Transmission Cycles

Transmission cycles vary considerably depending upon their complexity and the role of humans as hosts for the parasite. A vector-borne anthroponosis is a disease resulting from a parasite that normally infects only humans and one or more anthropophagic vectors (Fig. 2.1). Malaria, some forms of filariasis, and louse-borne typhus are examples of anthroponoses with transmission cycles that involve humans and host-specific vectors. Humans serve as reservoir hosts for these parasites, which may persist for years as chronic infections. Vectors of anthroponoses selectively blood feed upon humans and are associated with domestic or peridomestic environments. Widespread transmission of an anthroponosis with an increase in the number of diagnosed human cases during a specified period of time is called an epidemic. When human cases reappear consistently in time and space, transmission is said to be endemic.

Zoonoses are diseases of animals that occasionally infect humans. Likewise, ornithonoses are diseases of wild birds that are transmitted occasionally to humans. In most vector-borne zoonoses, humans are not an essential component of the transmission cycle, but rather become infected when bitten by a vector that fed previously on an infected animal host. Although humans frequently become ill, they rarely circulate sufficient numbers of parasites to infect vectors and thus are termed dead-end hosts. The enzootic transmission cycle is the basic, or primary, animal cycle (literally "in animals"). When levels of enzootic transmission escalate, transmission may become epizootic (an outbreak of disease among animals). Transmission from the enzootic cycle to dead-end hosts is called tangential transmission (i.e., at a tangent from the basic transmission cycle). Often different vectors are responsible for enzootic, epizootic, and tangential transmission. Bridge vectors transmit parasites tangentially between different enzootic and dead-end host species. Human involvement in zoonoses may depend on the establishment of a secondary amplification cycle among vertebrate hosts inhabiting the peridomestic environment.

WEE virus is a zoonosis that exemplifies primary and secondary transmission cycles and tangential transmission to man and equines (Fig. 2.3). In California, WEE virus amplification occurs in a primary enzootic transmission cycle that consists of several species of passerine birds and Cx. tarsalis mosquitoes. In addition to birds, Cx. tarsalis blood feed on a variety of mammals, including rabbits. Rabbits, especially jackrabbits, develop sufficient viremia to infect some Cx. tarsalis and Ae. melanimon mosquitoes, thereby initiating a secondary zoonotic transmission cycle. WEE virus activity in the secondary Aedes— rabbit cycle usually has been detected after

FIGURE 2.3 Components of the transmission cycles of a zoonosis such as western equine encephalomyelitis (WEE) virus. (Original by Margo Duncan)

amplification in the primary Cx. tay sails—bird cycle. Both Cx. tarsalis and Ae. meltmimon transmit the virus tangen-tially to humans and equines, which are dead-end hosts for the virus.

An important aspect of the ecology of vector-borne parasites is the mechanism(s) by which they persist between transmission seasons or outbreaks. Parasite transmission typically is most efficient when weather conditions are suitable for vector activity and population growth. In temperate latitudes, overwintering of parasites becomes problematic when vertebrate or arthropod hosts either enter a winter dormancy or migrate. Similar problems face tropical parasites when transmission is interrupted by prolonged dry or wet seasons. The apparent seasonality that is characteristic of most vector-borne parasites may be due to either the periodic amplification of a constantly present parasite or to the consistent reintroduction of parasites following focal extinction.

Mechanisms of parasite maintenance during periods of unfavorable weather include the following:

Continued transmission by vectors. During periods of unfavorable weather, vectors may remain active and continue to transmit parasites, although transmission rates may be slowed by cold temperature or low vector abundance. In temperate latitudes with cold winters, transmission may continue at a slow rate, because the frequency of blood feeding and rate of parasite maturation in the vector is diminished. In tropical latitudes, widespread transmission may be terminated during extended dry seasons that reduce vector abundance and survival. In both instances, transmission may be restricted spatially and involve only a small portion of the vertebrate host population. Human infections during adverse periods usually are highly clumped and may be restricted to members of the same household.

Infected vectors. Many vectors enter a state of dormancy as non—blood-feeding immatures or adults. Vertically infected vectors typically remain infected for life and therefore may maintain parasites during periods when horizontal transmission is interrupted. California encephalitis virus, for example, is maintained during winter and drought periods within the transovarially infected eggs of its vector, Ae. melanimon. Infected eggs of this floodwater mosquito may remain dormant and infected for up to several years and are able to withstand winter cold, summer heat, and extended dry periods. Inundation of eggs during spring or summer produces broods of adult mosquitoes that are infected at emergence.

Similarly, vectors that inhabit the nests of migratory hosts such as cliff swallows often remain alive and infected for extended periods until their hosts return.

Infected vertebrate hosts. Parasite maintenance may be accomplished by infected reservoir hosts that either continue to produce stages infective for vectors or harbor inactive stages of the parasite and then relapse or recrudesce during the season when vectors are blood feeding. Adult filarial worms, for example, continue to produce microfilariae throughout their lifetime, regardless of the population dynamics or seasonality of the mosquito vector. In contrast, some Korean strains of vivax malaria overwinter as dormant stages in the liver of the human host and then relapse in spring, concurrent with the termination of diapause by the mosquito vector(s).

Alternatively, parasites may become regionally extinct during unfavorable weather periods and then are reintroduced from distant réfugia. Two possible mechanisms may allow the reintroduction of parasites:

Migratory vertebrate hosts. Many bird species overwinter in the tropics and return to temperate or subarctic breeding sites each spring, potentially bringing with them infections acquired at tropical or southern latitudes. It also is possible that the stress of long flights and ensuing reproduction triggers relapses of chronic infections. In addition, many large herbivores migrate annually between summer (or wet) and winter (or dry) pastures, bringing with them an array of parasites. Rapid longrange human or commercial transportation is another possible mode for vector and parasite introduction. The seasonal transport of agricultural products and the movements of migratory agricultural workers may result in the appearance of seasonality.

Weather fronts. Infected vectors may be carried long distances by prevailing weather fronts. Consistent weather patterns, such as the sweep of the southeastern monsoon from the Indian Ocean across the Indian subcontinent, may passively transport infected vectors over hundreds of kilometers. The onset of WEE virus activity in the north central United States and Canada has been attributed to the dispersal of infected mosquitoes by storm fronts.

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