The bluetongue infection is an arthropod-borne viral disease of wild and domestic ruminants. This disease hits the livestock sector with significant economic losses in terms of productivity, morbidity and mortality. The economic losses attributed to BTV infections have not been explored accurately, but it estimated to be about 3 billion US dollar a year due to reduced milk yield, meat efficacy, weight loss, death, abortions, also export restrictions for live animals and their semen. The disease is caused by the Bluetongue virus (BTV)which belongs to the genus Orbivirus within the family Reoviridae. This disease is on the list of diseases notifiable to the World Organisation for Animal Health (OIE). The virus is transmitted by the midge Culicoides imicola, Culicoides variipennis, and other culicoids. These midges suck blood from infected animals and replication of virus occurs in the digestive tract. In later stages, salivary glands get affected and transmission of virus can occur. Almost 15 days are required for the whole cycle i-e from infection to transmission. Around the globe, 27 serotypes of BTV have been reported which in return complicate the vaccination strategies. In Pakistan serotype 2, 6, 8 and 9 has been reported recently. 

Bluetongue was first known as malarial catarrhal fever of sheep. It was first recorded in South Africa from import of sheep from Europe. Incubation period of BTV is usually 5–10 days. The virus is inactivated by 50°C for 3 hours or by 60°C for 15 minutes and sensitive to pH less than 6.0 and more than 8.0. The virus is also inactivated by ß-propiolactone; iodophores and phenolic compounds. Virus is very stable in the presence of protein (e.g. has survived for years in blood stored at 20°C).Although BTV is transmitted by arthropod vectors, transmission via seminal fluid through the placenta is also reported. Apart from biting midges some other vectors like arthropods; ticks and mosquitoes are also responsible for the transmission of BTV virus. Studies have shown that ingestion of BTV-infected bovine fetus is also one route of transmission. If the bull is viraemic its semen has the potential to transfer the virus. In both cattle and sheep BTV can cross the placental barrier that serves as another method of spread. The virus enters into the skin with the help of an infected midge bite. Replication of virus takes place in lymphocytes, macrophages and endothelial cells. 

Sheep are the most susceptible species for BTV infection but it can infect several other species of domestic and wild ruminants as well. The onset of disease may result in sub-clinical or signs of clinical illness that includes fever, abortion, lameness, oral lesions, excessive salivation and congenital deformities. In Pakistan, BTV infection is endemic and prevalence of its multiple serotypes has reported in Rambouillet sheep flock. In northern parts of the North West Frontier Province (NWFP)the level of BTV antibodies in other sheep flocksis not known. 

The most important preventive measure is to ban the import of animals from the countries where bluetongue is a major issue. Serological testing, virological testing and monitoring of vectors should be done. Vaccination and control of vectors can minimize the occurrence of disease. The protection of animals in sheds can be enhanced by door and window screens made of a fine mesh infused with insect repellent. The use of insecticides is another way to kill the midges as they are responsible for spread of virus, animals should be moved away from arthropod breeding and resting sites. Intradermal administration of ivermectins can also be used to protect the animals. It is impossible to completely eliminate the midges from the environment however; areas which are damp and favorable for midges must be dried out. Control of BTV is comparatively different due to multiple serotypes. The viral genome is having ten segments which allow the genes to exchange randomly between other viruses. As a result different generations of virus with mixed genes are produced. The best option for the control of disease is vaccination of animals. Recombinant vaccines, live attenuated and inactivated vaccines are usually available now a day. 

AMIR IFTIKHAR MALIK, DR. MUHAMMAD IJAZ,  

Lahore, October 6.