Animals can transmit West Nile virus: study
TORONTO â€” In a finding that challenges dogma about West Nile transmission, researchers have shown that infected mosquitoes can pass the virus to their non-infected, blood-sucking siblings as they feed on the same animal.
Scientists know that female mosquitoes become infected with West Nile while feeding on birds with high levels of the virus in their blood. The birds get West Nile after being bitten by infected mosquitoes, but it takes several days for the virus to build up in the blood.
Until now, most animals were considered dead-end hosts that did not pass along West Nile to new swarms of mosquitoes.
But researchers at the University of Texas Medical Branch found that when infected and non-infected mosquitoes fed simultaneously on a healthy mouse, the virus-free insects picked up West Nile within an hour – even though the blood of the lab animal showed no evidence of the virus.
“We were surprised. We didn’t expect to see any of these recipient mosquitoes infected,” principal investigator Stephen Higgs said Monday from Galveston.
“We don’t know how it’s getting from one mosquito to another.”
In five separate experiments, the scientists exposed anesthetized lab mice to scores of infected (donor) mosquitoes, then to about an equal number of non-infected (recipient) mosquitoes. Between two and six per cent of the recipient insects which fed on the animals’ blood were found to be infected after only an hour.
Higgs, whose study is published in this week’s Proceedings of the National Academy of Sciences, called the findings “scary.”
“Animals that we have never considered as relevant in West Nile transmission may now be implicated,” he said. “We know that thousands of horses are being bitten by infected mosquitoes because thousands have died.
“But nobody has considered that they have actually been a source of infecting more mosquitoes.”
Infected mosquitoes can also pass West Nile to humans. While most people have virtually no symptoms, about one in 150 will develop severe illness and may even die.
While people can take precautions to avoid being bitten, animals have no such defences and can be attacked by hundreds of the pesky bugs per hour, said Higgs. That means not just birds are possible hosts for the disease, but all kinds of animals – everything from elk and moose to racoons and field mice to cats and dogs.
“What this new method of transmission does is it accelerates the time, because you don’t need that period of the animal becoming sick or viremic (having high blood virus levels),” he said. “Mosquitoes can be basically infected instantaneously.”
The team plans to study other mosquito-borne diseases, such as dengue fever, which strikes up to 100 million people around the world each year and kills thousands.
“Maybe if this is happening with West Nile, maybe it’s happening with other viruses,” Higgs said. “We don’t know.”