West Nile Virus Emerging Infectious

West Nile Virus

Emerging Infectious Disease: West Nile Virus

The West Nile Virus (WNV) as in an infectious disease that has been historically determined as originating from the West Nile region of Uganda in 1937. The WNV, usually found in tropical and temperate regions, is characterized to cause non-neuroinvasive diseases such as asymptomatic infection, but could lead to serious neuroinvasive disease -- meningitis or encephalitis. The Center for Disease Control (CDC) traces the history of WNV for as early as 1937 in Uganda; in the 1950s, a virus characteristic of WNV was found in Egypt and Israel. From 1994 to 2003, WNV encephalitis in humans has occurred from Africa to Eastern Europe, and eventually, to United States (North America). Aside from humans, records of horses and birds dying from WNV encephalitis have been documented from 1996 to 2002 in Africa, Europe (France and Italy), and United States.

The West Nile virus belongs to the family Flaviviridae, and is a member of the Japanese encephalitis virus. Historically, WNV is known to infect birds, but cases of infection among other animals such as horses, cats and dogs have also occurred. Human infection of WNV is through a mosquito bite. CDC identifies this primary mode of transmission as resulting from mosquitoes feeding on birds infected with WNV. WNV symptoms can manifest differently, particularly in humans. At the very least, asymptomatic infection or non-neuroinvasive disease is manifested in humans infected with WNV. However, the most serious manifestation of WNV in humans is the neuroinvasive disease, leading to fatal meningitis or encephalitis, characterized by the following symptoms: fever and headache, general fatigue, enlarged nymph nodes, profound muscular weakness, and neurological signs such as "perivascular hemorrhage, dilation of the ventriculi of the brain, dislocation of the brain trunk, hydropericarditis and acute hemorrhagic pancreatitis" (Bourne, 2011). Incubation of WNV ranges from three to six days; similarly, recovery period could be within 3 to 6 days, although full recovery could extend up to 2 weeks.

Transmission of WNV infection, as mentioned earlier, mainly comes from mosquitoes feeding on infected birds. Human infection results from bites from WNV-infected mosquitoes. However, there have been reported cases in the CDC wherein transplanted organs, blood transfusion, mother-to-child (transplacental) and laboratory worker exposure have become alternative modes of transmission of WNV. Apart from the bite of an infected mosquito, humans could get infected with WNV when bare-handed contact with dead, infected animals occurs. However, infection among humans and animals (horses, birds, cats and dogs) are known to have not developed into other infectious diseases, thus lowering their susceptibility as potential hosts to WNV-caused diseases. WNV transmission "peaks" during rainy season in tropical and temperate regions, while WNV outbreaks or epidemics could occur during the summer months in the same regions.

Worldwide, the distribution pattern of WNV is mainly found in the northern, eastern and southern regions of Africa, parts of Eastern Europe, Central Asia, and South Asia. On a global scale, mortality rate of diseases caused by WNV human infection could range from 2.4% to as high as 47% (Bourne, 2011). In the United States, CDC reports its latest (2011) data showing that there have been a total of 432 WNV infections reported as of October 2011. Sixty-seven percent of this reported human infections are neuroinvasive cases (encephalitis / meningitis), while about 5% resulted to death. Across states, California has the most number of cases of WNV human infection at 87 cases (20%), followed by Mississippi as far second (46 cases, or 11% of total reported cases). The prevalence of WNV human infection in these states reflects the virus' characteristic as thriving in tropical / temperate regions. CDC has not published any reported case of WNV human infection in Oklahoma as of October 2011, indicating that among the U.S. states, the distribution pattern of the virus and diseases resulting from its infection are potentially not prevalent in the state at all (taking into account the fact that among U.S. states, only Hawaii, Alaska and Oregon do not report "WNV human, bird, veterinary or mosquito activity" since 1999 (Mostashari et. al., 1999).