In reviewing some of my medical journals, I found that subjects relating to the mosquito-borne West Nile virus (WNV) and other emerging diseases are cropping up more and more frequently. Although WNV appeared in the US only a little more than four years ago, clinicians are now being alerted to the possibility of having to diagnose this viral infection in their patients.

The virus was first identified in Uganda during the late 1930s and was later noted in epidemic proportions in Eastern Europe, circa the 1990s. It was not noted in America, however, until the heat wave and consequent drought of 1999. By 2002, again during hot, dry summer conditions, it had spread from the wetlands of New York across the US to 44 states, to the District of Columbia, and to five Canadian provinces. On October 9, 2003, the state health director announced the first probable human case of WNV in California.

During 2003, the total number of human cases exceeded 6,500, causing hundreds of deaths. Despite these mortality statistics, recovery from the disease is not without its serious consequences.

Additionally, once the virus settles into an area, other routes can spread it. There is concern over transmission through blood products, organ transplants, the placenta in utero, and breast milk.

The virus is capable of jumping species barriers. Currently it has spread to at least 230 species of animals, of which 130 are birds.

Death among wildlife and livestock or pets, including horses, is an area where few accurate statistics are kept; nonetheless, these numbers are presumed large. The virus is carried over large distances by migrating birds infected by Asian tiger or Culex mosquito bites. Once the virus is introduced into an area, local mosquitoes distribute it among the local nonmigrating bird population. These nonmigrating birds are then monitored along with sentinel (marker) hosts, such as caged chickens, to help track the spread. Crows and jays are particularly populous in the urban environment and are highly susceptible to WNV, and locating dead crows or jays is one way to track the disease. However, the virus has been detected in other wild birds, which are fed upon by other mosquito species.

Public health experts indicate that active surveillance of susceptible animal and bird populations is the first line of defense, along with control of standing water bodies that could provide breeding areas for the mosquito. In many ways, we are lucky that smaller jurisdictions have been required to develop stormwater plans, but even if such plans were not requisite, small jurisdictions should be aware that stormwater can supply excellent habitat for the mosquitoes as vectors of disease.

Mosquitoes are strong flyers but also can be moved long distances on winds. Accordingly, no jurisdiction is exempt from the virus. The mated but unfed female, capable of overwintering in a hibernating state, normally lives for about two weeks before being eaten by predators, but the lucky can survive for two to three months. Most mosquitoes, both males and females, are nectar feeders, but the female also needs a protein-rich blood meal for egg development.

For the two main vectors of WNV, Asian tiger and Culex (of which there are at least three important species), the female mosquitoes prefer to lay eggs on aged, stagnant, or putrid water—that is, the type often found at the outlet of stormwater drains. A single pool can generate tens of thousands of mosquitoes, as mosquitoes can produce numerous generations within one season.

The eggs can be washed into other water bodies by rain or merely by yard irrigation or other washings that find street drains. When the eggs hatch after about two weeks, the adult mosquitoes are ready to fly, feed on birds and other intermediate hosts, pick up the virus, and feed on unsuspecting human or livestock hosts. Thus, the cycle is repeated.

WNV has also been found in several species of floodwater mosquitoes, common in meadowlands, woodland pools, floodplains, and marshes. Again, these are areas receiving upland drainage, which often is nothing more than drain and wash water of municipal origin. During droughts or dry seasons, these waters also may be the sole source of drinking water for bird and animal hosts on which the mosquitoes feed. As a result, there is a tight circular multiplication among vector, virus, and animal-bird hosts that amplifies the viral risk to humans of the area.

The Asian tiger mosquito actively feeds throughout the daylight hours. It is also the vector for dengue fever, which in its hemorrhagic form is highly destructive. In contrast, the Culex mosquito bites from dusk to dawn and is also a vector for St. Louis encephalitis.

Because of its particular behavior, the Asian tiger is one of the most difficult mosquitoes to avoid. Indeed, the all-day feeding habits of the Asian tiger mosquito increase the risk of spreading West Nile virus; as the disease becomes established in an area, however, other local mosquito species may, as previously mentioned, become vectors.

Since both the Asian tiger and Culex mosquitoes prefer a stagnant-water habitat, they have a distinct competitive and survival advantage over other species that require cleaner water. Thus, stormwater plans need to examine this issue from several perspectives.

Edward McGowan has a degree in medicine and a doctorate related to water-quality control. He was the US Agency for International Development regional environmental officer for the eastern and southern half of Africa, an area covering 22 nations. In that capacity, he interacted with numerous governments, various United Nations agencies, WHO, US Department of Agriculture, USEPA, international donors, and US Foreign Service staff on issues of water quality and public health.

SW January/February 2004