US States Turn to Sentinel Chickens as West Nile Cases Rise Early

Tom Moran, head of the mosquito control section at the Delaware Department of Natural Resources and Environmental Control, described the birds as "the proverbial canary in the coal mine". Sentinel chickens, as they are called, serve as an early warning for mosquito-borne diseases, especially Eastern equine encephalitis and West Nile, and can often predict when virus activity begins ramping up.

That advance notice allows officials to target mosquito control efforts and warn the public about precautions. It also means doctors know to look out for patients with flu-like symptoms, particularly outside flu season, which can speed diagnosis, and that veterinarians can make sure horses are vaccinated.

Moran said the chicken itself is a "dead-end host": it can test positive for the viruses but cannot spread the diseases, which is what makes it an ideal sentinel.

Delaware keeps flocks of four chickens each at 20 locations throughout the state, a program that began in the early 1980s. Moran called it "kind of the old-school gold standard".

Some states have transitioned to testing mosquito pools, which can be a faster way to detect viruses, because it takes about a week for chickens to develop antibodies after being bitten. Delaware tried both methods but decided to use chickens only.

"The chickens sometimes pick up virus activity in an area that the mosquito pools did not," Moran said. Unlike the insects, the birds do not fly away, meaning officials can be certain that infected mosquitoes are active in that exact area. "It gives you a little more surgical information," he said. "You take that information and try to use it to lessen the specter of any human transmission."

Cases usually start appearing from north to south as migrating birds pass the viruses to mosquitoes, Moran said, and Delaware's season usually begins in early August. The chickens could also be tested for other mosquito-borne illnesses if they emerge, as happened when West Nile appeared in Delaware starting in 2000, when officials moved from testing dead wild birds to testing the sentinel chickens they already had. Some places test for illnesses such as St Louis encephalitis, and Moran said that if a neighbouring state detected such a pathogen, "we could probably pretty quickly get up to speed and start testing for that here too".

The decades of data collected from sentinel chickens on virus patterns can be invaluable for creating models that predict the severity of a season. That is the focus of Lindsay Campbell, associate professor of entomology and nematology at the University of Florida, who digitised decades of paper records from Florida's Department of Health and mosquito control programmes and drew on environmental patterns such as temperature, precipitation and land cover, as well as community science data like eBird to track bird migration.

Robert Guralnick, curator of biodiversity informatics at the Florida Museum of Natural History and a co-author with Campbell, said the sentinel chickens provide important data "telling us what happened, while we would like to know what might happen in the future". Campbell said the work offers the opportunity to see "the ebbs and flows, essentially, of these viruses over time" and then link them with different environmental characteristics to see whether elevated activity in the natural environment can be predicted.

Many factors are likely involved in how such diseases spread, including hosts such as mosquitoes, birds and other animals, as well as environmental conditions, which makes it complicated to predict how severe a year will be. The models examine, for instance, whether weather patterns or temperatures from four, six or twelve months earlier affect how bad a viral season is. Campbell said a strong association with a certain type of landscape linked to the mosquito vector has been seen for EEE virus, while for West Nile virus researchers have identified precipitation and temperature patterns a few months before chickens start testing positive.

"The information that these programs have been able to provide is really invaluable," Campbell said of sentinel chickens. Guralnick agreed: "It's a clever way to gather really good information about what's happening."

Editor's Summary

Delaware and other US states use flocks of sentinel chickens, which cannot transmit the viruses they carry, to detect West Nile and Eastern equine encephalitis and to guide mosquito control and public health warnings. The CDC has reported unusually early West Nile transmission this year, with the most early-summer cases since 2004. Researchers are using decades of sentinel chicken records, alongside environmental and bird migration data, to build models that may forecast the severity of future viral seasons.