Insect Vectors of Human Disease

Biting flies (insects of the arthropod order Diptera) are the most important of the aquatic vectors of human disease. Of the flies, mosquitoes (the family Culicidae) are the most important vectors because of the high mortality and morbidity of the many diseases they transmit and because of the range of diseases for which they can serve as vectors (Table 1). For example, certain species of mosquitoes can transmit many

Table 1 Examples of human diseases vectored by mosquitoes

Disease vectored Parasite transmitted

Table 1 Examples of human diseases vectored by mosquitoes

Disease vectored Parasite transmitted

Lymphatic filariasis

Brugia and Wucheria

nematodes

Malaria

Plasmodium protozoans

Yellow fever

Flavovirus

Dengue

Flavovirus

St. Louis encephalitis

Flavovirus

Japanese encephalitis

Flavovirus

Murray Valley encephalitis

Flavovirus

Russia spring-summer

Flavovirus

encephalitis

Omsk Hemorrhagic fever

Flavovirus

West Nile fever

Flavovirus

Kyasanur forest disease

Flavovirus

Louping Ill

Flavovirus

California encephalitis

Bunyavirus

La Crosse encephalitis

Bunyavirus

Rift Valley fever

Bunyavirus

Eastern equine encephalitis

Togavirus

Western equine encephalitis

Togavirus

Venezuelan encephalitis

Togavirus

Ross River fever

Togavirus

types of nematodes, protozoans, or viruses. However, it should be remembered that while —3000 described species of mosquitoes take blood for nourishment, most are nuisance biters (and some not even of humans) and do not transmit human diseases.

As with terrestrial insect vectors, disease transmission involves two different blood meals. Only adult females take blood meals, using the blood of their victims to provide protein for their eggs to develop and pass through their larval and pupal stages in the oftentimes nutrient-poor aquatic habitats that they occupy. Using mosquitoes as an example, during the first blood meal a female injects saliva into the feeding wound; the saliva contains substances to reduce blood clotting. Using muscle pumps in her head, she then ingests blood from an infected human. Adult mosquitoes generally cannot undergo ovarian development until they have taken a blood meal (although some, referred to as being autogenous, can do this). Females tend to be attracted to their hosts by the heat, moisture, and carbon dioxide that they emit but certainly other factors (e.g., volatile chemicals emitted through the skin) are involved. Species can be selective in terms of hosts; some choose only birds, while some others chose only mammals. In species where both are selected, disease transmission may be enhanced when one of the hosts can serve as a disease reservoir (e.g., birds in West Nile Fever; dogs, cats, and monkeys in Brugia lymphatic filariasis).

Depending on the disease agent resulting in disease, there are three processes that can occur within the mosquito as the ingested pathogen migrates from her gut to her salivary glands. The pathogen can multiply but not undergo developmental changes (e.g., as in yellow fever), the pathogen can undergo developmental changes but not multiply (e.g., onchocerciasis), or the pathogen can do both (e.g., malaria). In all cases, when a mosquito takes a blood meal from another host, the disease agent may be transmitted through the injection of the anticoagulant saliva.

Other Diptera are important as well in disease transmission to humans and their animals. Black flies (the family Simuliidae) transmit onchocerciasis (river blindness), which is caused by a nematode (roundworm). This filarial worm lives in the human body for over a decade, and adult females produce millions of larvae that migrate throughout the body causing a series of progressive symptoms as the number of parasitic worms increases, including rashes and lesions, intense itching, loss of skin pigmentation, general debilitation, and eventually blindness.

Many species of biting flies that occur in moist or semi-aquatic habitats as larvae or that are common riparian-dwelling species as adults also transmit diseases. For example, some sand flies (Ceratopogoni-dae) can transmit the protozoan causing leishmaniasis, tsetse (Glossinidae) transmit the protozoan causing African sleeping sickness, and some deer flies (Tabani-dae) can transmit Loa loa, the eye worm of humans.

Even biting flies that do not transmit disease can be major nuisances. In North America, horse flies and deer flies (both in the family Tabanidae) and black flies are irritating pests, oftentimes with painful bites that produce allergic reactions and infections that result from scratching the bites. There is also an economic cost associated with them in terms of lost tourist revenue and lower population densities in areas where they are a problem.

Other insects that are not important as direct transmitters of diseases (e.g., aquatic bugs, dragonfly larvae, beetle larvae, along with snails, small fish, and the even biofilm of aquatic plants) may serve as reservoirs of a water-associated disease, Buruli ulcer. Some naucorid bugs have even been shown experimentally to have the ability to transmit this disease.

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