West Nile virus (WNV) has emerged in recent years in temperate regions of Europe and North America, presenting a threat to public, equine, and animal health. The most serious manifestation of WNV infection is fatal encephalitis (inflammation of the brain) in people and horses, as well as mortality in certain domestic and wild birds.
History
WNV 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 first appearance of WNV in North America in 1999, with encephalitis reported in people and horses, and the subsequent spread in the United States over the past year, may be an important milestone in the evolving history of this virus.
WNV is a vector-borne virus. Invertebrate vectors, such as mosquitoes, circulate the virus among wild birds. Occasionally the virus is introduced into other vertebrate populations, such as people or horses, which serve as incidental hosts. Incidental hosts are infected animals that do not pass the virus on to vectors or other animals.
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.
Geographic Distribution
West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, Oceania (subtype Kunjin), and most recently, North America. Recent outbreaks of WNV encephalitis in people have occurred in Algeria in 1994; Romania in 1996-97; the Czech Republic in 1997; the Democratic Republic of the Congo in 1998; Russia in 1999; Israel in 2000; and the United States in 1999-2002. Epizootics of disease in horses occurred in Morocco in 1996; Italy in 1998; France in 2000; and the United States in 1999-2002; and in birds in Israel in 1997-2001 and in the United States in 1999-2002.
In the United States, WNV activity has been documented in 43 states and the District of Columbia from 1999 through September 17, 2002. The Centers for Disease Control (CDC) and Prevention continues to work with state and local health departments to help control the virus. Human illness from WNV is rare, even in areas where the virus has been reported. The chance that any one person is going to become ill from a mosquito bite is low. As of September 17, there had been 1,540 human cases of infection reported to CDC from 31 states and the District of Columbia in 2002, with 71 fatalities in 16 of those states, and new cases are surfacing daily.
WNV Infection in Animals
Horses are affected by WNV much more often than any other domestic animal. Many horses infected with WNV do not develop any illness, but of horses that become ill, about one-third (33 percent) die or need to be euthanized. Illness caused by WNV infection has been confirmed by the U. S. Department of Agriculture’s (USDA’s) National Veterinary Services Laboratories (NVSL) in equines from 32 states during the period January 1-September 12, 2002. The total number of equine cases of WNV tested at the NVSL or reported by state officials to date this year is 4,606, an increase of 2,245 during just the first ten days of September.
The number of equine cases reported in each of the 32 states include: Alabama (27), Arkansas (43), Colorado (139), Florida (130), Georgia (15), Illinois (249), Indiana (166), Iowa (225), Kansas (136), Kentucky (173), Louisiana (299), Maryland (1), Michigan (53), Minnesota (356), Mississippi (138), Missouri (174), Montana (32), Nebraska (709), New Jersey (13), New Mexico (19), New York (4), North Dakota (446), Ohio (55), Oklahoma (27), Pennsylvania (7), South Dakota (527), Tennessee (42), Texas (346), Vermont (3), Virginia (3), Wisconsin (26), and Wyoming (23).
Other livestock and poultry do not commonly show any illness if infected with WNV, including cattle. Infection studies in dogs suggest that they are susceptible to infection, but do not develop clinical signs of disease. The consequences of WNV infection in cats has not been previously studied, but WNV was isolated from a cat with neurological signs, suggesting that cats can develop clinical signs and neurological disease following infection.
Symptoms of WNV Infection
Most people who are infected with WNV will not have any type of illness or symptoms. It is estimated that 20 percent of people who become infected will develop West Nile fever: mild symptoms including fever, headache, and body aches, occasionally with a skin rash on the trunk of the body, and swollen lymph glands.
The symptoms of severe infection (West Nile encephalitis, meningitis, and meningoencephalitis) include headache, high fever, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, and paralysis. It is estimated that one in 150 persons infected with WNV (less than one percent) will develop a more severe form of the disease, which has an incubation period in people of three to 14 days.
WNV infection in horses may include both central nervous system and peripheral nervous system signs. These signs may be indistinguishable from those produced by other equine encephalitides including rabies, equine herpesvirus-1, equine protozoal myeloencephalitis, and eastern, western, or Venezuelan equine encephalomyelitis. The most common signs of WNV infection in horses have been ataxia, weakness of limbs, recumbency, muscle fasciculation, and death. Fever has been detected in less than one-quarter of all confirmed cases.
No evidence of person-to-person or animal-to-human transmission has been found; only an infected mosquito can transmit the virus.
Preventing WNV
Exposure to WNV through a mosquito bite is the principal means of acquiring infection. CDC continues to urge people to take steps to protect themselves from being bitten by mosquitoes. These steps include eliminating standing water from around the home where mosquitoes are likely to breed, wearing long sleeves and pants, and wearing insect repellents that contain deet.
One key to preventing or controlling future outbreaks of WNV among horses is to control mosquito populations and to prevent horses from being exposed to any adult mosquitoes that may be present. Similar recommendations would apply for other livestock or poultry should illness due to WNV in those types of animals ever come to be recognized.
In addition to the mosquito-related prevention measures, there is an additional action that can be taken to help prevent illness in horses caused by WNV infection: vaccination. On August 1, 2001, the USDA/Animal and Plant Health Inspection Services’ (APHIS’) Center for Veterinary Biologics issued a conditional license 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. The manufacturer of the vaccine recommends giving two intramuscular doses of one milliliter each, three to six weeks apart, followed by an annual booster. The booster should be given just prior to the start of the mosquito season in the area.
More information on WNV can be obtained from the USDA/APHIS Web site at www.aphis.usda.gov, or the CDC Web site at www.cdc.gov.
October 2002 Render