Infected Host Not Key to West Nile's Spread
Study suggests adjacent, feeding mosquitoes can infect each other
TUESDAY, June 8 , 2005 (HealthDay News) -- In a study that surprised its authors as much as anyone, scientists have discovered that the West Nile virus can be transmitted almost instantaneously between mosquitoes feeding side-by-side on an animal, even if that animal is not infected.
This may explain why West Nile traveled so quickly across North America, the researchers say. It may also support the continuing strong presence of the virus, although neither of these two theories is certain.
"I don't think this type of transmission would happen with humans, but it increases the number of animals out there which are driving the transmission," said Stephen Higgs, an associate professor in the department of pathology at the University of Texas Medical Branch, in Galveston.
"It's instantaneous transmission. The mosquitoes can be more quickly infected," he explained.
Higgs is lead author of the paper, published in this week's issue of Proceedings of the National Academy of Sciences.
While it's not yet clear if this same transmission process is taking place outside Higg's laboratory, "it suggests it can happen and there's no reason to think it might not be happening," the Texas researcher said.
Another expert agreed the finding could change how epidemiologists view West Nile's spread. "This makes us look a little closer at the ecology of West Nile virus, and it makes us really realize how complex the transmission cycle can be," said Richard Falco, co-director of the Vector Ecology Laboratory at Fordham University's Louis Calder Center in Armonk, NY. "It makes us look a little more closely at some other animals, particularly mammals, that we thought may not be involved. It suggests that they may be involved."
Since it first appeared in North America in 1999, West Nile virus has quickly made its way across the continent and is now found in virtually every contiguous state. In the intervening years, 15,000 humans have been infected with the virus and more than 650 people have died, according to the latest CDC data.
West Nile virus is normally passed from an infected mosquito to a bird then, from the bird, to other mosquitoes. This is known as viremic transmission, meaning that mosquitoes acquire the disease from birds that already have high levels of the virus in their blood. It usually takes several days for a bird to attain this level of virus.
Nonviremic transmission occurs when saliva from an infected insect reaches an uninfected inset. This process had been observed previously in ticks and black flies.
The current study grew out of a casual conversation Higgs had with his former boss, Ernest Gould, senior author of the paper. "We were just talking, 'I bet mosquitoes couldn't do this [nonviremic transmission],'" Higgs recalled.
The comment gave Higgs the idea to test the theory himself.
Infected mosquitoes were kept in one carton and uninfected mosquitoes in a second, adjacent carton. Both containers had mesh tops. An anesthetized mouse was then laid across the tops so that the mosquitoes could feed simultaneously on the animal.
Even though the mouse did not have time to acquire the virus in its blood, the uninfected mosquitoes became infected at rates of 2 percent to almost 6 percent, within an hour.
The experiment was repeated five times and, each time, much to the surprise of the investigators, it returned positive results.
In one instance, a single bite from a donor mosquito was enough to infect two out of 87 mosquitoes, a 2.3 percent infection rate.
"When the infected mosquitoes were feeding, somehow that virus got from them and infected the other mosquitoes," Higgs explained. "What we don't know -- and would love to know -- is how long a time can elapse between feedings for the mosquitoes to still get infected."
Until this new finding, scientists had assumed that birds -- which develop much higher levels of the West Nile virus in their blood -- were the only animals capable of passing the virus to uninfected mosquitoes. But the Texas discovery means all vertebrate animals, including horses -- once thought to be "dead-end hosts" -- may actually contribute to the spread of West Nile.
"We've always thought that the only way this virus is circulating in nature is by mosquitoes transmitting it to a bird, the bird developing viremia over the course of a few days and then [uninfected] mosquitoes coming in and feeding," Higgs said. But the new finding means mosquitoes "could be feeding on a whole variety of animals out there, people's pets and opossums and raccoons. If this works in those animals, all of those animals could be contributing to the life cycle."
Higgs is also interested in knowing whether the findings might help epidemiologists find new ways of controlling and monitoring the disease. Is it possible, for instance, that horses vaccinated against the virus could still transmit the disease from one mosquito to another, and thus to other animals?
The researcher's next step is to study the anatomical distance between the infected and uninfected mosquitoes feeding. In other words, if an infected mosquito feeds on the shoulder, could an uninfected mosquito pick up the virus at the leg or ear? He is also looking at potential time restrictions -- for example, if an infected mosquito dines at 6 p.m., could uninfected mosquitoes feeding at 10 p.m. still pick up the virus?
In the meantime, none of this alters current mosquito-protection guidelines, Falco stressed. "The recommendations for prevention of West Nile are the same: use insect repellant when you go out; make sure mosquitoes can't get into the house, make sure there's no standing water," he said. "This doesn't change that. From the basic person's point of view, it's still the mosquitoes."
For more on how you can protect yourself from the West Nile virus, visit the CDC.