Stephanofilarial dermatitis (humpsore) is a chronic obstinate skin disease of cattle and buffaloes, commonly seen in working bullocks of Assam, West Bengal, Tripura, Orissa, Bihar, Coastal area of Andhra Pradesh and Andaman Nicobar islands. The clinical disease exhibits a seasonal variation subject to factors like prevalence of vector flies, rainfall, temperature, humidity and predisposing injury. Therefore, the present work was undertaken to study the recent prevalence of humpsore in Assam.

A total of 1189 cattle were examined for prevalence study for one year (March 1999 to February 2000). These animals were categorised according to their breed, sex and age. They included 824 native (Bos indicus) and 365 pure and crossbreds and 837 males and 352 females. The animals were classified into seven different age groups viz., below 1 year, 1 to <3 years, 3 to <6 years, 6 to <9 years, 9 to <12 years and 12 to <15 years and 15 to<19 years.

Samples of skin scrapings were collected from the skin lesions from various parts of the body in vials containing 0.9% saline solution and were brought to the laboratory. Parasites recovered from the scrapings were fixed and preserved in hot 70% alcohol for identification. The prevalence rates in pre-monsoon, monsoon, post-monsoon and winter were observed and recorded.

To study the vector potential of different species of flies, a total of 622 flies associated with the lesions were collected during the period of the study. The flies thus collected were preserved in both dry and wet method and some representative samples were sent to the Zoological Survey of India, Kolkata for identification.

The flies were dissected under streomicroscope for larval stages. The larvae thus collected were identified following the description of Patnaik and Roy (1966). The number of flies positive for Stephanofilaria larvae and the number of larvae they harboured were recorded.

Out of 1189 animals examined, 523 were positive for S. assamensis and the overall percentage of prevalence was 43.98%. The prevalence was the highest in July (61.67%) and lowest in March (20.75%) (Table 1). The highest prevalence was observed in crossbred (50.41%) males (46.36%) than native cattle (41.14%) and females (38.35%). The highest infection was observed in the monsoon season (53.43%) followed by post-monsoon (43.16%), pre-monsoon (33.54%) and lowest in the winter (26.67%) (Table 1). The study revealed no infection in animals below 1 year of age. The highest percentage of infection was observed in animals of 3 years to below 6 years of age (57.08%). The infection rate then showed a gradual decline in higher age groups. The percentage infection recorded was 52.92, 38.08, 36.00 and 32.05 in 6-9, 9-12, 12-15 and 15-19 year old cattle, respectively.

The study on the prevalence of vector flies revealed that out of total 622 flies collected during the period, 305 were identified as Musca pattoni, 199 as Musca domestica and 118 belonged to the Subfamily Limnophaginae. Out of these species, only M. pattoni was found to carry the larval stages of S. assamensis. The incidence of S. assamensis larvae in M. pattoni (45 %) was the highest during the month of August and lowest (16%) in the month of December. No larvae could be found in January and February. The prevalence of flies was found to be highest in the month of July (118 out of 622) and lowest in the month of January (15 out of 622) (Table 2).

The larval load of S. assamensis in M. pattoni has been shown in Table 2. The highest number of larvae was recorded in the month of July (34 of 15 flies) with an average of 2.26 larvae per infected fly and the lowest in the month of December (6 of 3 flies) with an average of 2 larvae per infected fly.

Observations on the disease revealed more or less similar pattern, as reported by earlier workers in different districts of the state of Assam (Pande, 1936; Dutta, 1972 and Haider, 1961) and other parts of the country (Govinden et al., 2000 Patnaik, 1968). However, a marginal drop in infection rates suggest better management of the infected stock with increased awareness on the disease among the farmers and may be partly due to application of insecticides in malaria control programme undertaken by government and semi-government agencies resulting in reduction of fly population. Males had higher infection rate than females. Animals below one year of age were generally free from humpsore. Similar observation was made by Pal and Gupta (1988) and Rai et al. (1990). The highest infection rate (57.08%) was found in animals in the age group of 3 to < 6 years of age, when animals are mostly used for draught and agricultural operations. In animals over 6 years of age, there was a decreasing trend in the infection rate. Islam (1979) observed that animals above 11 years of age were relatively free from humpsore in Bangladesh. It can be presumed that the aged animals might have developed some degree of immunity resulting in low infection rates in this age group.

The disease was more pronounced in monsoon (53.43%) followed by post-monsoon (43.16%), pre-monsoon (33.54%) and winter (26.67%). Infection rate of the flies with larval filarids were also highest during this period (Table 2). Monsoon causes the latent lesions to flare up into the clinical form due to re-infection and increased number of vector bites, apparently raising the proportion of infected animals (Agarwal and Shah, 1984).

Amongst the three species of flies associated with the lesions of humpsore, M. pattoni was the predominant species (Table 2). Surprisingly, the recognised vector of S. assamensis, Musca conducens (Dutta, 1972; Patnaik, 1973) could not be found during the period of study. In the present investigation M. pattoni was found to carry the larval stages of S. assamensis. The larval burden was found to be the highest during the month of August, corresponding to the highest infection rate of the disease during the monsoon.

It is not unlikely that species other than M. conducens could be a vector of S. assamensis, as reported by Kabilov (1980) that S. assamensis could develop in Lyperosia titilans in Southern Russia. This appears to be the first finding on M. pattoni as a potential vector of S. assamensis and needs further investigation for confirmation. The occurrence of the disease therefore, can be correlated with the prevalence of the vector flies, rainfall, temperature, humidity and inflicting injury.

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