The birds are chirping, daffodils are popping up all over our yard (despite the demolition done to the bulbs this fall courtesy of a runaway stump-grinder) and the sun is shinning! March 20, 2012 marks the first day of spring in the northern hemisphere. So to kick off the new season here is the first installment of a 3-part series of blog posts. While we may all be excited to get back in the saddle and out on the trail, there are a few horse health topics we can’t forget, the darker side of this influx of warm air… And speaking of warm air, I don’t think anyone can argue that its been a mild winter and at least here in Indiana we seem to be moving right onto summer without much of a transition. Forget “In like a lion.” More like “In like an iguana baking in the desert sun!” Two weekends ago, I wore long-johns to practice and could barely keep my fingers from turning blue. Last Saturday, it was 80 degrees at the kick-off of our first game. That kind of sudden heat really makes you re-evaluate your fitness level.
But what else does this sudden heat wave bring to life, besides the flowers and grass (and we’ll get to the evils of the spring grass next week)? Bugs! With no more overnight frosts, mosquito season is upon us, and much earlier than normal. Now I know what that means for me, coating myself in OFF and calamine lotion and being driven to insanity by hundreds of itchy bites. But it also has implications for our horses and their mental well-being. Mosquitoes can carry a group of viruses that cause encephalitis, or infection in the brain. These viruses include West Nile Virus (WNV), Eastern Equine Encephalitis (EEE, often called Triple-E) and Western Equine Encephalitis (WEE). There is also a Venezuelan Equine Encephalitis (VEE) but that hasn’t been seen in this country since the 1970s, though some people in southern Texas and Florida consider it a threat.
Life Cycle of the Viruses:
The virus has a life cycle that mainly involves wild birds. Remember it was a veterinarian who discovered West Nile Virus in crows in New York City back in 1999. Mosquitoes pick up the virus when they feed on an infected bird and then transfer the virus to horses, or humans, when they bite them. These viruses cannot live outside of a host and do not survive in the environment for long and the virus does not exist in high enough levels in the horse’s blood stream to transmit to other mosquitoes, horses or humans. Therefore isolation of infected cases is not required.
EEE, WEE and WNV all act very similarly in the horse. The virus replicates in lymph nodes and infects the neurons in the brain. Clinical signs can appear anywhere from 2-3 days to as long as 3 weeks after infection. Signs may include: fever, depression, loss of appetite or neurologic signs (incoordination, staggering, hyperexitable, head pressing, leaning on walls, low head carriage, drooping lips/tongue or recumbency). Some horses may have intense puritis, or itchiness. Many horses infected with West Nile Virus will display very fine fasciculations of the muzzle that are often only noticed by seeing their whiskers quiver. The course of the disease is between 2 to 14 days and fatality rates range from 33% with WNV, to 50% with WEE and up to a staggering 90% with EEE. The only treatment is supportive care (IV fluids/nutrition, anti-inflammatories and corticosteroids). Horses that are lucky enough to recover may have residual neurologic deficits.
Prevention: Prevention includes vaccination and mosquito control (removal of stagnant water sources and liberal use of fly-sprays). When given properly (three initial shots 4-6 weeks apart, followed by an annual booster) the vaccines have been shown to be close to 100% effective. The American Association of Equine Practitioners
recommends that the, “annual re-vaccination must be completed prior to vector season in the spring. In animals of high risk or with limited immunity, more frequent vaccination or appropriately timed vaccination is recommended in order to induce protective immunity during periods of likely exposure. In areas where mosquitoes are active year-round, many veterinarians elect to vaccinate horses at 6 month intervals to ensure uniform protection throughout the year, although this practice is not specifically recommended by manufacturers of vaccines.”
So with the early onset of the warm weather, and hatching of billions upon billions of mosquito eggs, I would strongly recommend moving up your horse’s vaccination schedule to cover them against these devastating diseases as soon as you can. The bugs aren’t going to wait to bite your horse just because you’re not due for your spring shots till April!
And don't forget about Fido and Fluffy! While dogs and cats rarely get these viral encephalidities, mosquitoes carry heartworm disease and the warm weather will bring out other undesirable critters like fleas and ticks which carry a whole set of diseases of their own like tape worms and Lyme Disease. So, if you take a break from your flea/tick/heartworm prevention in the winter, now is the time to start back up again!!