Sentinel chicken records fuel models predicting viral season severity
The start of West Nile virus transmission began unusually early in some US states this year, with the highest early-summer case count since 2004, the US Centers for Disease Control and Prevention warned in July. Delaware’s response has leaned on a surveillance program dating to the early 1980s — flocks of sentinel chickens stationed across the state to flag mosquito-borne viruses before they reach humans.
“They kind of are the proverbial canary in the coal mine,” said Tom Moran, head of the mosquito control section at the Delaware Department of Natural Resources and Environmental Control. “It is kind of the old-school gold standard.”
Sentinel chickens serve as an early warning system for mosquito-borne diseases, especially West Nile virus and Eastern equine encephalitis (EEE) — viruses that can be deadly. They can detect virus activity before mosquitoes infect humans, allowing officials to direct mosquito-control efforts and warn the public about precautions. Doctors use the alerts to watch for flu-like symptoms in patients outside flu season, which speeds diagnosis. Veterinarians use them to confirm that horses are vaccinated.
The chicken is a “dead-end host,” Moran said — capable of testing positive for the viruses but unable to transmit them, which is what makes the birds safe sentinels.
Delaware keeps four chickens at each of 20 locations statewide in a program that began in the early 1980s. Some states have transitioned to testing mosquito pools, which can detect viruses more quickly because chickens take about a week to develop antibodies after being bitten. Delaware tried both methods and kept the chickens. “The chickens sometimes pick up virus activity in an area that the mosquito pools did not,” Moran said. The chickens also do not fly away, which means officials can be certain that infected mosquitoes are active in that exact area.
“It gives you a little more surgical information,” Moran said. “You take that information and try to use it to lessen the specter of any human transmission.”
Cases generally begin appearing from north to south as migrating birds pass the viruses to mosquitoes, Moran said, with Delaware’s season typically beginning in early August.
The program has adapted to new pathogens. West Nile appeared in Delaware starting in 2000, and officials shifted from testing dead wild birds for the virus to testing the sentinel flocks they already maintained. Some states also test the chickens for other local illnesses, such as St. Louis encephalitis. If a neighboring state detected a new pathogen, Moran said, Delaware “could probably pretty quickly get up to speed and start testing for that here too.”
The decades of records collected from sentinel chickens have become a resource for researchers modeling how severe future seasons may be. Lindsay Campbell, associate professor of entomology and nematology at the University of Florida, has digitized paper records from Florida’s Department of Health and mosquito-control programs and combined them with environmental data — temperature, precipitation, land cover — and community-science records of bird migration such as eBird.
“The information that these programs have been able to provide is really invaluable,” Campbell said. Robert Guralnick, curator of biodiversity informatics at the Florida Museum of Natural History and a co-author, said the sentinel flocks are “telling us what happened, while we would like to know what might happen in the future.” Guralnick called them “a clever way to gather really good information about what’s happening.”
“It gives us the opportunity to go ahead and see the ebbs and flows, essentially, of these viruses over time, and then we can try to link them with different environmental characteristics to see if we can maybe predict when there might be elevated activity in the natural environment,” Campbell said.
The models examine whether conditions from months earlier predict later viral activity — for instance, if weather patterns or temperatures from four or six or twelve months earlier affect how bad a viral season becomes. “For some viruses, like EEE virus, we’ve seen a strong association with a certain type of landscape that is associated with the mosquito vector — that’s really important for that virus,” Campbell said. For West Nile, the team has identified precipitation and temperature patterns that precede positive tests in chickens by a few months.
Delaware’s program evolved through trial and error. For the first few years, officials used white leghorn chickens, which Moran described as “very flighty” — the birds repeatedly escaped into surrounding woods. “It’s literally like a scene from Rocky, where you’re chasing the chicken around — you can’t catch these things,” he said. Tracking the birds sometimes took an hour. Once the program switched to Rhode Island red chickens, he said, the work went much more smoothly.
Researchers continue to look for faster or cheaper methods. One project, Moran said, has proposed using honey traps to catch mosquitoes. “We’re always looking: is there a better way, a faster way, a cheaper way, a more accurate way — it checks all those boxes?” he said. “Folks are always pushing the ball forward.” But for now, he said, “chickens rule the roost.”