Scientists are warning us that the West Nile Virus (WNV) could have a big impact in the northwest by summer of 2003. In Part I of this 3 part series I will review a history of the disease, how it’s transmitted and methods of prevention in horses. In April I will cover some important farm management techniques for minimizing mosquito populations as well as beneficial natural controls. In the May issue I’ll cover chemical controls as well as prevention in humans and I’ll provide you with additional resources.
As of last January, one county in Idaho and many in eastern Montana had submitted equine test results to the Centers for Disease Control and Prevention (CDC) that came back positive. There have been documented cases of WNV in a horse from the San Juan Islands boarding on Whidbey Island, Washington, and another in Bellingham, Washington. Though both survived, the virus is here, though how strong a showing it will make is unclear. Mosquito-borne diseases typically do not do well in the northwest and in B.C., but so far this disease has defied the best guesses of scientists and epidemiologists across both countries. For current information see http://cindi.usgs.gov/hazard/event/west_nile/west_nile.html
History and present status
Mosquito-borne diseases affect millions of people worldwide each year. In the United States, some species of mosquitoes can transmit diseases such as encephalitis and malaria to humans, and a variety of diseases to wildlife and domestic animals. WNV, first identified in Uganda in 1937, is primarily a bird disease. Birds in the corvid family (magpies, ravens, crows and jays) are especially susceptible. Interestingly, chickens appear to be immune and because of this are used in some areas as sentinel flocks to monitor the spread of WNV. By now most of us have learned that the carrier, or vector, of this disease is the mosquito. Mosquitoes become infected by feeding on an infected bird and can then pass the virus to humans, horses or other hosts. Horses and humans are dead-end hosts, which means they can’t transmit the disease to other animals or humans.
All 59 regularly occurring species of mosquitoes in our area can be carriers in all of their life stages – egg, larva, pupa and adult. The larva and pupa stages for all mosquitoes require stagnant water. Mosquito larva cannot survive in moving water. According to the CDC, by this past January 43 states had detected 3,989 confirmed human cases, 259 of whom died.
In a workshop on WNV held last January by the Washington State Department of Health, 14,515 equine cases had been reported nationwide by December 1 of last year. With a 40 per cent fatality rate, that's over 5,000 horses that have succumbed to the virus in just a few years.
Most people exposed to WNV don’t get sick. Some may develop mild flu-like symptoms, and in rare cases, WNV may cause encephalitis, or inflammation of the brain. The majority of humans who die from WNV are in their late fifties and older and may have a weakened immune system. It appears to be different for horses as horses of all ages have succumbed to the disease. Symptoms of WNV are similar to other neurological conditions and include rear limb buckling, knuckling over and ataxia. Horses doing poorly rarely recover.
Prevention in horses
Vaccinate your horse. It's the number one thing we can do for our animals. The vaccine, available only through veterinarians, produces an immune reaction in the animal's blood following two intramuscular injections, 3 to 6 weeks apart, of a surprisingly small amount of vaccine, usually about two cc’s. An annual booster shot is given in the spring.
In the warm regions of the United States where the mosquito season is longer, such as the Gulf Coast states where significant WNF activity was seen into December last year, booster shots are being administered more frequently or on a schedule similar to that recommended by the state veterinarian for Western, Eastern, and Venezuelan equine encephalitis (WEE, EEE, and VEE) boosters.
While no vaccine is 100% effective, the WNV vaccine appears to be performing well with few side effects.
Next month we'll talk about farm management techniques for minimizing mosquito populations as well as beneficial natural controls.
Alayne Renee Blickle, a life-long equestrian and creator/director of Horses for Clean Water, is an environmental educator working with horse and livestock owners. Contact Alayne by email at ARBlickle@aol.com or visit the HCW website at www.horsesforcleanwater.com.
Copyright © 2008 The NW Horse Source, LLC
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