Infectious Diseases - West Nile Virus

West Nile virus (WNV) is common in Africa, West Asia, and the Middle East, and it can infect birds, mosquitoes, horses, humans, and other mammals. It is spread by bites from infected mosquitoes, and although most people who become infected have few or no symptoms, some develop serious and even fatal illnesses. The virus first was reported in the United States in 1999, and the CDC has tracked its westward spread across the United States. During 2003 and 2004 WNV caused approximately 12,400 cases of human illness, including more than 360 deaths in the United States (http://www.cdc.gov/ncidod/dvbid/westnile/qa/cases.htm).

According to the CDC, the presence of West Nile virus in either humans or infected mosquitoes is permanently established in the United States. Although human illness from the virus is relatively rare, the disease, which is more likely to be fatal in elderly people and young children, was responsible for the death of 119 people in 2005 (http://www.cdc.gov/ncidod/dvbid/westnile/surv& controlCaseCount05_detailed.htm). Figure 7.4 shows the distribution of human WNV cases by state as well as infection of birds, animals, or mosquitoes.

The CDC advises taking precautions against mosquito bites, such as using insect repellent; wearing long pants and long-sleeved shirts treated with insect repellents; remaining indoors during dawn, dusk, and early evening, the hours when mosquitoes are most likely to bite; and removing standing water to prevent mosquitoes from laying eggs and breeding near homes and other populated buildings.

In 2006 there were at least two promising vaccine candidates against WNV. One, based on a yellow fever vaccine virus that contains two WNV genes, was being evaluated in human clinical trials. A second vaccine developed at NIH uses a virus into which WNV genes have been inserted. This vaccine protects monkeys and horses against WNV infection, and a clinical trial was underway. Several novel therapies also were being tested to treat persons already infected with WNV. The protective effect of an immunoglobulin product was being tested in hospitalized patients who have WNV encephalitis (viral infection of the brain). Investigators also were evaluating many naturally occurring and laboratory-made compounds to find out if they can combat WNV. The CDC reports in Emerging Infectious Diseases that as of February 2005 fifteen hundred compounds had been screened for antiviral action against WNV, and 2-3% were shown to have antiviral activity against WNV. Other research is focusing on the roles of mosquitoes FIGURE 7.4 West Nile Virus activity, 2005 "2005 West Nile Virus Activity in the United States (Reported to CDC as of January 10, 2006)," in West Nile Virus Statistics, Surveillance, and Control, Centers for Disease Control and Prevention, Division of Vector-Borne Infectious Diseases, 2006, http://www.cdc.gov/ncidod/dvbid/westnile/surv&control06Maps.htm (accessed January 17, 2006)and animals in virus transmission and testing new mosquito control methods.