West Nile Virus has spread throughout the United States since its discovery in New York City in 1939, when humans and horses were found to be infected after transmission from infected mosquitoes. This year, the incidence of human infections has soared, presumably due to increases in mosquito populations.
Figure 1 illustrates the prevalence of diagnosed cases throughout the country and human/ non-human transmission of West Nile Viral disease in the United States. Courtesy: Center for Disease Control.
As mentioned, horses are the most susceptible animal. However, whereas humans can be exposed by other routes such as blood transfusions, organ transplants, breastfeeding, and even during pregnancy from mother to baby, equine exposure is thought to be exclusively through the bite of an infected mosquito. Studies have not ruled out ticks as a vector, and they are thought to harbor viruses throughout colder months of the year, but they’ve not been proven to be infective by biting.
After transmission from the mosquito, the virus multiplies within the blood stream and then enters the brain to cause inflammation known as encephalitis. Not all horses bitten by infected mosquitoes develop disease symptoms, but when illness occurs we look for the following clinical signs:
--Loss of appetite
--Inability to swallow
--Walking in circles
--Rear leg weakness leading to paralysis
Not all horses with these symptoms have West Nile virus, so please consult your veterinarian to confirm with testing. There is no specific treatment for the disease beyond supportive measures designed to give the animal’s immune system time to combat the virus. There is likewise no vaccine effective in preventing infection, including equine vaccines for eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE).
The Center for Disease Control recommends the following measures to limit mosquito populations:
--Turn over plastic wading pools when not in use.
--Turn over wheelbarrows and don't let water stagnate in birdbaths.
--Empty and refill outdoor water troughs or buckets every few days.
--Aerate ornamental pools or stock them with fish. Water gardens can become major mosquito producers if they are allowed to stagnate.
--Clean clogged roof gutters every year. Millions of mosquitoes can breed in roof gutters each season.
--Eliminate any implements on the property that can contain standing water.
--Clean and chlorinate swimming pools when not in use. Mosquitoes may even breed in the water that collects on pool covers.
--Use landscaping to eliminate standing water that collects on your property, especially near manure storage areas. Mosquitoes may breed in any puddle that lasts for more than four days.
Additional steps can be taken to reduce the likelihood of exposure of horses to adult mosquitoes:
--Reduce the number of birds in and around the stable area. Eliminate roosting areas in the rafters of the stable. Certain species of wild birds are thought to be the main reservoir for the virus. (Although pigeons have been shown to become infected with West Nile Virus, they do not appear to act as reservoirs and therefore don't transmit the virus to mosquitoes).
--Periodically look around the property for dead birds, such as crows. Use gloves to handle dead birds and place the birds in plastic bags.
--Topical preparations containing mosquito repellents are available for horses. Read the product label before using.
The risk of disease transmission is highest in the fall months because mosquitoes carry the heaviest viral loads in late August to September. The risk of disease decreases when colder temperatures eliminate mosquitoes.
Exposure to infected horses cannot transmit the disease to humans; horses can, however, serve as a reservoir of infection to mosquitoes.
For help in assessing mosquito exposure risks on your property and for suggested control practices, please contact your county extension office, county Department of Environmental Protection, county Department of Health, or a mosquito and pest control company.
Other animals, including dogs and cats, can be infected with the virus via mosquito transmission, but they are not thought to be susceptible to disease; nor do they harbor the virus in numbers that would serve as reservoirs of infection. We do not consider this disease to be a consideration when household pets exhibit symptoms similar to those seen in other species.
Please email Dr Ed Mapes with any questions concerning West Nile Virus and its affects on animals.