I needed a stepladder to reach the top bookshelf and pull down my “large animal” textbooks from veterinary school. I don’t do equine work anymore, but my younger daughter loves horses, keeping me connected to that world of hay, leather tack, and manure that I, too, loved at her age. So when she called me this summer from a horse show, having been bitten by several mosquitos and worried about Eastern equine encephalitis, I dusted off the yellowing tomes.
Hagan’s “Infectious Diseases of Domestic Animals,” Blood & Henderson’s “Veterinary Medicine: A Textbook of the Diseases of Cattle, Sheep, Pigs, and Horses” were among the heavy books I checked out. I even have a vintage 1956 edition of “The Yearbook of Agriculture Animal Diseases” published by the USDA, stamped inside as having belonged to Robert Hale, Member of Congress, First District of Maine.
In late summer and early fall of 1912, an outbreak of an unidentified disease in horses struck Kansas, Nebraska, Colorado, Oklahoma, and Missouri, resulting in the death of an estimated 35,000 animals. Over the next few decades, thousands of horses across the U.S. died. The disease was dubbed equine encephalomyelitis, or encephalitis, as, at the time, it was thought to infect only horses and mules, and the characteristic lesion was inflammation of the brain. Scientists eventually identified three forms of equine encephalitis — Eastern, Western, and Venezuelan — each caused by a different strain of arbovirus. Our current concern, however, is limited to one.
Eastern equine encephalitis (EEE) is a rare but life-threatening disease transmitted by mosquitoes. Wild and domestic birds serve as “reservoir hosts,” carrying the virus without necessarily falling sick. Animals susceptible to the disease include several species of birds, opossums, rodents, goats, horses, and people. According to the CDC, an average of seven human cases are reported annually in the U.S. Massachusetts is one of the states with the highest incidence, with 10 human cases reported between 2009 and 2018. This year, we are facing another outbreak, with eight cases confirmed in seven horses and one goat, and 10 human cases, including two fatalities. Hence my daughter’s concerned telephone call from Fieldstone Show Park, right in the center of the outbreak, when she was bitten by mosquitoes.
In horses, EEE is also called sleeping sickness, staggers, or blind staggers. Infected horses first present with fever, which may resolve as the disease progresses. Animals soon exhibit deranged consciousness, appearing blind, staggering about, often walking in circles or crashing into things. When quiet, they display a sleepy demeanor, head hanging down, ears and lips drooping. Other symptoms may include loss of appetite, diarrhea or constipation, head pressing, tooth grinding, hypersensitivity to sound or touch, incoordination, and tremors. It often progresses to paralysis and death, with mortality exceeding 90 percent.
Although the clinical signs of EEE in horses may be dramatic, other serious central nervous diseases, such as equine herpesvirus, protozoal myelitis, and rabies can have similar presentation. Diagnosis depends on blood tests for virus-specific antibodies or direct testing of brain tissue on necropsy.
There is no known treatment other than basic supportive care, so it’s a good thing that back in the 1930s, scientists got to work developing a vaccine. Current recommendations in the U.S. are that horses receive a primary series of two to four doses of a commercial vaccine that protects against both EEE and WEE, followed by annual boosters, usually administered in the spring prior to the onset of mosquito season. In some situations, veterinarians may recommend even more frequent vaccination.
So is my daughter right to be extra-concerned, being around horses all the time? Doesn’t the fact they are vaccinated make her safer? Well, yes and no. EEE virus is passed among avian hosts by a species of mosquito that only feeds on birds. Transmission to horses, and to people, depends on the presence of a “bridge” species of mosquito that will feed on both birds and mammals. There is no risk of contracting EEE from direct contact with an infected horse or person. It can only be passed to people by the bite of an infected bridge mosquito. And if a mosquito bites a sick horse and then bites a person? No problem. Even in infected horses, the level of EEE virus in the bloodstream is so low that horse-to-horse or horse-to-human transmission via mosquitos is not considered a significant risk. What does increase human risk is being outside in areas where the species of mosquito has been shown to be carrying the virus. According to the Massachusetts Department of Public Health, 421 mosquitos have tested positive for EEE in our state this year — that’s a lot.
People infected with EEE can sometimes simply get an illness similar to the flu. Chills, fever, stiff neck, muscle aches, fatigue. It lasts a week, maybe two, and then you get better. You might not even know you had EEE. The second possibility, however, occurs when the central nervous system becomes involved, leading to fever, headache, irritability, restlessness, drowsiness, loss of appetite, vomiting, diarrhea, cyanosis, seizures, and coma. The mortality rate may be as high as 30 percent, and survivors often have permanent neurologic damage. Most cases occur in children under 15 or elders over 50.
An ounce of prevention is worth a pound of cure, especially when there is no cure. The best thing to do is to avoid mosquito bites. Use EPA-approved insect repellents. Wear long sleeves and pants. Check your screens. Eliminate standing water to reduce mosquito breeding environments. Many high-risk areas have started aerial spraying, and people are being advised to cancel outdoor activities, especially at dusk and dawn, when mosquitoes are most active. But I told my daughter to relax. “Go buy some good bug spray and stop worrying,” I said lightly. Then, of course, because I’m the mom, I quietly looked up the EEE incubation period — four to 10 days — and marked day 11 on my calendar.