Infection and disease in lymphatic filariasis: an epidemiological approach

Modeling the contribution of antibody attack rates to single and dual helminth infections in a natural system

Within-host models of infection can provide important insights into the processes that affect parasite spread and persistence in host populations. However, modeling can be limited by the availability of empirical data, a problem commonly encountered in natural systems. Here, we used six years of immune-infection observations of two gastrointestinal helminths (Trichostrongylus retortaeformis and Graphidium strigosum) from a population of European rabbits (Oryctolagus cuniculus) to develop an age-dependent, mathematical model that explicitly included species-specific and cross-reacting antibody (IgA and IgG) responses to each helminth in hosts with single or dual infections. Different models of single infection were formally compared to test alternative mechanisms of parasite regulation. The two models that best described single infections of each helminth species were then coupled through antibody cross-immunity to examine how the presence of one species could alter the host immune response to, and the within-host dynamics of, the other species. For both single infections, model selection suggested that either IgA or IgG responses could equally explain the observed parasite intensities by host age. However, the antibody attack rate and affinity level changed between the two helminths, it was stronger against T. retortaeformis than against G. strigosum and caused contrasting age-intensity profiles. When the two helminths coinfect the same host, we found variation of the species-specific antibody response to both species together with an asymmetric cross-immune response driven by IgG. Lower attack rate and affinity of antibodies in dual than single infections contributed to the significant increase of both helminth intensities. By combining mathematical modeling with immuno-infection data, our work provides a tractable model framework for disentangling some of the complexities generated by host-parasite and parasite-parasite interactions in natural systems.

Evidence of nuclear transport mechanisms in the protozoan parasite Giardia lamblia

Nuclear-cytoplasmic trafficking of proteins is a highly regulated process that modulates multiple biological processes in eukaryotic cells. In Giardia lamblia, shuttling has been described from the cytoplasm to nuclei of proteins during the biological cell cycle of the parasite. This suggests that a mechanism of nucleocytoplasmic transport is present and functional in G. lamblia. By means of computational biology analyses, we found that there are only two genes for nuclear transport in this parasite, named Importin α and Importin β. When these transporters were overexpressed, both localized close to the nuclear envelope, and no change was observed in trophozoite growth rate. However, during the encystation process, both transporters induced an increase in the number of cysts produced. Importazole and Ivermectin, two known specific inhibitors of importins, separately influenced the encysting process by inducing an arrest in the trophozoite stage that prevents the production of cysts. This effect was more noticeable when Ivermectin, an anti-parasitic drug, was used. Finally, we tested whether the enzyme arginine deiminase, which shuttles from the cytoplasm to the nuclei during encystation, was influenced by these transporters. We found that treatment with each of the inhibitors abrogates arginine deiminase nuclear translocation and favors perinuclear localization. This suggests that Importin α and Importin β are key transporters during the encystation process and are involved, at least, in the transport of arginine deiminase into the nuclei. Considering the effect produced by Ivermectin during growth and encystation, we postulate that this drug could be used to treat giardiasis.

Complex ecological dynamics and eradicability of the vector borne macroparasitic disease, lymphatic filariasis

BACKGROUND

The current global efforts to control the morbidity and mortality caused by infectious diseases affecting developing countries--such as HIV/AIDS, polio, tuberculosis, malaria and the Neglected Tropical Diseases (NTDs)-have led to an increasing focus on the biological controllability or eradicability of disease transmission by management action. Here, we use an age-structured dynamical model of lymphatic filariasis transmission to show how a quantitative understanding of the dynamic processes underlying infection persistence and extinction is key to evaluating the eradicability of this macroparasitic disease.

METHODOLOGY/PRINCIPAL FINDINGS

We investigated the persistence and extinction dynamics of lymphatic filariasis by undertaking a numerical equilibrium analysis of a deterministic model of parasite transmission, based on varying values of the initial L3 larval density in the system. The results highlighted the likely occurrence of complex dynamics in parasite transmission with three major outcomes for the eradicability of filariasis. First, both vector biting and worm breakpoint thresholds are shown to be complex dynamic entities with values dependent on the nature and magnitude of vector-and host specific density-dependent processes and the degree of host infection aggregation prevailing in endemic communities. Second, these thresholds as well as the potential size of the attractor domains and hence system resilience are strongly dependent on peculiarities of infection dynamics in different vector species. Finally, the existence of multiple stable states indicates the presence of hysteresis nonlinearity in the filariasis system dynamics in which infection thresholds for infection invasion are lower but occur at higher biting rates than do the corresponding thresholds for parasite elimination.

CONCLUSIONS/SIGNIFICANCE

The variable dynamic nature of thresholds and parasite system resilience reflecting both initial conditions and vector species-infection specificities, and the existence of hysteresis loop phenomenon, suggests that eradication of filariasis may require taking a more flexible and locally relevant approach to designing elimination programmes compared to the current command and control approach advocated by the global programme.

Population genetics of concurrent selection with albendazole and ivermectin or diethylcarbamazine on the possible spread of albendazole resistance in Wuchereria bancrofti

The Global Program for the Elimination of Lymphatic Filariasis (GPELF) intends to achieve its aims through yearly mass treatments with albendazole (ABZ) combined with ivermectin (IVM) or diethylcarbamazine (DEC). The use of ABZ and IVM separately to combat parasites of veterinary importance has, on many occasions, resulted in widespread drug resistance. In order to help predict the spread of potential ABZ resistance alleles through a population of Wuchereria bancrofti, we have developed a mathematical model that incorporates population genetics into EPIFIL, a model which examines the transmission dynamics of the parasite. Our model considers the effect of the combined treatments on the frequency of a recessive allele, which confers ABZ resistance. The model predicts that after 10 yearly treatments with ALB and DEC, 85% coverage and an initial resistance allele frequency of 5%, the frequency of the resistance genotype will increase from 0.25 to 12.7%. If non-random mating is assumed, the initial genotype frequency will be 2.34% and will increase to 62.7%. ABZ and IVM combination treatment may lead to weaker selection for this genotype. Treatment coverage, initial allele frequencies and number of treatments also affect the rate of selection.

Age-dependent prevalence of asymptomatic amicrofilaraemic individuals in a Wuchereria bancrofti-endemic region of India

The age-related prevalence of asymptomatic amicrofilaraemics, apparently uninfected individuals, was determined in a Wuchereria bancrofti-endemic region of Orissa, India during 2001. The prevalence of these uninfected individuals was highest in the younger age groups (aged < or = 15 years), decreased rapidly in those aged > 15 to 40 years, and then stabilized in those aged > 40 years.

A 26-year follow-up of bancroftian filariasis in two communities in north-eastern Tanzania

The results of surveys, for human bancroftian filariasis, carried out in 1975 and 1991 in endemic communities in north-eastern Tanzania have already been reported. In 2001, all consenting individuals from two of these communities (Tawalani and Kwale) were re-surveyed, and many of the individuals examined in the earlier surveys were re-identified. The findings revealed an extraordinarily static pattern of infection and disease over the 26 years of follow-up. By 2001, despite brief interventions introduced after the first two surveys, the community prevalences and mean intensities of microfilaraemia had almost returned to pre-treatment levels. The majority of re-identified individuals who had been found microfilaraemic in 1975 and 1991 were also microfilaraemic in 2001. Being found microfilaraemic in the first survey was a highly significant risk factor for being found microfilaraemic in the subsequent surveys. These observations strongly indicate that re-infection with Wuchereria bancrofti commonly takes place, and that, compared with an individual who has never been infected, an individual who has been infected in the past has a much higher chance of acquiring a new, detectable infection. As most of the re-identified individuals who were amicrofilaraemic in 2001 but microfilaraemic in one or both of the earlier surveys were still positive for circulating filarial antigens in 2001, it seems that, once an infection has been acquired, the chance of ever becoming free of infection is small. No relationship between past microfilaraemia and the development of chronic filariasis was observed but the number of clinical cases seen in 2001, among the re-identified individuals, was low. The significance of these findings to our understanding of the natural history of W. bancrofti infection is discussed.

Genetics of susceptibility to human helminth infection

Recent studies have shown that host genetics is an important determinant of the intensity of infection and morbidity due to human helminths. Epidemiological studies of a number of parasite species have shown that the intensity of infection (worm burden) is a heritable phenotype. The proportion of variance in human worm burden explained by genetic effects varies from 0.21 to 0.44. Human genome scans have identified a locus responsible for controlling Schistosoma mansoni infection intensity on chromosome 5q31-q33, and loci controlling Ascaris lumbricoides intensity on chromosomes 1 and 13, although the genes involved have not yet been identified. There is also evidence for genetic control of pathology due to S. mansoni, and linkage has been reported to a region containing the gene for the interferon-gamma receptor 1 subunit. There is some evidence for genetic control of filarial infection, though little information on filarial disease. Association studies have provided evidence for major histocompatibility complex control of pathology in schistosomiasis and onchocerciasis. Recent candidate gene studies suggest a role of other immune response genes in controlling helminth infection and pathology, but require replication. Identification of the genetic loci involved may be important in the understanding of helminth epidemiology and the mechanisms of resistance and pathology.

Transmission intensity and the immunoepidemiology of bancroftian filariasis in East Africa

Previous attempts to determine the interactions between filariasis transmission intensity, infection and chronic disease have been limited by a lack of a theoretical framework that allows the explicit examination of mechanisms that may link these variables at the community level. Here, we show how deterministic mathematical models, in conjunction with analyses of standardized field data from communities with varying parasite transmission intensities, can provide a particularly powerful framework for investigating this topic. These models were based on adult worm population dynamics, worm initiated chronic disease and two major forms of acquired immunity (larval- versus adult-worm generated) explicitly linked to community transmission intensity as measured by the Annual Transmission Potential (ATP). They were then fitted to data from low, moderate and moderately high transmission communities from East Africa to determine the mechanistic relationships between transmission, infection and observed filarial morbidity. The results indicate a profound effect of transmission intensity on patent infection and chronic disease, and on the generation and impact of immunity on these variables. For infection, the analysis indicates that in areas of higher parasite transmission, community-specific microfilarial rates may increase proportionately with transmission intensity until moderated by the generation of herd immunity. This supports recent suggestions that acquired immunity in filariasis is transmission driven and may be significant only in areas of high transmission. In East Africa, this transmission threshold is likely to be higher than an ATP of at least 100. A new finding from the analysis of the disease data is that per capita worm pathogenicity could increase with transmission intensity such that the prevalences of both hydrocele and lymphoedema, even without immunopathological involvement, may increase disproportionately with transmission intensity. For lymphoedema, this rise may be further accelerated with the onset of immunopathology. An intriguing finding is that there may be at least two types of immunity operating in filariasis: one implicated in anti-infection immunity and generated by past experience of adult worms, the other involved in immune-mediated pathology and based on cumulative experience of infective larvae. If confirmed, these findings have important implications for the new global initiative to achieve control of this disease.

Human antibody responses to Wuchereria bancrofti infective larvae

Human IgG antibody responses to Wuchereria bancrofti third stage infective larvae (L3) surface and somatic antigens were studied by indirect immunofluorescence (IFA) and immunoblot with endemic Egyptian sera (n = 115) with the aim of identifying targets of protective immunity. Human sera variably recognized 14 major bands in L3 by immunoblot. The statistical significance of group differences in antibody prevalence was assessed by the chi-squared test. Children and young adults (aged 10-20 years) tended to have antibodies to more L3 somatic antigens than older adults, with significant differences for bands at 66, 60 and 5 kDa. Infected subjects had more consistent antibody responses to antigens at 55, 50 and 6 kDa than endemic normal subjects with negative serum filarial antigen tests, who are presumed to be uninfected. A 5 kDa antigen was preferentially recognized by the latter group. Antibodies to L3 surface antigens were equally prevalent in uninfected children (75%) and adults (90%) but less prevalent in people with microfilaremia (38%) than in amicrofilaremic subjects with or without filarial antigenemia (81%) (P < 0.001). IFA-positive sera showed significantly enhanced recognition of antigens at 66, 40 and 14 kDa in immunoblots relative to IFA-negative sera. Additional studies are needed to further characterize antigens identified in this study and to establish whether they are indeed targets of protective immunity in humans.

Population biology of human onchocerciasis

Human onchocerciasis (river blindness) is the filarial infection caused by Onchocerca volvulus and transmitted among people through the bites of the Simulium vector. Some 86 million people around the world are at risk of acquiring the nematode, with 18 million people infected and 600,000 visually impaired, half of them partially or totally blind. 99% of cases occur in tropical Africa; scattered foci exist in Latin America. Until recently control programmes, in operation since 1975, have consisted of antivectorial measures. With the introduction of ivermectin in 1988, safe and effective chemotherapy is now available. With the original Onchocerciasis Control Programme of West Africa coming to an end, both the new African Programme for Onchocerciasis Control and the Onchocerciasis Elimination Programme for the Americas, rely heavily on ivermectin self-sustained mass delivery. In consequence, the need for understanding the processes regulating parasite abundance in human and simuliid populations is of utmost importance. We present a simple mathematical framework built around recent analyses of exposure- and density-dependent processes operating, respectively, within the human and vector hosts. An expression for the basic reproductive ratio, R0, is derived and related to the minimum vector density required for parasite persistence in localities of West Africa in general and northern Cameroon in particular. Model outputs suggest that constraints acting against parasite establishment in both humans and vectors are necessary to reproduce field observations, but those in humans may not fully protect against reinfection. Analyses of host age-profiles of infection prevalence, intensity, and aggregation for increasing levels of endemicity and intensity of transmission in the Vina valley of northern Cameroon are in agreement with these results and discussed in light of novel work on onchocerciasis immunology.

The relationship between infection and disease in Wuchereria bancrofti infection in Ghana

The relationship between infection and clinical disease in Wuchereria bancrofti infection was investigated in a community-based study in different endemic areas in Ghana. At the individual level, there was no association between acute adenolymphangitis and infection (microfilaraemia) status. There was a negative association between infection status and lymphoedema/elephantiasis, but a positive association with hydrocele; however, the intensity of infection was negatively associated with both elephantiasis and hydrocele. The community prevalence of infection was strongly associated with the prevalence of clinical filariasis (especially hydrocele). There was a strong positive association between the prevalence of infection in males and the odds of a case of hydrocele being microfilaraemic, suggesting that there is no acquired immunity to reinfection in cases of hydrocele. The pathophysiologies of elephantiasis and hydrocele may therefore differ from one another, and require further investigation.

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.

Prevalence of hydrocele as a rapid diagnostic index for lymphatic filariasis

The real burden of lymphatic filariasis in most endemic areas remains unknown even thought it is a major public health problem in many tropical countries, particularly in sub-saharan African. The nocturnal periodicity of the parasite requires parasitological examinations to be done at night. The aim of this study was to develop and validate rapid epidemiological assessment tools for the community diagnosis of lymphatic filariasis, that may be used in the future to determine the distribution of the disease and identify high risk communities in Ghana. Twenty communities with varying endemicity of filariasis were sampled from 3 endemic districts. Community members were selected for the study using a modified Expanded Programme for Immunization (EPI) cluster sampling technique. The prevalence of hydrocele was high (range 4.5-40.75%, mean = 17.78%) and the community prevalence of microfilaraemia correlated well with that of hydrocele (r = 0.84). The findings suggest that it is possible to obtain reliable and valid estimates of the community burden of lymphatic filariasis using the prevalence of hydrocele as a diagnostic index.

Wuchereria bancrofti microfilaraemia in children in relation to parental infection status

A total of 946 families with 2302 children was surveyed for microfilaraemia due to Wuchereria bancrofti. The prevalence of microfilaraemia among offspring born to microfilaraemic parents was significantly higher than in those born to amicrofilaraemic parents (P = 0.0049; relative risk = 3.40). However, there was no statistically significant difference between the prevalence of microfilaraemia in children born to microfilaraemic mothers or microfilaraemic fathers, suggesting that parental (not only maternal) infection is the important risk factor, and it may be exposure within the household which is important. Logistic regression analyses also confirmed that the risk of infection for offspring born to either microfilaraemic mothers or microfilaraemic fathers was higher than that for offspring born to amicrofilaraemic parents and indicated that infection in children < or = 20 years old was primarily dependent on parental infection status and minimally influenced by factors other than household exposure.

Past exposure and the dynamics of lymphatic filariasis infection in young children

This study utilizes parallel, longitudinal entomological and parasitological data collected during a 5-year vector control programme in Pondicherry, South India, to quantify Wuchereria bancrofti transmission from the vector to the human population. A simple mathematical model, derived from the standard catalytic model, is used to examine the hypothesis that current infection prevalence in young children is a dynamical function of their cumulative past exposure to infective bites. Maximum likelihood fits of the model to the observed data indicate a constant child infection rate with age, above a threshold representing the pre-patent period, or equivalently, the cumulative biting intensity required to produce patent infections. Extrapolation of the model allows the crude estimation of the equilibrium microfilaria age-prevalence curve due to control. The results suggest that vector control alone may have little impact on the overall age-prevalence of infection even when sustained for long periods. These observations are discussed in terms of the likely impact of density dependent mechanisms, such as acquired immunity, on model predictions.

Th1-like antifilarial immune responses predominate in antigen-negative persons

To characterize immune responses associated with the putatively immune state in bancroftian filariasis (that is, both microfilaria and antigen free), humoral and cellular responses were compared among antigen- and microfilaria-negative, antigen-positive and microfilaria-negative, and microfilaria-positive individuals. Antifilarial isotype levels were measured by enzyme-linked immunosorbent assay. Peripheral blood mononuclear cell responses were measured by proliferation, by bioassay for interleukin-2 (IL-2) and IL-10, and by reverse transcription-PCR for IL-4, IL-5, and gamma interferon. The absence of circulating filarial antigen was associated with Th1-like responses, including significantly higher proliferative (P < 0.001) and IL-2 (P = 0.008) responses and a higher prevalence of gamma interferon (0.02 < P < 0.1) responses. Significantly elevated antifilarial immunoglobulin G4 (IgG4) levels (P = 0.0035) were associated with antigenemia, whereas microfilaremia was associated with significantly decreased antifilarial IgG2 levels (P = 0.0014). IL-4 mRNA levels were not significantly different among the three groups; however, there was a subpopulation of microfilaremic individuals who did not make detectable levels of IL-4 mRNA and who produced low antifilarial IgG4 levels compared with those of individuals who had detectable levels of IL-4 mRNA. IL-5 mRNA levels also were not significantly different among groups; however, more microfilaremic individuals produced IL-5 mRNA in response to adult filarial antigens, and total parasite-specific IL-4 and IL-5 mRNA levels were significantly correlated (P = 0.05). Although longitudinal data are not currently available, the elevated Th1-like responses in antigen- and microfilaria-negative individuals are consistent with the hypothesis that these responses contribute to protection in putatively immune individuals.

Amicrofilaraemic carriers of adult Wuchereria bancrofti

To determine the extent to which Wuchereria bancrofti infections can be detected in asymptomatic amicrofilaraemic men in Greater Recife, Brazil, we studied 100 asymptomatic men who were long-term residents (> or = 15 years) of this filariasis-endemic area and who were amicrofilaraemic in 60 microL of capillary blood collected at night. Increasing amounts (1, 5, and 10 mL) of venous blood were collected in consecutive weeks, filtered, and examined for microfilariae; 27 men were eventually found to be microfilariaemic and 10 remained amicrofilaraemic but were found to be carriers of living adult W. bancrofti by ultrasound examination of the scrotal area. Thus, 37% of 'amicrofilaraemic' men (in 60 microL of blood) were found to be infected by more thorough investigation. Ultrasound is a valuable tool to identify adult worm infections in amicrofilaraemic persons, particularly for evaluation of serological assays and immunological studies in which the distinction between 'amicrofilaraemic adult worm carriers' and 'endemic normal subjects' is critical.

Re-assessing the global prevalence and distribution of lymphatic filariasis

This paper estimates the global burden of lymphatic filariasis based on a review of the published literature on infection and disease surveys. A method for aggregating and projecting prevalence data from individual studies to national, regional and global levels, which also facilitates the estimation of gender and age-specific burdens, is presented. The method weights in favour of the larger, and hence presumbably more reliable, studies and relies on estimated empirical relationships between gender, age, infection and disease in order to correct studies with incomplete data. The results presented here suggest that although the overall prevalence of filariasis cases is 2.0% globally (approximately totalling 119 million cases), the disease continues to be of considerable local importance, particularly in India and Sub-Saharan Africa. Estimates by age and gender clearly show that, unlike other helminth infections, filariasis is mainly a disease of the adult and older age-classes and appears to be more prevalent in males. This work suggests that the derivation of more accurate estimates of the burden of filariasis will require a better understanding of both the epidemiology and the spatial aspects of infection and disease. It also suggests that filariasis is preventable based on a geographically targeted strategy for control.

Bancroftian filariasis: analysis of infection and disease in five endemic communities of north-eastern Tanzania

Clinical and parasitological surveys for bancroftia filariasis were carried out in five endemic communities in north-eastern Tanzania, covering a population of 3086 individuals. High microfilarial (mf) prevalences (17.7%-34.7%) and mf geometric mean intensities (251-1122 microfilariae/ml) were observed in the communities. The mf prevalence generally increased with age, but often levelled out in the older age groups. Larger variability was observed in individual mf intensities and no clear association between mf geometric mean intensity and age or sex was seen. Hydrocele was the most common clinical manifestation (with a prevalence of 30.2%-40.0% in male subjects aged > or = 20 years) followed by leg elephantiasis (with a prevalence of 2.0%-6.8% in all subjects aged > or = 20 years). In four of the five communities, there was no significant difference in mf prevalence in males aged > or = 20 years between those with and without hydrocele. In all the communities, the mf geomtric mean intensities in microfilaraemic males with and without hydrocele were not significantly different. The present study therefore did not indicate any association between hydrocele in males (the most common type of chronic clinical manifestation seen) and presence or absence of microfilaraemia. In contrast, only two (4.4%) of the 45 subjects with leg elephantiasis were microfilaraemic. In children aged 1-15 years, mf prevalence was significantly higher among those with microfilaraemic mothers (18.0%) than among those with amicrofilaraemic mothers (7.9%). The children of microfilaraemic mothers were therefore at 2.3-fold higher risk of becoming microfilaraemic than the children of amicrofilaraemic mothers. No relationship between the mf prevalence of the children and the mf status of their fathers was observed.

A 16-year follow-up study on bancroftian filariasis in three communities of north-eastern Tanzania

The results of a 16-year, parasitological and clinical, follow-up study on bancroftian filariasis in three villages in north-eastern Tanzania are reported and analysed with respect to the long-term development of microfilaraemia and disease. The study revealed an extraordinary static situation, both at the community level and at the individual level. No significant differences were observed in any of the three villages, with respect to total or age-specific prevalences of microfilaraemia, hydrocele in males aged > or = 20 years or leg elephantiasis in all subjects aged > or = 20 years, when the results from 1975 were compared with those from 1991. Overall, 542 of the subjects (37.2% of the total) examined in 1975 were re-identified and re-examined in 1991. Of those microfilaraeic in 1975, 81.9% were also microfilaraemic in 1991. Similarly, 81.3% of those amicrofilaraemic in 1975 were also amicrofilaraemic in 1991. Thus, most subjects had the same microfilarial (mf) status at both surveys. The low mf loss rate indicates that re-infection of microfilaraemic individuals commonly takes place, and results in repeated sequences of microfilaraemia in the same individuals. The rate of gain of microfilaraemia was independent of age, thus indicating no age-related change in the examined population's susceptibility to develop or sustain microfilaraemia. Individual differences in susceptibility to develop mirofilaraemia therefore appear to be innately determined, or to be acquired in pre-natal or early post-natal life. Neither the presence of hydrocele in 1991, nor the development of new cases of hydrocele over the 16-year period were related to the mf status in 1975 or 1991, and no association between microfilaraemia and this chronic clinical manifestation was observed. The association between leg elephantiasis and mf status could not be analysed because of the small sample size.

Bancroftian filariasis: the pattern of microfilaraemia and clinical manifestations in three endemic communities of Northeastern Tanzania

Individuals from three villages in northeastern Tanzania, located 40 km inland from the Indian Ocean coast, were surveyed for parasitological and clinical evidence of bancroftian filariasis. Microfilarial (mf) prevalences ranged from 22.2 to 37.6%, and mf geometric mean intensities (GMI) ranged from 546 to 735 mf/ml blood, in the three villages. Microfilaraemia was rare in children below five years. The mf prevalences increased with age, reaching from 35.9 to 49.2% in individuals aged 45 years and above. No association between mf GMI and age was observed in any of the villages. Hydrocele was the most common chronic clinical manifestation, with prevalences ranging from 14.5 to 21.3% for all males, and from 52.9 to 62.1% for males aged 45 years and above. From 0.6 to 3.3% of the inhabitants in the three villages had elephantiasis, which most often affected the legs. Microfilaraemia was common in males with hydrocele, and in males of 45 years and above there was no significant difference in mf prevalence between males with (42.5%) and without (55.2%) hydrocele. In contrast, microfilariae were only detected in the blood from one of 18 individuals with elephantiasis. With respect to hydrocele, the present results do not support recently forwarded hypotheses assuming a general negative relationship between microfilaraemia and chronic clinical manifestations in bancroftian filariasis.

The association between microfilaraemia and disease in lymphatic filariasis

A standard tenet in the epidemiology of lymphatic filariasis is that patent infection is negatively related to chronic disease. We examine the empirical evidence for this relation by using published data from field studies carried out in a variety of bancroftian filariasis endemic areas. Meta-analysis of the individual study results for each disease category of hydrocele in males only, lymphoedema only, and both conditions combined (total chronic disease) indicate, contrary to expectation, no evidence for a negative association between infection and disease. Indeed, the trend of the empirical evidence is towards the opposite direction, with the majority of studies showing equal propensity of disease in microfilaraemics (mf+ves) and amicrofilaraemics (mf-ves), and more studies indicating a positive rather than a negative relation. There was also a trend for more positive studies for hydrocele compared to lymphoedema. Theoretical analysis suggests that between-study differences in blood sample volumes are unlikely to confound this finding. Analysis of between-study heterogeneity suggests that variations in the local incidence or prevalence of infection rather than unique geographical, including vector, differences might underlie the observed between-study variability in the microfilaraemia-disease association. These results are discussed in terms of dynamic explanations for infection-disease relations in lymphatic filariasis.

Density-dependent processes in the transmission of human onchocerciasis: intensity of microfilariae in the skin and their uptake by the simuliid host

The transmission success of Onchocerca volvulus is thought to be influenced by a variety of regulatory or density-dependent processes that act at various points in the two-host life-cycle. This paper examines one component of the life-cycle, namely, the ingestion of microfilariae by the simuliid vector, to assess the relationship between intake of larvae and the density of parasites in the skin of the human host. Analysis is based on data from three areas in which onchocerciasis is endemic and includes published information as well as new data collected in field studies. The three areas are: Guatemala (Simulium ochraceum s.l.), West and Central Africa (savanna members of the S. damnosum complex), and South Venezuela (S. guianense). The data record experimental studies of parasite uptake by flies captured in the field and fed to repletion on locally infected subjects who harboured varying intensities of dermal microfilarial infection. Regression analyses of log transformed counts of parasite burdens ingested by the flies plotted against log transformed counts of microfilariae per mg of skin revealed little evidence for saturation in parasite uptake by the flies as the intensity in the human host increased. There was a positive and highly significant rank correlation between both variables for the three blackfly species. In an alternative analysis a model was fitted to data on prevalence of flies with ingested microfilariae (mff) versus dermal mean intensities. The model assumed an overdispersed distribution of the number of mff/fly and a given functional relationship between intake and skin load. The results of both approaches were consistent. It is concluded that parasite ingestion by the vector host is not strongly density dependent in the three geographical areas and ranges of dermal loads examined. It therefore appears that this transmission process is of reduced importance as a regulatory mechanism in the dynamics of parasite population growth.

Maternal filarial infection — A persistent risk factor for microfilaremia in offspring?

The observation that children born to mothers that are infected with Wuchereria bancrofti ore more susceptible to filarial infection than those born to uninfected mothers, raises many questions, particularly regarding immune mechanisms. In this article, Allen Hightower, Patrick Lommie and Mark Eberhard discuss these issues and their implications for the epidemiology of filarial infection.