Hillsborough County's Sentinel Chickens: standing guard against West Nile Virus

October 22, 1:03 PMHillsborough County Mosquito Control ExaminerPamela Jacobson

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Sentinel chickens provide early warning of disease present in the mosquito population.
Thonotosassa Studios

Before the invention of carbon monoxide detectors, coal miners relied on canaries to alert them to the presence of the colorless, odorless yet deadly gas that sometimes forms in mine shafts after a fire or explosion. Canaries were preferred to mice, who might simply lie down and go to sleep. A canary would sway noticeably, then fall off its perch. Unfortunately for the canary, it usually didn't get back up.

Mosquito control and other public health professionals also rely on avian test subjects to detect the presence of disease in local mosquito populations. We call them, appropriately, Sentinel Chickens.

There is no actual species of chicken called “sentinels” -- although more than one neophyte has called around to various chicken farms in a fruitless attempt to procure some. The actual species of chickens commonly used as sentinels are Leghorn, Barrel Rock, Rhode Island Red, and Minorcan chickens, purchased at the age of 10-12 weeks (although some organizations raise their own).

The chickens are grouped into “flocks” of six, all female, and placed in wire cages sturdy enough to keep out raccoons, protect them from the elements, and yet allow free access to mosquitoes.

The flocks are then located in permanent sites away from public access to avoid vandalism and theft, and these locations are reported to the Bureau of Environmental Public Health Medicine each January (Florida Department of Health).

Before they are placed in their flock, the young chickens' blood is tested for anti-bodies to West Nile Virus, Saint Louis Encephalitis, Eastern Equine Encephalitis, and Highlands J Virus, to ensure they have not had any previous exposure.

Throughout the year, the chickens are re-tested every week for these anti-bodies. With 17 of these flocks spread throughout Hillsborough County, residents can be sure that if mosquitoes are carrying disease, our sentinels will give us an early warning.

Unlike the hapless canaries, sentinel chickens don't have to die for their cause. They don't even get sick from the viruses; they merely develop antibodies. And although being cramped in a cage with five other chickens and poked with a needle once a week probably isn't a whole lot of fun for a chicken, after they have put in a year of service, they get to “retire” to Steppin' Stone Farm, a “Christian residential home for at-risk teenage girls,” where they spend their days as free-range chickens, providing eggs for the residents. Not a bad deal for a chicken, if you ask me.

Chickens that turn up positive for one of the viruses go to the farm early. Since they don't actually contract the virus, their meat and eggs are entirely safe for consumption.

Out of 67 counties in Florida, 34 are participating in the state's Sentinel Chicken Program, sending weekly blood samples to the Florida Department of Health Laboratory in Tampa for testing.

Other states have established sentinel programs as well, due in large part to the arrival of West Nile Virus in the United States in 2001, however Florida's program was iniated much earlier, in 1978. Floridians have long had to deal with large periodic outbreaks of Saint Louis Encephalitis, and frequent transmission of Eastern Equine Encephalomyelitis virus (try saying THAT five times fast), both every bit as troublesome as West Nile, if not more so.

Yet on the horizon is still another mosquito-borne alphavirus, Chikungunya, which causes an illness with symptoms similar to dengue fever. The name itself translates to “that which bends up” referring to the way a victim tends to double over in pain from the fever and arthritis. The virus is indigenous to tropical Africa and Asia, however in 2007 a gentleman visiting from Africa brought the virus with him to Italy, resulting in 130 confirmed cases in the province of Ravenna.

Interestingly enough, the species of mosquito responsible for perpetuating the outbreak in Italy is not native to that country. Aedes albopictus, the Asian Tiger Mosquito, first showed up in Italy in the early 1990's. It is believed to have arrived in a shipment of used tires from Atlanta.

That's right, Aedes aegypti and Aedes albopictus, the mosquito vectors for Chikungunya, are widespread in the United States.

Florida, however, with our well-organized mosquito control and sentinel chicken program, is well equipped to deal with this and any other mosquito-borne illnesses that are the inevitable result of burgeoning globalization. Of 46 states reporting human cases of West Nile Virus in 2008 – some into the hundreds -- only 6 had fewer than Florida, which had only 3.

Thank you, sentinel chickens!