| Insect Vectors |
Insects provide a system that can deliver pathogens directly to the bloodstream and are essential to the spread of some infections. From a pathogen's perspective, moving from host to host is essential to survival; yet, the skin presents a barrier. Wounds, burns, and catheters provide opportunities for entry for some pathogens, but insect-borne bacteria have an advantage. Still, arthropod-transmitted microbes must be able to survive in the arthropod's gut, proliferate, and then become positioned (such as in the insect's salivary gland) for delivery to the animal host.
Members of the protozoal species Plasmodium, which cause malaria, have evolved a successful relationship with their arthropod vector, the Anopheles mosquito. Malaria is prevalent in areas where this mosquito thrives - in parts of Africa, Asia, and China. Three million people die every year from Plasmodium infections. Between 1950 and 1970, efforts to eradicate malaria involved the use of the insecticide DDT. Unfortunately, mosquitoes developed resistance to the spray. Now considered a reemerging disease, malaria incidence is on the rise as eradication programs failed and drug-resistant strains of the parasite have evolved. The complex life cycle of the parasite makes development of vaccines difficult, and efforts to reduce malaria by controlling its insect vector continue (Fig. 5).
Approximately eighty viruses depend on insects for transmission. The virus that causes dengue and dengue hemorrhagic fever has the broadest distribution, comparable to that of malaria. Approximately 2.5 billion people live in areas at risk for dengue, and millions are afflicted each year. The fatality rate is about five percent, with most fatalities occurring in children and young adults. Transmitted by the mosquito Aedes aegypti, dengue, or "breakbone fever," causes a range of symptoms: nausea and weakness, severe bone and joint pain, and high fever.
Four immunologically distinct types of the virus exist, so individuals can contract the disease four times during their lifetime. An infection with a second subtype of the virus may result in a severe hemorrhagic disease, involving leakage of blood or fluid from mucous membranes. The hemorrhage seems to involve an immune reaction, resulting from sensitization in a previous infection. A global pandemic of dengue began in southeast Asia after World War II. In the 1980s dengue hemorrhagic fever began a second expansion into Asia, with epidemics in Sri Lanka, India, the Maldive Islands and, in 1994, Pakistan. During the 1980s epidemic dengue arose in China, Taiwan, and Africa. Aedes aegypti and an alternate mosquito vector, Aedes albopictus, are present in the United States. Two outbreaks of dengue were reported in Texas during the 1980s, which were associated with epidemics in northern Mexico.
The dramatic global emergence of dengue relates in part to the lack of effective mosquito control in afflicted countries. Often, deteriorating public health infrastructures are to blame.