West Nile Virus Update

By Dave Sauter, DVM, Kulshan Veterinary Hospital

West Nile Virus first entered the United States in 1999, causing an isolated problem in the New York area.  Since that time, the disease has continued to spread across North America and is now permanently established.  As it has spread through the country, the highest number of cases has often occurred in late summer.  Consequently, it is an appropriate time to review the disease.

Background

West Nile virus was first isolated in 1937 in the West Nile District of Uganda. Mosquitoes transmit the virus and West Nile virus has been isolated from over 40 different species.  Mosquitoes acquire the virus from infected birds, which are the natural “reservoir” for the virus.  Many species of birds can be infected with West Nile virus.  Corvid species (crows, blackbirds) and birds of prey seem especially vulnerable to the disease.  Humans and horses are “dead-end” hosts, which means they can be infected but do not transmit the disease.  Other domestic and wild animals can be infected but are not as vulnerable to the disease, including cats, dogs, sheep, goats, bats, skunks, rabbits, and even alligators. 

West Nile virus first entered North America in 1999.  It only affected a small area in the New York City area, where it caused 62 human cases including 7 fatalities and a handful of equine cases.  It has continued a steady march across North America.  In 2003, the human total case count reached 9,862 with the highest number of cases in Colorado, Nebraska, and South Dakota.  In the horse population, the pattern has been similar with new states typically experiencing the highest number of cases.  The table below is a summary of the data on equine cases since 1999:

Several interesting observations can be made from this data.  First is the steady westward movement of the virus.  Another is the tendency for high case counts when the virus first moves into a new area.  In general, the case counts go down in subsequent years, but there have been exceptions to that trend (for example, another high count in Pennsylvania in 2003 after first striking that area in 2000).  2003 was an encouraging year because there was a large drop in the number of equine cases.  This first time reduction in case counts did not occur in the human population.  In fact, in 2003 the human population case count was for the first time higher than the number of equine cases (9,862 versus 5,181).  This might be related to the fact that there is an effective equine vaccine but no current human vaccine for the disease.

CLINICAL SIGNS OF WEST NILE VIRUS

Once bitten by an infected mosquito, the virus spreads through the blood stream and spreads into the central nervous system, including the brain and spinal cord.  This results in inflammation and impaired function of the nervous system.  There are a variety of symptoms of the disease, depending on the location and severity of the inflammation in the central nervous system.  Signs of disease may include:

1.       Listlessness

2.       Weakness, especially of the hind limbs

3.       Muscle tremors

4.       Fever

5.       Twitching of the muzzles and ears

6.       Stumbling, incoordination

7.       Falling

8.       Inability to stand

9.       Death in 30% or more

There are certainly numerous other diseases that can cause similar signs to these.  Some examples include EPM (equine protozoal myeloencephalitis), the neurological form of Rhino, botulism, moldy corn poisoning, sleeping sickness (WEE, EEE), and even Rabies.  Blood tests are available to help confirm the diagnosis.

TREATMENT AND PREVENTION

Supportive care is the only treatment available for West Nile virus disease.  Medications to reduce inflammation help to alleviate symptoms.  More severe cases might require intravenous fluid therapy and nutrient support.  For horses that are unable to stand, a sling might be an option. 

Prevention is the best means to protect horses from West Nile virus disease.  There are presently two effective vaccines available for horses.  Primary immunization requires two vaccinations given 3 to 6 weeks apart.  Thereafter vaccination should be repeated annually prior to mosquito season, to provide the greatest protection when risk to exposure is the highest. 

Reducing exposure to mosquitoes is another key element to prevention. 

1.       Spot-on fly repellents are showing very promising results (e.g. Freedom, Liberty). 

2.       Flysheets. 

3.       Be aware of peak mosquito hours.  For many species, the greatest activity is around dusk and dawn.  Mosquitos are less likely to come into the barn, so keeping horses in during peak hours can help.  Fans on the stalls are also helpful. 

4.       Measures to reduce mosquito populations can be taken.  Mosquitoes lay their eggs in standing water, so limit the number of such places around your home and barn.  Get rid of items that hold water (containers, lids, old tires, pot holes, etc).

Finally, report unusual numbers of dead birds to local authorities.  Increased numbers of dead birds is an early indication that West Nile virus has entered a new area.  The earlier the virus is detected, the quicker a community can respond and take protective measures.  It is helpful to be aware and attentive, but it is also important not to overload officials with unnecessary reports and misinformation.