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8-02-2002
A quick summary of WNV: Everyone has likely followed the westward progression of WNV during 2002, with considerably more cases reported this year than during 2001. The furthest westward progression is approximately on a north south line through central Manitoba, eastern North and South Dakota, Nebraska, Kansas, Oklahoma and Texas (about as far west as 98-100th meridian). Expectation is that the virus will make its way into the west later this year or next year. So far there is still no evidence that horses or humans can serve as competent reservoirs - that is, mosquitoes must bite a viremic bird before they are capable of transmitting the disease. Though lots of species have been reported with antibodies to WNV (from infected mosquito bites), only human, equine and avian species are known to suffer disease. Antibody reports in other mammalian species should be expected but it means nothing relative to development of disease. Based on the distribution of the virus in mosquitoes and birds, and clinical cases in horses and people, it appears most infections in horses and people do NOT result in disease. However, if clinical disease does occur, the case fatality rate in horses (including those euthanatized) is running about 35-38%. The number of affected horses is a miniscule percent of the population at risk. In the USDA/NVSL confirmed 47 cases, 17 (36%) of which have died or were euthanatized (see report below). Including unconfirmed state reports, about 71 cases of equine WNV are known for 2002. I have seen no efficacy data on the provisionally licensed Ft. Dodge vaccine. However, a recent report showed cross protection to WNV by using Japanese Encephalitis and St. Louis Encephalitis vaccines in rodents. To date WNV has been reported in 34 states and DC since first found in New York in 1999.
Human cases of WNV number around 45, with all age classes affected (most recent cases having affected teenagers) but the overwhelming numbers in older than 50 age groups. Thus far only 1 mortality. Only Louisiana (32 cases), Mississippi (5) and Texas (8) have reported human WNV cases in 2002. Interesting that Florida, with the highest equine and avian case load has yet to report a human case. In 2001, Florida reported 12 human WNV infections. CDC is still estimating that less than 1% of infections result in significant clinical disease. They also indicate that for those people severely affected by WNV, case fatality rate is about 3-15% with highest mortality in the elderly.
Thomas J. Roffe, PhD, DVM
Northern Rocky Mountain Science Center
USGS-BRD
FWP Bldg, 1400 S. 19th Ave.
Bozeman, MT
T: 406-994-5789
F: 406-994-4090
Cell: 406-539-4955
Supporting
Documents:Update on the Current Status of West Nile
Virus
Equine Cases of West Nile Virus Infection in 2002: 1 January through
25 July
Equine illness caused by West Nile virus (WNV) infection has been confirmed by the National Veterinary Services Laboratories (NVSL) in 47 horses from 7 states: Florida (14), Kentucky (1), Louisiana (17), Mississippi (1), North Dakota (2), Tennessee (1), and Texas (11). The equine cases in North Dakota as are the first ever detected in those states.
Of the 47 horses classified as having a case of WNV infection, 17 (36 percent) were reported to have died or been euthanatized. The numbers of equine deaths associated with WNV infection by state are: Florida, 4; Kentucky, 1; Louisiana, 5; Mississippi, 1; North Dakota, 2; and Texas, 4. In addition to the NVSL tested cases, various state officials have reported equine WNV cases. The total number of equine WNV cases reported from either NVSL or a state source is 56. The horses are from 7 states: Florida (14), Kentucky (2), Louisiana (17), Mississippi (4), North Dakota (2), Tennessee (1), and Texas (16).
NVSL Confirmed Equine Cases of Clinical WNV Infection
(n=47)
1 January through 25 July, 2002:
Florida (10 counties, 14 equine)
Columbia County (1 horse)
Duval County (1 horse)
Lake County (1 horse)
Levy County (1 horse)
Marion County (4 horses)
Okeechobee County (1 horse)
Orange County (1 horse)
Polk County (1 horse)
Seminole County (1 horse)
Volusia County (2 horses)
Kentucky (1 county, 1 equine)
Nelson County (1 horse)
Louisiana (5 parishes, 17 equine)
Calcasieu Parish (8 horses)
Cameron Parish (4 horses)
Iberia Parish (1 horse)
St. Tammany Parish (2 horses)
Vermilion Parish (2 horses)
Mississippi (1 county, 1 equine)
Harrison County (1 horse)
North Dakota (2 counties, 2 equine)
Cass County (1 horse)
Grand Forks County (1 horse)
Tennessee (1 county, 1 equine)
Jefferson County (1 horse)
Texas (3 counties, 11 equine)
Harris County (5 horses)
Montgomery County (1 horse)
Orange County (1 horse)
County not reported (4 horses)
U.S. Department of the Interior | U.S. Geological Survey
URL: http://biology.usgs.gov/cro/WNV8-02-02.htm
Page Contact Information:
tlwilliams@usgs.gov
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