On July 6, 2001, the presence of West Nile (WN) virus in Florida was confirmed when a dead crow found in Jefferson County was determined to be positive by the Florida Department of Health. In the late summer 1999, the identification of WNV in New York marked the first recognition of the virus into the Western hemisphere. During the 1999 outbreak, cases of encephalitis caused by WN virus were documented in birds, humans, and horses in a limited geographic area in New York State. In 2000, the identification of WN virus in mosquitoes, birds, humans and horses proved that the virus had successfully overwintered and was now present in a wider geographic area extending from Vermont to North Carolina. In 2001, WN virus was again identified in mosquitoes and birds in several Eastern states.

Equine cases  In 1999, WN encephalitis was diagnosed in 25 horses living in neighboring counties in NY. The disease was fatal in 9 horses, while the rest of them recovered over weeks to months. In 2000, veterinary surveillance identified WN virus infections in 63 horses with neurologic disease in 7 states (28 horses in NJ; 21 in NY; 7 in CT; 4 in DE; and 1 each in MA, PA, and RI). Twenty-three of these horses either died or were euthanatized. Horses were affected with no predilection for any category of age, gender or breed.

Clinical signs of WN virus infection in horses  WN virus causes encephalitis, an inflammation of the brain that is accompanied by a variety of neurologic disorders including:

• Ataxia (most common clinical sign, affecting rear limbs more often) characterized by stumbling, staggering, wobbly gait, or incoordination
• Weakness
• Going down with difficulty and inability to rise
• Muscle fasciculation
• Facial paralysis or twitching
• Teeth grinding
• Blindness

Fever was documented in approximately 1/3 of affected horses in previous outbreaks.

Transmission cycle  Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit WN virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness. The disease is not directly transmitted between horses, from birds to horses or from horses to humans.

Diagnosis  A test that detects antibodies to the virus in the blood is available and should be used in horses presenting clinical signs of encephalitis. The same test may be used on cerebrospinal fluid. Other tests that identify the presence of the virus in nervous tissues may be performed after post mortem examination of deceased animals.

Prevention  An equine vaccine to prevent WN virus infection is being developed and might be available by late summer. Currently, prevention relies on avoidance of exposure to mosquitoes, which includes

• Eliminating mosquito-breeding sites such as stagnant water contained in bird baths, flower pots, old tires, unused pools, plastic tarp and the likes
• Keeping horses away from ponds, marshes or wetlands
• Keeping horses indoors at dusk and dawn, the periods of greatest mosquito activity
• Use fans or screens in stalls and barns
• Apply insect repellents as per manufacturer’s recommendations

These measures may also protect horse against other deadly diseases carried by mosquitoes such as Eastern Equine Encephalitis and St Louis Encephalitis.

Surveillance  An extensive surveillance program involving monitoring of mosquito pools, sentinel flocks of chicken and sentinel horses is already established in Florida. Based on experience from previous years, one of the first indications that WN virus might be present in an area is the unusually high death rate of birds, in particular crows or blue jays. Anyone who discovers a dead bird is encouraged to report it to the local health department or fish and wildlife conservation commission office, or to use the form found online at http://wld.fwc.state.fl.us/bird/. In addition, any horse presenting clinical signs consistent with encephalitis should be promptly examined by a veterinarian.

What are the risks for horses in Florida?  Like in 1999 and 2000, cases of WN encephalitis are expected to occur in 2001 in humans and horses. However not all horses exposed to the virus develop the disease, and amongst horses with clinical signs, the outcome is favorable in the majority of the cases. Altogether, a low percentage of the equine population is considered at risk of developing the disease.