LIFE CYCLES, TRANSMISSION, AND DISTRIBUTION
As is evident from the previous discussion, many parasites require but a single host species for the completion of their life cycles. The method by which the parasite is transmitted from individual to individual within that species is determined in large part by its viability in the external environment and, in the case of helminths, by the conditions required for the maturation of offspring. The mode of transmission, in turn, determines the social, economic, and geographic distribution of the parasite. A few examples are described in Table 51–4.
The protozoan T. vaginalis does not produce protective cyst forms. Although its active, or trophozoite, form is relatively hardy, it can survive only a few hours outside of its normal habitat, the human genital tract. Thus, for all practical purposes, transmission requires the direct genital contact of sexual intercourse. As a result, trichomoniasis is cosmopolitan, occurring wherever human hosts engage in sexual activity with multiple partners.
Another protozoan, E. histolytica, inhabits the human gut and produces hardy cysts that are passed in the stool. Transmission occurs when another individual ingests these cysts. Like T. vaginalis, the organism can be passed by direct physical contact, in this case by oral–anal sexual activity. This mode of transmission, in fact, accounts for the high incidence of amebic infections in male homosexuals. Unlike T. vaginalis, however, the cysts can survive prolonged periods in the external environment, where they may eventually contaminate food or drinking water. Thus, in environments such as mental institutions, where the level of personal hygiene is low, or in populations in which methods for the sanitary disposal of human wastes are not available, amebiasis is common.
The intestinal helminth Ascaris lumbricoides illustrates still another transmission pattern. In this infection, highly resistant eggs are passed in the human stool. Unlike the situation with E. histolytica described previously, the eggs are not immediately infective but must incubate in soil under certain conditions of temperature and humidity before they are fully embryonated and infectious. As a result, this parasite cannot be transmitted directly from host to host. The organism spreads only when indiscriminate human defecation results in deposition of eggs on soil and subsequent exposure of that soil to the climatic conditions required for embryonation of the eggs. For this reason,Ascaris infections are most prevalent in poorly sanitated areas of the tropics and subtropics.
A few protozoa and many helminths require two or more host species in their life cycle. To avoid confusion, it is customary to refer to the species in which the parasite reproduces sexu-ally as the definitive host and that in which asexual reproduction or larval development takes place as the intermediate host. When there is more than one intermediate, they are known simply as the first and second intermediate hosts. In some cases, such as that of Taenia sagi-nata, the beef tapeworm, both host species are vertebrates; humans serve as the definitivehost and cattle as the intermediate. Among parasites that inhabit the blood and tissues of hu-mans, it is more common for a blood-feeding arthropod to serve as a second host and as the transmitting vector. An example is malaria, in which the causative plasmodium is transmittedfrom person to person by the bite of an infected female mosquito of the genus Anopheles. Inthis particular instance, sexual reproduction occurs in the mosquito, making it the definitivehost and relegating the human host to the role of a mere intermediate.
The distribution of parasites requiring a nonhuman host is limited to the ecologic nicheoccupied by this second host. Thus, the areas in which malaria is endemic are restricted bythe distribution of Anopheles mosquitos. The area of disease distribution is, in fact, gener-ally smaller than that of the nonhuman host, because conditions favoring parasite transmis-sion may also differ. For example, both the abundance of Anopheles and the speed withwhich the malarial parasite completes its development within them are directly related to the ambient temperature and humidity. Among temperate zone Anopheles, the number of infected mosquitoes may be insufficient to sustain parasite transmission. In tropical areas,transmission is more likely to be constant and intense. In another more obvious example,infections with T. saginata are found only in areas where cattle are raised for human con-sumption and, within those areas, only where indiscriminate human defecation and the in-gestion of raw or undercooked beef are common.