Disease Information
13 April 2001
Vol. 14 - No. 15
WEST NILE FEVER IN THE UNITED STATES OF AMERICA
Follow-up report No. 2 (end of the outbreak)
See also: 6 October 2000, 1 September 2000, 3 March 2000, 29 October 1999
Text of an e-mail received on 5 April 2001 from Dr Alfonso Torres, Deputy Administrator, Veterinary Services, United States Department of Agriculture, Washington, DC:
End of previous report period: 22 September 2000 (see Disease Information, 13 [39], 178, dated 6 October 2000).
End of this report period: 22 March 2001.
The West Nile virus (WNV) outbreak has ended.
For the year 2000, WNV was identified in 60 horses in the following States:
State No. of horses Connecticut 7 Delaware 4 Massachusetts 1 New Jersey 27 New York 19 Pennsylvania 1 Rhode Island 1
All horses had onset of clinical illness between 17 August and 30 October 2000. The last equine case of WNV had clinical onset on 30 October 2000 in New Castle County, State of Delaware.
A total of 36 horses were males (32 geldings, 3 stallions, 1 colt) and 24 were females (all mares). At least 11 breeds of horses were infected. The genders and breeds of horses infected with WNV appear to reflect the general population of horses in the area.
The age of WNV-case horses averaged 14.0 years and ranged from 4 months to 38 years.
Diagnostic details:
- The most common clinical signs were ataxia, weakness of limbs, recumbency with difficulty or inability to rise, and muscle fasciculation.
- A total of 23 (38%) of the 60 horses died or were euthanised.
- Virus was isolated from brain tissues of 7 of the 60 ill horses.
- All 7 equine brain samples from which virus was isolated were also positive by RT-nPCR(1), as were 3 additional brains of horses from which virus was not isolated.
- A total of 59 horses were positive for IgM antibody in serum tested by IgM-capture ELISA(2).
- A total of 55 horses had serum titres to the PRNT(3). One horse showed a four-fold increase in PRNT titre on comparison of acute and convalescent serum samples.
Surveillance in birds: bird surveillance focused on dead bird collections, primarily of American crows (Corvus brachyrhynchos) and other wild birds. WNV did not affect any commercial poultry.
More than 4,300 wild birds were found positive for WNV in 12 States and the District of Columbia. Positive birds were collected from April to November 2000. Surveillance in wild birds has continued into the winter of 2000-2001 in several States. No WNV positive birds have been reported so far in 2001.
Surveillance in mosquitoes: overwintering mosquitoes have been collected in several areas where WNV activity was documented in 2000, but none have been found positive for WNV in 2001.
Conclusion: WNV activity in the United States of America has ceased because of various factors, including climate and vector control activities.
More information on WNV in the United States of America can be found on the USDA-APHIS(4) web site at: http://www.aphis.usda.gov/oa/wnv/index.html
(1) RT-nPCR: reverse transcriptase - nested polymerase chain reaction.
(2) ELISA: enzyme-linked immunosorbent assay.
(3) PRNT: plaque reduction neutralisation test.
(4) USDA-APHIS: Animal and Plant Health Inspection Service of the US Department of Agriculture.
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