect of Wuchereria bancrofti on its vector Culex quinquefasciatus

This paper investigates a cohort of 2187 laboratory reared Culex quinquefasciatus fed on 69 human volunteers, including 59 persons with different levels of Wuchereria bancrofti microfilariae and 10 without microfilaria. Mosquitoes were followed until death. Mosquito survival was analysed in relation to the level of microfilaria in the human and larval count in the dead mosquito. Vector mortality during the extrinsic incubation period (12 days post-engorgement) was significantly higher in mosquitoes fed on microfilaraemic volunteers (50%) than in those fed on amicrofilaraemics (29%). Both the percentage infected and the geometric mean parasite density was significantly higher among mosquitoes which died before 13 days (45% infected and 10 larvae per infected mosquito) than those surviving beyond 13 days (39% and 2.2), suggesting a parasite loss of more than 80% during the extrinsic incubation period. A large proportion (62%) of the mosquitoes that died during the early of phase of parasite development were infected (36% in low, 26% in medium and 90% in high human Mf-density). Survival analysis showed that the parasite load in mosquitoes and the human Mf-density for a given parasite load are independent risk factors of vector survival. Overall, the hazard of dying was found to be 11-15 times higher among mosquitoes fed on microfilaraemic volunteers than those fed on amicrofilaraemics. The hazard doubles for every increase of about 60-70 parasites in the vector. As a consequence of the parasite-induced reduction in vector survival, the transmission success of the parasite is reduced. The implication of the results on control/elimination of lymphatic filariasis using mass-drug administration is discussed.

Transmission dynamics of lymphatic filariasis: density-dependence in the uptake of Wuchereria bancrofti microfilariae by vector mosquitoes

Gaining a better understanding of parasite infection dynamics in the vector mosquito (Diptera: Culicidae) population is central to improving knowledge regarding the transmission, persistence and hence control of lymphatic filariasis. Here, we use data on mosquito feeding experiments collated from the published literature to examine the available evidence regarding the functional form of the first component of this parasite-vector relationship for Wuchereria bancrofti (Filarioidea: Onchocercidae) causing Bancroftian filariasis, i.e. the rate of microfilariae (mf) uptake from the blood of infected humans by the feeding mosquito vector. Using a simple logarithmic regression model for describing the observed relationships between the mean numbers of mf ingested per mosquito and parasite load in humans in each study, and a linear mixed-effects meta-analytical framework for synthesizing the observed regressions across studies, we show here for the first time clear evidence for the existence of density-dependence in this process for all the three major filariasis transmitting mosquito vectors. An important finding of this study is that this regulation of mf uptake also varies significantly between the vector genera, being weakest in Culex, comparatively stronger in Aedes and most severe and occurring at significantly lower human mf loads in Anopheles mosquitoes. The analysis of the corresponding mf uptake prevalence data has further highlighted how density-dependence in mf uptake may influence the observed distributions of mf in vector populations. These results show that whereas strong regulation of mf uptake, especially when it leads to saturation in uptake at low human parasite intensities, can lead to static distributions of mf per mosquito with host parasite intensity, a weaker regulation of mf ingestion can give rise to changes in both mean mf loads and in the frequency distribution of parasites/mosquito with increasing human parasite intensity. These findings highlight the importance of considering local vector infection dynamics when attempting to predict the impacts of community-based filariasis control. They also emphasize the value of developing and applying robust meta-analytic methods for estimating functional relationships regarding parasitic infection from population ecological data.

Modelling the epidemiology, transmission and control of lymphatic filariasis

Wuchereria bancrofti transmitted by Culex quinquefasciatus accounts for >90% of the global burden of lymphatic filariasis (LF). Recent advances in diagnostic and control tools and a better epidemiological understanding of the disease have led to hope that LF is eradicable. The World Health Organization has helped a number of member countries to launch nation-wide programmes of mass treatment with antifilarial drugs such as diethylcarbamazine, albendazole and ivermectin, for the elimination of this disease. In order to make rational decisions about control strategies, reliable predictions of the long-term impact of such treatment, and of alternative interventions, need to be made, and these can only be based on a sound, quantitative understanding of the population biology of the parasites. Mathematical models have proven valuable in gaining quantitative insights into the population dynamics of the parasites, and may be used to make credible predictions of the likely outcomes of various control strategies. This article provides an overview of the development of the relevant mathematical/statistical models and of their application in studies of the epidemiology, transmission and control of lymphatic filariasis.

The transmission dynamics of bancroftian filariasis: the distribution of the infective larvae of Wuchereria bancrofti in Culex quinquefasciatus and Anopheles gambiae and its effect on parasite escape from the vector

The anatomical distribution of the infective larvae (L3) of Wuchereria bancrofti in Culex quinquefasciatus and Anopheles gambiae, and its influence on L3 escape, was evaluated by exposing the vectors to human individuals infected with W. bancrofti. After the extrinsic incubation period of W. bancrofti, a random sample of the infected mosquitoes was dissected to determine the distribution of infective larvae in the body of the mosquitoes and the proportion of mosquitoes that were infected. The remaining mosquitoes were exposed to an experimental definitive host (mouse skin). The infective larvae on and in the host tissues were counted. The engorged mosquitoes were dissected to determine the proportion infected and the distribution of L3 in mosquitoes after exposure. The results show that 53.4% of L3 escaped from Cx. quinquefasciatus to enter the experimental host whereas only 4% of L3 escaped from An. gambiae. Analysis of the results indicates that the number of L3 in the vector and the proportion of L3 in the head and mouthparts influence the number of L3 escaping from the vector to enter the definitive host and consequently the number that gain access to host tissues. The implication of these findings on the transference of the parasite from the vector to the definitive host is discussed.

Density-dependent processes in the transmission of human onchocerciasis: relationship between microfilarial intake and mortality of the simuliid vector

In order to construct an analytical model of onchocerciasis transmission, it is necessary to elucidate the functional relationships of the various population rate processes taking place within the human and vector hosts. Two previous papers have explored the evidence for density-dependent regulation in relation to microfilarial intake by, and larval development within, the Simulium host. This paper investigates the survivorship of wild-caught blackfly samples fed on subjects with different intensities of Onchocerca volvulus microfilarial infection. Analyses were based on data for Guatemalan S. ochraceum s.l. (possessing a well-developed cibarial armature), West African S. damnosum s.l. (forest species), and South Venezuelan S. guianense (the latter two lacking a toothed cibarium). The mean survival times of samples of the 3 species, kept under laboratory conditions, decreased as parasite intake increased, the rate of mortality being dependent on the fly's age (measured as time post-feeding) and on the worm load acquired. An empirical, time-dependent hazard function was fitted to observed death rates/fly/day which rose very shortly after engorgement, declined subsequently, and rose again throughout the extrinsic incubation period of the parasite. The parameters of this hazard model were all positively correlated with the density of microfilariae in the bloodmeal. Expressions of survivorship and life-expectancy as explicit functions of time post-feeding and mean parasite intake were derived. The average expectation of life at engorgement for uninfected flies in the laboratory was estimated to be around 1 week for both, armed and unarmed blackflies. Residual life-expectancy decreased with time post-feeding and microfilarial load in both categories of vectors. This decline (resulting from age- and parasite-dependent mortality rates) was much more pronounced in those species lacking a toothed fore-gut. Whilst a fraction of heavily infected S. ochraceum was able to survive the latent period of the parasite, being therefore potentially capable of transmitting the infection, equivalent worm loads in S. guianense resulted in a drastic reduction of the expectation of infective life. These results provide additional evidence to support the hypothesis that, in the case of intrinsically susceptible vectors, unarmed simuliids are more efficient at low microfilarial loads, when the transmission rate from human to vector host is higher, and parasite-induced fly mortality is negligible. The opposite takes place in armed flies, which perform poorly at low parasite burdens and better at heavier loads, with little parasite-induced vector death.

Density-dependent processes in the transmission of human onchocerciasis: relationship between the numbers of microfilariae ingested and successful larval development in the simuliid vector

A previous paper reported that the intake of Onchocerca volvulus microfilariae (mff) by different species of Simulium is essentially proportional to the parasite load in the skin of infected carriers. This paper examines the fate of the ingested mff in susceptible vectors to assess the relationship between parasite intake and infective larval output in blackfly species with and without well-developed cibarial armatures. Analysis is based on data from 3 onchocerciasis endemic areas: Guatemala (S. ochraceum s.l.), West Africa (S. damnosum s.l./S. sirbanum) and the Amazonian focus between South Venezuela and Northern Brazil (S. guianense and S. oyapockense s.l.). The data, which include published and unedited information collected in the field, record experimental studies of parasite uptake by wild flies maintained in captivity until the completion of the extrinsic incubation period. The relationship between L3 output (measured as the mean number of successful larvae/fly or, as the proportion of flies with infective larvae) and average microfilarial intake, was strongly non-linear. This non-linearity was best represented by a sigmoid function in case of armed simuliids (S. ochraceum s.l., S. oyapockense s.l.), or by a hyperbolic expression in that of unarmed flies (S. damnosum s.l., S. guianense). These results are compatible, respectively, with the patterns of 'initial facilitation' and 'limitation' described in culicid vectors of lymphatic filariases. A maximum mean number of 1-3 L3/fly was observed in all 4 vectors. It is concluded that O. volvulus larval development to the infective stage is regulated by density-dependent mechanisms acting at the early phase of microfilarial migration out of the blackfly's bloodmeal. Damage by the bucco-pharyngeal armature may also be density dependent. A hypothesis, based on this density dependence is forwarded to explain initial facilitation, so far only recorded in vectors with well-developed cibarial teeth. Our results provide quantitative support for the conjecture that chemotherapy alone is likely to have a greater impact on reducing onchocerciasis transmission in endemic areas where the main vector has a toothed fore-gut than in foci where the vectors have unarmed cibaria.

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 4. Facilitation, limitation, proportionality and their epidemiological significance

Quantitative understanding of the transmission dynamics of lymphatic filarial parasites is essential for the rational planning of control strategies. One of the most important determinants of transmission dynamics is the relationship between parasite yield, the success rate of ingested microfilariae (mf) becoming infective larvae in a mosquito vector, and mf density in the source of the human blood meal. Three types of relationship have been recognized in human filaria/mosquito couples--limitation, facilitation and proportionality; facilitation has hitherto been observed only in the couple Wuchereria bancrofti/Anopheles gambiae in Burkina Faso, in experimental studies on a high density mf carrier. The present paper demonstrates facilitation in W. bancrofti/An. gambiae and W. bancrofti/An. arabiensis in lower mf density carriers in The Gambia and Tanzania, and in W. bancrofti/An. funestus in Tanzania. Facilitation was not found in An. melas in The Gambia nor in An. merus in Tanzania. Analysis of published data shows limitation at low level mf densities in W. bancrofti/Culex quinquefasciatus in Sri Lanka, and in the same couple in India. Limitation also occurs in Brugia malayi/Aedes togoi in experimental cats; proportionality occurs in B. malayi/Mansonia bonneae in Malaysia. The epidemiological significance of these host/parasite relationships is discussed, and supporting evidence for its validity is presented from the published results of large-scale control programmes.

Factors affecting transmission of Wuchereria bancrofti by anopheline mosquitoes. 1. Uptake of microfilariae

Ingestion of Wuchereria bancrofti microfilariae (mf) from humans by 639 Anopheles gambiae, 557 An. arabiensis, 117 An. melas and 9 An. funestus was investigated. The mf densities in blood fell into 3 groups; 0-32mf/ml, 107-122 mf/ml and 421-1140 mf/ml. In An. gambiae and An. arabiensis percentage of mosquitoes ingesting mf was strongly associated with mf density in host blood; in An. melas the association was much weaker. Mean number of mf ingested per mosquito was also strongly correlated to mf density in An. gambiae and An. arabiensis but not in An. melas. At low mf densities both An. gambiae and An. arabiensis concentrated mf, with concentration decreasing as density increased. From regression analysis, observed and expected uptake of mf would be equal at 622.9 mf/ml in An. gambiae and 391.6 mf/ml in An. arabiensis.

Infection of Aedes aegypti with Brugia pahangi administered by enema: results of quantitative infection and loss of infective larvae during blood feeding

A technique for infecting mosquitoes with known numbers of Brugia pahangi microfilariae by enema is described. Virtually all mosquitoes receiving three microfilariae or more by this route contained infective larvae 10 days later. Within a range of 1 to 40 microfilariae, numbers of infective larvae recovered (Y) were related to the numbers of microfilariae administered (X) by the equation log10Y=0.04 + 0.84 log10X. Mosquitoes feeding on sugar for up to 20 days did not lose a significant number of infective larvae. A blood meal 10 days after infection reduced the proportion of females still infected, as well as the number of worms remaining in those still infected. A second blood meal on day 17 after infection only reduced the proportion of mosquitoes still infected. Adults reared on a low larval diet were less susceptible to infection, and a refractory mosquito strain did not support the development of third-stage larvae. Filarial worms had no effect on mosquito mortality until more than 30 microfilariae were administered.

Some observations on filariasis in Western Samoa after mass administration of diethylcarbamazine

An extremely efficient diethylcarbamazine administration campaign to eradicate Wuchereria bancrofti has been carried out in Western Samoa. The use of the membrane-filtration technique has shown that a large number of people exhibit extremely low microfilarial densities, often with less than 10 in 1 ml of venous blood. It was found that one of these low level microfilaria carriers readily infected the local vector Aedes polynesiensis and that development took place to the infective stage. It was estimated that 497 infective larvae of W. bancrofti will enter the human population of Western Samoa daily from these vectors. Resumption of filariasis transmission is possible and surveillance of the human and mosquito populations should be continued for a number of years and control measures taken quickly if further transmission occurs.