15.4. Generalized disease cycles
In all biologically transferred diseases, a biting (blood-feeding or sucking) adult arthropod, often an insect, particularly a true fly (Diptera), transmits a parasite from animal to animal, human to human, or from animal to human, or, very rarely, from human to animal. Some human pathogens (causative agents of human disease such as malaria parasites) can complete their parasitic life cycles solely within the insect vector and the human host. Human malaria exemplifies a disease with a single cycle involving Anopheles mosquitoes, malaria parasites, and humans. Although related malaria parasites occur in animals, notably other primates and birds, these hosts and parasites are not involved in the human malarial cycle. Only a few human insect-borne diseases have single cycles, as in malaria, because these diseases require coevolution of pathogen and vector and Homo sapiens. As H. sapiens is of relatively recent evolutionary origin, there has been only a short time for the development of unique insect-borne diseases that require specifically a human rather than any alternative vertebrate for completion of the disease-causing organism’s life cycle.
In contrast to single-cycle diseases, many other insect-borne diseases that affect humans include a (non-human) vertebrate host, as for instance in yellow fever in monkeys, plague in rats, and leishmaniasis in desert rodents. Clearly, the non-human cycle is primary in these cases and the sporadic inclusion of humans in a secondary cycle is not essential to maintain the disease. However, when outbreaks do occur, these diseases can spread in human populations and may involve many cases.
Outbreaks in humans often stem from human actions, such as the spread of people into the natural ranges of the vector and animal hosts, which act as disease reservoirs. For example, yellow fever in native forested Uganda (central Africa) has a “sylvan” (wood-land) cycle, remaining within canopy-dwelling primates with the exclusively primate-feeding mosquito Aedes africanus as the vector. It is only when monkeys and humans coincide at banana plantations close to or within the forest that Aedes bromeliae (formerly Ae. simpsoni), a second mosquito vector that feeds on both humans and monkeys, can transfer jungle yellow fever to humans. In a second example, in Arabia, Phlebotomus sand flies (Psychodidae) depend upon arid-zone bur- rowing rodents and, in feeding, transmit Leishmania parasites between rodent hosts. Leishmaniasis, a disfiguring ailment showing a dramatic increase in the Neotropics, is transmitted to humans when suburban expansion places humans within this rodent reservoir, but unlike yellow fever, there appears to be no change in vector when humans enter the cycle.
In epidemiological terms, the natural cycle is maintained in animal reservoirs: sylvan primates for yellow fever and desert rodents for leishmaniasis. Disease control clearly is complicated by the presence of these reservoirs in addition to a human cycle.