West Nile Virus, WNV, is a disease mosquitoes transmit in horses. The virus is transmitted when a mosquito makes a meal of blood taken from infected bird and then feeds on a horse. Mosquito infects the bird with the virus, then the mosquito feeds on the infected bird, and then feeds on and infects the horse. If horses recover, they do so in about a week. Horses show signs of disease 3-15 days after being infected.
Horses are not the only ones who can get it; birds, llamas, goats, sheep, dogs, bears, various reptiles and humans as well as more species. The strains of WNV are capable of causing disease in certain domestic and exotic species of birds. Crows and blue jays especially, in which the infection was usually fatal.
WNV infection in mammals does not come in big amounts of the virus in the bloodstream, as it does in birds. There is a very small amount of virus in the blood of a infected horse. Mosquitoes are unable to transmit the virus from horse to horse or from horse to human. There are some clinical signs that a horse may show; stumbling, toe dragging, twitching muscles in the neck/shoulder area and more. Aged horses tend to get the disease worse. It's more common for horses (and humans) to get WNV in warmer weather.