FLEAS AND DISEASE1

Authors

William L. Jellison, Rocky Mountain Laboratory

Document Type

Article

Date of this Version

1959

Citation

The U.S. Government Workers

Comments

Annual Review of Entomology Vol. 4, 1959

Abstract

In the broad field of disease caused by arthropod-borne agents there are many instances wherein the pathogenic organism appears to be well adapted to the vector and may even pass an essential part of its life cycle therein, as do the malaria parasites of man within the anopheline mosquitoes. The pathogen may be carried from stage to stage or even passed from one genera­tion to another through the egg (transovarial passage). These adaptations of parasite to arthropod vector are thought to result from a long host-parasite association. The arthropod, in such instances, is conveniently referred to as a biologic vector of the pathogen.

In a few instances there is a closer relationship between vector and pathogen in which groups of related pathogens are transmitted by groups of related arthropods. Both pathogens and arthropods show about the same relative degree of diversity and homogeneity. Considerable host-parasite specificity is exhibited by both vector and parasite. An outstanding example of this degree of relationship is the relapsing-fever group 0'£ spirochetes of the genus Borrelia and their tick vectors of the genus Ornithodoros. Both are distributed in all the major faunal regions, in temperate and tropical zones, and occur in definite tick-spirochete combinations. These spirochetes exhibit a high degree of vector specificity, are maintained in part through transovarial passage, and develop in the body cavity of the tick. Only a single species is not carried by ticks, i.e., Borrelia recurrentis (Lebert), the cause of louse-borne relapsing fever of man.

The malarial parasites also fall in this category and are transmitted only by mosquitoes: human and other primate malaria by Anopheles, bird malaria by Culer and other genera. Transmission in nature is accomplished by no other means than the bite of mosquitoes.

The rickettsial diseases of man and animals are in general associated with ixodid ticks, but here there are more exceptions. The tick-borne rick­ettsiae are beautifully adapted to their vectors and are even maintained by transovarial passage, a phenomenon not well established for any pathogen of vertebrates, either virus, bacteria, or protozoan, carried by an insect.