Although the vast majority of infections have been identified in birds, through September 2000 the Centers for Disease Control and Prevention (CDC) has received reports of West Nile virus infection in horses, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and raccoons.
CDC scientists believe the virus has probably been in the eastern United States since the early summer of 1999, possibly longer. Although not confirmed, there is evidence that an elderly man was infected with West Nile virus in October 1998 in Marshall Virginia. He suffered some memory loss due to his illness.
Background
West Nile virus was first isolated from a febrile adult woman in the West Nile District of Uganda in 1937. The ecology was characterized in Egypt in the 1950s. The virus became recognized as a cause of severe human meningoencephalitis (inflammation of the spinal cord and brain) in elderly patients during an outbreak in Israel in 1957. Equine disease was first noted in Egypt and France in the early 1960s. The appearance of West Nile virus in North America in 1999, with encephalitis reported in humans and horses, may be an important milestone in the evolving history of this virus.
West Nile virus has been commonly found in humans and birds and other vertebrates in Africa, Eastern Europe, West Asia, and the Middle East, but until 1999 had not previously been documented in the Western Hemisphere. West Nile virus has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, and animal health. The most serious manifestation of West Nile virus infection is fatal encephalitis (inflammation of the brain) in humans, horses, as well as certain domestic and wild birds.
Transmission of West Nile Virus
Mosquito-borne diseases affect millions of people worldwide each year. In the United States, some species of mosquitoes can transmit diseases such as encephalitis, dengue fever, and malaria to humans, and a variety of diseases to wildlife and domestic animals.
Infected mosquitoes are the primary source for West Nile virus. Although ticks infected with West Nile virus have been found in Asia and Africa, their role in the transmission and maintenance of the virus is uncertain. However, there is no information to suggest that ticks played any role in the cases identified in the United States.
The West Nile Virus is spread over distances by migrating birds, and locally by mosquitos. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting to take blood. The virus is located in the mosquito's salivary glands. During blood feeding, the virus may be injected into the animal or human, where it may multiply, possibly causing illness.
One of the species of mosquitos found to carry West Nile virus is the Culex species which survive through the winter, or "overwinter," in the adult stage.
Following transmission by an infected mosquito, West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.
Even in areas where mosquitoes do carry the virus, very few mosquitoes-much less than 1%-are infected. When humans are infected with West Nile virus, less than 1% of those will develop severe illness. The chances you will become severely ill from any one mosquito bite are extremely small. Among those with severe illness due to West Nile virus, case-fatality rates range from 3% to 15% and are highest among the elderly (persons older than 50 years).
There is no documented evidence of person-to-person or animal-to-person transmission of West Nile virus. Normal veterinary infection control precautions should be followed when caring for an animal suspected to have this or any viral infection.
Prevention of West Nile Virus
Stay indoors (or inside mosquito-netting) at dawn, dusk, and in the early evening. Wear long-sleeved shirts and long pants whenever you are outdoors. Spray clothing with repellents containing permethrin or DEET since mosquitoes may bite through thin clothing. Apply insect repellent sparingly to exposed skin. An effective repellent will contain 35% DEET (N,N-diethyl-meta-toluamide). DEET in high concentrations (greater than 35%) provides no additional protection. Repellents may irritate the eyes and mouth, so avoid applying repellent to the hands of children. To protect head and face, cover with a hat and headnet.
Whenever you use an insecticide or insect repellent, be sure to read and follow the manufacturer's DIRECTIONS FOR USE, as printed on the product.
Several companies are developing a vaccine. Clinical trials are planned to start in late 2002. It is expected that 2004 or 2005 would probably be the earliest that a vaccine would reach the market.
Symptoms
Most infections are mild, and cause flu-like symptoms, but for those with a weak immune system, the disease can be devastating.
Symptoms include fever, headache, and body aches, occasionally with skin rash and swollen lymph glands. More severe infection may be marked by headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, paralysis, and, rarely, death.
The incubation period in humans (time from infection to onset of disease symptoms) for West Nile encephalitis is usually 3 to 15 days.
If you or your family members develop symptoms such as high fever, confusion, muscle weakness, and/or severe headaches, you should see your doctor immediately.
-- TrekFit
Resources and Acknowledgments
- American Mosquito Control Association (AMCA)
- Annals of Internal Medicine - REVIEWS: Mosquitoes and Mosquito Repellents: A Clinician's Guide
- CDC - Arboviral Encephalitides
- CDC West Nile Virus National surveillance system, 2000: final plan, May 26, 2000.
- CDC West Nile encephalitis Questions and Answers
- Environmental Protection Agency (EPA) - Information on insect repellents containing DEET
- Links to State and City Web Sites - Special West Nile virus surveillance program - contact these jurisdictions for information on their specific programs.
- The New York City Department of Health - West Nile Virus Information
- Pesticides and Mosquito Control
- The Virginia Department of Health West Nile Virus Infection
- US Department of Agriculture Animal and Plant Health Inspection Service. Summary of West Nile virus in the United States.
- USDA Guidelines for Investigating Suspect West Nile Virus Cases in Equine