A model for the dynamics of human lymphatic filariasis

The Domestic Dog as a Laboratory Host for Brugia malayi

Of the three nematodes responsible for lymphatic filariasis in humans, only Brugia malayi is actively maintained in research settings owing to its viability in small animal hosts, principal among which is the domestic cat. While the microfilaremic feline host is necessary for propagation of parasites on any significant scale, this system is plagued by a number of challenges not as pronounced in canine filarial models. For this reason, we investigated the capacity in which dogs may serve as competent laboratory hosts for B. malayi. We infected a total of 20 dogs by subcutaneous injection of 500 B. malayi third-stage larvae (L3) in either a single (n = 10) or repeated infection events (125 L3 per week for four weeks; n = 10). Within each group, half of the individuals were injected in the inguinal region and half in the dorsum of the hind paw. To track the course of microfilaremia in this host, blood samples were examined by microscopy biweekly for two years following infection. Additionally, to identify cellular responses with potential value as predictors of patency, we measured peripheral blood leukocyte counts for the first year of infection. A total of 10 of 20 dogs developed detectable microfilaremia. Peak microfilaria density varied but attained levels useful for parasite propagation (median = 1933 mL⁻¹; range: 33–9950 mL⁻¹). Nine of these dogs remained patent at 104 weeks. A two-way ANOVA revealed no significant differences between infection groups in lifetime microfilaria production (p = 0.42), nor did regression analysis reveal any likely predictive relationships to leukocyte values. The results of this study demonstrate the competence of the dog as a host for B. malayi and its potential to serve in the laboratory role currently provided by the cat, while also clarifying the potential for zoonosis in filariasis-endemic regions.

Considering humans as habitat reveals evidence of successional disease ecology among human pathogens

The realization that ecological principles play an important role in infectious disease dynamics has led to a renaissance in epidemiological theory. Ideas from ecological succession theory have begun to inform an understanding of the relationship between the individual microbiome and health but have not yet been applied to investigate broader, population-level epidemiological dynamics. We consider human hosts as habitat and apply ideas from succession to immune memory and multi-pathogen dynamics in populations. We demonstrate that ecologically meaningful life history characteristics of pathogens and parasites, rather than epidemiological features alone, are likely to play a meaningful role in determining the age at which people have the greatest probability of being infected. Our results indicate the potential importance of microbiome succession in determining disease incidence and highlight the need to explore how pathogen life history traits and host ecology influence successional dynamics. We conclude by exploring some of the implications that inclusion of successional theory might have for understanding the ecology of diseases and their hosts.

This study explores the analogy between ecological succession in terrestrial ecosystems and infections in a human-host landscape over time, showing how the ecosystem of long-term multi-pathogen dynamics within and among hosts may be a critical missing consideration in understanding epidemiology.

Protective CD4+ Th1 cell-mediated immunity is reliant upon execution of effector function prior to the establishment of the pathogen niche

Intracellular infection with the parasite Leishmania major features a state of concomitant immunity in which CD4+ T helper 1 (Th1) cell-mediated immunity against reinfection coincides with a chronic but sub-clinical primary infection. In this setting, the rapidity of the Th1 response at a secondary site of challenge in the skin represents the best correlate of parasite elimination and has been associated with a reversal in Leishmania-mediated modulation of monocytic host cells. Remarkably, the degree to which Th1 cells are absolutely reliant upon the time at which they interact with infected monocytes to mediate their protective effect has not been defined. In the present work, we report that CXCR3-dependent recruitment of Ly6C+ Th1 effector (Th1EFF) cells is indispensable for concomitant immunity and acute (<4 days post-infection) Th1EFF cell-phagocyte interactions are critical to prevent the establishment of a permissive pathogen niche, as evidenced by altered recruitment, gene expression and functional capacity of innate and adaptive immune cells at the site of secondary challenge. Surprisingly, provision of Th1EFF cells after establishment of the pathogen niche, even when Th1 cells were provided in large quantities, abrogated protection, Th1EFF cell accumulation and IFN-γ production, and iNOS production by inflammatory monocytes. These findings indicate that protective Th1 immunity is critically dependent on activation of permissive phagocytic host cells by preactivated Th1EFF cells at the time of infection.

Caring for patients in the global programme to eliminate lymphatic filariasis

Clinical lymphatic filariasis (LF) is a debilitating, disfiguring medical condition with severe psychosocial consequences for patients and their families. Addressing these patients’ medical needs is a major component of the global programme to eliminate lymphatic filariasis (GPELF). In the 20 y of providing a minimal package of care many thousands of surgical operations to correct LF hydrocoeles been performed and national programmes in >90% of LF endemic countries have received the training needed to care for their patients. The creation of educational materials detailing appropriate patient care, together with increased funding, have been key catalysts in increasing awareness of clinical LF in recent years. Nevertheless, the implementation of care for these patients has often faced challenges that have led to delays in fully implementing the patient care component of GPELF; these include locating these often stigmatised individuals, maintaining provision of the necessary consumables (e.g. soaps and creams) and maintaining programme support within already overstretched national LF teams. As the LF global programme moves to achieve success by 2030 it will be vital to continue to focus efforts on the care and rehabilitation of those suffering from lymphoedema and hydrocoeles, learning from the experiences of the past 20 y.

Advances in Vaccine Development for Human Lymphatic Filariasis

According to the World Health Organization, over 880 million people are currently at risk of acquiring lymphatic filariasis (LF) in over 52 countries worldwide. Current approaches to control LF by 2020 are short of the anticipated goal. Several studies suggest the existence of protective immunity against LF in humans. Thus, it is possible to develop a prophylactic vaccine against LF in humans. Several potential vaccine candidates were identified and tested for their potential against LF. To date, preclinical studies suggest that it is possible to develop a prophylactic vaccine against LF. Much work needs to be done, but it is clear that a prophylactic vaccine, combined with targeted chemotherapy, is critically required for eliminating LF worldwide.

Understanding the relationship between prevalence of microfilariae and antigenaemia using a model of lymphatic filariasis infection

Background

Lymphatic filariasis is a debilitating neglected tropical disease that affects impoverished communities. Rapid diagnostic tests of antigenaemia are a practical alternative to parasitological tests of microfilaraemia for mapping and surveillance. However the relationship between these two methods of measuring burden has previously been difficult to interpret.

Methods

A statistical model of the distribution of worm burden and microfilariae (mf) and resulting antigenaemic and mf prevalence was developed and fitted to surveys of two contrasting sentinel sites undergoing interventions. The fitted model was then used to explore the relationship in various pre- and post-intervention scenarios.

Results

The model had good quantitative agreement with the data and provided estimates of the reduction in mf output due to treatment. When extrapolating the results to a range of prevalences there was good qualitative agreement with published data.

Conclusions

The observed relationship between antigenamic and mf prevalence is a natural consequence of the relationship between prevalence and intensity of adult worms and mf production. The method described here allows the estimation of key epidemiological parameters and consequently gives insight into the efficacy of an intervention programme.

Chronic parasitic infection maintains high frequencies of short-lived Ly6C+CD4+ effector T cells that are required for protection against re-infection

In contrast to the ability of long-lived CD8⁺ memory T cells to mediate protection against systemic viral infections, the relationship between CD4⁺ T cell memory and acquired resistance against infectious pathogens remains poorly defined. This is especially true for T helper 1 (Th1) concomitant immunity, in which protection against reinfection coincides with a persisting primary infection. In these situations, pre-existing effector CD4 T cells generated by ongoing chronic infection, not memory cells, may be essential for protection against reinfection. We present a systematic study of the tissue homing properties, functionality, and life span of subsets of memory and effector CD4 T cells activated in the setting of chronic Leishmania major infection in resistant C57Bl/6 mice. We found that pre-existing, CD44⁺CD62L⁻T-bet⁺Ly6C⁺ effector (T_EFF) cells that are short-lived in the absence of infection and are not derived from memory cells reactivated by secondary challenge, mediate concomitant immunity. Upon adoptive transfer and challenge, non-dividing Ly6C⁺ T_EFF cells preferentially homed to the skin, released IFN-γ, and conferred protection as compared to CD44⁺CD62L⁻Ly6C⁻ effector memory or CD44⁺CD62L⁺Ly6C⁻ central memory cells. During chronic infection, Ly6C⁺ T_EFF cells were maintained at high frequencies via reactivation of T_CM and the T_EFF themselves. The lack of effective vaccines for many chronic diseases may be because protection against infectious challenge requires the maintenance of pre-existing T_EFF cells, and is therefore not amenable to conventional, memory inducing, vaccination strategies.

Naturally acquired resistance to reinfection by numerous infectious pathogens including Leishmania, Plasmodium, Mycobacterium, and parasitic worms, typically coincides with an ongoing primary infection. This natural resistance to reinfection, termed concomitant immunity, is often referred to as a memory response and provides the rationale for the vaccine effort against these infectious pathogens. However, immune memory is mediated by populations of long-lived cells that do not require an ongoing primary infection to mediate protection. The requirement for chronic infection to maintain concomitant immunity suggests that the critical cells that mediate this immunity are not memory cells. In the present study we define short-lived effector T cells that pre-exist secondary challenge, not memory cells, as the critical cells that mediate concomitant immunity. These observations provide direct evidence on a cellular level that conventional vaccination strategies against chronic infectious diseases, whose development is predicated upon the belief that concomitant immunity can be mediated by long-lived memory cells, are unlikely to succeed.

Early Decision: Effector and Effector Memory T Cell Differentiation in Chronic Infection

As effector memory T cells (Tem) are the predominant population elicited by chronic parasitic infections, increasing our knowledge of their function, survival and derivation, as phenotypically and functionally distinct from central memory and effector T cells will be critical to vaccine development for these diseases. In some infections, memory T cells maintain increased effector functions, however; this may require the presence of continued antigen, which can also lead to T cell exhaustion. Alternatively, in the absence of antigen, only the increase in the number of memory cells remains, without enhanced functionality as central memory. In order to understand the requirement for antigen and the potential for longevity or protection, the derivation of each type of memory must be understood. A thorough review of the data establishes the existence of both memory (Tmem) precursors and effector T cells (Teff) from the first hours of an immune response. This suggests a new paradigm of Tmem differentiation distinct from the proposition that Tmem only appear after the contraction of Teff. Several signals have been shown to be important in the generation of memory T cells, such as the integrated strength of “signals 1-3” of antigen presentation (antigen receptor, co-stimulation, cytokines) as perceived by each T cell clone. Given that these signals integrated at antigen presentation cells have been shown to determine the outcome of Teff and Tmem phenotypes and numbers, this decision must be made at a very early stage. It would appear that the overwhelming expansion of effector T cells and the inability to phenotypically distinguish memory T cells at early time points has masked this important decision point. This does not rule out an effect of repeated stimulation or chronic inflammatory milieu on populations generated in these early stages. Recent studies suggest that Tmem are derived from early Teff, and we suggest that this includes Tem as well as Tcm. Therefore, we propose a testable model for the pathway of differentiation from naïve to memory that suggests that Tem are not fully differentiated effector cells, but derived from central memory T cells as originally suggested by Sallusto et al. in 1999, but much debated since.

Immunopathogenesis of lymphatic filarial disease

Although two thirds of the 120 million people infected with lymph-dwelling filarial parasites have subclinical infections, ~40 million have lymphedema and/or other pathologic manifestations including hydroceles (and other forms of urogenital disease), episodic adenolymphangitis, tropical pulmonary eosinophilia, lymphedema, and (in its most severe form) elephantiasis. Adult filarial worms reside in the lymphatics and lymph nodes and induce changes that result in dilatation of lymphatics and thickening of the lymphatic vessel walls. Progressive lymphatic damage and pathology results from the summation of the effect of tissue alterations induced by both living and nonliving adult parasites, the host inflammatory response to the parasites and their secreted antigens, the host inflammatory response to the endosymbiont Wolbachia, and those seen as a consequence of secondary bacterial or fungal infections. Inflammatory damage induced by filarial parasites appears to be multifactorial, with endogenous parasite products, Wolbachia, and host immunity all playing important roles. This review will initially examine the prototypical immune responses engendered by the parasite and delineate the regulatory mechanisms elicited to prevent immune-mediated pathology. This will be followed by a discussion of the proposed mechanisms underlying pathogenesis, with the central theme being that pathogenesis is a two-step process-the first initiated by the parasite and host innate immune system and the second propagated mainly by the host's adaptive immune system and by other factors (including secondary infections).

T-cell activation and the balance of antibody isotypes in human lymphatic filariasis

Human filarial infection presents a spectrum of clinical states with two major poles: asymptomatic microfilaraemia and amicrofilaraemic chronic disease. Microfilaremia is associated with a Th1-type tolerance, and maximal IgG4 antibodies, while elephantiasis patients react across a broad range of immune parameters. In this review, Rick Maizels and his colleagues discuss recent advances in the immunology of human filariasis and present a summary of their latest studies in an endemic area of Indonesia.

The dynamics ofWuchereria bancroftiinfection: a model-based analysis of longitudinal data from Pondicherry, India

This paper presents a model-based analysis of longitudinal data describing the impact of integrated vector management on the intensity ofWuchereria bancroftiinfection in Pondicherry, India. The aims of this analysis were (1) to gain insight into the dynamics of infection, with emphasis on the possible role of immunity, and (2) to develop a model that can be used to predict the effects of control. Using the LYMFASIM computer simulation program, two models with different types of immunity (anti-L3 larvae or anti-adult worm fecundity) were compared with a model without immunity. Parameters were estimated by fitting the models to data from 5071 individuals with microfilaria-density measurement before and after cessation of a 5-year vector management programme. A good fit, in particular of the convex shape of the age-prevalence curve, required inclusion of anti-L3 or anti-fecundity immunity in the model. An individual's immune-responsiveness was found to halve in ~10 years after cessation of boosting. Explanation of the large variation in Mf-density required considerable variation between individuals in exposure and immune responsiveness. The mean life-span of the parasite was estimated at about 10 years. For the post-control period, the models predict a further decline in Mf prevalence, which agrees well with observations made 3 and 6 years after cessation of the integrated vector management programme.

Preadult stage parasites and multiple timed exposure to infective larvae are involved in development of limb edema in Brugia malayi-infected Indian leaf monkeys (Presbytis entellus)

The pathogenesis of filarial limb edema is not known. The role of parasitological variables and parasite-mediated phenomena in the development of limb edema was investigated in the Presbytis entellus-Brugia malayi model. Infection was initiated with subcutaneous inoculation of infective third-stage larvae (L(3)), and the animals were reexposed to different doses of L(3) at the prepatent, patent, and diminishing microfilaremia (0 to 5% of peak microfilaremia count) stages of infection. A large L(3) inoculum size and repeated inoculation in the ankle region during the prepatent, patent, and diminishing microfilaremia stages of infection were found to be necessary for reproducible induction of limb edema. The preadult stage of the parasite was found to be the most potent inducer of limb edema, followed by L(5) and L(4). The presence of the proinflammatory cytokines tumor necrosis factor alpha, interleukin-1beta, and interleukin-6 in edema fluid in the leg receiving the parasite challenge indicated that the limb edema development was due to parasite-mediated cytokine responses. The absence of bacterial infection or anti-streptolysin O titer in the edema fluid and blood indicated that bacterial infection is not necessary for the development of limb edema.

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.

Host protective immunity and vaccine development studies in lymphatic filariasis

Lymphatic filariasis caused mainly by infection fromWuchereria bancrofti andBrugia malayi remains as the major cause of clinical morbidity in tropical and subtropical countries. Development of vaccine against filarial infection can act as additional measure to the existing therapeutic and vector control methods in the control of this disease. The main hurdles in the development of anti-filarial vaccine are the strict primate specificity ofWuchereria bancrofti, the paucity of parasite material, the diversity of clinical manifestations and their associated complex immune responses, lack of clear understanding on host-parasite interactions and the mechanisms involved in protective immunity. However in the past few years, the information generated in immuno-epidemiological studies, correlated with observations in experimental animals suggests that a filarial vaccine is feasible. Initially live irradiated infective larvae have been successfully used to induce high level of protective immunity in several animal models. Applying diverse strategies, variety of purified or recombinant filarial antigens have been explored for their ability to induce protection in different host-parasite systems. Some of these targeted filarial antigens induced high level of resistance in experimental animals against challenge infections. More focussed studies on thorough characterization of parasitological and immunological changes associated with resistance induced by such candidate protective antigens and on delivery mechanisms and safety aspects will be crucial in their selection for possible use 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.

Immune Dynamics in the Pathogenesis of Human Lymphatic Filariasis

Despite the longstanding recognition of the spectral nature of human disease due to lymphatic filariasis, immunologists interested in pathogenesis have mostly examined patients classified as being at either one extreme pole or the other. While the clinically asymptomatic individuals with microfilaremia who sit at one pole always have active infection, it has been difficult to define who else on the clinical spectrum is actively infected with living adult worms. In this review, David Freedman discusses how the ability to measure circulating filarial antigen in patient serum has advanced our ability to understand the immunopathogenesis of lymphatic filariasis by improving the precision of patient classification. Recent work suggests that the presence (or absence) of antigenemia, rather than overt clinical manifestations of disease, is closely associated with specific cytokine responses. A framework for patient classification based on these findings is proposed.

Modelling variability in lymphatic filariasis: macrofilarial dynamics in the Brugia pahangi--cat model

A striking feature of lymphatic filariasis is the considerable heterogeneity in infection burden observed between hosts, which greatly complicates the analysis of the population dynamics of the disease. Here, we describe the first application of the moment closure equation approach to model the sources and the impact of this heterogeneity for macrofilarial population dynamics. The analysis is based on the closest laboratory equivalent of the life cycle and immunology of infection in humans--cats chronically infected with the filarial nematode Brugia pahangi. Two sets of long-term experiments are analysed: hosts given either single primary infections or given repeat infections. We begin by quantifying changes in the mean and aggregation of adult parasites (inversely measured by the negative binomial parameter, kappa in cohorts of hosts using generalized linear models. We then apply simple stochastic models to interpret observed patterns. The models and empirical data indicate that parasite aggregation tracks the decline in the mean burden with host age in primary infections. Conversely, in repeat infections, aggregation increases as the worm burden declines with experience of infection. The results show that the primary infection variability is consistent with heterogeneities in parasite survival between hosts. By contrast, the models indicate that the reduction in parasite variability with time in repeat infections is most likely due to the 'filtering' effect of a strong, acquired immune response, which gradually acts to remove the initial variability generated by heterogeneities in larval mortality. We discuss this result in terms of the homogenizing effect of host immunity-driven density-dependence on macrofilarial burden in older hosts.

Generalized linear modelling for parasitologists

Typically, the distribution of macroparasites over their host population is highly aggregated and empirically best described by the negative binomial distribution. For parasitologists, this poses a statistical provlem, which is often tackled by log-transforming the parasite data prior to analysis by parametric tests. Here, Ken Wilson and Bryan Grenfell show that this method is particularly prone to type I errors, and highlight a much more powerful and flexible alternative: generalized linear modelling.

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.

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.

Vaccine research and development for the prevention of filarial nematode infections

The development of vaccines for the prevention of filarial nematode infections is in a state of relative infancy in comparison to vaccines for other parasitic diseases, such as schistosomiasis and malaria. There are many reasons for this slow start. Some of the principal problems are: (1) the lengthy and complex life cycle of these organisms with attendant complex immune responses, (2) the unique characteristics associated with a relatively large number of different pathogens, (3) the lack of suitable model systems for study of medically important infections, (4) the paucity of parasite material for antigen discovery and recombinant library construction, (5) the lack of substantial evidence suggesting the natural occurrence of protective immune responses, and (6) the limited data on mechanisms responsible for protective immunity. As technical hurdles are considered, it is also critical to focus on the characteristics of a vaccine necessary for its eventual utility. In the case of a vaccine for D. immitis a completely successful product will need to approach a 99+% efficacy. This is because of the 99+% efficacy of competitive chemotherapeutic products and the fact that microfilaremia observed on blood examination, resulting from as few as two worms, would present as a vaccine failure. Although very low worm burdens in large dogs could be perceived as success in the context of protection from clinical disease, because of the option of virtually complete chemoprophylactic protection, the typical veterinary practitioner would probably fail to appreciate less than complete vaccine protection. In contrast, a vaccine that produced a reduction in adult worm burdens without complete protection in either lymphatic filariasis or onchocerciasis would be very important. Highly effective chemoprophylactic agents are not widely available for prevention of the human filariases, and dramatically reduced clinical disease provided by less than a completely effective vaccine could occur as the result of fewer adult worms. The importance of developing these vaccines has outweighed the obstacles to this research. There has been a great deal of epidemiological and experimental evidence to suggest a vaccine is feasible and antigen discovery has progressed relatively rapidly within just the past few years. Efforts to generate appropriate larval cDNA libraries are beginning to yield dividends and a variety of fascinating vaccine candidates have been cloned. Additional antigen discovery, research on appropriate modalities for overexpression of genes from these parasites, and the complex tasks associated with vaccinology remain as significant research and development obstacles.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Microfilariae recurrence in Polynesian Wuchereria bancrofti carriers treated with repeated single doses of 100 micrograms/kg of ivermectin

Forty-six Polynesian carriers of Wuchereria bancrofti were treated with 3 successive single doses of ivermectin, 100 micrograms/kg, given every 6 months. Immediate microfilaricidal activity of ivermectin was excellent in all carriers, since residual mean microfilaraemia levels, 2 d after each of the 3 treatments, were less than 1% of pretreatment levels. Before initial treatment, geometric mean microfilaraemia was 500 microfilaria (mf)/ml for the whole group (range 21-6398 mf/ml); 6 months after each successive treatment it was 197, 108 and 87 mg/ml, respectively, 39.4, 21.6 and 17.4% of the pre-initial treatment level. By considering the mean percentage recurrent level at 6 months after the 3rd treatment (36.8%) as a threshold, it was possible to classify the carriers into 2 groups: 17 in whom the percentage recurrent level was > 36.8% and who were considered as 'fast repopulating' individuals, and the remaining 29 who were considered as 'slow repopulating' individuals. In the latter group, 6 months after each of the 3 treatments, the recurrent microfilaremia levels were 22.7%, 8.0% and 4.9% of the pre-initial treatment level, respectively, while they were 95.1%, > 100% and > 100% in the former. The constant recurrence of mf suggests that ivermectin, at a dosage of 100 micrograms/kg, had no effect on adult worms in 'fast repopulating' individuals, whereas the progressive lessening in recurrence of mf suggests some activity (sterilizing or killing) of ivermectin on W. bancrofti macrofilariae in 'slow repopulating' individuals.(ABSTRACT TRUNCATED AT 250 WORDS)

IgE responses in cats infected with Brugia pahangi

Passive cutaneous anaphylaxis tests were used to examine the IgE responses of cats repeatedly infected with the filarial nematode Brugia pahangi. Specific IgE was usually detected only in those cats that killed their adult worms and rarely in those cats in which adult worms survived for long periods. We suggest that this specific IgE is actively involved in killing adult worms in the lymphatics.

Antibodies against somatic antigens and excreted/secreted products of Brugia pahangi in rats with patent and non-patent infections

The humoral responses of Sprague-Dawley rats infected with Brugia pahangi were examined for up to 6 months after infection by ELISA, immunoblotting, and IFAT. In 2 experiments, 50% and 62.5% of rats developed patent, microfilaraemic infections. Mean adult worm burdens at autopsy were approximately 2% of the inoculum, and only patent rats yielded living adult worms. IgG antibody levels against crude somatic extracts (CSE) of all parasite stages and against adult excreted/secreted (ES) products were significantly higher in patent than non-patent rats. Both patent and non-patent rats produced anti-microfilarial surface antibody, as revealed by immunofluorescence. Immunostaining of Western blots by early infection sera showed no consistent difference in recognition of infective larval (L3) antigenic components by IgG or IgM antibody between eventually-patent and eventually-non-patent rats. By 26 weeks, however, patent rats recognized more components. The data suggest that antibodies against L3, adult, and microfilarial somatic antigens, ES antigens and microfilarial surface antigens do not correlate with the subsequent development of microfilaraemia in any individual rat.

Infection and disease in lymphatic filariasis: an epidemiological approach

A major question in the study of any parasitic disease is the relationship between infection and clinical disease. The public health importance of lymphatic filariasis has generated a large body of research in this area, both in laboratory studies (Ottesen, 1984,1989; Maizels & Lawrence, 1991) and in the field (Hayashi, 1962; Hairston & Jachowski, 1968; Denham & McGreevy, 1977; Vanamail et al. 1989 b; Bundy, Grenfell & Rajagopalan, 1991; Srividya et al. 1991 b). Despite this, there is still no conclusive explanation for the apparently complex relationship between infection and clinical disease observed in human communities. At least part of the problem may lie in the current impossibility of measuring adult worm burden in vivo (Pichon et al. 1980; Denham & Fletcher, 1987; Das et al. 1990; Grenfell et al. 1990). Although there has recently been significant progress in the development of immuno logical markers for infection status in humans (Ottesen, 1989; Day et al. 1991 a), microfilaraemia is still the most reliable measure of current infection in the field. Studies in endemic areas indicate that, far from there being any simple direct relationship between microfilaraemia and disease status, it is possible to find some individuals with microfilariae in their blood but no disease, and indeed with all other combinations of infection and disease status (Hairston & de-Meillon, 1968; Hairston & Jachowski, 1968; Beaver, 1970; Bryan & Southgate, 1976; Denham & McGreevy, 1977; Pani et al. 1991). Furthermore, the proportions of people in different categories are often observed to vary between endemic areas (Denham & McGreevy, 1977; Day et al. 1991 a).

Animal models for the study of immunity in human filariasis

A major challenge to the development of vaccines against human lymphatic filariasis and onchocerciasis is to direct the immune response toward elimination of the early, prepathogenic larval stages and away from responses that mediate pathology. In this review, James Lok and David Abraham discuss the various animal models that have been used to investigate the pathways leading to immunity, immunological tolerance and chronic pathology in these diseases. Owing to the strict host specificities of the human-dwelling filariae, no single model serves to duplicate exactly all these aspects. Nevertheless, it has been possible to demonstrate a protective immune response invoked by and directed against incoming third-stage larvae of both lymphatic and skin-dwelling filariae. The fact that subsets of the sequelae of human filarial infection can be duplicated in animal systems should also aid in unravelling the mechanisms determining the course of infection and in ensuring that vaccine candidates do not produce an inappropriate immunopathological response. A proposed scheme for using animal models in screening candidates for a vaccine against Onchocerca volvulus is presented.

The endemic normal in lymphatic filariasis: A static concept

Residents of areas endemic for lymphatic filariasis are continually exposed to infection with mosquito-transmitted infective larvae (L3), some of which survive to become adult worms and subsequently produce micro filarial (mf) transmission stages. The question of whether naturally acquired resistance occurs in adult residents of endemic areas has recently become of interest as the development of molecular vaccines against filarial parasites is being considered(1,2). There have been two epidemiological approaches to demonstrate acquired resistance to Filariasis in human populations. In this review Karen Day examines both approaches in the context of an immunoepidemiological study of bancroftian filariasis in Papua New Guinea (PNG). The merits of each as a conceptual framework for studies of protective immunity in lymphatic filariasis will be discussed.