West Nile Virus (WNV) is a mosquito-borne virus that causes encephalitis (inflammation of the brain) and/or meningitis (inflammation of the lining of the brain and spinal cord). WNV primarily infects and multiplies in birds, which serve as reservoirs for the virus. The severity of the disease in birds varies among species, with American Crows frequently dying from the infection. WNV can also infect and cause illness in horses and humans, although only a small percentage of infected animals or people become sick. The virus also has been shown to infect several other types of mammals (including bats, cats, chipmunks, squirrels, skunks, and domestic rabbits) often without causing clinical signs.

Animals must be bitten by a virus-carrying mosquito in order to become infected with WNV. Mosquitoes become infected when they bite, or take a blood meal, from a bird carrying WNV. Some species of mosquitoes bite mammals as well as birds, and this is how the virus may be transmitted from a bird to a horse, human, or other mammal. There is no evidence that infected horses, humans, or other animals are able to transmit the virus to other animals, humans, or mosquitoes.

The incubation period, or the time between the bite by an infected mosquito and the onset of clinical signs, ranges from 7-14 days. Birds that are infected with WNV may show signs such as the inability to fly, incoordination and abnormal movements. Most crows are simply found dead. Clinical signs of horses which do become ill range from very mild to fatal illness. Typical signs include abnormal movements that either appear suddenly or appear gradually and worsen, sleepiness, dullness, facial paralysis, and inability to rise. Some horses may develop mild fevers, blindness, seizures, and many other signs.

Currently there is no specific treatment for the disease caused by West Nile Virus. Approximately 65% of horses will recover from the disease and will

have gained immunity, which is believed to be long lasting.

A vaccine is available as an aid in control of this disease in horses. Due to the rapidly emerging nature of this disease and its spread across North America, the vaccine has been approved for release under a provisional license until effectiveness can be shown. Horse owners are advised to consult with their veterinarian concerning West Nile Virus vaccination of their horses.

In addition to the equine vaccine, standard insect control measures should be utilized. Consider the use of insect repellents, and place horses in barns/stables under fans during dawn, dusk, and other times when mosquitoes are present. Eliminate opportunities for mosquito breeding by draining wet areas of pasture, draining puddles, repairing eave troughs and gutters, clearing any containers that might hold even small pools of water, and draining water tanks once or twice weekly.

Michigan, as well as many other states, has set up a surveillance program for West Nile Virus. There are several ways local health departments, Michigan Department of Agriculture, Michigan State University, Michigan Department of Community Health, and local Nature Centers are testing for the prevalence of the virus:

• Live traps for mosquitos
• Live wild bird blood testing
• Dead crow and blue jay testing
• A partnership with private practice veterinarians to locate suspicious equine cases
• Testing of suspicious human cases

So far in Michigan, 65 dead birds testing positive for West Nile Virus have been found in nine counties (Barry, Calhoun, Ingham, Jackson, Macomb, Muskegon, Oakland, Ottawa, Washtenaw, and Wayne Counties). Once WNV is established as being present within a county, not all of the dead birds submitted for testing from that area are tested. There have been no horse or human cases in Michigan.

With the colder weather of winter, mosquitoes are less common and therefore there is not a significant amount of infection of WNV found in Michigan. As the weather becomes warmer in early spring, and the mosquitoes are active once again, there will be an increase of WNV infections.

In 2001 different State surveillance programs have found 6,403 positive WNV birds in 27 states (including Washington DC and Canada), 564 horses found in 20 states, and 53 human cases in 10 states.  (Case numbers are through Dec 31, 2001)