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First
year WNV Experience in Missouri Dr.
Gayle Johnson, DVM, DACVP, Steve Kleiboeker, DVM, PhD, Christie Loiacono,
DVM, PhD West
Nile Virus (WNV) is a member of the flavivirus family, along with yellow
fever, Dengue fever and several other human and animal pathogens. WNV is not related to other common mosquito-borne
encephalitis viruses affecting horses in the US, but is distantly
related to St. Louis and La Crosse encephalitis viruses, which are human
pathogens. Other
notable relatives world-wide include Japanese encephalitis virus.
In a similar manner to other arboviruses, transmission is
predominantly by Culex mosquitoes between hosts that maintain the
infection in nature, in this case, various species of birds. Effective
bird reservoir hosts have adequate viral blood levels to allow
mosquitoes to successfully infect other hosts.
Horses, humans and other mammals are incidental hosts that have
less viremia and cannot successfully infect mosquitoes, so that the
cycle cannot continue when they are infected. WNV
has spread rapidly throughout the US since first diagnosis in New York
City in 1999, presumably due to numerous susceptible species that had no
experience with infection. The
isolates in the United States are relatively homogeneous, and are most
similar to isolates from Israel that caused avian and human mortality.
Large numbers of cases occurred in the mid-western states in
2002, and further westward spread is expected in the coming year. That
viral infection took 2 years to spread throughout the eastern flyway
allowed the VMDL at the University of Missouri to have testing
procedures in place prior to the outbreak. In
Missouri in the summer of 2002, the Veterinary Medical Diagnostic
laboratory found the following:
For
horses, a case of WNV was defined on the basis of clinical signs and
diagnostic testing or the presence of histologic lesions compatible with
viral encephalitis and diagnostic testing. To
understand which horses were becoming ill, patient population profiles
were compared with populations of horses gathered from information
submitted with EIA test samples to the VMDL prior the WNV occurrence
(n=18,000) and from data collected from horse owners by the Missouri
Horse Project (n = 200,000, 2001). It was found that equine patients were much more likely to be
horses than mules, jacks or donkeys. There was over representation of
the Arabian and Quarter horse breeds compared to the general horse
population, and also over representation of older horses among patients.
There was no bias related to biological sex. Cases
of WNV were confirmed in Missouri between July and November of 2002.
Examination of the county-wide distribution of cases in Missouri
indicated that nearly all areas in the state had some equine cases. In
comparison to the distribution at the end of the first 10 weeks of the
outbreak, near the end of September, there was apparent intensification
of cases in the northeastern quadrant of the state between by late
November, when the last case was diagnosed. This trend remained, even after correction for reported horse
numbers by county. Soil
moisture plots indicated that this area had persistent low soil moisture
contents, and plots of rainfall data for individual counties suggested
that rain had occurred 2-4 weeks before peaking cases of clinically
affected horses. As a
whole, rainfall did not predict the occurrence of cases in the state as
a whole, however. In
living horses, recent infections were most economically and readily
determined by recent seroconversion, as assessed by testing for IgM
antibody specific for WNV. Viral sequences could also be detected in
blood samples, but only in a few cases did this test provide diagnostic
information above that which resulted from the IgM capture-ELISA test.
Unvaccinated clinically normal herdmates of sick horses were
sampled after positive diagnoses were made in 103 herds. Between 23 and
34% of these horses were positive in various group sizes, indicating
that clinical illness did not always result from infection. The
diagnosis of WNV infection also had to be confirmed in dead horses, and
in some instances RT-PCR determination of viral genetic sequences was
the only method available. Tissues
from 65 field or university-performed postmortems were available for
testing. After developing
tests using different primers for unfixed and fixed tissue, we were able
to confirm the presence of WNV infection in about 70% of these suspect
horses. In another about 20% of dead horses, another cause of death
appeared likely, either as a result of findings that indicated another
illness, other structural diseases of the brain, or as a result of
finding no structural brain disease in cases where only brain was
available for examination. In less than 5% of the cases, IgM-negative or
untested horses had lesions or signs consistent with viral encephalitis
and were PCR negative, after Herpes, EEE and usually rabies were ruled
out. The etiology of these
cases is unresolved. Altogether, the WNV outbreak had an economic impact on the state during 2002. According to the Missouri horse project, owner estimates of the value of Missouri equines averaged $2100 per horse. Assuming that mortality figures are correct for the entire population of the state and that cases were reported accurately, the economic burden of replacement costs alone would be about $450,000. Operating expenses excluding utilities and insurance, which would be impacted by an animal’s death were estimated state-wide to be about 40% of the purchase cost, which would suggest an additional $180,000 economic hardship to industries supplying feed, routine health care, boarding facilities and travel-related services. This is in addition to the environmental impact on wild bird species and the potential impact on human and animal health. |