Vector (Mosquito) Control

The largest group of animals on the face of the planet, the arthropods (meaning "jointed feet"), includes a very highly successful class of organisms, Insecta, the insects. It is estimated that insect species outnumber all other known species of animals and plants on earth combined. One kind of insect, the mosquito, a two-winged insect of the family Culicidae, has particular importance with respect to the activities of humankind. Mosquitoes are a type of fly that have adaptive traits that make them very successful, and very devastating pests. Most importantly, female mosquitoes of key species must consume blood from vertebrates to provide nourishment for egg production. Female mosquitoes will ingest blood-meals from amphibians, birds, or mammals. In the process of consuming blood to provide the protein necessary for egg development, the mosquito becomes an important, and dangerous, transmitter of disease. When an organism is capable of transmitting a disease, it is said to be a vector of that disease. Mosquitoes are perhaps the most important arthropod vectors of human disease in the world.

Mosquitoes of various species can transmit viral, protozoan, or bacterial organisms that cause disease in human beings. Viruses (complex constructions of protein and DNA or RNA) are, by definition, dependent upon living cells for replication and transmission. Some protozoans, single celled organisms possessing nuclei, have become dependent upon other organisms for their survival. Likewise, some bacteria, single-celled organisms without nuclei, cannot survive outside of a particular host organism. Mosquitoes act as vectors, or vehicles for such disease-causing organisms to jump from host to host to complete life-cycles in order to continue survival.

Mosquito vectors transmit viral strains and protozoal parasites that cause immense human suffering worldwide. Globally relevant viral diseases transmitted by mosquito vectors include Dengue fever and Yellow fever. Recently, in the United States, West Nile virus , Eastern Equine Encephalitis (EEE), La Crosse Encephalitis, and St. Louis Encephalitis are important arthropod-borne viruses, or "arboviruses." The largest impact that mosquito vector species have on humans is in the transmission of malaria , a disease caused by protozoan infection of red blood cells that results in massive, cyclic bouts of very high fever and in many cases, death. Vector control, or the management of disease-spreading species of mosquitoes, is a large concern among global communities. Once thought to be a historic problem, renewed interest has emerged concerning vector-borne disease as a potential threat to contemporary and future civilization.

Global mosquito control consumes tremendous financial resources and is costly in terms of time, effort, and environmental impact. Different parts of the world attempt vector control by various methods, but the most effective way of controlling mosquitoes is a combination of scientific knowledge and common sense. Integrated pest management combines the use of biological or chemical products, knowledge of the mosquito's habitat , and information about the mosquito's life cycle to control the potentially dangerous pests. Larviciding is a successful component of mosquito control. Larviciding uses an insecticide to spray areas of stagnant where mosquitoes lay their eggs and the larvae grows. Adulticiding is ground spraying for adult mosquitoes and is applied by spraying a fog of insecticide around the premises where they live. Common sense methods of mosquito control are emptying dishes or birdbaths where water collects, keeping grass mowed and bushes trimmed, and filling in low-lying areas so that water is not left to stagnate there.

Even with global vector control, malaria is reemerging in most countries where the disease is endemic. Malaria is caused by any of four species of protozoan parasites. Plasmodium falciuparum and Plasmodium vivax are the most deadly. The parasite is transmitted by a particular genus of mosquito, Anopheles. For many years, malaria was a problem in the United States. In the 1940s, a massive malaria vector control program was initiated that used immense quantities of the potent pesticide , DDT, to kill the mosquitoes that transmit malaria. DDT is an effective agent in the control of mosquito vectors of disease, but two problems have plagued its use. The first is its persistence in the environment and harmful effect on fish and wildlife and its implication as a mammalian neurotoxin that could potentially affect human beings. The second concern is the emergence of DDT-resistant strains of mosquitoes. Despite this, DDT is still used in many underdeveloped areas of the world to control mosquito infestations that pose a threat to human health and wellbeing. In the United States, the Centers for Disease Control (CDC) monitors and surveys for malaria and other vector-borne diseases that endanger the population. The CDC also advises U.S. citizens on appropriate drug therapies to utilize while visiting countries where disease-bearing vector species of mosquitoes are prevalent. In the year 2000, 246 cases of imported malaria were reported to the CDC.

One effort to control vector spread of malaria has been the Roll Back Malaria initiative which seeks to reduce the world's malaria burden by one half by the year 2010. One aspect has been the distribution of insecticide-treated mosquito nets in areas where malaria vectors are especially problematic for pregnant women and children, such as the country Nairobi. As part of the effort, 60,000 insecticide-treated mosquito nets were distributed in 2002 in cooperation with the United Nations Children's Fund. This form of vector control incorporates physical barriers with chemical insecticides and social education programs to effect change in the prevalence of this devastating disease.

Mosquito control, however, involves many diseases other than malaria, however. Dengue fever is a viral infection transmitted by mosquito vectors that causes paroxysmal joint and muscle pain, headache, vomiting, and rash. Commonly, because of its symptoms, Dengue fever is confused with malaria or typhoid fever, a bacterial infection. Dengue fever is a potential threat in Africa, China, Southeast Asia, the Middle East, Central and South America and the Caribbean. A historically notorious arbovirus, Dengue fever is returning. According to the CDC Division of Vector-Borne Infectious Disease, 1997 was the year in which Dengue fever was the most important mosquito-borne viral disease affecting humans. At that time, an estimated 2.5 billion people lived in areas of risk, making Dengue fever a mosquito (vector) control problem en par with malaria. Four major reasons for the emergence of Dengue fever as an important mosquito-borne disease have been identified. First, sufficient mosquito control efforts are practically absent in areas where the risk of contracting Dengue fever is most acute. Second, uncontrolled population expansion in such areas has led to a deficit in effective sewer and waste management systems, the result of which only contributes o the spread of vector-borne disease. Demographic changes leading to inadequate water management systems have exacerbated this problem. Thirdly, accessible and rapid transportation via airplanes had created a new, swift means for spread of the Dengue vector mosquito. Stowaway infected mosquitoes spread disease to new areas with newfound speed. Finally, limited financial resources have forced nations challenged by such vector-borne disease to choose between effective vector control and other, greatly needed endeavors. Until and unless Dengue and other vector-borne diseases are recognized as threats, funding for vector control programs may remain secondary to more pressing social dilemmas in undeveloped nations. While no Dengue vaccine exists, vigorous research employing biotechnology continues.

More recently, mosquito-borne illnesses have been witnessed in North America. West Nile virus has surfaced in America, formerly only attributed to central Asia, Europe, Africa, and the Middle East. By 2001, the West Nile virus had been documented by the CDC to be present in Alabama, Arkansas, Connecticut, Delaware, Florida, Georgia, Illinois, Indiana, Iowa, Kentucky, New York, Ohio, Pennsylvania, Wisconsin, and other mid-Atlantic states. As of 2001, 149 cases of West Nile virus illnesses had been reported to the CDC, which includes 18 deaths. The West Nile virus is a variety of pathogen called a flavovirus that can cause encephalitis, coma, and death.

Two additional arbovirus pathogens transmitted by mosquito vectors in the United States are LaCrosse Encephalitis and Eastern Equine Encephalitis. The LaCrosse virus belongs top the Bunyviridae subgroup and has approximately 70 cases of infection in the United States annually. While fatality rate of infection with LaCrosse is less than 1%, hospitalization and neurological complications that occur with infection are serious. Children under the age of 16 years are particularly at risk of contracting LaCrosse Encephalitis in endemic areas. A similar vector-borne disease is Eastern Equine Encephalitis (EEE). The manifestations of EEE range from influenza-like symptoms to severe swelling of central nervous tissue, coma, and death. The etiologic agent for EEE is a family of mosquito-borne viruses called the Togaviridae. Since 1964, there have been approximately 153 confirmed cases of EEE in the United States.

Vector mosquito control in the United States remains a priority in many urban areas. Mosquito control is most often accomplished by chemical insecticide spraying of select areas where known vector breeding areas exist. This approach is viewed by some experts to be reactive rather than proactive. Still, potent chemical means of mosquito control remain the most cost-effective alternative for vector disease transmission in this country and abroad. However, much research is underway to find alternative methods of controlling dangerous mosquito populations. For example, now standard biotechnological techniques such as gene cloning and transposon insertion are presenting new and exciting ways of genetically altering mosquito vectors to become immune to the diseasecausing microorganisms , thus rendering them mere pests, and not potential transmitters of life-threatening disease.

[Terry Watkins ]