Human,
animal cases of West Nile continue to climb
Virus identified
in small number of new animal species
Every week, health
officials learn something new about the West Nile virus that exploded across
the United States this summer.
When the virus first appeared in New York City in 1999, only humans, some bird
species, and horses were thought to be susceptible to the exotic encephalitis
virus. Now, dogs, squirrels, sheep, goats, even alpacas, aren't safe. Veterinary
diagnostic laboratories in several states are seeing illness and death in mammalian
species not known to have been susceptible to it.
This has been the worst year in the virus' brief history in the United States.
At the end of 2001, West Nile virus had spread to the Midwest and as far south
as Louisiana. Between 1999 and 2001, a total of 149 human cases and 18 fatalities
had been documented in the United States.
As of Oct. 9, all but seven states have detected the virus within their borders,
with nearly all of them reporting human cases. The Centers for Disease Control
and Prevention is attributing 146 deaths and more than 2,500 infections to the
virus, with Illinois leading the nation with 38 fatalities.
In August and September, the CDC awarded $18.6 million to several cities and
states to help combat the virus. This year alone, the agency provided an estimated
$35 million toward this end. Since 2000, more than $58 million has been made
available to state and local health departments to develop or enhance epidemiologic
and laboratory capacity for West Nile virus and other arboviral diseases.
With news of the apparent epidemic, recent discoveries about the virus having
been making headlines. Four recipients of transplanted organs have contracted
West Nile virus from a single infected donor. After five recipients of blood
transfusions tested positive for the virus, the CDC, along with other federal
agencies and blood collection groups, began an investigation into the risks
of passing the virus through blood. An infant appears to have been infected
through breast-feeding.
In addition, six people in Mississippi and Louisiana sick with the virus developed
an acute flaccid paralysis, leading the CDC to encourage clinicians to check
patients with AFP for West Nile virus infection. Now, the Food and Drug Administration
has approved the first national trial of a drug to treat the infection.
Between Jan. 1 and Oct. 6 of this year, some 9,000 equine cases in 36 states
had been confirmed by the Department of Agriculture's National Veterinary Services
Laboratories or reported by state officials.
"As this disease makes its way across the continental United States, we
are seeing an increasing number of West Nile virus cases in horses," said
Bobby R. Acord, director of the USDAs Animal and Plant Health Inspection Service.
"Our best estimates are that one in three WNV-affected horses will die."
The USDA is urging horse owners to protect their horses from West Nile virus
through use of the approved vaccine or other preventive measures.
Veterinarians are making discoveries of their own about West Nile virus. Since
1999, the general consensus had been that only certain birds and a small number
of mammalian species were at risk. Early in the outbreak, however, there were
signs that other species—specifically raccoons, cats, and bats—were
susceptible to the virus (see JAVMA, Oct. 15, 2000, page 1127).
And now there is growing evidence that the virus is, in fact, killing small
numbers of other animal species.
In Illinois, West Nile viral infections were identified in several gray squirrels,
a three-month-old wolf, and an eight-year-old dog with an immune-mediated disease.
A mountain goat in Wyoming tested positive for the virus, and there are reports
of mass die-offs in the gray squirrel population in a region of the state where
West Nile virus is active.
The NVSL confirmed the virus in goat and sheep tissue samples submitted by Nebraska.
And, finally, a fox squirrel in Michigan was infected, as well as an adult alpaca
in Minnesota.
Since September, veterinarians in St. Tammany, La., have been conducting serologic
surveys on dogs and cats that the CDC will examine to determine how the virus
is affecting the pet population there. The parish is a hotbed of virus activity
with numerous human and equine cases, explained Dr. Brent Robbins, director
of animal services in the parish.
Around 400 samples will be collected from cats and dogs during routine examinations.
Free testing is also available for pet owners worried about the health of their
animal. "What [the
CDC] is really interested to know is if [dogs and cats] can be used as sentinels,"
Dr. Robbins said.
Although there is no dog or cat case of West Nile virus infection in Louisiana,
some veterinarians in St. Tammany have seen unusual fevers and neurologic signs
they can't explain in cats and dogs, according to Dr. Robbins.
He thinks that, like people, many animals are infected but don't show any
signs of clinical illness. Of the small group that do, an even smaller number
actually die as a result. "You would think with as active as the virus
has been in the area, there would be numerous cat and dog illnesses and deaths,"
he said."We don't see that."
The results of the serologic survey in the parish are expected to be ready in
December. For continued updates about West Nile virus, visit the AVMA Web site
at www.avma.org.
JAVMA, Vol 221, No. 9, November 1, 2002
Veterinary Forum March 2005
West Nile Virus Infection in Captive Birds
JAVMA 224(10):!640-1643, 2004. D'AgostinoH, Isaza R. Kansas
State University College of Veterinary Medicine, Manhattan, and University of
Florida College of Veterinary Medicine, Gainesville
In 2002, five zoologic institutions in Kansas reported West
Nile virus (WNV) infection in a total of 11 birds: four golden eagles, one cinereous
vulture, one sandhill crane, one snowy and one great-horned owl, one African
grey and one thick-billed parrot, and one emu. All of them became ill at the
onset of a WNV outbreak in Kansas. The most common clinical signs were neurologic
(eight of 11 birds), including sudden onset of recumbency, mild ataxia, tetraparesis,
and tremors. In some of the birds, other common signs were anorexia, depression,
and rapid weight loss. Two of the birds that did not display clinical signs
died suddenly.
Laboratory testing confirmed leukocytosis in five birds and heterophilia in
five birds, along with monocy-tosis in two birds and reactive lymphocytes in
one bird. Serum samples were submitted for plaque reduction neutralization testing
(PRNT), which detects antibodies against WNV. Only two of eight samples tested
positive. Oral swabs were submitted for reverse transcription poly-merase chain
reaction (RT-PCR) to detect WNV RNA. Four of five samples tested positive.
Nine birds died or were euthanized within 3 days of the onset of clinical signs.
A 10th bird was euthanized 3 weeks later because of lack of response to supportive
care. The 11th bird, the sandhill crane, was the sole survivor. Postmortem examination
of the 10 dead birds using RT-PCR and immunohistochemical staining on brain
tissue confirmed the diagnosis of WNV in all cases.
Since the identification of WNV in the northeastern United States in 1999, the
virus has spread across the country, reaching the West Coast in 2002. Bird collections
in zoologic institutions are particularly vulnerable because birds are natural
hosts of the virus. Unlike humans and dogs, which can contract the virus without
becoming ill, most infected birds become ill and die within days.
Because the PRNT and RT-PCR testing available for antemortem diagnosis of WNV
are not definitive, the authors of this case report recommend using both tests
to increase the chances of diagnosis. In addition, early detection is critical,
especially in large zoologic collections. If WNV infection is suspected, measures
should be taken to isolate infected birds to decrease exposure of the remaining
birds. In addition, strict mosquito control techniques should be implemented.—SEP
COMMENT: West Nile virus is here to stay and is now a major clinical
problem. Over 284 avian species were reported to the CDC West Nile Virus Avian
Mortality Database since 1999 (www.cdc.gov/ncidod/dvbid/
westnile/birdspecies.htm). Some questions often asked about mosquitoes and
their control appear in the box Client Education: Dealing with Mosquitoes and
West Nile Virus on page 20.—JDH