What is West Nile virus?

Causes and Symptoms

For most of the twentieth century following its discovery in Uganda, West Nile virus, an organism that can cause severe inflammation of the spinal cord and brain, was known only in Africa, southeast Europe, and southwest Asia. As the century came to a close, however, the potentially fatal disease emerged in the Western Hemisphere, in the New York City metropolitan area. Since then, it has spread across the continental United States, seven Canadian provinces, Mexico, the Caribbean, and parts of Central America. One of the hallmarks of the disease is dead birds, such as crows, who are also susceptible to the virus.

The first North American cases of the mosquito-borne West Nile virus were found in patients from New York in 1999, and its resurgence in following summers indicates that this disease has become established. The virus is a member of the Japanese encephalitis complex most closely related to St. Louis encephalitis, which also occurs in North America. This group of encephalitis viruses includes the Japanese, Murray Valley, and Stratford complex of viruses. The West Nile virus is also called West Nile encephalitis or West Nile meningitis.

The West Nile virus is called an arbovirus, which is an abbreviation for “arthropod-borne virus.” Mosquitoes are the only known agents of transmission of the virus. Other blood-feeding animals, such as ticks and flies, may carry the virus, but so far none of them has been shown to transmit the viral disease to humans. The disease cannot be transmitted from one person to another or between animals and humans other than through a vector mosquito.

A potential transmission cycle begins when a mosquito becomes infected with the disease by feeding on an infected animal, usually a bird or mammal. The virus enters the mosquito’s bloodstream and is eventually carried to its salivary glands. The virus incubates within the salivary glands for an unknown period of time. During the mosquito’s next blood meal, some of the saliva containing the virus may be injected. Once inside the individual, the virus multiplies and is carried via the bloodstream to all the tissues of the body. In about 1 in 150 individuals, some of the virus makes its way past the blood-brain barrier and into the brain. In humans, the pathology of the West Nile virus disease manifests as a type of encephalitis, a swelling of the meninges, or tissue linings, of the brain.

The first symptoms of infection usually appear within three to fifteen days following exposure, and they may include fever, headache, general body aches, swollen lymph glands, and sometimes a skin rash. In more serious cases, symptoms such as high fever, stiffness, disorientation, stupor, coma, tremors, convulsions, and muscle weakness and paralysis may progressively occur.

Treatment and Therapy

Severely infected individuals are hospitalized and treated with a battery of life-support therapies, including intravenous fluids, ventilation to aid breathing, and inoculations to prevent the occurrence of pneumonia and other secondary infections. In many cases, the symptoms may last several weeks. In severe cases, brain damage and other neurological disorders may be permanent. Fatalities result from swelling of the brain and associated neurological pathologies. At present, there is no treatment for the disease.

Perspective and Prospects

West Nile virus is a classic emerging disease. It was first isolated in 1937 in the West Nile Valley of Uganda in humans and animals, notably horses and birds. The virus quickly spread throughout much of interior Africa, following the course of the Nile River southward into Uganda and neighboring countries, then westward into the Republic of the Congo, then still farther south into Botswana and Zimbabwe. Spreading northward along the Nile River, the virus reached Egypt in 1950, then spread across the Middle East, Eurasia, and Oceania. By 1997, the virus was documented in Italy, France, and Romania. The first cases of West Nile virus in North America were reported in the early summer of 1999 in New York. Twelve states reported cases in 2001, and all states but Alaska and Hawaii had reported the disease by 2005. The numbers of both infections and fatalities in the United States decreased between 2008 and 2011. However, in 2012, there were over 5,600 reported cases of West Nile in the United States, including over 280 deaths—the deadliest year on record for this disease in the United States. After this spike, the numbers subsequently declined again, with around 2,500 cases in 2013 and around 2,100 in 2014. The number of reported cases in Canada in 2003 reached about 1,400; by 2005, only 200 persons were diagnosed. In 2012, approximately 450 cases of West Nile were reported in Canada. Canadian cases of the disease also declined after 2012, with the number reaching twenty-one in 2014.

The transmission and fatality rates in humans are actually very low, for several reasons. First, the incidence of infected mosquitoes in an area is typically small, generally on the order of about 1 percent or less, even in regions that have had a long history of West Nile virus. Second, acquiring the virus from the bite of an infected mosquito is not certain, as particles of the virus may or may not be transmitted into the human bloodstream during the blood meal. Finally, small amounts of virus may not produce the “full-blown” disease in humans. The Centers for Disease Control and Prevention (CDC) suggest that about 1 in 150 people bitten by an infected mosquito becomes severely ill with the disease. Healthy people may simply fight off the viral infection without manifesting any symptoms at all. Others experience a variety of generally mild, flulike symptoms, which may include malaise, aching bones, and headaches.

CDC estimates of fatality rates are uncertain but suggest that one in one thousand infected people actually dies from the disease, but some estimates place the fatality rate at 3 to 15 percent. As with many viral diseases, the effect of West Nile virus increases with age; the young and healthy seem to develop very few symptoms and almost no residual traces of the disease, but people fifty and older or individuals with compromised immune systems are most at risk. Individuals who survive exposure apparently develop a long-term immunity to the virus.

At present, the only way to prevent becoming infected with West Nile virus is to avoid being outdoors in areas where mosquitoes are plentiful or, alternately, to confine outdoor activities to midday hours, when mosquitoes are least active. When outdoors, one should use a recommended mosquito repellant and wear long pants and a long-sleeved shirt.

Among mammals, horses seem most susceptible to the West Nile virus, but the virus has also been documented in dogs, cats, rabbits, skunks, bats, chipmunks, mice, and squirrels. At least seventy species of birds have been found with the disease, of which sparrows, ducks, pigeons, starlings, and crows are the best-known examples. Of these animals, crows seem most at risk, or at least seem to show the highest incidence of infection and mortality, but this may reflect their size and coloration, both factors making them easily seen. Despite public concern over the incidence of West Nile virus in birds and other mammals, tests have shown that the virus cannot be transmitted simply by handling dead birds. There are no records of animal-to-animal or animal-to-human transmission of West Nile virus. However, it is always advisable to wear gloves and avoid the animal’s body fluids when handling animals found dead or known to be infected with West Nile virus.

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