West Nile Encephalitis in Horses

Background
West Nile virus a mosquitoborne virus was detected for the first time in the United States in August 1999. The virus can cause encephalitis which is characterized by inflammation of the brain and spinal cord. The first outbreak of West Nile encephalitis in the United States occurred in the New York City metropolitan area where 62 people were diagnosed with the disease. Seven of those people died. The infection also was diagnosed initially in several breeds of horses a variety of zoo birds and various native bird species especially crows.

In October 1999 a total of 25 horses all from Long Island New York had confirmed signs of West Nile encephalitis of which nine died or were euthanized. The first confirmed case of West Nile virus in a Pennsylvania horse was reported in October 2000. Since then positive detections in horses have been made in most mainland states throughout the United States and provinces in Canada. Horses may be exposed to West Nile virus and not show signs but of those that are infected and become ill about 35 percent die or are euthanized. With the continuing health threat of West Nile virus to horses ongoing research to learn more about the disease and develop effective prevention and control measures becomes even more important.
Transmission of West Nile Virus
Infected birds serve as the reservoir host of the West Nile virus. These birds develop viremiaa large number of virus particles in their circulatory system. Mosquitoes become infected after taking a blood meal from an infected bird. They in turn infect horses by biting them.
The incubation period the time between exposure to the virus and appearance of the first signs of diseaseis estimated to be between 3 and 15 days. Humans and horses are known as deadend or terminal hosts. These hosts have so

few virus particles in their bloodstream that a mosquito cannot accumulate enough of the virus while taking a blood meal to subsequently transmit the infection.
No evidence suggests that horses can transmit West Nile virus to other horses birds or humans. In fact evidence exists that the virus can be found in the horses bloodstream for only a few days during the entire course of the infection. Despite this fact one should use caution when handling blood spinal fluid or nervous tissue from suspect animals since these may contain the virus.
Signs of West Nile Encephalitis
The West Nile virus can affect the central nervous system and cause encephalitis. Clinical signs may include one or more of the following weakness hind end front end or both characterized by falling to their knees or difficulty in remaining standing ataxia incoordination muscle twitching or tremors altered mental state somnolent or aggressive circling or stallwalking hypersensitivity to touch or sound cataplexy or narcolepsy seizures blindness cranial nerve deficits facial paralysis tongue weakness difficulty swallowing recumbency and fever.
Horses with severe clinical signs can die as a result of the infection or are euthanized due to secondary complications. Current data indicates that the mortality rate in clinically affected horses is about 35 percent and even higher in recumbent horses. The West Nile virus has been confirmed in foals as young as three weeks. However the risk of infection and death seem to increase with age. Research indicates that elderly horses have decreased antibody titers because horses greater than ten years of age tend to have an age dependent decrease in neutralizing antibody response after vaccination.
It is important not to presume that horses with clinical signs of encephalitis have West Nile encephalitis. A definitive diagnosis requires ruling out other diseases with similar neurological signs. Rabies botulism equine protozoal myeloencephalitis