Horses and West Nile Virus

West Nile virus is passed from host birds to animals, and to humans by mosquitoes. "Any animal could become infected with the virus – birds and mammals, wildlife and pets, as well as people," says Dr. Millicent Eidson, state public health veterinarian for New York.

As a horse owner, this is something you should be concerned about. In humans, the virus can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord), which can be deadly. Most human cases have been in the elderly.

The virus also could sicken and kill your horse. The West Nile virus was first detected in the United States in the summer of 1999 and has spread from the northeast to Florida. Confirmed cases of the virus have been documented in animals and humans. Here are the most commonly asked questions about West Nile Virus:

When and where does it usually break out?

"When mosquitoes are active, that's when the virus is spread," says Nicholas Komar, Ph.D., vertebrate ecologist at the Arboviruses Diseases Branch of the Centers for Disease Control and Prevention. "In the temperate zone of the world (i.e., between latitudes 23.5 degrees and 66.5 degrees north and south), West Nile virus cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round."
Public health officials believe the virus is likely to spread to other parts of the United States in the future. "Probably within a few years it will turn up in most cities in the country," Eidson says. "It is probably in all of the wild bird species right now and as they move around the country with natural migration patterns, the virus will be introduced to new areas."

How is it transmitted?

The virus is transmitted when mosquitoes feed on infected host birds, which may circulate the virus in their blood for a few days. After an incubation period of 10 days to 2 weeks, infected mosquitoes can then transmit West Nile virus to humans, horses and other animals while biting to take blood. During blood feeding, the virus can be injected into the animal or human, where it may multiply, possibly causing illness.

You cannot contract West Nile virus from person-to-person contact such as touching or kissing a person who has the disease. There is also no evidence of a horse-to-person or horse-to-horse transmission of West Nile virus. This means you cannot get infected with West Nile virus by caring for an infected horse, nor can a horse infected with West Nile virus infect horses in neighboring stalls.

What are the symptoms?

Symptoms in horses with West Nile virus vary. "Affected horses may develop an unsteady gait, difficulty with walking, lack of appetite, and muscle weakness of the hind limbs," Eidson says.

"In the beginning, horses with West Nile Virus infection may present with an unusual single leg lameness, where pain cannot be localized," explains Dr. Andrew Hoffman, a faculty member in Large Animal Internal Medicine at Tufts University. In 2000, Hoffman saw two cases and consulted on several others. "Within 48 hours, there can be severe problems with coordination and behavior changes. The horse goes down or has difficulty rising.

"Fever is not a prominent finding initially, which may distinguish this disease from rabies and Eastern encephalitis. The typical signs of herpes myelitis (fever, rear leg incoordination and weakness, and bladder paralysis) are not all there. However, herpes myelitis, rabies, equine protozoal myelitis, Eastern and Western encephalitis, and other forms of encephalitis all have to be considered in each case.

"Neurologic diseases are not easy to sort out, requiring careful clinical examination, cerebrospinal fluid analysis, and submission of samples to state laboratories for further analysis. A necropsy should be done in every case suspected of infectious neurologic disease, as a neurologically sick horse serves as a marker for potential disease to other horses and humans.

"The most important thing you can do is to call your veterinarian when you think you have a horse with neurologic disease, especially if there is a sudden onset," says Hoffman.

How is it treated?

Supportive care is given to horses affected with West Nile Virus. "The most important thing you can do to try to keep your horse healthy is to limit his exposure to mosquitoes," Komar says.

What steps can you take to protect your horse against the virus?

When it comes to the health and well being of your horse, it's better to be safe than sorry.