Viruses, parasites, fungi, and bacteria have evolved with their reservoirs from ancestral forms. The evolution of plants and animals in the tropics has generated high species diversity in many taxa. It follows that microorganisms also are diverse in the tropics because each has evolved with specific reservoir hosts. The more diverse the hosts, the more diverse will be the infectious agents. This diversity is also accompanied by focal-ity, an increased degree of spatial localization. Of course, there are exceptions to such focality, namely in birds and bats, which may fly long distances, and human beings, who travel and take along their domestic animals. The microorganisms associated with widely dispersed animals or plants will be less focal.
This focality in the tropics also means that there are probably numerous as yet undescribed agents infecting wild vertebrates and vectors in tropical forests that have the potential to cause disease in people. In 1976, in 1995, and subsequently, Ebola virus emerged from cryptic forest foci in Zaire to cause fatal hemorrhagic human disease. These episodes are a reminder that tropical zoonotic agents may be very focal and hidden in geographically and ecologically limited transmission cycles until people intrude.
Most disease agents are very closely adapted to their vector or vertebrate host. Agents do not easily jump genus and species barriers and thus cannot readily adapt to new environmental conditions when their vector or vertebrate host becomes restricted by a change in environment.
In 1876, Wallace classified and bounded continental and faunal regions.13 These regions are Nearctic, Neotropical, Palearctic, Oriental, Ethiopian, and Australasian. Theiler and Downs14 studied the distribution of 280 arboviruses and rodent-associated viruses, and they showed that 247 existed in only one of these regions (Fig. 2-1). Presumably this meant that their vector or vertebrate host was quite specific, and they either were not transported to other regions or there was no available vector or vertebrate in another region to support their cycle of transmission. Thirty were found in two regions and only three in more than two regions. All but one of the viruses that had been discovered in more than one geographic region infected domestic animals, domiciliary mosquitoes (Aedes aegypti), or birds and thus had a means of transportation to another region. Viruses that were adapted to rodents were destined to have a very focal distribution because rodents do not fly and, with the exception of the house mouse and the wharf rat, have limited geographic interchange.
The same concepts hold for most bacterial and parasitic tropical agents that have three- or four-factor complexes. Each of the three major species causing human schistosomiasis, for instance, evolved with a different snail. The distribution of each species is limited to that of the snail: Schistosoma mansoni with Biomphalaria in tropical Africa and Brazil, Schistosoma haematobium with Bulinus in Africa and the Middle East, and Schistosoma japonicum with Oncomelania in the Far East. Although the snail is sometimes more widely dispersed than the schistosome, the potential for disease exists wherever the snail resides. Note also that new areas of disease are being created with the construction of dams that provide ecological conditions to support snails. The schistosomes are subsequently introduced to the dam sites by an influx of human populations that bring the parasite with them.
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