Is Alaska the next stop for West Nile Virus?
A summer of testing suspect Alaska birds for the presence of West Nile virus, turned up none found to be carrying the disease, which is transmitted in the wild through infected mosquitoes. The virus so far this year has killed 28 people and sickened more than 1,000 in the United States. Will it ever make it to Alaska? Hector Douglas reports.
In less than six years, West Nile virus has reached all the way across the United States from New York City to Los Angeles. West Nile virus, also known as WNV, has appeared in all but a few states.
But whether the West Nile virus will travel north to Alaska depends on the biology of birds and mosquitoes, says James Kruse, an entomologist with the U.S. Forest Service in Fairbanks.
“The disease needs to replicate in the salivary glands of the mosquito, and then the mosquito needs to go forth and bite new birds,” explains Kruse. “Once the bird is sick, that is when other mosquitoes can bite it, and the disease would be transferred to those new mosquitoes.”
The virus extends its range when wild birds contract WNV during migration and carry it into new areas. This summer, West Nile virus has expanded into central Canada. That has some scientists wondering whether WNV could make its way to Alaska—including scientists like Louisa Castrodale, an epidemiologist with the state health department. She says a lot of events would have to line up for an outbreak of West Nile virus to occur:
“Can we get a bird up here with virus? Can the mosquito bite the bird and pick up enough virus? Can the mosquito live long enough to amplify this virus? Is that mosquito the same kind of mosquito that likes to bite a human?
“If you can get all those lined up together, that’s when we would see it up here.”
Castrodale helps coordinate the state’s interagency monitoring program for West Nile virus. As part of that program, the state is keeping a lookout for dead birds. Studies have shown that certain species of birds like ravens, magpies, jays, and hawks start dropping dead when West Nile virus moves into an area.
Ravens, crows and other birds are part of a natural alarm system scientists use to detect the spread of West Nile virus. If the virus comes to Alaska, sick and dying birds will probably be one of the first clues.
“What we are focusing on,” says Castrodale, “is that family of birds that includes the crows and the magpies and the jays, and looking for evidence that they have the virus, and that is going to give us an indication of whether it could spill over into the human population.”
Don Ritter is former head of the Alaska State Virology Lab, which diagnoses the gamut of public health diseases from rabies and flu to HIV. Last year the lab analyzed 22 dead birds from around the state. Researchers extracted genetic material from the birds’ brains and amplified it, making many identical copies. This makes it easier to search for the specially coded single strands of genetic material that are West Nile virus. When this stuff gets into the brain, it can cause severe inflammation, which can bring about rapid death. Ritter says the lab also screened human blood samples for viral antibodies—specialized proteins that defend the body against infection and disease.
The size and complexity of the virus antibody, called an “IGM,” helps to document exposure, but an additional test is required to confirm presence of West Nile virus. That test is performed by the Centers for Disease Control and Prevention in Fort Collins, Colorado. Last year, the labs in Alaska and Colorado confirmed two human cases of West Nile virus in Alaska, but these were visitors, not Alaskan residents. They became ill with symptoms of WNV while in Alaska, but they had been exposed to areas with infected mosquitoes in the Lower 48 before their trip north. According to the state health department, there is no evidence—either from the humans or birds tested to date—that West Nile virus is active in Alaska.
James Kruse of the Forest Service doesn’t think West Nile virus will become established in Alaska. He says conditions here are just not right. Only four out of the 35 mosquito species in Alaska are known to transmit West Nile virus. Lots of those mosquitoes have to bite lots of infected birds in order for the virus to spread. But mosquitoes in Alaska produce just one generation per year, says Kruse. “Therefore, there’s limited opportunity for the disease to ramp up in the bird population through repeated mosquito attacks throughout the summer.”
Kruse says that even if a sick bird does arrive in Alaska, there are few mosquitoes that have emerged in early spring, the time when migratory birds are making their way across the state. And West Nile virus is not spread to mosquitoes after about 10 days from the time the bird is bitten. Kruse says that this would limit transmission of the disease between birds and mosquitoes, and therefore limit the spread of West Nile virus.[Read more at Arctic Science Journeys]
Iowa State University Entomology Department | Posted 09.14.04 at 5:12 pm