West Nile virus (WNV) is an infectious viral disease that attacks the neurological system. Fever and depression are common clinical signs, along with limb weakness. Some horses become ataxic (uncoordinated) in the hind limbs. Stumbling, toe dragging and listing to one side are possible signs. Muscle twitching or tremors often develop in the face, muzzle and lips, and/or in the neck, shoulders and chest. Swallowing might be difficult, and vision can be impaired.
Horses that are normally calm might become hypersensitive to touch and/or sound.
Severely affected horses might wander aimlessly, develop convulsions, or become paralyzed in the rear quarters and go down, unable to rise.
Death occurs in 30-40% of cases. Of those that survive, as many as 40% of cases persist with neurologic abnormalities of gait and behavior for six months, and some deficits can persist indefinitely.
Transmission of WNV relies on interaction between birds and mosquitos, particularly Culex species of mosquitos. Birds are the reservoir for the virus, and many avian species (corvids such as crows, ravens, jays; house finches; owls and hawks) often succumb to the infection.
If a mosquito has taken a blood meal from an infected bird, it can carry the virus to horses or humans and to other birds when it takes its next blood meal.
Horses and humans are dead-end hosts, meaning that a mosquito biting an infected horse or human cannot acquire sufficient virus loads to pass infection on to other individuals of any species.