—Christine, via e-mail

A.Eastern equine encephalomyelitis (EEE) is transmitted to horses (and humans) by certain species of mosquitoes. The disease persists from year to year as there is a natural reservoir of the EEE virus in birds. Blood feeding by infected female mosquitoes transmits infection to birds, which then pass the infection to other mosquitoes who subsequently bite them. This completes an annually occurring cycle of virus spread from the mosquitoes to the reservoir species and back again.

Horses become ill when infected mosquitoes from this natural mosquito-bird cycle bite them. It is highly unlikely that spread from horse to horse, horse to human, or human to horse can occur as virus levels do not get sufficiently high post infection in these species to infect a mosquito that subsequently bites. Horses and humans are, therefore, known as ‘dead-end hosts’ as there is no potential for further spread.

Occurrence of EEE each year and the degree of spread geographically are dependent upon how favorable environmental conditions are for mosquitoes and reservoir species. This will vary from year to year. Environmental change, whether short term or as part of a more generalized climate alteration to a warmer level, allows further northerly spread of mosquito populations and a longer season of favorable conditions for activity. This may allow infected mosquitoes to contact more susceptible horses where effective vaccination is not generally practiced.

Your state, Florida, has a high population of horses in a region of very favorable conditions for rapid mosquito population increases. Warm years of high rainfall will lead to greatly increased mosquito populations as there is more standing water in which to breed. Increased mosquito numbers equals more potential for spread of EEE into the horse population. Reports of infection increase along with mosquito numbers when prevention by vaccination is not correctly and uniformly applied.

Vaccination programs are best formulated with knowledge of the aforementioned biology and area-specific risk factors. The American Association of Equine Practitioners has published vaccination guidelines, which can be found at aaep.org/guidelines/vaccination-guidelines/core-vaccination-guidelines/easternwestern-equine-encephalomyelitis.

In high-risk areas, such as Florida, increased vaccination frequency, initiation of vaccination at younger ages, and earlier annual vaccination administration is prudent. Your primary care veterinarian is best placed to advise you on your situation. Vaccination should be initiated in the spring prior to the onset of the risk period (i.e., rapid mosquito population increase). Because mosquitoes transmit EEE to horses—it’s not spread horse to horse—a retired horse or one who does not travel is still at risk; don’t neglect to vaccinate just because a horse doesn’t travel.

Other steps to minimize chances of infection involve reduction of mosquito numbers where possible (e.g., remove all containers of standing water where mosquitoes can breed) and minimizing opportunities for mosquitoes to bite horses (e.g., using insect repellents, fly sheets, and fans in stalls, and removing litter and debris from the barn).