Indigenous Disease Agents of North America

Within this varied habitat a myriad of organisms lived and died with the usual range of interrelation ships ecologists have led us to expect. Birds and mammals must have had their parasites, and, it is assumed, sylvan cycles of infection and death took place. To identify these in the lack of any, as yet, human presence is difficult, but insofar as today's known zoonoses cycles in North America have been elucidated, some endemic conditions can be postulated.

These would include the bacteria known to cause tularemia, botulism, relapsing fever, anthrax, and pasteurellosis (Cockburn 1971; Meade, Florin, and Gesler 1988) as well as the highly localized Babesia microtia of ticks infesting rodents on Nantucket Island. Additionally, Rickettsia rickettsii of the tick Ixodes infested small mammals, and possibly Rickettsia typhii was carried by the fleas on native rodent species already present (Newman 1976). More certainly, the viral encephalitis group is nidal in the region (MacLean 1975), as are a number of helminthic conditions such as the tapeworm Diphyl-lobothrium pacificum, whose definitive host is the Pacific seal (Patrucco, Tello, and Bonavia 1983), the pinworm Enterobius vermicularis, and the thorny-headed worm Acanthocephala; all are parasites of wild animals (Fry, Moore, and Englert 1969; Fry and Moore 1970). The parasitic flatworm of migratory waterfowl, whose intermediate host is the snail, is endemic among the bird populations of the North American flyways, and has probably been so for eons before unfortunate humans encountered the cercariae and experienced "swimmers' itch" (Du-Four 1986).

Other organisms associated with water and the faunal population include Leptospira (although the agent of Weil's disease was not present until the introduction of its natural host, the brown rat - Rattus norvegicus) and a number of Salmonella organisms carried by seagulls and other sea-living animals including oysters and fish (Ffrench 1976). Furthermore, the free-living ameba Naegleria flowleri, inhabiting soil, water, and decaying vegetation (but only entering medical literature as a disease agent in 1965), along with other similar organisms, has long been in the environmental complex of Nearctica (Craun 1986).

Still another group of organisms associated (in at least 27 cases) with a free-living existence in soil and water is the mycobacteria (Runyon 1971). As is well known, some species of the genus Mycobacterium are associated with human tuberculosis and leprosy. Others, however, called "environmental," vary in frequency, depending upon locale, soil conditions, and climate; and modern American studies have shown that human exposure to these is quite common (Steinbock 1987). Furthermore, Mycobacterium bovis and occasionally Mycobacterium avium produce disease in humans that is clinically identical to that caused by Mycobacterium tuberculosis. Native Nearctic guinea pigs, mice, bison, and birds may well have harbored these varieties. Thus organisms capable of causing human tubercular lesions were almost certainly in the New World before the advent of humans (Anderson et al. 1975).

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