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Alligators can transmit West Nile Virus

GAINESVILLE SUN (Florida) 03 August 03 Gators may play role in West Nile - The reptiles are possible transmitters (Greg C. Bruno)
Be thankful alligators can't fly.
Aside from the horror a winged-crocodilian would instill on the average nature lover, scientists at the University of Florida say there is another reason to credit evolution for keeping alligators grounded: new evidence that the reptile may be as effective as birds at spreading West Nile encephalitis.
Since West Nile first entered the United States four years ago, the virus has infected a growing list of avian and terrestrial species, including chickens, crows, horses and humans.
Four people in Florida have already tested positive for the disease this year, according to the Centers for Disease Control and Prevention.
But while the number of known species afflicted with the disease has climbed since 1999, researchers have long thought it was the high levels of virus found only in birds that served as the primary route for infection of mosquitoes, which are known to carry the disease to other victims.
Now, it seems, alligators may also play a role in the transmission of West Nile.
"We believe that gators may serve as an amplification host based upon the levels of virus in blood," said Elliot Jacobson, an expert in reptile disease at the University of Florida, referring to the term used to describe a host's ability to pass West Nile to others.
"They have levels overlapping with that of some birds and a certain level needs to be reached in order to infect mosquitoes," he said. "Horses and humans do not have these levels."
The findings are based on research following a mass die-off of approximately 300 alligators at a captive-gator farm in Christmas, Fla., last year.
During routine necropsies conducted on three of the farm's reptiles, investigators detected viral loads of West Nile in the 1-meter long yearlings that exceeded the threshold needed to infect Culex pipiens, a common house mosquito. It is unclear if such an infection occurred, however.
What is also unknown is how the farmed Florida reptiles were first introduced to the disease. A similar die-off of West Nile-infected alligators at a farm in Georgia in 2002 found that horse meat fed to the reptiles tested positive for the virus, but the alligators in Christmas were fed primarily beef, Jacobson says.
Therefore, mosquito bites are the most likely route of initial infection, he suspects. Contrary to the long-held belief that mosquitoes were incapable of biting through the tough skin of an alligator, recent studies have found that the insects regularly feed on the reptile's softer regions, including eyelids, tongues and mouths.
Once infected, Jacobson said, he believes the gators transmitted the virus amongst themselves via water in their holding tanks.
And while the blood levels found in Florida reptiles are lower than those exhibited by some birds (crows, for example, exhibited blood levels significantly higher than alligators), Jacobson says alligators may be able to live for days, if not weeks, after infection, passing the virus onto larger numbers of mosquitoes.
Birds, by contrast, often die within hours of contracting the disease.
Mike Bunning, a CDC epidemiologist in Fort Collins, Colo., who has extensively studied the transmission of West Nile, said such a distinction could be significant.
From a scientific standpoint, an animal that maintains lower levels of virus in the bloodstream for longer periods of time "would be more of a problem than an animal that reached (higher concentrations) but stayed alive for a short period of time."
It is unclear what Jacobson's findings may mean for human health. Bunning said while it appears likely that gators could serve as amplification hosts for West Nile, their role in transmitting the virus to mosquitoes, and subsequently people, is probably trivial.
"If we thought that alligators were a significant problem, I would have expected that the areas that had alligators would have seen a human epidemic that exceeded other areas that didn't have alligators," which hasn't been the case, Bunning said.
The American alligator is found primarily in the Southeast, and ranges from the southern Virginia-North Carolina border to the Rio Grande in Texas. Cases of West Nile reported this year have been found as far north as Minnesota and as far west as Colorado.
In addition, both Jacobson and Bunning agree that alligators' susceptibility to West Nile may be enhanced by captive environments. At the farms in Georgia and Florida, alligators were housed in temperature-control buildings with water heated to 90-plus degrees, a practice that may have helped the virus spread, the scientists said.
A UF study is now under way to determine the virus' presence in wild Florida alligators.
Still, despite the uncertainties, the CDC has begun its own study to determine the reptile's West Nile tolerance and its role in West Nile transmission. Results are expected later this month.
"The preliminary work that we've done here matches quite nicely with what Elliot has identified," Bunning said.
Gators may play role in West Nile