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West Nile Virus Update, October 2000"
by Helen Prinold

By now, most equine-owners know that West Nile virus (WNV) is a mosquito-borne disease that can cause encephalitis (inflammation of the brain) or meningitis (inflammation of the lining of the brain and spinal cord). We also know that it's a relatively new disease in North America - outbreaks of WNV have occurred before in Egypt, Asia, Israel, South Africa, parts of Europe and Australia. We also know roughly how it spreads, and that it may be coming to Canada in the future.
But what do equine owners need to know and watch out for?

First - the chart shows how WNV is spread to humans and equines by the bite of an infected mosquito (infected ticks have also been reported outside North America).

The virus is located in the mosquito's salivary glands. During the bloodmeal, the virus is injected into the blood system of the equine.

Where WNV is now is being closely monitored. As a result of this intensive active surveillance, a considerable amount of information regarding the distribution of West Nile Virus (WNV) has accumulated over the summer months. Based on reports as of October 5, 2000 eight states in the northeast have reported the presence of the virus: New York, New Jersey, Connecticut, Massachusetts, Rhode Island, New Hampshire, Maryland and Pennsylvania plus - most recently - Washington. In 1999, 25 cases of illness in horses on Long Island, New York, were found to be attributable to WNV. Nine of those equines died or had to be euthanatized. An additional 36 equines on Long Island were found to have been exposed to WNV and developed antibodies to the virus, but did not develop clinical illness. The virus continues to be reported most frequently in dead birds and mosquitoes, but in 2000 there were to date 29 equine cases in the U.S. - 16 of those horses died or were euthanized. Equine fatalities attributable to WNV have also recently been reported in the south of France and human cases have been confirmed in Israel.

So what danger faces your area from the virus? Well, if you are in Canada or on the eastern seaboard of the U.S. there is a chance your area may in the future have the virus. For example, Health Canada noted in an October 18th release that "it is likely that the virus is now more widespread in those States and the District than the reports suggest." WNV has been found in mosquito pools (their groups are not known as herds) among several species - including mosquito species that are active at different times of the day (dawn, daytime and evening). It's a common assumption to think that because mosquitos die out over the winter, the virus won't spread or will be contained. However, infected migrating birds that go south can carry the virus to the warm southern states, spreading the disease among mosquitos there and continuing to infect birds as they come north in the spring. Also, studies are being done on the probability that overwintering female mosquites in underground areas (subway tunnels, basement crawl spaces) can continue to be infectious when they emerge. Owners and show organizers should know what to watch for, how to reduce their risk and use caution when taking equines back and forth from affected areas of the United States.

Recognizing WNV's symptoms are complicated because it takes a very short time for an affected horse to go from minor symptoms that could be a number of things to a serious and life-threatening infection. After transmission by an infected mosquito, WNV multiples in the horse's blood system and crosses the blood brain barrier infecting the brain. The virus interferes with normal central nervous system functioning and causes inflammation of the brain. There is no documented evidence of person-to-person, or animal-to-person transmission of WNV - but that does not mean it could not occur as other viruses can transmit when fluids are exchanged - such as saliva from the infected animal in an open cut. It does not appear that one equine can infect another in a neighbouring stall - but mosquitoes can certainly move between stalls easily

So what are the symptoms, and how do you tell if you horse could be infected? Basically, you and your veterinarian should consider WNV a possible suspect (along with rabies and other viral diseases) if at least one of the following signs is present:

- apprehension;
- depression;
- listlessness;
plus any two of these signs:
- head shaking;
- flaccid paralysis of the lower lip;
- ataxia (including stumbling);
- weakness of hind limbs;
- inability to stand;
- limb paralysis; or paresis (disorientation or convulsions).
Keep an eye out for signals your horse is gradually losing coordination. As an example, a 12 year-old Arabian gelding from Massachusetts had hind limb lameness noticed on August 26th, had slight muscle spasms and severe lameness, and by August 29th was paralyzed and could not get up, he was euthanized shortly after the paralysis.

Equines with suspected WNV should be isolated from mosquito bites, if at all possible. There is currently no approved vaccine - a veterinarian's regular "vaccines" do not protect your horse, but there is no reason to destroy an animal just because it has been infected with WNV. Data suggest that at least half of the horses recover from the infection.

Essentially - monitoring for WNV is no different from what most equine owners currently do - watch out for symptoms of illness and if the symptoms look like they are getting worse - call the vet! There are also some "barn-keeping" items you can do over the fall, winter and spring to be safer in 2001. Essentially they focus on reducing mosquitoes. Mosquitoes lay their eggs in standing water. Weeds, tall grass, and bushes provide an outdoor home for the common adult mosquito which is most often associated with West Nile virus. Certainly mosquito control on a farm is difficult, however, here are a few tips that may reduce your risk in the future:

  • Remove all discarded tires from your property.
  • Dispose of tin cans, plastic containers, ceramic pots, or similar water-holding containers.
  • Make sure gutters drain properly. Clean clogged gutters in the spring and fall.
  • Clean and chlorinate swimming pools, outdoor saunas and hot tubs. If not in use, keep empty and covered. Drain water from pool covers.
  • Change the water in bird baths at least once a week - you may also wish to consider whether the birds you are attracting to your feeders may in future years be virus carriers.
  • Turn over plastic wading pools and wheelbarrows when not in use and drill holes in other containers with standing water (except water troughs!) - aerate or consider using fish in pools for water features you wish to keep.
  • Eliminate standing water that collects on your property and consider using a larvicide if eliminating the water is impossible.
  • Wash water troughs out frequently - at least monthly.
  • Remind or help neighbors to eliminate breeding sites on their properties.
  • Mosquitoes can enter barns through unscreened windows or doors, or broken screens. Make sure that doors and windows are closed whenever possible and that windows have tight-fitting screens.

Information for this update was derived from: Health Canada, New York State Department of Health, New York City Department of Health, New Jersey Department of Health and Social Services, Connecticut Department of Environmental Protection, Massachusetts Department of Public Health, Rhode Island Department of Environmental Management, New Hampshire Department of Health and Human Services, Maryland Department of Health and Mental Hygiene, Pennsylvania Department of Health, Vermont Department of Health, Virginia Department of Health and the Government of the District of Columbia. the National Atlas of the United States at http://www.nationalatlas.gov/virusmap.html, Promed at http://www.promedmail.org, USDA at http://www.aphis.usda.gov and the US Geological Survey, National Wildlife Health Center.

Helen Prinold is CD-L subscriber and who has been driving for four years (although she notes she has loved and ridden almost all of her life). She owns a Welsh Mtn. pony and a Morgan cross, and has done combined driving trials, pleasure and Welsh breed show driving, is a member of several driving organizations and is on the Canadian Carriage Driving Classic's Board of Directors.



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