Another way that a disease agent can be introduced is by introduction of an infected vector. Vectors may be either biological vectors that are persistently infected and allow the pathogen to develop and reproduce, or mechanical vectors on which the pathogen resides for a short period of time. Because they are persistently infected, biological vectors are more likely to introduce exotic disease agents of animals to new areas than are mechanical vectors. An example of a disease agent suspected to be introduced to a new area by a biological vector is West Nile Virus introduction into the northeastern U.S. by a mosquito vector.

West Nile Virus (WNV) causes encephalitis in humans, horses, and birds. The disease is present in Africa, Europe, the Middle East, west and central Asia, and Oceania, but was only recently introduced to the U.S. Mosquito vectors spread WNV; in the U.S., both Culex spp. and Aedes spp. are capable of transmitting the virus. As with the recent FMD outbreaks, officials are asking how WNV was introduced into the U.S. Scientists with the Centers for Disease Control and Prevention suspect that the virus was present in the U.S. by at least early summer 1999. The virus’ point of origination is unclear but isolates from the U.S. resemble most closely virus strains from the Middle East. A possible scenario for introduction is that an infected mosquito vector from an endemic area traveled to North America on an airplane and was inadvertently released to introduce the virus into native bird populations.

Eggs and a larva of Culex spp., a mosquito vector of West Nile virus.

Various species of birds can serve as reservoirs of WNV. To date, over 70 species of birds, predominately corvids, have tested positive for WNV since the disease was first recognized. Birds with severe infections suffer high morbidity and mortality rates, but typically develop life-long immunity after exposure and a short viremia. Mosquitoes acquire the virus when they feed on the infected reservoir birds. Alarmingly, some adult Culex spp. in the northeastern U.S. survive the winter and thus are able to overwinter the virus. Many mammals, including humans and horses, are incidental hosts that become infected when fed upon by an infected mosquito. Although mammals do not develop levels of viremia sufficient to infect mosquitoes, and thus cannot serve as reservoirs, infections in mammals may result in severe, potentially fatal meningoencephalitis.

Visit the National Atlas of the United States to view distribution maps of WNV detection in humans, mosquito, wild birds, and other animals at: http://nationalatlas.gov/virususa.html