Eastern, Western and West Nile Encephalomyelitis
When we have a wet spring, there is no doubt that enhanced growth of the Delaware state bird, the mosquito, will be occurring. Delaware Mosquito Control, located in Milford, Delaware, monitors the mosquito population with mosquito traps year round in Kent and Sussex counties. Additional monitoring is accomplished by sentinel birds. These are chickens, placed in coops in locations around the state, monitored weekly by blood sampling for the presence of mosquito-transmitted diseases which might be transmitted to humans and animals. If mosquitoes or birds are found to be harboring disease, the Delaware State Department of Health is notified to begin public announcements about preventative measures for humans and horses. These two techniques serve as an early warning system to inform us of the risk to the horses and to humans. This allows spraying programs to target the known risk areas. So what diseases are they looking for?
Any disease with all these initials (EEE,WEE,WN) for a start. (This is not a creative way to say “We Win”) During the height of the mosquito season in our region of the United States it is high risk time for the encephalitis diseases. These diseases, including a few other human encephalitic diseases, are the reason for the monitoring system that is in place. The acronyms in the title stand for Western Equine Encephalomyelitis, Eastern Equine Encephalomyelitis, and West Nile Encephalomyelitis. All of these diseases can be debilitating or fatal depending on the virus, the horse’s immune status and how early you as an owner and caretaker identify the illness and begin treatment. This discussion will be limited to EEE and WN since WEE is not a risk on the East coast of the US.
EEE and WN are in the group of viruses known as arboviruses. They are spread to horses by mosquitoes. There is no direct transmission of the virus from horse to horse. The mosquito is a necessary vector in the life cycle of the virus. The other important part of transmission is that birds serve as a reservoir to supply the virus to the mosquitoes which then transmit virus to the horse. Both of these viruses can cause potentially fatal disease in people as well, by the same bird-mosquito vector system. EEE is 95% fatal in horses. WN is 40% fatal in horses. The birds give the virus wings to spread over large distances. As infected birds, made sick by mosquitoes, move to new feeding areas or begin their fall migratory patterns they can move the virus a few miles to thousands of miles. The mosquitoes, on the other hand, usually only move about 500 feet from where they hatch. Multiple species of birds and mosquitoes are involved in the life cycle of the virus.
Although all species of birds are susceptible to the encephalitis viruses, larger species are typically the birds of concern. Birds of prey like eagles, hawks, and owls, because of their size pose more of a viremic threat than the smaller birds. But crows, blue jays, robins, and cardinals can develop enough viremia to be fatal to the bird and to serve as an amplification reservoir for the virus making human and horse exposure risk more problematic. If you find any of these birds dead for unexplainable reasons you should report them to the State Veterinarian’s office.
We as owners and veterinarians can help protect the horses and ourselves by improving management practices such as eliminating free standing pools of water, use of insect repellants and fly sheets, as well as avoiding dusk and dawn turn out times or other high exposure mosquito times of day unique to your farm or stable area. Vaccination programs greatly enhance the protection against the infected mosquitoes. Horses need to be vaccinated multiple times to afford the best protection. Vaccine products currently available require a 2-shot initial series 30 days apart. Most vaccines are labeled for once per year or annual administration but with our proximity to marshland, twice per year or biannual vaccination will provide a stronger antibody response. States along the Gulf of Mexico have been vaccinating every 4 months do to the increased threat created by the more tropical climate resulting in continuous mosquito exposure. Weanlings should receive a 3-shot initial series starting at 3 months of age to give them the best possible protection.
Although WN was first found in wild birds in Delaware in 2000, it was first found in sentinel birds on July 26, 2004.The finding is part of a statewide surveillance program conducted by the Mosquito Control Section of the Department of Natural Resources and Environmental Control. Historically the virus amplification process from mosquito to bird to mosquito to horse reaches high enough levels to start seeing cases of EEE or WN about mid August extending until a killing frost. This year (2013) as of this revision we have had 3 confirmed WNV cases and one EEE case in Delaware. Giving horses boosters in July, about 30 days before this amplification gets high enough to spread to horses, provides excellent protection until killing frosts and bird migration dilutes the virus load in our region, about mid-November. For the most recent reported cases within the state contact the State Veterinarian’s office in Dover, DE (Office – 302-698-4500) or USDA/APHIS web site.
Contact Brenford Animal Hospital to have your horse vaccinated against these preventable diseases, or if you suspicious about your horse’s behavior or neurologic symptoms. --PFH (1.14.14)