Flocking together may
be problem for crows
14 of 16 crows in
experiment die from
West Nile virus
It turns out that a mosquito bite isn’t the only way for West Nile virus to be transmitted, at least among crows.
Results of a recent experiment conducted by the United States Geological Survey (USGS) at its National Wildlife Health Center in Madison, Wisc., appear to show the virus can be transmitted bird-to-bird without the aid of mosquitoes in a laboratory setting.
"In the experiment, 16 crows were housed in a 16-by-20-foot flight room with 12-foot ceilings. There, they shared food and water and sat on common perches. The room was cleaned daily. Nine infected birds died within five to eight days. Four healthy or ‘control’ birds died from the virus five to eight days later. A fifth ‘control’ bird died 11 days after that, meaning the virus was transmitted from once healthy birds to another healthy bird," according to information posted on the agency’s Web site (usgs.gov).
The experiment was done in collaboration with the Wildlife Conservation Society which also helped fund the study.
Lisa Reed, a scientist at Rutgers University, New Brunswick, specializing in corvids, the bird family that includes crows, said scientists suspected transmissions through means other than mosquitoes in March when a red-tailed hawk was found dead of the disease in New Jersey well before mosquitoes would provide a means of contracting it.
Reed said there are several ways the disease may be passed from bird to bird without mosquitoes. She and officials at the USGS noted that it is very unlikely humans could get West Nile virus from birds without mosquitoes as a "vector," or means of transmitting the disease.
With the onset of colder weather, mosquito activity in the region has just about ceased. While the experiment showed the virus could be transmitted in a closed setting, results from the wild could be available early next year.
Reed noted there is a large roost of crows in Staten Island, N.Y., just on the other side of the Outerbridge Crossing during the winter months with thousands of birds spending at least some time there.
She said there is a student counting the crows at the roost now, but it is only the beginning of the season so the information is limited because there is little context.
"The roost seems to have shifted, but there is construction in the area that could have caused that," Reed said, adding that last year there was some unusually warm weather at the start of the season that also likely could have had an effect.
The population of crows in the roost peaks in January and February, according to Reed, and when that happens, more significant data should be available.
Roughly 1,100 birds in New Jersey, the vast majority of them crows, have died of West Nile virus so far this year, according to Reed.
Among the possible means of transmission from bird to bird other than mosquitoes that are being considered are ticks, lice and through the birds’ feces, she said.
The Rutgers scientist noted that the virus has been found in the feces of the birds, and it is not unusual for birds to have feces on them. Their means of preening, that is using their bill for cleaning their feathers, also is being considered as a means for the disease to enter their systems, according to Reed.
"This has been pretty much a guessing game for a lot of us," Reed said. "We have a few answers for some questions, but it will take a lot of time to get other answers."
Dr. Robert McLean, of the USGS, said even though the research is significant, it means more to the wildlife community than the public health community as the threat of humans contracting the virus directly from birds is slim. He emphasized that anytime someone finds a dead animal, regardless of whether it is a dead bird or a neighborhood pet, they should avoid handling it, or use gloves or a plastic bag turned inside out to prevent contact with the carcass.