Bancroftian filariasis in Pondicherry, south India--epidemiological impact of recovery of the vector population.

An Integrated Vector Management (IVM) strategy was implemented from 1981 to 1985 in one part of Pondicherry, South India, for the control of the bancroftian filariasis vector Culex quinquefasciatus (the IVM area). The rest of the town (the comparison area) received the conventional larvicidal input. After 1985 both the areas were managed conventionally. The switch to conventional strategy resulted in an increase of vector density in both areas. The microfilaraemia prevalence in humans showed a general decline (P less than 0.05) from 1986 to 1989 only in the IVM area whereas its intensity did not change significantly in either area. While the age-specific rate of gain of infection was generally unchanged in the IVM area, an increase in all age classes was observed after 1985 in the comparison area, where the Annual Transmission Index was high during the previous years. In both areas the rate of loss of infection increased during 1986-9 compared to 1981-6. The results suggest that 3 years is too short a period to relate the changes in entomological parameters to those in the microfilaraemia status of the population.

Immunoepidemiology of lymphatic filariasis: the relationship between infection and disease.

Lymphatic filariasis is manifested by a spectrum of symptoms that range from microfilaraemia to gross immunopathology. The geographical variations seen in this disease have been explained by heterogeneity in genetically determined host responses. By modelling the disease through time, Don Bundy, Bryan Grenfell and P.K. Rajagopalan can provide a simple, unified explanation for the observed heterogeneity. Their model shows that there is a sequential progression from infection, microfilaraemia, amicrofilaraemia to obstructive disease in all individuals who experience microfilaraemia. Only the probability of developing microfilaraemia is geographically variable, being dependent on the local incidence of infection.

The dynamics of infection and disease in bancroftian filariasis.

This study examines the relationship between the dynamic of Wuchereria bancrofti infection and the development of chronic lymphatic disease. Data sets from Pondicherry, south India, and Calcutta are used to estimate the age-specific proportion of the endemic population which has converted from microfilaria positive to amicrofilaraemia, and is assumed to be at risk of disease. For men, but not women, the age-prevalence profile of the estimated population 'at risk' is shown to correspond closely to the observed age-prevalence of chronic lymphatic disease in the same community. For both sexes, and independent of age, approximately 11% of the population at risk eventually develop lymphoedema. These observations suggest that filariasis endemic populations consist of those individuals who remain amicrofilaraemic and asymptomatic, and those who progress through the sequence: uninfected, microfilaraemic, amicrofilaraemic, to develop irreversible obstructive lymphatic pathology.

Clinical manifestations of bancroftian filariasis in relation to microfilaraemia and diethylcarbamazine therapy.

We studied the clinical manifestations of Bancroftian filariasis in relation to microfilaraemia and diethylcarbamazine (DEC) therapy in three groups of individuals in Pondicherry. In 3170 persons examined in a door-to-door survey (Group I), the prevalence of disease was found to be independent of microfilaria (mf) status (the disease rate was 13.4% in mf carriers and 13.6% in amicrofilaraemic persons). There was no association between the occurrence of disease (and individual manifestations), and mf status and its intensity in 1103 mf carriers examined in the filariasis clinic (Group II). Clinical manifestations, however, were age and sex dependent in Group II. Change in mf status in a five-year period (1981 to 1986) with and without DEC therapy did not influence the clinical manifestations in 1024 individuals (Group III). Though the disease rate in mf carriers who discontinued DEC (22.2%) and in those who completed one course of DEC (14.8%) was higher than those who did not receive DEC (10.6%), statistical analysis did not show any significant difference between the groups. The study confirms that the dynamics of infection and filarial disease are complex, and other associated factors need to be investigated.

Frequency distribution of lymphatic filariasis microfilariae in human populations: population processes and statistical estimation.

This paper uses simple mathematical models and statistical estimation techniques to analyse the frequency distribution of microfilariae (mf) in blood samples from human populations which are endemic for lymphatic filariasis. The theoretical analysis examines the relationship between microfilarial burdens and the prevalence of adult (macrofilarial) worms in the human host population. The main finding is that a large proportion of observed mf-negatives may be 'true' zeros, arising from the absence of macrofilarial infections or unmated adult worms, rather than being attributable to the blood sampling process. The corresponding mf distribution should then follow a Poisson mixture, arising from the sampling of mf positives, with an additional proportion of 'true' mf-zeros. This hypothesis is supported by analysis of observed Wuchereria bancrofti mf distributions from Southern India, Japan and Fiji, in which zero-truncated Poisson mixtures fit mf-positive counts more effectively than distributions including the observed zeros. The fits of two Poisson mixtures, the negative binomial and the Sichel distribution, are compared. The Sichel provides a slightly better empirical description of the mf density distribution; reasons for this improvement, and a discussion of the relative merits of the two distributions, are presented. The impact on observed mf distributions of increasing blood sampling volume and extraction efficiency are illustrated via a simple model, and directions for future work are identified.

A mathematical analysis of various factors involved in transmission of bancroftian filariasis in Pondicherry.

The variation of clumping factor in microfilaraemia among different age groups was observed in Pondicherry for 1981 and 1986. The clumping factor was minimum in the age group 0-5 yr and it was maximum in 16-20 yr group in 1981 and 21-25 yr in 1986. The variation in clumping factor resembled the changes in microfilaria (mf) rate. Multiple linear regression analysis showed that the multiple correlation between mf rate and the other parameters i.e., median microfilarial density (MFD50), clumping factor in human population, and, infection rate and infectivity rate in vector population was highly significant. The relationship between filariometric indices in human host and vector population may provide the basis on which a mathematical model on transmission of filariasis could be developed.

Estimation of fecundic life span of Wuchereria bancrofti from longitudinal study of human infection in an endemic area of Pondicherry (south India).

The fecundic life span of adult female W. bancrofti was estimated by longitudinal study of microfilaraemia in a cohort of population (7,525) in Pondicherry. The estimation was based on a deterministic model, using the rate of loss in infection. The life span of the parasite was 10.2 yr without chemotherapy, while it was reduced to 5.3 yr following diethyl-carbamazine therapy. The analysis of mean microfilarial counts in microfilaraemic persons without chemotherapy indicates that the rate of production of microfilaria by the adult female is stable at least for a period of five years.

Parasitological aspects of malaria persistence in Koraput district Orissa, India.

A sample survey in 37 villages covering 10,733 people in 1986-87 in the Koraput district, Orissa showed that the malaria prevalence is of a much higher order than that reported by the National Malaria Eradication Programme (annual parasite incidence between 14.3 and 26.8 during 1981-86). Out of 833 positives detected 714 had Plasmodium falciparum, 86 had P. vivax, 12 had P. malariae and 21 had mixed infections. There were 650 asymptomatic parasite carriers and 127 gametocyte carriers. The infant parasite rate was 15.82 per cent and average enlarged spleen (AES) in 2 to 9 yr old children was 1.98. In a year round fever survey in 22 villages, 5520 blood smears were collected and 1364 were found positive for malaria, with 77.3 per cent P. falciparum. In a mass blood survey conducted in a labour camp at an irrigation project, 610 people were examined, and 181 were positive. Nearly 40 per cent of migrants and 22 per cent of locals were positive, P. falciparum being dominant.

Bancroftian filariasis in Pondicherry, south India: 1. Pre-control epidemiological observations.

A 5-year Integrated Vector Management (IVM) project was implemented in Pondicherry, South India, for the control of Bancroftian filariasis. The efficacy of the IVM strategy was compared with routine control strategy under the national programme. The present paper describes the pre-control epidemiological features of filariasis as determined by a mass blood survey in 1981. Of 24946 persons examined 8.41% were microfilaraemic. Microfilaraemia prevalence was homogeneous throughout the study area. The prevalence and intensity of microfilaraemia were age dependent, and increased monotonically until about 20 years, following which there was a decline until about 40 years to become relatively stable in older age classes. The gender profiles of both prevalence and intensity of microfilaraemia showed no difference between the sexes until about 15 years of age, following which both were higher in males compared to females. The distribution of microfilarial counts was overdispersed, indicating aggregation of adult worms.

Bancroftian filariasis in Pondicherry, south India: 2. Epidemiological evaluation of the effect of vector control.

This article examines the evaluation of a bancroftian filariasis control programme undertaken in Pondicherry from 1981-5. Integrated vector management was applied in one half of the town, and routine operations under the national programme (larviciding and chemotherapy) continued in the comparison area. The programme was evaluated by monitoring relative change in the epidemiological statistics of both populations. The results indicate that there was significant reduction in prevalence of microfilaraemia in juveniles in the controlled area. An apparent reduction in intensity of microfilaraemia was also observed but this was a consequence of the reduction in prevalence, since the density of microfilariae remained unchanged. The results suggest that primary constraints on the epidemiological evaluation of the vector control of filarasis are the longevity and the population characteristics of the parasite.

A simple deterministic model for host-parasite relationship in Wuchereria bancrofti infection & its relevance to parasite regulation in human host.

A deterministic immigration-death model, which reflects the population dynamics of W. bancrofti in human host has been applied to study the relationship between vector and human infections. Application of the model showed that the rate of acquisition and loss of human infection were approximately equal (L = 0.130 and M = 0.129). The relationship of infective resting density (IRD) in vector population with maximum intensity (Imax) of infections and microfilaria prevalence (MFP) in human population were examined by using the least squares polynomial regressions. The fifth order polynomial regressions were found to be adequate to describe the observed pattern (Imax vs IRD: R2 = 0.8464, P = 0.0015; MFP vs IRD: R2 = 0.7246, P = 0.019). The observed relationships indicated that at an infective resting density of 0.26 per man hour or above, the density-dependent factors start regulating the human infections, which showed a declining trend, following this level.

Impact of 50 years of vector control on the prevalence of Brugia malayi in Shertallai area of Kerala state.

This article examines the long term effects of vector control on the prevalence of B. malayi infection and disease, by comparing the results of 3 earlier studies (1934, 1955, 1976) in one area of south India with a recent (1986) survey. The data indicate that disease and infection prevalence have declined continuously over the last 50 years. Infection has declined (from 21 to 2%) more markedly than disease (from 24 to 10%). Age-specific data indicate that this difference is due to the irreversibility of the clinical signs and the long term survival of diseased cohorts. The results indicate that the prevalence of clinical brugian filariasis can be reduced using vector control and that such control programmes cannot be evaluated in short term or by using crude morbidity statistics.

Estimation of age-specific rates of acquisition and loss of Wuchereria bancrofti infection.

This study uses a reversible catalytic model to estimate the age-specific rates of gain and loss of Wuchereria bancrofti infection from data collected during a control programme in Pondicherry, South India. The data describe the infection status in 1981 and 1986 of two cohorts of individuals, one living in an area where vector reduction had been achieved, and the other in a comparable endemic area. The rate of loss of infection in the absence of reinfection is estimated for the cohort in the control area, and the rate of gain of infection by the cohort in the endemic area estimated by substitution in the model. The mean expected life span of patent infection is estimated to be 5.4 years. The instantaneous rate of loss of infection is independent of age, while the rate of gain of infection exhibits a convex age-profile, peaking in the 16-20 year age-class. The reduced rate of gain in adults is largely attributable to the increasing proportion of potentially resistant individuals with clinical disease. The results suggest that the age-distribution of bancroftian filariasis is primarily determined by age-dependency in the rate of acquisition of infection.

Malaria in Koraput district of Orissa.

Incidence and prevalence of malaria was studied in the predominantly tribal district of Koraput, Orissa state. In the mass blood surveys in 61 malarious villages, a total of 12,122 persons were examined and 1,604 (13%) were found positive for malaria. Infant parasite rate was 23% and young children 2-4 years old were the worst affected (parasite rate 27.2%). P. falciparum, P. vivax, and P. malariae accounted for 80, 10 and 3% respectively of all infections. A sample of 2,554 children below 10 years of age were examined for splenomegaly. The spleen rate in 2-9 year old children was 22% and the average enlarged spleen (AES) was 2.06. There was no association between size of the spleen and species of parasite. Fever survey was done for a year in 22 villages and 5,511 blood samples were collected of which 24.8% were positive for malaria parasite. The annual parasite incidence (API) was 32.4 per 1000 infants and 89.5 per 1000 population. Children, 2-4 years old recorded the highest incidence (457/1000). The age pattern of prevalence and incidence indicate high level of acquired immunity in the population. Prevalence and incidence varied among different tribal communities. Prevalence of infection and splenomegaly was higher among children of Bonda, Kondh and Poroja tribes. The role of genetic factors in malaria is discussed.

Detection of Plasmodium ovale in Koraput district, Orissa state.

We report for the first time the detection of P. ovale infection in three patients in two hilly villages of Koraput district, Orissa state, India. The identification of the parasite was confirmed by the Imperial Colleage of Science and Technology, London.

Control of malaria and filariasis vectors in South India.

Community participation is increasingly seen as a major component of successful control programmes against parasitic diseases. One of the strongest exponents of this has been the Vector Control Research Centre (VCRC) in Pondicherry, South India (Fig. 1), who have successfully motivated and involved village communities in vector control activities with considerable success against malaria and filariasis vectors. Their largest single programme, the Filariasis Control Demonstration Project in Pondicherry Town, has now been handed to the Pondicherry State Government, while the VCRC's work on malaria and filariasis control is now expanding throughout other parts of India. As this article shows, much has been learnt from these projects, not just about control techniques but also about education, administration and decision-making.

Population movement and malaria persistence in Rameswaram Island.

The role of population movement on the persistent transmission of malaria in Rameswaram Island was studied. Majority of the inhabitants of the island are fishermen, who engage in perennial fishing. They move from one coastal place to the other for fishing and stay in temporary camps depending on season and fish availability. Such seasonal fishing camps attract fishermen from the mainland coastal villages also. The parasitological and entomological studies carried out in these places reveal that some of the camps are highly vulnerable to the movement of individuals with malaria infection and highly receptive. Rameswaram being a holy place, receives pilgrims from all over India and Nepal. Plasmodium falciparum cases recorded from the pilgrims of North India indicate the danger of the possible introduction of chloroquine-resistant parasite in the island. Also, a large number of passengers in transit from various countries, many of which are at risk of malaria transmission, stay in the island before or after visiting Sri Lanka. Such population movements being a continuous and regular feature are significant and result in failures in the operational programmes.