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West Nile Virus in New Jersey Equine:
Preview of 2001, Review of 2000

What we're doing in 2001
How it Looked in 2000
What We've Learned

REMEMBER: ALL SUSPECT CASES OF WNV, EEE/WEE
OR RABIES INFECTION IN HORSES MUST BE REPORTED TO THE
DIVISION OF ANIMAL HEALTH (609-292-3965) WITHIN 24 HOURS

What we're doing in 2001
NJDA will improve its ability to respond to the needs of the equine community and industry by expanding available services for the surveillance, field assistance and diagnosis of WNV infections in horses. Part of the effort includes hiring a veterinarian to oversee the newly established equine neurologic disease unit in the Division of Animal Health. This program will facilitate the flow of information to the public, veterinarians, state and federal agencies, national and international organizations, the pharmaceutical industry and academicians involved in the study and control of WNV.

The Division's laboratory will also provide additional and confirmatory tests that should significantly reduce the time between clinical signs of disease and definitive diagnosis.

NJDA has also been actively supporting efforts to rapidly develop an effective vaccine against WNV. We are cautiously optimistic that a vaccine may be conditionally approved very soon. The Division of Animal Health will cooperate with all efforts to obtain data from field trials and with any other activities that will demonstrate the efficacy of a safe vaccine for our equine residents.

Because WNV has already had an economic impact on our equine industry, NJDA has recommended that the OIE (Office of International Epizooties) develop a scientifically-sound chapter on WNV that can minimize future international equine travel restrictions applicable to the industry in New Jersey and throughout the nation. In New Jersey alone, the $3.2 billion equine industry includes the United States Equestrian Team; four race tracks hosting 6,500 races annually including the prestigious Hambletonian (the Standardbred equivalent of the Kentucky Derby); 7,600 equine facilities; 24 breed groups; and hundreds of show and equestrian events each year; in addition to the much loved backyard horse.

Here's what you can do to help protect your horses:

  1. Minimize the mosquito habitat on your property by eliminating any water that stands more than three days

    • Empty or flush water troughs at least every 3 days
    • Clean gutters, birdbaths, and swimming pools
    • Monitor low areas on property and tire ruts --- if water stands there for more than 3 days, mosquitoes will be able to breed
    • Remove all old tires from your property - including tire jumps and tires used to hold down tarps

  2. Contact your county mosquito agency for assistance in identifying and treating high-risk areas on your property such as ponds, streams and marshes.

  3. Contact your vet as soon as possible if you observe any of the symptoms described above in your horse. The only treatment for this condition is supportive but prompt treatment may be life saving! Don't wait!

  4. Vaccinate your horse for Eastern and Western equine encephalitis (EEE/WEE) and rabies to protect horses from these diseases since they may produce symptoms that can be confused with those of WNV.

  5. Where possible, keep your horses stalled during dusk and dawn to minimize mosquitoes' feeding on your horses. This means don't turn your horses out at night, or turn them out only when it is completely dark, i.e., between 10pm and 4am. The most important months to observe this restriction are August, September, and October.

  6. On large breeding farms or other facilities where keeping horses stalled at night is not feasible, the horses should be pastured in areas away from standing water and woodlands.

  7. If you notice mosquitoes biting you or your horse in a particular pasture on your farm, don't use that pasture for turn-out. Likewise, avoid riding in areas where you notice mosquitoes biting.

  8. Help us understand the disease even more by keeping a record of the infection in your horse. While this will be difficult to do at a time when you are concerned about your horse's health, your observations may lead to additional preventive measures or recommendations to better protect your horses. Include behavioral changes, changes in appetite, daily temperature, first sign of infection, response to supportive treatments, etc., in your notes. We are asking your veterinarians to do the same.

How it Looked in 2000
A total of 63 clinical cases of West Nile virus (WNV) in horses were reported from seven northeastern states in 2000. The first case occurred in New York on August 17 and the last case was reported from Delaware on October 30. Of the 63 horses, 23 did not survive (a 37% mortality rate).

In New Jersey, 28 horses from 11 counties developed clinical signs of infection throughout the season. An additional 15 horses on the positive farms developed antibodies to the virus without showing any sign of illness. Of the 28 horses with clinical signs, eight did not survive the infection. The case mortality rate in New Jersey was 29%.

The breakdown of cases per county in New Jersey was as follows:

 
Affected County Number of Cases
Atlantic
3
Bergen
1
Burlington
2
Cape May
1
Gloucester
1
Hunterdon
2
Middlesex
1
Monmouth
10
Ocean
2
Salem
2
Sussex
3

NJDA's Division of Animal Health implemented a plan developed in the spring of 2000 to help identify and diagnose WNV in horses. The Division consulted with veterinarians in more than 100 cases resulting in over 330 laboratory tests in 81 cases and provided field support in 28 case investigations.

With the help of local veterinarians, the Division also retrieved nearly 1,000 blood samples from stablemates of all positive horses as well as from horses on farms where there were no clinical infections. These results, when analyzed with pertinent data collected at the farms, will help identify risk factors that might be modified in the future to provide better protection against WNV infection.

What We've Learned
The results of much of this extensive testing and investigation are still being analyzed, but the following information has been extracted from the data:

  1. The only current treatment for this condition is supportive but prompt treatment may be life saving! Call your vet if you suspect that your horse has WNV!
  2. No breed appears more susceptible than any other.
  3. Slightly more geldings/stallions than mares were infected.
  4. The age range of infected horses was 6 months to 32 years with an average age of just over 13 years.
  5. The age range of horses that died was 7 to 32 years with an average age of just over 15.
  6. The clinical signs observed by owners and veterinarians included:

    1. Abnormal use of their hind limbs
    2. Loss of balance
    3. Depression
    4. Apprehension
    5. Generalized or hind limb muscle weakness
    6. Muscle twitching
    7. Falling down
    8. Inability to rise
    9. Grinding teeth
    10. Appear colicky

  7. Many horses continued to eat grain while sick, although a few refused to eat anything.
  8. As the season progressed, more horses survived infection. We don't have an explanation for this, but it may be that owners became more concerned more quickly, resulting in more rapid treatment by their veterinarians.
  9. Elevated temperatures do NOT occur in most cases (only 25 percent had elevated temperatures).

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