West Nile Virus
West Nile virus has recently emerged as a public health concern, with heavy media coverage of both increased infections, as well as deaths. Until very recently (1999, to be precise), the disease caused by this virus was virtually unknown in North America: it was thought that this disease was confined to Africa, the Middle East, west and central Asia, Oceania, and Europe. Now, cases have been documented in nearly every state of this country, particularly in late summer and early fall.
West Nile virus is a flavivirus that is carried by mosquitoes. When carrier mosquitoes transmit the virus to birds via a "blood meal," the virus multiplies. Sometimes this viremia kills the host bird, but more often the bird lives. This actually serves the virus' life cycle better, since the bird host, when bitten by subsequent mosquitoes, can then pass the virus back to the mosquitoes, which then go on to infect other birds, and so on. While mosquitoes can also infect horses, dogs, cats, and humans, they are not ideal hosts, since high-level viremia does not usually result in these animals.
This, of course, does not mean that people do not get sick from the West Nile virus. Although most people who are infected (by mosquitoes) do not even manifest significant symptoms, 20% do develop mild disease, called "West Nile fever." This is characterized as a febrile illness of sudden onset, often accompanied by malaise, anorexia, headaches, myalgia, nausea, vomiting, rash, eye pain, and lymphadenopathy. West Nile fever typically lasts only a few days, with no permanent sequelae.
However, approximately 1 in 150 infections is more severe, resulting in encephalitis or meningitis. Those most at risk for encephalitis or meningitis are individuals above 50 years in age. West Nile encephalitis/meningitis is often accompanied by fever, weakness, gastrointestinal symptoms, and mental status changes; some patients also demonstrate rashes, muscle weakness, paralysis, seizures, and a variety of other neurological symptoms.
Diagnosis requires a high level of clinical suspicion and clinical correlation. Helpful hints to the correct diagnosis include the local presence of West Nile virus activity, otherwise unexplained encephalitis or meningitis in adults older than 50 years in summer or early fall, and a suspicious travel history. Of course, none of these guidelines is absolute, since patients of all ages have been infected in all seasons.
Laboratory tests can help point clinicians in the right direction, as well as clinch the diagnosis. Lab abnormalities include elevated leukocyte counts with lymphocytopenia and anemia, hyponatremia, CSF pleocytosis with a predominance of lymphocytes, elevated CSF protein, and normal CSF glucose. These are non-specific findings, so an ELISA test directed against IgM antibody in serum or cerebral spinal fluid serves as confirmation. IgM antibody to the West Nile virus is approximately 90-95% sensitive in serum or CSF early in the infection. However, false positive results can occur due to cross-reactions with other viruses (such as those that cause yellow fever, dengue and Japanese encephalitis). IgM titers can remain elevated for as long as six months following the acute illness.
Treatment is supportive, including IV fluids, respiratory support, and prevention of secondary infections.
Preventive measures include use of DEET-containing insect repellant, either on bare skin or clothes worn over the skin; staying indoors during peak mosquito biting hours; and eliminating standing water sources (prevents the laying of mosquito eggs).
It is important to remember that 1) the chance of being bitten by an infected mosquito is low, 2) the chance of coming down with mild disease is lower, and 3) the chance of being stricken by severe disease is sharply lower, still.
David Cheng, M.D.
Staff Pathologist