It’s Time to Start Looking For West Nile Virus Again

By Tim Burton

CSU Extension Agent

May 21, 2004

We are looking for dead crows, ravens, jays, and magpies to collect and send to the Colorado Department of Public Heath’s Virus Laboratory for West Nile virus (WNV) testing. Unlike St. Louis encephalitis (SLE), and Western equine encephalitis (WEE) which circulate in a bird-mosquito-bird cycle without harming the bird host, some native bird species infected with WNV have experienced up to a 90% mortality rate. This has been particularly true for corvids (crows, ravens, and magpies). Although 76 bird species have been infected with WNV we will focus on corvid species because they are distributed around the state and tend to have a limited home range not to mention they are easily recognized.

West Nile Virus can cause encephalitis, an inflammation of the brain, or meningitis, an inflammation of the membrane surface of the brain and spinal cord in humans, horses, cats, dogs and domestic and wild birds. It is closely related to St. Louis encephalitis, commonly found in the United States for years. Mosquitoes acquire the virus from birds and pass it on to other birds, animals and people. Mosquitoes spread this virus after they feed on infected birds and then bite people, other birds and animals. It is not spread by person-to-person contact and there is no evidence that people can get the virus by handling Infected animals. West Nile virus cases occur primarily in the late summer or early fall, although the mosquito season is April through October.

People can only become infected with WNV after being bitten by an infected mosquito. There is no evidence that people can get WNV from infected animals or people, or that people can transmit the West Nile virus to other animals, birds, or people. The incidence of mosquito-transmitted disease in Colorado is quite low and closely monitored by public health agencies.

Protect Yourself from Bites. Mosquitoes can travel 2 - 3 miles from their breeding site in search of a blood meal. The following tips can help to reduce your risk of being bitten by a mosquito: •Use the proper type of light outside: incandescent lights attract mosquitoes, Install screens in windows; make sure screens are "bug tight" •Stay indoors at dawn, dusk, and in the early evening when mosquitoes are most active. If you must go outdoors, wear a long-sleeved shirt and long pants •Use mosquito repellents sparingly and strictly according to the label instructions •Adult mosquitoes rest in shrubbery and other shaded areas during the day. These areas can be treated with approved insecticides. Foggers for flying insects can also be used, but will provide only short-term relief. Various aerosol insecticides are available for controlling mosquitoes indoors.

Repellents - Insect repellents can be effective at reducing mosquito bites and, therefore, may reduce the risk of getting some diseases, such as West Nile Virus. When applied (sparingly) to exposed skin insect repellents deter mosquitoes from biting. Spray thin clothing with repellent because mosquitoes can bite through them.

Repellents commonly available to consumers contain the active ingredients DEET (N, N-diethyl-m-toluamide). All insecticides must have the name and amount of active ingredient (AI) on the label. Whenever you use an insecticide or insect repellent, be sure to carefully read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the pesticide product label. No pesticide is 100% safe and care must be exercised in the use of any pesticide.

Insect repellents do not provide complete protection against West Nile Virus or other insect-borne diseases. You should avoid areas with high populations of mosquitoes when possible, especially at peak biting times; use protective clothing (light-colored, long-sleeved shirts and long pants with bottoms tucked into boots or socks).

DEET - For more than 40 years, DEET has been the standard in mosquito repellents. DEET concentrations range from a low of about 5% up to 100%. Skin reactions (particularly at DEET concentrations of 50% and above) and eye irritation have been the most frequently reported adverse effects. The use of DEET products primarily results in exposure from skin contact, although unintentional exposure by breathing and ingestion can also occur.

By using products with lower concentrations of DEET and by applying as little of the product as needed for your outdoor activities, you can reduce your exposure to DEET. The American Academy of Pediatrics recommends that repellents used on children contain no more than 10% DEET.

DEET products can be applied to clothing, but may damage some synthetic fabrics and plastics, especially products with very high DEET concentrations. Launder treated clothing separately from other clothing. Frequent reapplication or saturation is unnecessary for effectiveness. Use only what is required to maintain protection.

Horses become infected the same way people do-by the bite of infected mosquitoes. The virus is located in the mosquito's salivary glands and when the mosquito "feeds" on the horse, the virus is injected into its blood system. In the bloodstream, the virus multiplies and may cause illness.

What are the signs of West Nile virus in horses? Not all horses become clinically ill. In those that do, following transmission by an infected mosquito, the virus multiplies in the horse's blood system, crosses the blood brain barrier and infects the brain. The virus interferes with normal central nervous system functioning and causes inflammation of the brain. Clinical signs include loss of appetite and depression, fever, weakness or paralysis of hind limbs, muscle fasciculations or muzzle twitching, impaired vision, lack of coordination (ataxia), head pressing, convulsions, difficulty in swallowing, circling, hyperexcitability or coma.

Do these signs always indicate West Nile virus? No. Some other mosquito-borne viral encephalitic diseases of horses caused by Eastern, Western and Venezuelan encephalitis viruses can cause a horse do demonstrate signs similar to WNV. Additionally, diseases such as rabies, botulism and equine protozoal myeloencephalitis (EPM) also have signs similar to WNV. If you think your horse is exhibiting signs of encephalitis, contact your veterinarian. Only laboratory tests can confirm a diagnosis of West Nile encephalitis.

Can an infected horse transmit the virus to horses in neighboring stalls? No. There is no documented evidence that WNV is transmitted from horse to horse. However, precaution should be taken by removing the horse from further potential mosquito-borne infection and tested for the virus.

What is the treatment for an infected horse? There is nor specific treatment for West Nile virus in horses. Most horses recover from the infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent. Caregivers should prevent the animal from injuring itself.

Can my horse be vaccinated against WNV? Yes. A West Nile virus vaccine for horses has been approved and is available. The vaccine consists of two intramuscular doses administered 3-4 weeks apart, and then revaccination is required annually. Check with your veterinarian for an explanation of the process and how the vaccine works.

My horse is already vaccinated against Eastern Equine Encephalitis, Western Equine Encephalitis and Venezuelan Equine Encephalitis. Will these vaccinations protect against WNV? No. These viruses belong to another family of viruses for which there is no cross-protection.

Two important types of mosquitoes in Colorado are the Aedes and the Culex/Culiseta.

Do all mosquitoes carry the West Nile virus? No. Mosquitoes are a major insect pest problem in Colorado, but not all types of mosquitoes carry the virus. The principal transmitter of the virus is the Culex pipiens, or the northern house mosquito. Once the mosquito is infected, it can transmit the virus to people or other animals.

Culex/Culiseta Mosquitoes - Mosquitoes in the Culex/Culiseta group actively feed only at night. The Culex mosquito makes the typical whining noise for which mosquitoes are known. The Culex tarsalis mosquito transmits Western equine encephalitis virus. These mosquitoes pass the winter and other unfavorable periods as adults inside houses and other sheltered areas. In spring and summer, adults lay floating egg "rafts" on water. Wrigglers and pupae are found in permanent or semi permanent standing water, which includes wading pools, birdbaths and clogged gutters. Development from egg to adult usually takes 10 to 14 days. Single and multigenerational species occur in Colorado. For detailed information on mosquito management around the home, family protection, insecticides and mosquito habitat modification, visit www.ext.colostate.edu/PUBS/INSECT/05526.html.

What should you do with dead birds? Monitor your property for dead birds such as crows, ravens, jays and magpies (corvids species). If you find a dead crow, raven, jay, or magpie, call the CSU Cooperative Extension Office in Eads at (719) 438-5321 as soon as possible so someone can come and get it. Call only if the bird is freshly dead. Don’t call if the carcass is decomposed or maggot-infested. Crows, ravens, jays, and magpies with an obvious cause of death (road-kill, predation) are acceptable for submission provided the carcass is still fresh, in New York 38% of the WNV positive birds showed signs of trauma probably because the birds were more vulnerable due to illness.