Arthropod Vectors as Reservoirs of Microbial Disease Agents

Abstract

The subject of arthropod vectors as reservoirs of agents of animal diseases is difficult to review because of divergent viewpoints of the definition of a reservoir. Some investigators would reserve this term for vertebrate hosts and the term vector for intermediate hosts (141). Others would consider vectors to be reservoirs if they participate in an essential phase of the disease cycle, or show cyclic propagation or transmission of the patho­ gen [a comprehensive, tabular review of ticks in this role is supplied by Neitz (126)]. Other factors also complicate precise definition of the word reservoir. These include: (a) length of life and types of metamorphosis of the arthropods; (b) method of transmission by biting or by contamination from fresh or old feces; and (c) premunition (latent persistence of organ­ isms after initial infection) in the vertebrate host and the too often specula­ tive role this plays in providing a source of infection for ectoparasites. In this review, the reservoir mechanism is envisioned as a flexible bio­ logical adjustment of the vertebrate and arthropod hosts to pathogens, in which the relative importance of either host as a reservoir varies in different diseases. Space permits only restricted selection of evidence that emphasizes the role of arthropods beyond that of simply being vectors (e.g., yellow fever virus and mosquitoes). The terms transovarial (or hereditary) and transstadial refer to passage of disease agents through ova and between active stages of a given vector, respectively.