West Nile Virus

WNV is a vector-borne virus that was recognized in the Western Hemisphere in 1999. The virus circulates in wild birds, transmitted by insect vectors such as mosquitoes. To date, 111 species of captive and domestic birds have been diagnosed with West Nile virus infection.

Occasionally, the virus is introduced into vertebrates, most notably humans and horses, which can serve as incidental hosts. These incidental hosts do not amplify the virus, and no cases have been reported of direct virus transmission between mammals.

Since 1999, the only vectors found to be associated with the disease in the United States have been mosquitoes. At least 30 species of mosquitoes have been found positive for WNV, but not all of those are active in the bird-to-bird or bird-to-mammal transmission cycle.

Horses

Equines have been most significantly affected by the introduction of WNV into the Western Hemisphere. In 2002, there were over 15,000 cases of clinical disease due to WNV in U.S. horses. The largest concentration of cases were found in the Midwest, with over 800 cases each seen in Minnesota, Iowa, Nebraska, Illinois, Missouri, Oklahoma, and Texas. Virginia had 45 cases last year, with 19 of those in Fauquier Co. (see the Virginia Department of Health Office of Epidemiology’s website at http://www.vdh.state.va.us/epi/newhome.htm) for the detailed statistics.

WNV infection can cause encephalitis, but many horses infected with WNV do not show any signs of illness. The clinical attack rate has been estimated to be approximately 10%. Of those that do develop clinical disease, approximately 1/3 die or are euthanized. Clinical signs in horses include ataxia, depression or apprehension, weakness, partial paralysis, muscle fasciculations, and death. Fever has not been commonly reported as a clinical sign.

In February of 2002, full licensure was granted to a WNV vaccine for use in horses. This killed virus product is to be given in 2 separate 1ml doses, 3 to 6 weeks apart, followed by an annual booster. Full immunity is not achieved until 3-4 weeks following the second initial dose. The manufacturer (Fort Dodge Animal Health- see http://www.equinewestnile.com/default.htm) reports 95% of fully vaccinated horses challenged after 12 months showed no evidence of viremia. A recent study of over 500 infected horses in Colorado and Nebraska revealed only 13 cases of WNV in fully vaccinated horses, and no fatalities. ( www.cvmbs.colostate.edu/aphi).

Testing for WNV in horses is done by an IgM Capture ELISA serologic assay. This will detect recent infection, up to several weeks, but does not pick up antibodies from vaccination or natural infection of a longer duration. Other serologic assays, such as serum neutralization, have been rendered difficult to interpret by the use of the vaccine since the fall of 2001, and no doubt by the natural exposure of many horses to the virus. Tissue, preferably brain, can be tested for WNV by virus isolation or polymerase chain reaction.

In Virginia, IgM Capture ELISA testing is available through the Virginia Department of Agriculture and Consumer Services’ laboratory in Warrenton. The cost is $10, although submissions during the peak WNV season will this year be subsidized by a grant from the Virginia Department of Health. Please contact the VDACS diagnostic laboratory in your region for more information at http://www.vdacs.state.va.us/animals/labservices.html.

PCR testing and isolation on tissue from necropsies of suspect horses can be performed at the Division of Consolidated Laboratory Services (DCLS) in Richmond, or at the National Veterinary Services Laboratories in Ames, IA. Again, please contact your local diagnostic laboratory for more information. DCLS in Richmond will also survey equine brains submitted for rabies testing through local health departments (and found negative for rabies) using WNV PCR.

Other mammals:

Clinical disease in mammals other than horses and humans is extremely rare. No cases of WNV have been seen in cattle, although antibodies were detected in 2 cows in Nebraska in 2002. Virus has been detected in sheep (a Suffolk in NE), an alpaca (IA), a llama, domestic dogs, a domestic cat (feral-NJ) and a rabbit. Native wildlife found to have exposure to or infection with WNV include: gray squirrel (IL and Fairfax Co., VA), fox squirrel, eastern chipmunk, black bear (6% seropositive-NJ), big brown bat, little brown bat, white-tailed deer (1% seropositive-NJ), timber wolf (captive-IL), mountain goat (captive-NE) and striped skunk.A few cases of clinical disease have been described in cats and dogs:

The cat was a feral kitten from New Jersey that became clinically ill with encephalitis.

Almost all dogs infected show no clinical signs whatsoever. A small serosurvey last year in Rockford, IL revealed an infection rate of 23.5% in otherwise healthy dogs (as measured by the presence of neutralizing antibodies). Earlier, a serosurvey of healthy dogs in New York City and Nassau County, NY indicated a 5-11% exposure rate there.

There were reportedly several clinical cases in domestic dogs in 2002, including cases from Illinois, Nebraska and Louisiana. Detailed information on these cases has not been published to date, but most reportedly had suspicious abnormal neurologic signs and increased serologic titers. Previously, the only canine case reported was from Botswana in 1982. As in other species, clinically affected animals develop encephalitis, but not enough cases have occurred in canines to allow a description of any additional characteristic signs. Treatment, as in clinical disease in other species, would be supportive.

Research work done by the Centers for Disease Control on WNV in canines has shown that dogs infected with the virus show a mild leukopenia, but otherwise no evidence of clinical disease. The virus is capable of replicating, but not to a level that would allow dogs to become amplifying hosts. Virus was not isolated from the saliva of infected dogs. See http://www.cdc.gov/ncidod/dvbid/westnile/conf/pdf/Bunning4th03.pdf for research results on canine infection and work in other species.

Serologic testing of samples from mammals other than equines (and humans) is generally limited to serum neutralization testing. Correlation of clinical signs of encephalitis and a four-fold rise between acute and convalescent titers are necessary to determine infection. Even then, cross reactivity with other flaviviruses can result in a positive SN, and testing by plaque reduction neutralization testing is necessary for confirmation. Testing can be coordinated through your local state diagnostic laboratory, and is offered at NVSL, Cornell University Diagnostic Laboratory, and other locations.

Prevention

In all cases, prevention of WNV centers on vector control. Reducing the population of mosquitoes can help to reduce or eliminate the virus in a given geographic area. The most important step is to remove all man-made sources of stagnant water in which mosquitoes might breed. This includes old tires, wheelbarrows, clogged roof gutters, plastic wading pools, ornamental pools, and other sources of stagnant water. Mosquitoes can potentially breed in any stagnant puddle that lasts more than 4 days.

Preventing access to adult mosquitoes will also help to reduce infection. Screened animal housing, insect repellants, and limiting outdoor exposure during times of high mosquito feeding activity all can help reduce the infection rate.

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