West Nile fever (WNF) is a zoonotic mosquito-borne viral disease caused by infection with the west nile virus (WNV). The virus can infect birds, humans and horses. WNF has a wide geographical range which includes the Americas, Asia, Africa, Australia (presented as Kunjin virus), Middle East and portions of Europe. The disease is spread through migration of birds. There are a few vaccines available for horses to prevent the disease.
Clinical signs of WNF are often highly variable, including their duration. Of those horses that survive the course of the disease and appearing as if they fully recovered, 30% will have a relapse in clinical signs within 7 to 10 days. Overall, 30% of horses affected by WNF will eventually progress to complete paralysis of one or more legs. Most horses end up getting euthanized for humane reasons or they may suddenly die. Clinical signs most often observed in horses with WNF include:
- Fever: Mild to moderate fever of 102-103°F (38.6-39.4°C) concurrent with loss of appetite and depression.
- Muscle fasciculations: Fine and course fasciculations of the muscles of the horse's face and neck are most common. Fasciculations often involve all four legs and body; sometimes they occur on the muzzle and eyelids. When severe, fasciculations may impede the horse from performing normal activities such as walking, eating, and interacting with other horses and people.
- Personality changes: Often involving complete change in personality of the horse. Horses that are usually quiet may have periods of hyperexcitability and apprehension, sometimes to the point of aggression. Horses that are usually aggressive may become oddly compliant and quiet. During periods of hyperexcitability, affected horses will sometimes develop a narcolepsy-like sleep disorder.
- Cranial nerve defects: Frequently are abnormal for short periods of time. Tongue weakness, head tilt and muzzle deviation are the most common. Difficulty breathing as a result of nervous system dysfunction can sometimes occur as a result of respiratory failure and gastrointestinal disturbances.
- Spinal abnormalities: Horses may develop ataxia and paresis involving different combinations of forelimbs or hindlimbs, or by a flaccid paresis that is localized to one or more legs. The condition may only last a short period, or horses may suddenly become recumbent.
WNV is primarily transmitted by many different mosquito species. Other arthropods such as ticks and lice might have minor roles in transmission. Birds are the primary vertebrate reservoir hosts for WNV. Mosquitoes transmit the virus to birds who transmit it back to mosquitoes. Humans and horses are the dead end hosts.
Over 25,000 clinical cases of the disease have been diagnosed in horses. Cases occur during peak mosquito activity months in northern climates and is year-round in tropical and subtropical climates.
The incubation period for WNV is 3 to 15 days. The majority of infected horses don't present any clinical signs. The mortality rate is approximately 1 in 3 clinically affected unvaccinated horses.