October 3, 2003
Contact: Peper Long 202-673-0206
Sharp Decline in West Nile Virus Infections
at the Smithsonian’s National Zoo
West Nile virus, which sickened and killed 20 birds in the Smithsonian’s National Zoo last year, has not infected a single animal in the Zoo’s collection so far this year, according to test results through the end of September. The sharp decline is likely due to a combination of factors. The number of infections could change by December, when the Zoo will complete testing for West Nile virus.
As of mid-September, a total of 52 Zoo and wild birds have been tested for West Nile virus. Of those, all of the 20 Zoo birds were negative, and eight of the 32 wild birds tested positive – two black-crowned night herons and six crows. During the same period last year, 148 Zoo and wild birds were tested for the virus, and 81 of those were infected with West Nile. The Zoo tests animals that are clinically ill or have died. Samples are sent to the Animal Health Diagnostic Laboratory at Cornell University College of Veterinary Medicine.
There are several possible reasons for
the significant drop in numbers this summer, including
a natural immunity. “Last year, the disease was spreading through
the area and susceptible birds were dying. West Nile is now considered endemic
in the area and most animals have already been exposed and have some level of
immunity,” said Suzan Murray, the Zoo’s chief veterinarian.
Peter P. Marra, a senior scientist and animal ecologist at the Smithsonian Environmental Research Center in Edgewater, Md., agrees some natural immunity may be helping curb the infections this year. In 2002, Marra tested several thousand wild birds, including about 500 around the National Zoo.
Tests showed many of the wild birds had West Nile virus antibodies in their blood, indicating they had been exposed to the virus. “Last year, we found over 50 percent of the birds had antibodies, which is huge,” Marra said. “It’s possible that those animals are showing some immunity.” Marra also tested wild birds this year; those results will be ready in early 2004.
Another possible factor affecting the lower infection
rate at the Zoo this year could be the reduced populations
of crows, which are considered targets for the virus,
says Richard J. Montali, the Zoo’s chief pathologist.
Preventative methods, such as vaccinations and mosquito control, could also be helping curb the virus. A few hundred Zoo animals – mostly birds, along with rhinos, zebras and tapirs – have been vaccinated again this year. But the Zoo’s medical staff cautions that it is difficult to determine if the vaccination is helping prevent the disease. “Although we aren’t seeing the disease in our vaccinated animals, we are not seeing much in the wild birds either, and they weren’t vaccinated,” said Donald K. Nichols, associate pathologist.
The Zoo stepped up its mosquito control measures this year, using bug traps and removing standing water throughout the complex. “We also are using a biological control method, available commercially, derived from otherwise harmless bacteria that eliminate mosquito larvae,” Montali said.
The Zoo is working with the following local and national programs that are monitoring West Nile virus: the Washington, D.C. Department of Health’s Animal Disease Division; the Army Center for Health Promotion and Preventative Medicine at Fort Meade, Md.; and the Zoo West Nile Surveillance Program at Cornell University in Ithaca, N.Y.
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