By Suzanne Bush
Okay, there may be icicles hanging off the barn roof, but that doesn't mean it's too early to start thinking about mosquitoes and all the problems they bring. Last September, a Warren County, PA horse developed serious neurological symptoms and was euthanized. Laboratory testing confirmed that the filly had Eastern Equine Encephalomyelitis (EEE), otherwise known as "sleeping sickness." The young Percheron was one of two Pennsylvania horses known to have contracted EEE last year.
The disease is spread from birds to mosquitoes and then to horses. According to the Centers for Disease Control (CDC), horses are considered "dead end" hosts of the disease. That's because mosquitoes that bite an infected horse are not likely to become infected since there is an insufficient concentration of the virus in the horse's blood. It's the mosquito that has picked up the virus through feeding off an infected bird that we have to worry about.
According to Dr. Craig Shultz, DVM, director of the Bureau of Animal Health and Diagnostic Services in the state Department of Agriculture, different types of mosquitoes feed off different animals. "The mosquitoes that generally feed off the horses don't carry it. It's the bird mosquito that presents a concern about carrying it bird to bird. It builds it in the environment and the other mosquitoes carry it to people. The bird mosquitoes are unable to feed on horses or people."
EEE has been a recurrent scourge for America's equines since at least the 17th Century. After a major outbreak of EEE in coastal areas ranging from New Jersey to the Carolinas in 1933, 1934 and 1935, epidemiologists confirmed the connection between mosquitoes and the disease. It was not until years later that birds were identified as hosts of the disease, according to the US Department of Agriculture's Animal and Plant Health Inspection Service (APHIS).
While EEE has occurred in Pennsylvania in the past, Shultz says that these cases were unusual. "They came right at the very end of mosquito season. There was sort of a spike of EEE cases nationwide and five other states reported cases in the same time period."
Shultz says the mortality rate for horses that contract EEE is quite high. The CDC estimates that it's close to 90 per cent "They have to be treated symptomatically, and some survive, but it's not a good prognosis," Shultz explains. "It would require very prompt anti-inflammatory treatment very early in the disease." He says horses that do survive almost always have permanent and significant neurological damage.
Pennsylvania's Department of Environmental Protection (DEP) is part of the system in place to defend residents and their livestock from EEE. Shultz says that DEP had few options last year when the two horses with the virus died. It was the end of mosquito season, so there was no point in spraying. "But DEP is going to monitor the population of mosquito that spreads virus from bird to bird."
But because the birds that carry the virus don't seem to be affected by it, it's hard to know where to look for the species of mosquito that is the culprit in the cycle of EEE. "When we know that horses have been exposed," he explains, the DEP has a starting point. The exposed horses are sentinels, able to provide a fairly precise location for where there are EEE carriers.
The most effective weapon available to protect horses from EEE is a vaccination. "The EEE vaccine in horses is a pretty well-tested long-standing vaccine that has been in use for a long time," Shultz says. "Neither of the dead horses had been vaccinated against the virus." Horses that are infected with the virus will present increasingly alarming symptoms, beginning with fever, depression and a loss of appetite. Neurological symptoms include weakness, problems with coordination, blindness and abnormal sensitivity to light and sound.
As frightening as this virus is, Shultz says that the presence of mosquitoes does not always mean that the EEE virus is also present. "You have to have that perfect combination of the bird to bird mosquitoes, and the other mosquito species that feeds on humans and equines. It's not a direct situation. All these stars have to be aligned."