doi:10.5455/vetworld.2013.196-199 Seroprevalence of bluetongue in north eastern Indian state- Assam 1

Aim: The study was undertaken to assess presence of sub-clinical bluetongue in the state of Assam, one of the un-affected north-eastern state of India. Materials and Methods: Serum samples were collected from apparently healthy as well as suspected sheep, goat and cattle from different districts of Assam encompassing various agro-climatic zones. Anti-BT antibodies were screened in sera using indirect enzyme linked immunosorbent assay (iELISA). Results: Out of total 313 animal serum samples screened (sheep-68, goat-195, cattle-50), 58.82% of sheep, 31.79% of goat and 70.00% of cattle serum samples were found positive. The prevalence of anti-BT antibodies in different agro climatic zones ranged between 31-50%. Conclusion: This study revealed high seroprevalence of bluetongue in cattle, sheep and goats in Assam. Incidence of bluetongue in animals of Assam being not reported so far; the present seroprevalence status of bluetongue in Assam indicates presence of sub-clinical infection in the state for the first time.


Introduction
emerged within Europe, events that have been linked to international trade and climate change in the region, Bluetongue is an economically important disease raising concerns about possible future threats posed by affecting susceptible ruminants in the semi-tropical bluetongue and other related orbiviral diseases [8].and temperate regions of the world.Bluetongue virus BTV has now been identified on all continents except (BTV), the type species of the Orbivirus genus and Antarctica [9].The first confirmed outbreak was Reoviridae family, is the causative agent of bluetongue reported in island of Cyprus [10].In India, the first [1].BTV can infect ruminants, camelids, and occasionally report of the disease was from Maharashtra in 1963 that large carnivores [2].The virus is transmitted by biting cause a heavy loss in sheep [11].Since then Southern midges (Culicoides spp.) in which it also replicates.It and Western states of India experienced several can sometimes also be transmitted either via an oral incidences and/ or outbreaks of BT from time to time.route, or vertically in sheep and cattle [3,4].BTV is Out of 26 serotypes distributed globally, 21 serotypes only enzootic in areas where continuous series of virus have been reported from various parts of the country infection cycles in vector and vertebrate host are [12].Major part of India being tropical with high to maintained.In ruminants, it may cause severe systemic moderate rainfall, the huge population of ruminants is disorders with moderate to high mortality.The infected susceptible to BT infection.bovines exhibit prolonged viraemia compared to sheep However, eastern and north-eastern part of India and may act as reservoir host for BT virus [5].BTV are considered un-affected region in terms of active infection of ruminants is often sub-clinical, but outbreaks of severe disease occur with regular frequency especially disease prevalence as no such disease incidence/ outbreaks at the upper and lower limits of the virus' global range are reported so far in any Government documents or where infection is highly seasonal-occurring in the late literature [13,14].Absence of any bluetongue disease summer and autumn [6].
incidence/outbreak in entire eastern and north-eastern Twenty four distinct BTV serotypes have been India was documented in the Progress Report (2002recognized for decades, any of which is thought to be 2004) of All India Network Programme on

Indirect enzyme linked immunosorbent assay
The seroprevalence of BT in sheep, goat and (iELISA): For detection of anti-bluetongue antibodies cattle in different states of India showed wide variation, in test serum samples, iELISA was performed using the as they represented different agro-climatic zones, test protocol as per De and coworkers [20] with slight 0 covering the sub-temperate south, semi-arid north and modification in blocking time i.e. for overnight at 4 C. north-west, humid and sub-humid east and sub-The BTV antigen was supplied from the collaborating temperate Himalayan region [22][23][24][25][26][27] and different centre of All India Network Programme on Bluetongue methods of investigation [17,28].However, in our (AINP-BT) at Mukteswar.Briefly, 50µl of diluted BT study we found a significant difference between the viral antigen (rVP7) was added to coat each well of the species.Cattle population has high seropositivity plate.After blocking the uncoated portions of the wells followed by sheep and goat.This data indicates that the with blocking buffer, 50µl of diluted known positive, cattle and sheep population are highly vulnerable in known negative and test serum were put into the wells comparison to goats.This also possibly implies that the and kept in room temperature for 1 hour.After washing cattle population acts as major carrier of virus and thus the plate three times, 50µl of diluted conjugate was plays an important role in its dissemination.Cattle are added to all wells except the conjugate control and kept considered to be the reservoir hosts of BTV because the for 1 hour at room temperature.Then, 50µl of viraemia is prolonged and the majority of infections are chromogen-substrate (ortho-phenylene diamine, subclinical [29].Oberoi and others [30] demonstrated OPD) solution was added to all wells.It was kept for which were having a resemblance with our findings BT antibodies, iELISA is one of 492 nm.The average optical density (O.D.) values of the specific and sensitive tests [18].Almost from all negative control is calculated and compared with the parts of Indian subcontinent seroprevalance of BT was test O.D. values.The O.D. values of tests that were reported except from the North-Eastern states higher than the average O.D. values of the negative including Assam [19].To know the presence of subcontrol were considered as positive for anti-BT clinical infection, if any, prevalence of anti-BT antibodies.antibodies in sheep, goat and cattle of different districts Meteorological information: The overall climate of the of Assam was ascertained in the present study using study area is temperate.The average temperature of the serum samples of randomly collected suspected as well Bluetongue capable of causing BT in ruminants.However, two Disease [15].further BTV serotypes, BTV-25 (Toggenburg orbivirus, Della-Porta and co-workers [16] reported BT from Switzerland) and BTV-26 (from Kuwait) have infection in sheep, goat and cattle by serological recently been identified in goats and sheep, methods.With the rapid development in serological respectively [7].Since 1998, multiple BTV types have techniques like AGPT, CIE, SNT and more recently Materials and Methods and relative humidity lies between 65-85%.[21].Sera: Total 313 numbers of serum samples were

Table - 1
. Seroprevalence of bluetongue in different animal species in Assam

Table - 2
. Seroprevalence of bluetongue in animals of different Agro-climatic zones of Assam Preventive Veterinary the BTV antibodies in 70% of cattle sera in Punjab state Medicine Doi:10.1016/j.preventmed.2011.04.005.