The avian influenza H7N3 outbreak in South Central Asia

A highly pathogenic avian influenza virus subtype H7N3 was first diagnosed in Pakistan in 1995. The outbreak initially started in the cold northern region of the country where the migratory birds move in every year from China and Russia at the onset of winter. The disease affected mainly a population of 3.2 million birds, primarily breeders and broilers within a radius of 100 Km. The flock-level morbidity rates ranged from 13.9-86.7 % and within flock mortality ranged from 51 to 100 %. Although mostly breeder stocks are reared in this region and they were hit badly, some broiler and layer farms also got affected at some other places. The severity of disease started declining within 5 months as it descended towards the southern parts of the country. A mass vaccination as well as biosecurity program was launched in and around the affected area, which eventually helped to overcome the disease. Since August 1995 no confirmed case of avian influenza has been recorded in the country. A selectively employed serosurveillance program is still underway in the country. The characterization of various field isolates by amino acid sequencing of HA cleavage site indicated some changes in the sequence over a period of 4 months. On the other hand, serologic analysis has indicated the emergence of non-pathogenic subtypes out of the original highly pathogenic virus. The disease has not been reported from other countries in the region.