Mississippi leads the nation in West Nile Virus cases Published July 10, 2007 By Capt. Michael Cuomo 14th Medical Operations Squadron COLUMBUS AIR FORCE BASE, Miss. -- Mississippi is currently leading the nation in cases of West Nile virus and has suffered one fatality. However, none of the cases have been in Lowndes County or any of the surrounding Counties. West Nile virus, known since the late 1930s, is widely distributed throughout the world. It has been found in parts of Africa, the Middle East, Europe, Western Asia, and North and South America. It was not until this century that epidemics began to occur more frequently with subsequent increases in human fatality rates similar to rates observed when the virus was introduced into North America. As of September 2004, West Nile virus activity had been detected in all of the contiguous 48 states. Infection with WNV may cause mild flu-like symptoms or no symptoms at all in humans. Clinical disease can progress to a lethal inflammation of the brain and spinal cord. The incubation period of WNV in humans is typically three to 14 days. According to the Centers for Disease Control and Prevention, an estimated 20 percent of people infected will develop symptoms. The severe form of West Nile virus occurs in one in 150 people showing symptoms. Currently no specific treatment exists for WNV infection in humans other than supportive therapy. Two antiviral drugs administered in high doses have shown some activity in vitro, but controlled case studies have not been completed. Early stage clinical trails are in progress to assess safety and effectiveness of treating patients with immunoglobulin G antibodies. Development of a protective vaccine for humans is underway and is not yet available. It is currently thought that West Nile virus is maintained primarily in the bird population by mosquitoes. Birds are the natural host for the virus, although viral infection has been fatal to many native and exotic bird species in the United States. The most important birds involved in virus amplification have not been identified, although recent studies have demonstrated that perching birds, certain gulls, and two birds of prey species had a greater ability to pass on the virus to a mosquito host than other bird groups tested. At least 33 known mosquito species occur on Columbus AFB, and at least 19 of these species have tested positive for WNV infection by virus isolation, detection of viral nucleic acids or detection of viral antigen. One must be careful not to interpret this as evidence that all of these mosquitoes are involved in virus transmission. It simply means that these mosquitoes have fed upon an infected host at least once. Some mosquitoes can be infected with the virus but are not capable of transmitting it to humans. Several criteria must be met before incriminating a mosquito species as a vector. These established standards involve careful scientific scrutiny. Conditions for incrimination of a vector species can be summarized into four broad categories: 1) there must be repeated virus isolations from field-caught mosquitoes; 2) the alleged mosquito species must be susceptible to virus infection in a carefully controlled laboratory setting and be able to transmit the virus to a susceptible host; 3) the mosquito species must feed on vulnerable hosts in nature; and 4) an association between mosquito activity and virus transmission must exist in nature. Based on the number of positive field-collected mosquito pools and our limited knowledge of mosquito host feeding preferences, a few mosquito species appear to be more important for virus maintenance in birds, while others are more likely to be involved in transmission of virus to humans, horses and other animals as carrying hosts. Many suspected vectors are species that breed in discarded tires and similar habitats. Of the suspected mosquito vectors the following species occur on Columbus AFB and are known to breed in artificial containers: Culex p. pipiens, Culex restuans, Culex salinarius, Ochlerotatus triseriatus, and Aedes albopictus. Culex p. pipiens prefers to feed on birds. Laboratory studies have demonstrated that this species is a moderately efficient vector of West Nile virus. Based on our knowledge of Culex p. pipiens bird-feeding habits, numerous field isolates, and laboratory demonstration of vector competence, this species is likely to be the most important vector in maintaining West Nile virus in birds in a peridomestic habitat. Culex restuans also shows potential for being involved in virus transmission between birds. Ochlerotatus triseriatus, Aedes albopictus, and Culex salinarius are opportunistic feeders and have been shown to be competent vectors in the laboratory. Additional field and laboratory studies are essential to clarify what role these and other mosquito species play in virus transmission to humans, horses, and other animals. Do your part to eliminate mosquito-breeding sites around your home, because many of the suspected vectors are those that utilize natural habitats and artificial containers found around residential areas. Control targeted at the larval stage is best both economically and ecologically because mosquitoes are confined to one space at this stage of their development. It has been shown that heavy rainfalls can at times be detrimental to container-breeding mosquitoes. Heavy and frequent rain can flush out egg rafts, larvae, and pupae from containers and can be detrimental to localized populations in the microhabitat. Mosquitoes are cold-blooded animals, and their development depends upon ambient temperature. Generally, the warmer it is the more quickly the mosquito develops from egg to adult stage. Replication of the virus in the mosquito is also temperature-dependent. Cooler ambient temperatures slow virus replication, while warmer temperatures increase replication. There is no way to visually determine whether a mosquito is infected, thus it is prudent to avoid any bites. Check with the 14th Medical Group Public Health Office on virus activity in this area. Protection from mosquito bites involves making individual choices.