© Sandee Molenda 2003
West Nile Virus (WNV) is an arbovirus (arthropod borne virus) spread by mosquitoes and primarily affects humans, equines, and birds. Birds are the reservoir for WNV. The virus was first identified in the West Nile District of Uganda in 1937, but did not appear in the United States until the outbreak of 1999 in the Northeast. The virus has progressively moved westward and now is present in most of the 48 contiguous states. The disease has now been confirmed in two humans in California with epidemiological studies in progress to determine where the infection was acquired. There is still no additional evidence of additional WNV activity in California. As of 2002, more than 4,000 human cases of WNV have been reported with almost 300 deaths.
In birds, corvids and raptors are the most susceptible to WNV. In many areas, these birds have been wiped out of existence where WNV is prevalent. Currently, more than 100 species of birds are known to be susceptible to WNV including many exotic and pet bird species. Dr. Susan Clubb, who is in Florida and has a private practice as well as is the treating veterinarian at Parrot Jungle, has not seen the numbers of possibly infected birds that we were afraid would be happening. She stated she had treated less than a dozen birds believed to be infected and many recovered with supportive care. Dr. Clubb went on to state that in her opinion the disease has both a low morbidity and low mortality rates in parrots. There have been documented cases of bird to bird transmission, however, this has only been proved in corvids and jays — not exotic birds.
Symptoms in birds include: seizures, ataxia, uncoordination, weight loss, diarrhea, tremors, general neurological problems, brain lesions and death.
The only vectors found to be associated with outbreaks of WNV in the United States since 1999 are mosquitoes. At least 30 species of mosquitoes have been found positive for WNV, although several of those species are likely not involved in active transmission of the virus from bird-to-bird or from bird-to-mammal. Given that mosquitoes are associated with WNV transmission, one key to preventing or controlling future outbreaks of WNV is to control mosquito populations and to prevent exposure to any adult mosquitoes that may be present.
On August 1, 2001, a conditional license was issued by the USDA-APHIS’ Center for Veterinary Biologics for an equine WNV vaccine. The vaccine is a killed virus product. Conditional licensing means that the product has been shown to be safe, pure, and have a reasonable expectation of efficacy in preventing illness caused by WNV. Each state veterinary authority must also approve the use of the product in their state. This vaccine is restricted to veterinarians and currently only authorized for private use in horses although this vaccine is also being used in zoos and biological parks to protect rare and exotic birds from WNV. However, it is unknown whether or not the vaccine will be effective in preventing WNV in animals other than horses. So far, there has been no adverse reactions to the vaccine and it did produce measurable antibodies in all birds even in low dosages. In chickens, the vaccine has produced kidney damage and neurological lesions although there were no clinical signs in the chickens.
Reducing the population of mosquitoes, especially species that are apparently involved with bird-to-bird transmission of WNV, such as some Culex species, can help to reduce or eliminate the presence of virus in a given geographical area. The most important step any property owner can take to control such mosquito populations is to remove all man-made potential sources of stagnant water in which mosquitoes might breed. Dispose of any water-holding containers, including discarded tires. Drill holes in the bottom of containers that are left outdoors. Clean clogged roof gutters annually. Turn over plastic wading pools or wheelbarrows when not in use and do not allow water to stagnate in bird baths. Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can breed in the water that collects on swimming pool covers. Aerate ornamental pools and use landscaping to eliminate standing water that collects on your property; mosquitoes can potentially breed in any stagnant puddle that lasts more than 4 days. Thoroughly clean livestock watering troughs monthly. Local mosquito control authorities may be able to help in assessing the mosquito breeding risks associated with a specific property.
Placement of a board or a piece of sheet metal on the top of this feeding cage extension will also keep the birds droppings from falling into the dishes when they hang on the front of the flight area. In outdoor situations having this “roof” in such close proximity to the dishes will also help keep the feed from becoming rain soaked.
Decreasing Exposure to Adult Mosquitoes
Housing animals in structures with well-maintained insect screening can be useful to reduce exposure to adult mosquitoes. Use of such mosquito-resistant structures may actually lead to mosquito exposure unless precautions are first taken to eliminate mosquitoes from inside the structure. This may be accomplished through a number of means including the use of mosquito adulticides. In addition, use of fans may reduce the potential ability of mosquitoes to feed on animals.
Use of insect repellents may be of some value in decreasing exposure to adult mosquitoes. Due to practical limitations in the coverage area with a particular product formulation, and due to limited duration of effectiveness of some formulations under certain conditions, repellents should not be solely relied upon to prevent mosquito exposure. Repellents should be used according to their label instructions regarding appropriate species, method of application, and other precautions. Topical application of a product containing a synthetic pyrethroid compound (e.g., permethrin) as the active ingredient may offer the best combination of safety and efficacy.
Although some species of mosquitoes feed at dusk or dawn, others are daytime feeders or feed at any time of the day or night. As it is not yet clear which mosquitoes are responsible for the transmission of WNV making recommendations as to when certain animals should avoid outdoor exposure may not be particularly useful at this time. However, a recently completed epidemiologic study of WNV suggests that keeping animals indoors at night may be helpful in reducing their risk of infection.
For More Information: (updated 4/06)
To contact APHIS Veterinary Services’ Emergency Program Staff, call 800-940-6524
USDA/APHIS West Nile Virus Web-Page
Centers for Disease Control