U. ILLINOIS (US)—The primary culprit in spreading West Nile virus—the common mosquito, Culex pipiens—appears to have a significant, albeit unwitting, accomplice: the robin.
“Robins are what we call super-amplifiers of the disease,” says Jeff Brawn, an ornithologist at the University of Illinois.
“Crows and jays just can’t handle West Nile virus and they die when they get it, but robins do just fine. They are a competent host of the disease.
“And, because robins are common in urban environments, they might be part of the smoking gun for why and where the disease is and isn’t.”
Brawn believes robins may hold the key to understanding the transmission dynamics of the disease, and like detectives at the scene of a crime, he tracks the birds.
“We want to find out how they move, where they roost, and what they do at night. We found roosts of hundreds of birds. You have a competent host all together at night when the mosquitoes feed.”
Rounding up the usual suspects was one of the first research projects. “We took a census of the birds, so we know what birds are out there,” Brawn says. Using blood meal samples from mosquitoes, they were able to establish the preference of the mosquito.
“Some birds may have anti-biting adaptations such as thick feathers,” Brawn says. “We counted the birds to see how common they are out there. And then we did the blood meal analysis to find out what the mosquitoes were actually feeding on. That’s how we established preference, and robins started to poke out in that as well.”
Brawn’s group catches robins in mist nets, takes a blood sample and then lets them go. From the blood, they can tell if the bird has developed antibodies and how long the bird has been carrying the virus.
“The virus tends to only be in the blood stream and available to the mosquitoes for a few days, when the bird is ‘viremic.’ That’s when the virus is replicating like crazy.”
If another mosquito bites the bird, the mosquito gets the virus and can then transmit it to another host—which could be another bird or a human.
In one experiment, Brawn’s group looked at all song birds and disproved a theory about infection rates. “This study was on the community level to see if a variation in the bird community leads to variation in the infection rates in the mosquitoes,” he says.
“The hypothesis is that in an area that has a very diverse bird population, there will tend to be a lower infection rate. The thinking is that if you’ve got low diversity of birds, and one of the species happens to be a great host, then infection rates amongst mosquitoes will go up a lot because they’re focusing on that species. So, it’s sort of like spreading the risk of infection.
“That’s the hypothesis. But, our study did not find it to be true.”
Birds that had not left the nest yet were the topic of another study. Brawn figured that the nestlings were like bait for hungry mosquitoes.
“They’re fairly helpless and they don’t even open their eyes for a week. So, they’re sitting still and then don’t have feathers and they don’t have a well-developed immune system. That was our hypothesis that they are immunologically naïve,” Brawn says.
But once again, the trail ran cold. “We didn’t get a lot of infection rates in the nestlings. It just doesn’t look like nestlings themselves were contributing by being the super amplifiers on this.
“On the other hand, with hatch-year birds we found more corroborative evidence,” he says. Hatch-year birds are just birds that are born that year—the first year it leaves the nest. There are a lot of them, and they remain viremic longer.
“We found a large percentage of hatch birds that tested positive for the virus. The results of that study suggest that the abundance of hatch-year birds facilitates rapid amplification of West Nile virus.
“I would hope birds don’t get persecuted because of it, because they shouldn’t. The best way to get rid of West Nile virus is to take care of the mosquito.
“Birds are involved, certainly, but we shouldn’t get rid of all of our nature preserves in order to prevent West Nile virus. Birds are passive participants in the process of amplification and transmission of the disease,” Brawn explains.
More news from the University of Illinois: http://www.aces.uiuc.edu/news