The Eastern equine encephalitis virus counts on the presence of two things to maintain its lifecycle: a species of mosquito, Culiseta melanura, and that mosquito's favored blood source, wild swamp-loving birds. Several other mosquito species can bridge the gap between infected birds and humans, though the chance of our becoming infected with the virus is quite low. The Center for Disease Control and Prevention tells us an average of six human cases of EEE are reported each year, typically from eastern and Gulf Coast states. That said, because of the high fatality rate associated with EEE, it is perhaps the most serious mosquito-borne human disease in the country.
Photo courtesy of Alfred Borchard/sxc.hu.
The EEE virus is also of interest to us because other mammals and domestic poultry can become infected with it. It does predominantly affect horses: between 2003 and 2009, the average number of equine cases per year was 283, and roughly 90 percent of those animals succumbed to the disease. That said, cases of infected cattle, sheep, swine, dogs, emus, pheasants, chukar partridges and turkeys have trickled in over the years. Then, in 2004, the list of susceptible species lengthened. A necropsy performed at Tufts University's Cummings School of Veterinary Medicine revealed EEE was in fact the cause of an alpaca's death.
Further research at the university revealed several prior cases of camelids, both llamas and alpacas, succumbing to EEE. Then, in September and October of 2005, three New Hampshire alpacas succumbed to the disease in a matter of days, and the cause of a fourth alpaca's death was strongly suspected to be EEE, but was never confirmed. Maine, Massachusetts and New York also reported EEE alpaca deaths that year.
This news rocked the world of alpaca farming. Before 2004, the steadfast belief was that camelids were not susceptible to the virus, and llama and alpaca owners need not be concerned. Which begs the question: had animals been succumbing to EEE all along without our knowing it?
EEE causes swelling of the brain, and clinical signs in a llama or alpaca can be difficult to interpret and/or confused with those of many other diseases. An infected animal may have a fever, lack coordination and the ability to rise, exhibit bizarre behaviors, experience seizures and rapid eye movement, and repeatedly flex its head and neck backwards. Some or all of these symptoms can be mistaken for other diseases, including a meningeal worm infection, rabies, West Nile virus or bacterial meningitis.
To confirm the presence of the EEE virus, blood samples must be analyzed and a spinal tap may be required. If EEE was not considered a threat to llamas and alpacas prior to 2004, given what we know now it is perfectly reasonable to assume the occasional animal succumbed to EEE but another disease took the credit.
Let's put this in perspective. The incidence of EEE in livestock is relative to the prevalence of the virus in a given region. Take for example the year 2005, when 21 human cases of EEE were confirmed in the U.S. More than half of them occurred in New Hampshire (seven) and Massachusetts (four). It then comes as no surprise that that region's alpacas took a hit from EEE that year.
For those alpaca and llama farms in regions that have a high exposure risk (i.e., coastal regions including those bordering the Great Lakes), it's worth having a good old-fashioned conversation with your veterinarian, who may bring up the idea of EEE equine vaccines. They are not technically approved for use in camelids, but a small study funded by the New England Alpaca Owners and Breeders Association and the Alpaca Research Foundation concluded they were effective in alpacas, particularly those animals less than 2 years of age. Antibody titers rose post vaccination, but the rate was slower than that experienced in horses, and levels attained were not as high as those attained in horses. One of the study's principle investigators, Dr. Amy Bright, DVM, of Candia, N.H., has since developed a vaccine protocol and lays claim to the fact that she has not lost a single alpaca to EEE since herds under her care were vaccinated.
Aside from vaccination, consider key elements of mosquito biology. Since all reported EEE cases have occurred between July and October, stabling during that time period - just before dusk and only releasing to pasture when the sun is well over the horizon - might be wise. If it is a relatively wet year, stabling is particularly wise. Vet-approved insect repellents are another means to help reduce the risk of infection, and get rid of any pools of stagnant water that could potentially serve as a mosquito nursery.
Eliminating mosquitoes is impossible, but reducing the risks associated with their presence is not. Keep abreast of the news and stay in touch with your local livestock organizations for up-to-date reports on EEE prevalence in your region. No doubt about it, the best thing you can do for your livestock is to stay informed.
The author, a regular contributor to Farming, is a biologist who lives and farms in Vermont's Northeast Kingdom.