Trend of Wuchereria bancrofti infection in Pondicherry urban agglomeration after the withdrawal of a five year vector control programme.

A study was undertaken to investigate the changes in the prevalence of bancroftian filariasis after six years of withdrawal of a five year (1981-85) vector control programme in Pondicherry and the results are presented in this paper. Though there was a drastic increase in the intensity of transmission of filariasis after the withdrawal of vector control operations in 1986, the prevalence of microfilaria (mf rate) of the population declined from 6.35% in 1986 to 4.80% in 1992 (P < 0.05). However, there was an appreciable increase in the mf rate and intensity in the children (0.5 year) born after the withdrawal of the control operations. The overall loss rate of infection was much higher than the gain rate. Many people found to be microfilaraemic six years before, became amicrofilaraemic now. The overall reduction in mf rate, higher loss rate of infection and microfilaraemic people turning amicrofilaraemic indicate that chemotherapy, which has been intensified during the last five years under the National Filaria Control Programme (NFCP), was playing a major role in reducing the infection. However, implementation of mass chemotherapy with annual single dose of DEC (6 mg/kg), combined with vector control may yield better results in further reducing the mf rate as the current approach of detection for treatment may become increasingly difficult with gradual fall in endemicity level. Mass chemotherapy will provide an opportunity to further reduce the mf rate and eliminate filariasis within a few years from Pondicherry urban agglomeration. The increase in the incidence of infection in children, born after the withdrawal of the vector control programme clearly indicates that transmission is continuing and if immediate action is not undertaken it would be difficult to sustain the gains.

Spread of lymphatic filariasis, re-emergence of leishmaniasis & threat of babesiosis in India.

The impact of unabated population growth and consequent ecological changes is felt in the spread of vector-borne diseases also. Filariasis, once considered to be more associated with urban areas and urbanization, is rapidly emerging as a major problem in rural areas. The need to delimit the endemic areas through a new rapid assessment procedure and the possible control methods of rural filariasis are discussed. Leishmaniasis has re-emerged with a vengeance in some parts of the country and the reasons for this are analyzed. Babesiosis, though zoonotic, has the potential to invade the domain of man in India.

Cumulative exposure and its relationship with chronic filarial disease in bancroftian filariasis.

Several hypotheses have been put forth about the factors influencing the dynamics of infection and disease in lymphatic filariasis. However, appropriate validation of these hypotheses by real situation analyses of epidemiological data is lacking. The present analyses examine the relationship between cumulative exposure to infection and prevalence of disease by utilizing the existing entomological and clinical data collected between 1981 and 1986 in Pondicherry, South India, endemic for bancroftian filariasis. While there was a significant negative association when the cumulative exposure was correlated with total prevalence of disease (r = 0.70, p = 0.024) as well as hydrocele alone (r = 0.74, p = 0.014), a significant positive association was found with prevalence of lymphedema (r = 0.72, p = 0.018). These results suggest that hydrocele development follows early after exposure, but prolonged exposure could result either in development of lymphedema or immune tolerance resulting in microfilaremia. These could also suggest that the pathomechanisms in development of hydrocele and lymphedema could follow different pathways. Implications of the present findings are discussed in light of the various hypotheses put forward by earlier studies.

Spatial and temporal variations in prevalence, intensity and aggregation of microfilaria in the human host.

The prevalence, intensity and the frequency distribution of microfilaria (mf) of Wuchereria bancrofti in the human blood and their variation in time (pre-vector control, control and post control periods) and space have been examined. The prevalence and intensity showed significant variation between time and different areas of urban Pondicherry. The significant reduction in the prevalence and intensity of microfilaremia in the areas of high endemicity indicated that the effect of vector control was well marked in these areas. The distribution of microfilaria in human population was adequately described by the truncated negative binomial prior to interruption of transmission, and the dispersion parameter 'k' of the negative binomial varied between 0.17 and 0.68. A change in the distribution was observed after the 5 years of vector control, during which the transmission was completely interrupted. The increased 'k' values suggest that the distribution of mf is influenced by the degree of exposure to infection. Therefore, comparison of changes in the parameter 'k' can be considered as an additional tool for evaluating the control operations aimed at interrupting the transmission.

Observations on population density of Culex quinquefasciatus and transmission indices of Bancroftian filariasis during and after Integrated Vector Management strategy.

An Integrated Vector Management strategy, implemented as an alternative to the conventional control operations that include mainly chemical control in Pondicherry, South India, reduced very substantially the population density of Culex quinquefasciatus. This resulted in drastic decrease in the intensity of transmission of bancroftian filariasis transmitted by Culex quinquefasciatus and consequently the incidence of new infections in children of 0-5 age group was minimized. When the IVM strategy was withdrawn after five years of implementation and conventional control measures were re-adopted, resilience of Culex quinquefasciatus population was observed and human exposure to the risk of infection increased. The results suggest that maintenance of vector density at reduced levels for prolonged periods, is necessary to control infectious diseases like filariasis, which is difficult in the present day urban situations in developing countries. Hence the emphasis should be on chemotherapy to achieve control of lymphatic filariasis.

Efficacy of bi-annual administration of DEC in the control of bancroftian filariasis.

The efficacy of bi-annual administration of DEC at the dose of 6 mg/kg body weight was evaluated on the microfilaraemia prevalence, density and vector filarial infection rates. Administration of four doses (4 x 6 mg/kg) of DEC significantly (P < 0.05) reduced the microfilaria rate of the community from 6.02 per cent to 2.31 per cent, microfilaria density from 0.66 to 0.17 and infectivity rate of the vector population from 0.8 per cent to 0.39 per cent.

Biting periodicity index of Culex quinquefasciatus & its relationship with microfilaria periodicity in Pondicherry.

The biting periodicity index of the bancroftian filariasis vector, C. quinquefasciatus, was calculated on the basis of a large longitudinal data base and compared with the periodicity index of microfilariae of Wuchereria bancrofti. The biting periodicity index of the vector population was 29.78 and found to be close to the periodicity index of the microfilariae. The peak biting activity of the vector and peak appearance of microfilariae in the peripheral blood occurred at about 01.00 h, which accounts for the optimum infection of the vector population. The early night biting density of the vector population showed a highly significant (P less than 0.0005) linear relationship with all night biting density.

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.

Prevalence of bancroftian filariasis & its control by single course of diethyl carbamazine in a rural area in Tamil Nadu.

The prevalence of microfilaraemia, clinical spectrum of bancroftian filariasis and vector potential were studied in Vettavalam village in North Arcot district of Tamil Nadu. The effectiveness of selective therapy with diethyl carbamazine (DEC) in controlling filariasis in rural areas was also evaluated. The prevalence of microfilaraemia (mf rate) and disease (disease rate) was found to be 11.7 and 11.09 per cent respectively. Hydrocele was the dominant clinical sign in males and lymphoedema in females. The density of the vector Culex quinquefasciatus was 25.44 females/man-hour, and the infection and infectivity rates were 18.16 and 1.09 per cent respectively. All mf carriers detected after the mass blood survey were given a single course of DEC at the dosage of 6 mg/kg of body wt/day for 12 days. Only 61.6 per cent of them took the full course of DEC treatment. DEC therapy brought down the mf rate from 11.7 to 5.84 per cent after one month. In the absence of further treatment, there was no significant change in mf prevalence after one year.