Fall 2003

Table of Contents

Habitat fragmentation may help spread WNV
(Excerpted from an article by Laura Spinney)

Researchers have found that the mosquitoes that transmit West Nile Virus (WNV) to birds are quite particular about the species they feed on. One of their favorites, the brown-headed cowbird, happens to be increasing in numbers and pushing westwards through the US as a result of the fragmentation of its habitat by humans, showing how we might be driving new epidemics towards ourselves.

The primary hosts of WNV are birds. Mosquitoes that normally feed only on birds maintain a cycle of infection within them, and the virus only breaks out of that cycle to infect other species when “bridge vectors” - mosquitoes that bite both humans and birds - come into contact with an infected bird.

For that reason, the degree of contact between bird and mosquito – or horse and mosquito in the case of another, far more vicious neurological disease called Eastern Equine Encephalomyelitis (EEE) - is thought to be a major factor determining whether the virus crosses the species barrier. When contact is high, the virus amplifies itself more quickly and there is a higher chance that the bridge vector will come into contact with it.

To investigate how the degree of contact affects viral levels, and hence the risk of infection for humans, Thomas Unnasch of the University of Alabama at Birmingham and colleagues analyzed the stomach contents of bird-biting mosquitoes in three states: New Jersey, New York and Tennessee. They used a reverse transcriptase-polymerase chain reaction to detect the presence of WNV, and another sensitive assay to determine the species of origin of the mosquitoes’ blood meals.

At all the test sites, they found that of the 24 bird species the mosquitoes fed on, three accounted for more than 50 percent of the blood they had ingested. Of these, the most notable was the brown-headed cowbird.

Similarly, in Alabama’s Tuskagee National Forest, which saw an epidemic of EEE in 2001, the first year of the study, the mosquitoes favored two species: the American robin and the brown-headed cowbird, with the cowbird accounting for more than 40 percent of their blood meals.

The American crow seemed not to be to the mosquitoes’ liking at any of the sites, although American crows are regarded as “sentinels” for the arrival of WNV because they are highly susceptible to it and die off quickly once infected.

In both the EEE and WNV studies, the researchers were surprised to find that the birds the mosquitoes preferred to bite were not endemic to the swamps they themselves inhabited. The birds’ usual habitats were grasslands or higher altitude ecosystems.

According to Unnasch, that suggests the mosquitoes’ habitat could be larger than was previously thought, and they might forage outside swamp areas before returning to them to digest their meals and lay their eggs. At the same time, forest clearance could be enabling grasslanddwelling cowbirds to stray closer to mosquito-ridden areas. “What we are seeing is a consistent pattern of these arboviral vectors targeting just a very few species,” he says.

His team also found that in July and August a higher proportion of the mosquitoes’ blood meals came from hosts other than birds. Before that July dropoff, however, birds were their main targets.

Unnasch thinks that the mosquitoes might be zeroing in on fledglings of certain species. He suggests that young birds are a dead-end population. Because they are virally naive, they die quickly and provide no reservoir for transmission to other birds or humans.

However, fledglings abound early in the season and transmission generally peaks in late summer, he says, so more research is needed to explain the delayed, late summer peak.

“This to me is really interesting because of the increases we have seen in brown-headed cowbirds with the fragmentation of the landscape,” said Sharon Collinge of the Department of Environmental, Population and Organismic Biology and the Environmental Studies Program at the University of Colorado, Boulder. “They’ve moved westwards and they tend to forage more around forest edges.”