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Filariasis

National Filariasis Control Programme In India and New Strategies for Its Control

III. Trend and Present Endemicity of the Problem

1. W. bancrofti nocturnal periodic infection: The problem of bancroftian filariasis from 1953 to 2001is given in Table 1.

Table 1- Problem of W. bancrofti infection at different points of time (in mill.)

Though the disease has been prevalent since antiquity, no organized survey had been made to estimate the problem in the country.  Megaw and Gupta were the first to publish a filaria map of India in 1927 based on night blood surveys conducted in different parts of the country.  Jaswant Singh and Raghavan highlighted the problem in 1953 and prepared an endemicity map, based on replies to a questionnaire circulated to different states.  The estimates made in 1962, 1970 and 1976 revealed the problem to be much higher than what had been estimated earlier.  In 1981, the delimitation surveys throughout the country showed about 304 million people lived in endemic areas.  The latest estimates in 2001 indicate that about 473 million people are exposed to the risk of bancroftian infection; of these about 125 million live in urban areas and about 348 million in rural areas.  About 31 million people are estimated to be harbouring microfilaria (mf) and over 23 million suffer from  filaria disease manifestations.

Table 1 indicates that the problem has increased manifold during the last five decades.  The increase is mainly due to (i) extension of delimitation surveys in hitherto unsurveyed districts (ii) natural growth of population in the endemic areas and (iii) spread of infection to new areas previously known to be non-filarious.

The estimates made in recent years were based on sample surveys conducted during the last five decades in different districts as the delimitation surveys in a district take two to three years to complete.  Modern statistical methods are to be adopted to complete the delimitation surveys as early as possible so that the estimates are based on recent surveys instead of surveys conducted two to five decades back.  Recently repeat surveys conducted in some towns showed a declining trend in bancroftian infection because of better environmental sanitation and increased use of personal prophylactic measures.

State-wise estimated distribution of population at risk, number of mf carriers and number of persons with filaria disease manifestations in 2001 are given in Table-2.

It would be seen that the State of Bihar has  highest endemicity of over 17% followed by Kerala (15.7%), Uttar Pradesh (14.6%).  Andhra Pradesh and Tamil Nadu have about 10% endemicity.  Goa showed the lowest endemicity of less than 1% followed by Lakshadweep (1.8%), Madhya Pradesh (above 3%) and Assam (about 5%).  The latter two States have pockets showing high endemicity.  Of over 23 million disease cases in the  country, Bihar, Jharkhand and Uttar Pradesh together contribute more than 15 million cases, which constitute nearly two thirds of the total number of cases in the country. The seven States namely Andhra Pradesh, Bihar, Kerala, Orissa, Uttar Pradesh and West Bengal, where MDA pilot trials are being undertaken, contribute over 86% of mf carriers and 97% of disease cases in the country.

Filaria endemicity map stratifying the country based on mf rate as on 31^st December, 1995 is shown in Fig.1.  Northern districts of Kerala, Tamil Nadu, Andhra Pradesh, coastal districts of Orissa and eastern parts of Uttar Pradesh showed mf rate above 6%. Small pocket in Maharashtra, Karnataka, Madhya Pradesh, Bihar, West Bengal and Assam also have mf rate above 6%.  The problem of filariasis is yet to be delimited in most of the districts in Maharashtra and a few districts in Karnataka, Tamil Nadu, Madhya Pradesh, Orissa, Uttar Pradesh, Bihar, West Bengal and Assam. The present estimates reveal that bancroftian filariasis is endemic in 15 States and five Union Territories.

Table-2 State-wise estimated population exposed to the risk of filariasis and estimated number of mf carriers and filaria cases as on 31.12.2001 (in million)

NA: Not Available            * Provisional

The North-Western States/UTs namely, Jammu & Kashmir, Himachal Pradesh, Punjab, Haryana, Chandigarh, Rajasthan, Delhi & Uttaranchal and North-Eastern States namely Sikkim, Arunachal  Pradesh, Nagaland, Meghalaya, Mizoram, Manipur and Tripura are known to be free from indigenously acquired filarial infection.

2. B.malayi Nocturnal Periodic Infection: The infection is prevalent in the States of Kerala, Tamil Nadu, Andhra Pradesh, Orissa, Madhya Pradesh, Assam and West Bengal.  The single largest tract of this infection lies along the west coast of Kerala, comprising districts of Trichur, Ernakulum, Alleppey, Kottayam, Quilon and Trivandrum, stretching over an area of 1800 sq. km.  The infection in the other six States is confined to a few villages only.  Surveys undertaken recently in Kerala and a few villages in other States revealed either reduction of foci or complete elimination of the parasite as well as the vector in many villages which were known to be endemic for B. malayi  infection four decades back.  The declining trend of this infection is due to (1) filling of Mansonia breeding places for real estate: (2) removal of host plants for lotus and fish culture; (3) replacement of Pistia stratiotes by Salvinia auriculata, a less hospitable host plant for the principal vector; (4) use of residual insecticidal spray under NAMP which has markedly reduced B.malayi vectors, and (5) increased use of microfilaricidal drugs as well as personal prophylactic measures due to better health education of general public about the causation of disease.

Presently over 2.5 million people are exposed to the risk of B.malayi  filariasis with about two lakh mf carriers and 1.25 lakh cases of filarial disease manifestations.

Nocturnal subperiodic B.malayi  infection prevalent in some South-East Asian countries has not been found to be present in India.

3. W. bancrofti Diurnal Subperiodic Infection:  During 1958 the National Institute of Communicable Diseases (formerly known as Malaria Institute of India) conducted filaria survey in A&N Islands and found circulating mf in day time also.  Subsequent surveys by NICD brought out the presence of diurnal subperiodic W.bancrofti infection among the local inhabitants of Nicobar Group of Islands.  The surveys revealed that a few islands namely Car Nicobar, Chowra and Kamorta – Nancowrie were endemic for this infection.  Since the survey was very limited, it was not possible to know the trend of this infection.  Entomological evidence indicates that Aedes (Finlaya) niveus group of mosquitoes play the vectorial role.  It was found that the infection is limited to a few islands; the total population of these endemic islands is less than 10,000.

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