What Wisconsin Horse Owners Should Know About |
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| WEST NILE VIRUS |
What Wisconsin Horse Owners Should Know About |
|
| WEST NILE VIRUS |
What is West Nile Virus? Although West Nile virus (WNV) has received a great deal of attention due to its ability to cause illness in both horses and humans, it is primarily a viral disease of birds. In its normal life cycle, the virus is passed in the blood of infected birds to biting mosquitoes. These mosquitoes then carry the virus in their salivary glands and pass it on to other susceptible birds when they ingest another blood meal. The cycle then repeats itself. Unfortunately, other animals - including horses and people-can also acquire the disease if bitten by an infected mosquito.
Where Did West Nile Virus Come From? West Nile virus was first identified as a disease-causing agent in 1937 when it was isolated from the blood of a woman in the West Nile district of Uganda (hence the name "West Nile virus"). It was first diagnosed in horses in Egypt and France in the 1960s. Since its discovery, the disease has appeared throughout Africa, Eastern Europe, the Middle East, and West Asia, but it was not seen in the western hemisphere until 1999 when an outbreak of West Nile virus encephalitis -- a life-threatening inflammation of the brain -- occurred in the New York City area. This outbreak, which was centered in Queens and limited to areas located within a 75 mile radius of New York City (New York State, New Jersey and Connecticut) resulted in the deaths of 7 people and 9 horses as well as large numbers of wild birds (especially crows and jays which are particularly susceptible to the disease). No one is certain how the virus entered the U.S., but it may have been transported from an area where WNV was already present to the port of New York via an infected mosquito, bird or other animal carried on an international flight or ship. (We know through genetic analysis that the strain of virus that entered the U.S. in 1999 closely resembles that found in a WNV-infected goose in Israel in 1998, and WNV activity had increased in both the Middle East and Europe just prior to the New York outbreak.)
Where in the US is West Nile Virus Now? Since its introduction into the U.S., West Nile virus has spread from its original locus in the New York City area to other regions of the country. In the year 2000 WNV was reported in 12 states along the Eastern seaboard (Connecticut, Delaware, Maryland, Massachusetts, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Vermont and Virginia) and in the District of Columbia. By 2001 the virus had extended its range into the Midwest and was reported in 27 states including Wisconsin, Michigan, and Illinois, as well as Ontario, Canada and the Cayman Islands in the Caribbean. As of August 30, 2002, West Nile virus has been confirmed in all of the states of the Eastern, Southern and Midwestern United States as far west as Montana, Wyoming, Colorado and New Mexico. (Click here to view current map of West Nile Virus in the US) Carried by migratory birds, WNV is expected to eventually reach the West Coast. Which Birds Carry West Nile Virus? West Nile virus had been found in more than 100 species of birds, including many commonly found in Wisconsin. With the exception of crows and their close relatives (jays and ravens) which are particularly susceptible to the disease and experience a high mortality rate from it, most birds infected with WNV recover completely and, once recovered, are probably immune to re-infection. Domestic poultry can become infected with WNV but generally do not become ill and are not thought to be able to infect mosquitoes or other animals. (For a complete listing of bird species that have been found positive for WNV to date, see the links at the end of this article) What is the Extent of Equine WNV Disease in the US? Last year 20 states reported a total of 738 cases of WNV disease in horses. The vast majority of these cases (492) were reported from the state of Florida. Other states reporting equine cases of WNV in 2001 include Alabama, Connecticut, Delaware, Georgia, Indiana, Illinois, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, New Hampshire, New Jersey, New York, North Carolina, Pennsylvania, Rhode Island, Tennessee and Virginia. So far this year infected horses have been reported from the majority of states reporting West Nile infection in birds. For updated information on equine WNV infections in the US visit http://www.aphis.usda.gov/oa/wnv. To view a map of the states reporting cases of WNV in horses go to http://www.aphis.usda.gov/oa/wnv/equimap02.jpg. What is the Status of West Nile Virus in Wisconsin? Last year (2001) 58 dead birds (56 crows, a blue jay, and a blackbird) found in 5 Wisconsin counties (Kenosha, Racine, Milwaukee, Waukesha and Dane counties) tested positive for WNV. So far this year (as of 29 August 2002), West Nile virus has been confirmed in birds in 32 Wisconsin counties including Adams, Brown, Dane, Columbia, Eau Claire, Green, Iowa, Jefferson, Kenosha, Langlade, Marathon, Marquette, Milwaukee, Oneida, Outagamie, Ozaukee, Portage, Racine, Rock, Sauk, Sheboygan, St. Croix, Taylor, Vilas, Walworth, Washburn, Washington, Waukesha, Waupaca, Waushara, Winnebago and Wood Counties. Confirmed equine cases of West Nile virus have been reported in Dane, Fond du Lac, Jefferson, Washington, Manitowoc, St. Croix and Waushara counties, and a possible case of equine WNV has been reported in Pepin County. Possible human cases of the disease have been reported in Winnebago and Adams Counties. Updated information on West Nile virus activity in Wisconsin can be obtained by visiting the Wisconsin State Health Department's web site at http://www.dhfs.state.wi.us/dph_bcd/westnilevirus Wisconsin now has a 1-800 hotline number for obtaining information on the disease and for reporting sick or dead birds (crows, jays or ravens) suspected of being infected with the virus. .Although there is no evidence that WNV can be transferred directly from dead birds to humans, people are advised to use a protective barrier - rubber or latex gloves or an inverted plastic bag - when handling them. Dead birds not sent to the state for testing should be double bagged and placed in an outdoor trash container with a secure lid to avoid ingestion of the dead animals by other birds, cats or dogs.Should I Vaccinate My Horse Against WNV? In 2002, an equine vaccine against WNV received a one year conditional licensing * for use in horses and is currently available from veterinarians in Wisconsin. This vaccine has been shown to be effective in laboratory studies (serum from vaccinated horses significantly reduced viral infection of horse cells in culture, indicating that protective antibodies were produced in response to vaccination ). Definitive studies of vaccine efficacy in live horses are currently in their final phase and should be reported in the near future. Side effects of the WNV vaccine appear to be similar to those of other equine vaccines; the most significant adverse reaction to date has been soreness and stiffness at the site of injection.
( Studies have shown that in areas where WNV is present, horses most at risk were those located within 1/2 mile of large communal bird roosts or large congregations of water fowl. If your horse resides in such an area you should consult with your veterinarian regarding the advisability of administering the WNV vaccine.)
Since WNV has been confirmed in several Wisconsin counties, many people are now considering vaccinating their horses against this disease. If you are considering vaccinating your horse you should know that two doses of vaccine, given 3 to 6 weeks apart, are essential for protection against WNV: One dose of the vaccine will not protect your horse against WNV infection. In addition, if you are vaccinating your horse prior to travel to an area where WNV activity is high, both doses of the vaccine should be completed at least 4 weeks prior to travel. Although duration of protection from the vaccine is not yet known, completion of the first two dose series this spring should provide sufficient protection for horses living in Wisconsin and other areas where mosquito activity ends in late fall. The manufacturer expects that vaccination against WNV will eventually be recommended once a year in conjunction with other spring vaccines in northern states where mosquito activity and the highest risk of WNV infection peak in late summer and early fall. However, in areas where mosquito activity remains high throughout the year, booster doses of WNV vaccine may be recommended. (It is currently recommended that horses residing in Florida be vaccinated 3 times a year) If you plan to take your horse into such an area this winter, check with your veterinarian regarding recommendations for a booster shot.
Use of the vaccine in pregnant mares and foals: WNV vaccine has been used extensively in pregnant mares with no reported ill effects to either the mare or foal; in fact, foals born to vaccinated mares may be born with protective antibodies against the disease. Foals from unvaccinated mares living in high risk areas can receive the WNV vaccine as early as 6-8weeks of age, but foals vaccinated at this young age should receive 3 doses of the vaccine instead of the usual 2 dose series. . Foals born to vaccinated mares in high risk areas and foals not living in or travelling to high risk areas can be given the usual 2 dose series of WNV vaccine when they are 3 to 5 months old.
How Else Can I Protect My Horse From WNV? WNV is transmitted to horses via the bite of infected mosquitoes ; the risk of acquiring WNV can be decreased by reducing mosquito populations and by limiting exposure to adult mosquitoes. The most important step any property owner can take to control mosquito populations is to remove all man-made sources of stagnant water in which mosquitoes might breed. The following actions are recommended:
Limiting your horse's exposure to adult mosquitoes will also help prevent infection with WNV, although this may be more difficult than reducing mosquito populations. The following actions may be helpful:.
How Do I Know if My Horse Has WNV Disease? Symptoms of West Nile virus infection in horses include listlessness, stumbling and loss of coordination, , muscle tremors, limb weakness or inability to stand, partial paralysis and death. Fever is not usually present (only 25% of affected horses develop a fever). If your horse demonstrates any of these symptoms, your veterinarian can test him for WNV infection. .
If My Horse Gets WNV, Can He Infect Me or My Other Horses? There is no evidence that infected horses can pass the disease on to other horses, people, birds or other animals. However, owners of horses exhibiting symptoms of West Nile virus infection (neurological symptoms) are advised to take precautions against the possible transmission of rabies since this disease may present with symptoms similar to those of WNV. (Rabies will be covered in a later issue of Eastern Wisconsin Horseman's Newsletter -- it is spread by contact with the saliva of an infected animal) . It is currently thought that horses and animals other than birds are not capable of infecting mosquitoes since the virus is present in low numbers and only for a short period of time in their blood ; however, this is not known for certain. (In one study horses infected with WNV experimentally did not transmit the disease to mosquitoes; however, there is some evidence that at least one mammalian species can do this).
Will I Have to Euthanize My Horse if He Gets WNV? The mortality rate for horses exhibiting symptoms of WNV disease is approximately 40%; however, horses that live through the 2 to 3 weeks of WNV infection generally recover fully with no long-term side effects. Because infected horses do not transmit the disease to other animals or to mosquitoes, there is no reason to euthanize an infected horse except for humane reasons if the animal is severely ill.
Can My Other Animals Become Infected With WNV? In addition to birds, several species of mammals have tested positive for WNV including dogs, cats, rabbits, squirrels, skunks, bats, chipmunks and raccoons. However animals other than birds rarely, if ever, develop severe WNV disease, and most are probably not capable of infecting mosquitoes..Dogs and cats are readily infected with WNV by mosquitoes; cats can also develop low-level infections by eating WNV-infected mice. (Studies of oral infection in dogs have not been conducted). Some cats do develop a mild, self-limited clinical illness following infection by mosquitoes; to date, clinical illness has not been seen in dogs. Neither dogs nor cats are thought to be able to transmit the disease to other animals or to mosquitoes. Because of the possibility of oral transmission, it is best to keep cats and dogs from preying on potentially infected animals or from scavenging carcasses.
People Can Get WNV Disease Too -- What Can I Do to Protect My Family? Like horses and other mammals, most people infected with WNV will not develop symptoms of the disease. However, a small proportion of infected people will experience a mild illness that may include fever, headache, body aches, skin rash and swollen lymph glands. Less than 1% of infected people will develop encephalitis - a potentially fatal inflammation of the brain - and of these, a small proportion will die (death is estimated to occur in less than 1 out of 1000 infections) Symptoms of encephalitis include headache, high fever, neck stiffness, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis.. Medical care should be sought as soon as possible for persons exhibiting these symptoms.
There is currently no vaccine available to prevent WNV disease in humans. In addition to reducing mosquito populations on their property (see "How Else Can I Protect My Horse From WNV" above) individuals can reduce their risk of infection with WNV by limiting their exposure to biting mosquitoes. The following actions are recommended for persons living in high risk areas (These actions are particularly important for the elderly and for people with impaired immunity - those with cancer, chronic illnesses or taking steroid medications - since they are most at risk for developing severe disease if infected with WNV):
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My sincere thanks to Linda Glaser of the Wisconsin State Department of Health, Kathy Converse of the US Geological Survey's National Wildlife Health Center, Dr. John Tuttle of the Fort Dodge Animal Health Company, and Dr. Dan O'Leary of the Centers for Disease Control for their assistance with the preparation of this article.
WNV in States Neighboring Wisconsin and Helpful Links:
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