Eastern/Western/Venezuelan equine encephalitis

This encephalitis causing group of viral diseases of wild birds including pheasants, chickens, turkeys, ducks and pigeons with a high morbidity and mortality has become a matter of concern due to its transmissibility between birds and their zoonotic nature. All 3 types of infections are caused by members of the family Togaviridae, genus Alpha virus. These conditions currently occur only in North America and northern part of South America. The natural hosts are wild birds and rodents. T he viruses can also infect horses and humans. The viruses, which begin their life cycles as parasites of wild birds, are transferred to horses and humans, through saliva of mosquitoes. The most important mosquito species in maintaining the bird-mosquito transmission cycle is Culiseta melanura, which reproduces in freshwater hardwood swamps.

The symptoms of equine encephalitis in humans are fever, drowsiness and incoordination, often followed by paralysis and death. The mortality rate is 90%, usually within 2 to 3 days in the most virulent Eastern type and as high as 50% in the Western type. Equine encephalitis is a serious public-health problem in northern part of South America.

Symptoms and lesions: The birds show nervous symptoms like ataxia, paresis, paralysis, flaccid neck, circling, tremors etc. Sometimes, the infection may also be asymptomatic. On PM examination of the dead birds, no gross lesions are seen and microscopic lesions are also not pathognomonic.

Diagnosis: Isolation in baby mice, tissue culture, and developing chicken embryos can be confirmatory.

Prevention and control: General biosecurity and mosquito control policy should be adopted. Basically this is not a disease of domestic poultry however; strict vigil and early detection of any suspected mortality in backyard poultry may prevent further losses.