West Nile virus

Transmission and Spread of West Nile virus

West Nile virus was first isolated in Uganda in 1937. Since then, there have been numerous epidemics of the virus in Israel (1950s),France (1962), South Africa (1974), and Romania (1996). Most recently, there were two epidemic outbreaks in 1999: One in Russia, and the second in the New York City area (NY99);the first time that the virus has appeared in the Western hemisphere. In NYC, 62 people were infected and six died from the disease. The lineage of the NY99 strain of the WNV is though to be related to the strains found recently in North Africa, Romania, Kenya, Italy, and the Middle East.

It is not yet known how the virus was introduced to the US, though it has been hypothesized that the cases found in the US can be attributed to the strain of the WNV that had been spreading in the Mediterranean region since 1998. Since the initial outbreak in 1999, there have been 23,975 reported human cases by the CDC of the virus and 962 deaths around the world through 2006.

The virus is primarily transmitted to humans by the Culex pipiens and Culex. quinquefasciatus species of mosquitoes . Birds serve as a natural reservoir, with humans, horses and other mammals acting as incidental, dead-end hosts. Although primarily transmitted to humans by mosquitoes, the virus can be transmitted from human to human by blood transfusion, organ tranplantation, and by pregnant women may pass the infection to their child via intrauterine delivery. West Nile virus infects at least 300 bird species, 60 mosquito species, and 30 animal species. After research was conducted on the species of mosquitoes that carry the virus, scientists claimed that the rapid spread may be due to the hybridization of the Culex pipiens species of mosquitoes. It is believed by some researchers that the mosquitoes in Europe feast on either humans or animals, but not both, thereby limiting the spread of the virus. They hypothesis that the hybridization of US mosquitoes has led to their feasting of both humans and animals, and allows for the spread of the virus from species to species.

Clinical symptoms track closely with the particular strain of the West Nile fever infection and two major lineages have been described. Lineage II strains are found primarily in Africa and Madagascar while lineage I strains are widely distributed across North America, Europe and Africa. Virus strains found in North America are particularly neuroinvasive. At present, no vaccines for humans or antiviral agents exist for WNV and its spread can, therefore, not be stopped, but only slowed. However, there are vaccines available for horses and birds. For now, recovery from the virus is dependant on the body’s own immune response and production of IgM antibodies.