Arthropodborne Diseases

Table II lists the principle groups of insects and arachnids involved in arthropod-borne diseases and the associated types of pathogens. Among the wide variety of arthropods that transmit pathogens to humans and other animals, mosquitoes are the most important, followed by ticks. Viruses and bacteria (including rickettsiae) are the most diverse groups of pathogens transmitted by arthropods, followed by protozoa and filarial nematodes.

All of the viruses listed in Table II are arthropod-borne viruses, usually referred to as arboviruses, indicating that they are typically transmitted by insects or other arthropod hosts. The study of arboviruses is termed ar-bovirology. These and related terms are discussed in more detail in Chapter 2, on the epidemiology of vector-borne diseases.

Pathogens are transmitted by arthropods in two basic ways, either biologically or mechanically. In biological transmission, pathogens undergo development or reproduction in the arthropod host. Examples of diseases that involve biological transmission are malaria, African trypanosomiasis, Chagas disease, leishmaniasis, and lymphatic filariasis. In mechanical transmission, pathogens are transmitted by arthropods via contaminated appendages (usually mouthparts) or regurgitation of an infectious blood meal. Examples of diseases that involve mechanical transmission are equine infectious anemia and myxomatosis. Biological transmission is by far the more common and efficient mechanism for pathogen maintenance and transmission.

A wide range of life-cycle patterns and degrees of host associations is characterized by arthropod vectors. Some ectoparasites, such as sucking lice, remain on their host for life. Others, such as mosquitoes and most biting flies, have a more fleeting association with the host, with some being associated with it only during the brief acts of host

TABLE ri

Examples of Arthropod-Borne Diseases of Medical-Veterinary Importance

Arthropod vectors

Diseases grouped by causative agents

Mosquitoes

Black flies Biting midges

Sand flies

Horse flies and deer flies

Tsetse flies

Triatomine bugs

Lice

Fleas

Ticks

Mites

Viruses: yellow fever, dengue, Rift Valley fever, myxomatosis; eastern equine encephalomyelitis, western equine encephalomyelitis, Venezuelan equine encephalomyelitis, St. Louis encephalitis, LaCrosse encephalitis, Japanese encephalitis, Murray Valley encephalitis, Chikungunya fever, O'nyong nyong fever, Ross River fever, West Nile fever. Protozoans: malaria. Filarial nematodes: Wuchererian filariasis, Bancroftian filariasis, dog heartworm

Filarial nematodes: human onchocerciasis (river blindness), bovine onchocerciasis

Viruses: bluetongue disease, epizootic hemorrhagic disease, African horse sickness, leucocytozoonosis,

Oropouche fever. Filarial nematodes: equine onchocerciasis, mansonellosis Viruses: sand fly fever, vesicular stomatitis. Bacteria: Oroya fever (Veruga Peruana). Protozoans: leishmaniasis Viruses: equine infectious anemia, hog cholera. Bacteria: tularemia. Protozoans: surra (livestock trypanosomiasis). Filarial nematodes: loiasis, elaeophorosis Protozoans: African trypanosomiasis, nagana Protozoans: American trypanosomiasis (Chagas disease)

Viruses: swine pox. Bacteria: epidemic typhus, trench fever, louse-borne relapsing fever Viruses: myxomatosis. Bacteria: plague, murine (endemic) typhus, tularemia

Viruses: tick-borne encephalitis, Powassan encephalitis, Colorado tick fever, Crimean-Congo hemorrhagic fever, African swine fever. Bacteria: Lyme disease, Rocky Mountain spotted fever, Boutonneuse fever, tick-borne ehrlichiosis, Q fever, heartwater fever (cowdriosis), anaplasmosis, tick-borne relapsing fever, avian spirochetosis, theileriosis (East Coast fever), bovine dermatophilosus. Protozoans: babesiosis Bacteria: tsutsugamushi (scrub typhus), rickettsialpox

Note: For more Comprehensive coverage, see the individual chapters devoted to each arthropod group.

location and blood-feeding. Between these two extremes is a wide range of host associations exhibited by different arthropod groups.

Literature references on vector-borne diseases, together with their epidemiology and ecology, are provided under Arthropod-Borne Diseases at the end of this chapter.

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