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News

2001/12/18

Overview Of West Nile Virus In Horses by Dr. Frederick Harper and Dr. Fred Hopkins West Nile Virus (WNV) is a new disease in the United States and Tennessee. It can infect both people and horses even causing death in both. WNV results in an encephalitis which is an inflammation of the brain. No specific treatment is available for WNV but vaccination and mosquito control should aid in preventing WNV in horses. As with similar diseases, WNV is principally a disease in birds that is transmitted from infected birds to other susceptible birds, people or horses by mosquitoes and occasionally ticks. WNV was discovered in 1937 in Uganda and is commonly observed in Africa, Western Asia, the Middle East and occasionally in Eastern (southern) Europe; however, the first cases in the United States were reported in late summer of 1999 in dead crows in the New York City area. WNV infections were soon diagnosed in humans and horses in this area. The disease spread from birds to people and horses in the Northeast U. S. The WNV is incredibly adaptable. WNV has been detected in a large number of domestic and wild birds, various mosquito species and causes illness and death in birds, humans and equine. Based on limited evidence, a few other species are also affected such as: bats, raccoons, chipmunks, cats, rabbits, squirrels and skunks. Apparently, birds, humans and equine are more susceptible to get WNV. Apparently, bird migration has resulted in the spread of the disease along the Atlantic seaboard and westward and even north into Canada. Due to the public health concerns, there is a monitoring program on birds (crows, blue jays and hawks) and mosquito pools if WNV is found in birds. Any one observing sick or dead crows, blue jays or hawks (only these species are wanted) should contact their local health officials. Submit only sick or fresh dead specimens. Sick or fresh dead specimens can be placed in a plastic bag by inverting the bag over one's hand to retrieve the bird, or one can use protective plastic gloves. There is no evidence that a human can be infected from contact with a bird; this is just a precautionary measure. Crows are extremely susceptible to WNV and become very sick and die within a short period of time. WNV is transmitted by an infected mosquito. There is no evidence that WNV can be transmitted from horses to humans or visa versa. Humans and horses are terminal or deadend hosts. They do not have a sufficiency viremia, blood level of the virus, of adequate magnitude or duration for a mosquito to get the WNV from a horse or a human and infect either other mammals or mosquitoes. There is no evidence that humans or horses can get the virus from contact with sick humans or horses, either. However, bird-to-bird transmission has been proven. This disease is similar to Eastern and Western Equine Encephalomyelitis, which are also transmitted from birds by mosquitoes, but are not transmitted from horses to humans either. In 1999, there were 25 WNV equine cases with nine dying or being euthanized (36%). Of the 60 equine cases in 2000, 23 died or were euthanized (38%). In two years, there were 85 confirmed equine WNV cases with 32 deaths or horses being euthanized for a death rate of 38%. There has been 20 cases confirmed in humans with only two deaths through 2000. A man in Northern Florida is the only human to be diagnosed with WNV in 2001. Three studies conducted on humans in areas with high levels of human and equine infections showed the following levels of infections: 0 in a population of 800, 1 in a population of 500 and 1 in a population of 1,000. This is only 0.13% or 1.3 infected persons per 1,000 individuals. In infected humans, only 1 in 150 (0.66%) gets the encephalitis. So the probability of a person even contacting this disease in an endemic area is very low. This translates to less than one person per 100,000 individuals getting an encephalitis even in a "hot bed" area. Of individuals getting the disease, the death rate in humans has been low. These data should reassure individuals that this is not a disease with a high risk death rate in humans. Culex salinarius is of concern as it feeds on mammals, including humans more than other mosquitoes species associated with WNV. The Culex pipiens complex of mosquitoes closely associated with WNV breed in standing water, especially water polluted with organic matter. These mosquitoes also may prefer to bite birds and are active from dawn to dusk. So far in 2001, WNV has been detected in larger numbers and over a wider range than in 1999 or 2000. Twice as many birds have been confirmed with WNV this year as last year. WNV has been detected as far south as Florida and as far west as Louisiana. WNV is appearing earlier too. The first positive horse case was July 12, 2001 in FL. Last year the first cases were August 17. The epicenter of WNV activity seems to have moved south. In 2001, there have been 33 human cases of WNV mostly in New York and Florida, but only one death. Most WNV infections produce no symptoms in humans or horses. People who become infected may have a fever, headaches and body aches which last only a few days within two weeks of exposure to WNV. A few individuals will have more severe symptoms such as neck stiffness, incoordination, disorientation, tremors, convulsions, paralysis and rarely death. There have been 191 positive WNV cases diagnosed in horses in 15 states in 2001. The states are: Alabama, Connecticut, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, New Jersey, New York, Pennsylvania and Virginia. About 70 percent of the equine cases in 2001 have been diagnosed in Florida. Tennessee can be added to this list. A positive WNV horse case was confirmed on October 23, 2001, in Greene county. The eight year old mare was euthanized on October 6, due to neurological problems. The horse had not been vaccinated for WNV and had been in Tennessee for the past several years. Horses with WNV lack muscular coordination. They are weak in their hindquarters, stumble and go down with difficulty in rising. They may tilt their heads, have facial or muzzle witching, paralyzed or drooped lips, grind their teeth and may become blind. They may be lethargic, lack awareness, or be hyperexcitable. Fever has been detected in about 1/4 of all confirmed cases. Some horses are subclinical. These signs are often confused with other diseases such as Equine Protozoal Myeloencephalitis (EPM). Horses that live through two-three weeks of WNV usually recover without any long-termside effects. In those parts of the world where WNV is endemic, a percentage of the equine population has been exposed to the virus, developed antibodies and become immune to the disease. On August 1, 2001, the USDA conditionally licensed a WNV vaccine for horses. Conditional licensure of a vaccine means that it is pure and safe, and it can reasonable be expected to be effective. This conditional license is good for one year. The Tennessee Department of Agriculture has granted permission for the use of the vaccine for horses by an accredited veterinarian. Owners planning to ship horses to other countries should be alerted that WNVvaccination may not meet the import regulations of some countries due to the presence of certain antibodies in the horse's blood. These antibodies are not permanent but could interfere with international shipment of horses. Outbreaks normally subside in the fall and winter as mosquitoes die off. With WNVspreading south, there may be areas where the disease occurs year-round if mosquitoes are active in the winter months. Efforts should be implemented to reduce the presence of mosquitos, especially eliminating their breeding grounds. This can be accomplished by removing any materials such as old tires that can contain standing water from farms. Consult with local mosquito control agencies or health agencies for methods to control mosquitoes in standing water such as occurs in streams that partially dry-up in summer, low swampy areas, etc. Due to the human health concerns, local health organizations may have greater interest in mosquito control in such areas. Use recommended mosquito repellants on horses and insecticides in and around stabling and congregating areas, especially at dusk, dawn and night. Many mosquito species are dusk, night or dawn feeders, but some are daytime feeders. Unfortunately, WNV has been noted in 43 different species of mosquitoes. Horse owners who take their horses to states/areas where WNV has been diagnosed should use caution. Vaccinate your horses if recommended by your veterinarian. Precautions should include the use of recommend insecticides on horses and in trailers. Keep all equipment clean. Use fans when horses are stalled to reduce mosquito contact. The best possible protection is to prevent exposure of humans and animals to mosquitoes. The key preventive message is to eliminate mosquito breeding areas around homes, horse barns/facilities and prevent mosquito bites. Horse owners can contact their county Extension office for the latest information on WNV in horses.

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