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September 2002 Open Breed Issue |
CONTENTS Ride
The West West Nile Virus Encephalitis Want To Trail Ride Close To Home?
Gallop Pole
REAL ESTATE Baxter
Black
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West
Nile Virus Encephalitis PRAIRIE EQUINE
HOSPITAL, PLLC Definition and Etiology: West Nile Virus (WNV) encephalitis is a mosquito-borne viral disease that affects a broad range of animals including humans, horses, mice, and many species of birds. It is considered an emerging infectious disease. Other animals, including dogs and cats, may be infected with the virus, but little if any work has been done to characterize their disease process. WNV is an RNA virus that is closely related to the viruses that cause St. Louis encephalitis, Japanese encephalitis, and Murray Valley encephalitis. Clinical Signs: Clinical disease arising from infection with WNV has been best studied in horses and donkeys during natural outbreaks of disease or after experimental infection. Most infections appear to be subclinical but roughly 10% of animals develop severe disease. The incubation period (the time from exposure to clinical signs) ranges from 6-10 days, and some horses have been reported to show a secondary onset of disease approximately 20 days after infection. Some but not all equines develop a fever with the onset of clinical disease. Initial signs tend to occur abruptly and include depression, listlessness, ataxia (incoordination), and paresis (weakness), particularly of the rear limbs. Some horses have been reported to show apprehension and anxiety. The disease progresses over 1-3 days to manifest such signs as head shaking and incessant chewing, paralysis of the lower lip or tongue, severe ataxia, ascending paralysis, and terminal recumbency. These clinical signs represent a severe encephalitis and widespread viral replication in the brain and spinal cord. Animals with less severe forms of the disease may recover fully within 5-15 days. Epidemiology: WNV has been known for many years in several areas of Africa, southwestern Asia, and the Middle East, and periodic epizootics have occurred in European countries, including France and Italy. WNV was first recognized in N. America in 1999 as the etiologic agent responsible for an outbreak of fatal disease in crows and humans in New York. Subsequently, several equine deaths that year were attributed to WNV infection. Continuing virus activity in New York, New Jersey, and adjoining states indicates that the virus overwintered in N.E. US and likely has become endemic in that area. As with other arboviruses, the peak period of transmission in temperate regions is the late summer and early fall, with cessation of cases during the winter months when mosquito populations disappear. However, WNV recently was isolated from a dead hawk in the middle of winter in NY, suggesting that non-vector transmission, possibly by predation, occasionally may occur. Treatment and Prognosis: No specific treatment is available for animals infected with WNV. The prognosis is very poor for horses that show clinical signs. However, some animals do recover, and standard supportive treatment (as with other neurologic diseases) is warranted. Prevention: The standard means for control during epidemics of WNV is vector abatement via elimination of mosquito breeding sites and application of insecticides (larvacides and adulticides). There is a vaccine currently available for horses, but it is not completely FDA approved (meaning that all the safety and efficacy studies have not been completed). The vaccine is a killed product that should be given once and boosted in 3-4 weeks. It is then given annually. It is the opinion of PRAIRIE EQUINE HOSPITAL that the vaccine should lessen the severity of clinical signs if your horse were to become infected. If you have any questions about the vaccine or the disease, please call Dr. Gray at the hospital. |
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