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Tracking West Nile
WSU lab detects virus’ spread among animals
By Linda Weiford, WSU News
WADDL microbiologist Kathy Noble
after she tested blood samples taken
from two horses. Both were negative
for West Nile Virus.
(Photos by Linda Weiford, WSU News)
Scientists at the Washington Animal Disease Diagnostic Laboratory, or WADDL, have been testing blood samples taken from suspect horses to determine if the virus is what’s making them sick. In late August, after getting a positive reading on a sample drawn from an ailing horse in Benton County, WADDL staff contacted state veterinarian Leonard Eldridge at the state capital.
Knowledge of that infected horse, coupled with five positive mosquito samples from the south central part of the state, "told us the virus was starting to spread in Washington,” said Eldridge. "We immediately got the word out to residents urging them to take precautions and vaccinate their horses.”
The 2-year-old quarter horse had to be euthanized after its condition worsened, said Eldridge. Then, a week later, a Yakima man in his 30s tested positive for the same disease.
"By then, I wasn’t surprised. It was in an area where we knew the virus was active,” Eldridge said.
He knew West Nile was active, in part, because of WADDL, where scientists use sophisticated molecular tools to identify infectious agents that turn up in Washington and elsewhere in the Northwest. Some of the diseases they test for affect animals only, such as distemper. Others, such as West Nile, avian influenza and the plague, attack animals and humans alike.
West Nile produces no symptoms in some humans and flu-like ailments in others. At worst, 1 in 150 develops a high fever, brain inflammation and paralysis that sometimes leads to death, according to the U.S. Centers for Disease Control. The disease not only sickens humans and certain wild birds but also animals such as horses, mules, llamas and squirrels. A vaccine exists for horses but not for people.
Across the nation, the virus has flourished during record heat and drought, keeping health officials on high alert and scientists at labs like WADDL testing for the virus’s presence in animal blood and tissue samples.
"By all indications, the virus is still spreading,” said WADDL executive director Terry McElwain, also a professor of veterinary pathology at WSU’s veterinary college.
Recent surge in cases
Nationwide, the rates of human infection are three times what they were a year ago, according to the CDC, with 2,636 cases reported so far this summer compared to 1,993 during all of 2011. In the agency’s most recent surveillance report, infections climbed nearly a third in a single week ending Sept. 11.
More in the Northwest
In Washington state, which had no reported cases last year, three residents have tested positive since late August, two of whom may have contracted the virus while traveling out of state, according to the state’s department of health. In neighboring Idaho, the virus has sickened eight people - three seriously with neurological symptoms – as health officials advise residents to avoid exposure to mosquitoes.
Virus on wings
Animals’ antibody levels are measured
to determine if they’re infected by West
Nile. A positive reading would turn a
"At first they said West Nile would remain concentrated along the Eastern Seaboard,” he recalled. "Then, as it moved westward, they said it wouldn’t spread beyond the Midwest. In 2006 - seven years after the virus was detected in New York - it infected several residents here in Washington. Now it’s been reported in every state except Hawaii and Alaska.”
Birds, mosquitoes drawn to water
Most surprising this summer is the surge of infections. What’s making these microscopic agents a bigger threat?
Most likely, a mixture of ecological factors, including the hot, drought-filled summer, said McElwain.
"Mosquitoes and birds are being drawn to smaller amounts of water where they’re congregating, perhaps at higher levels,” he said.
And so, a mosquito bites an infected bird during a blood feeding, flies off and injects the virus into a horse or human with the effectiveness of a loaded hypodermic needle, minus its painful sting.
All the while, it’s important to keep in mind that this transmission "is a system involving mosquitoes, virus and birds,” said Jeb Owen, assistant professor of entomology at WSU. It’s not just the number of mosquitoes circulating that determines the levels of infection, he said. It’s also the number of infected birds and how climate conditions – in this case, drought – bring bug and bird together.
"One line of evidence suggests that West Nile is worse in drought years because surface water becomes scarce, resulting in birds and mosquitoes congregating in high densities around the remaining water,” he said. "Another explanation is that high temperatures in drought years promote more rapid mosquito and virus replication.”
Quick diagnosis crucial
In the meantime, scientists at WADDL will continue examining samples to help determine where West Nile exists and where it’s spreading. The sooner they detect the virus’s presence in horses and other animals, the sooner efforts can made to warn the public and contain it, said state veterinarian Eldridge.
"Believe me, they make my job easier,” he said. "With remarkable efficiency, they test for infectious diseases that could impact animal health, human health and the economic welfare of our state. West Nile has hit states like Texas and South Dakota especially hard. We don’t want that. Ever.”
For more information on the West Nile outbreak and how to avoid infection:
Centers for Disease Control: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm
Washington State Department of Health: http://www.doh.wa.gov/YouandYourFamily/IllnessandDisease/WestNileVirus.aspx