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The parasite transmission cycle

The epidemiological pattern of T. cruzi reveals that primitive transmission was restricted to established cycles in tropical environments. The parasite was spread via anal gland secretions and urine from opossums and later via triatomine insects that fed on small mammals in broad areas of the South American continent, with no human intervention in this natural cycle.22

Because of long periods of adaptation, the parasite has a wide range of wild mammal reservoirs, including Didelphis marsupialis, Philander opossum, Dasypus novemcinctus, Tamandua tetradactyla, Saimiri sciurius, Chiropotes satanas, and bats of the genus Phyllostomus and others. The same situation persists today in the wild, where the disease maintains an enzootic character. The presence of T. cruzi does not seem to significantly affect triatomines nor does it impact mammals that have been naturally infected. Humans might then have become infected as a single addition to the already extensive host range of T. cruzi, which also includes other primates.23

Estimates have been made about the age of the order Xenarthra (which contains armadillos, anteaters, and sloths), one of the four major clades of placental mammals reported to be hosts for T. cruzi. All four of these clades were isolated in South America following its separation from the other continental land masses. Xenarthrans diverged over a period of about 65 million years, leaving more than 200 extinct genera and only 31 living species.

The next placental mammal T. cruzi reservoirs to emerge in South America were the caviomorph rodents and platyrrhine primates during the Eocene.24 These clades appear following colonizations by rafting or island hopping across the Atlantic Ocean from Africa by their respective most recent ancestors. The continental isolation ended when the Isthmus of Panama land connection between South and North America emerged approximately 3.5 Mya in the Pliocene, marking the beginning of the Great American Biotic Interchange.25

The northern invaders of South America included carnivores, insectivores, noncaviomorph rodents, lagomorphs, artiodactyls, and perisodactyls. Bats contribute to 20% of the mammalian species diversity. Based on molecular dating, interordinal diversification occurred in Laurasia during the Cretaceous, including the appearance of bats estimated at 85 Mya.26

T. cruzi was originally transmitted directly between marsupials, but was subsequently vectored to other mammals through the advent of the blood-sucking Hemiptera (Triatominae) that are now considered the major vectors. This transfer from marsupials to other mammals may have been the main factor promoting adaptation of the parasite from the original widespread form (T. cruzi I) to a range of other lineages.

Current estimates suggest that the first divergence from T. cruzi I to T. cruzi II (mainly human), occurred about 10 Mya.2 The epidemiological pattern of T. cruzi reveals that primitive transmission was restricted to established cycles in tropical forest environments. Triatomine insects fed on small mammals in broad areas of the South American continent, with no human involvement in the natural cycle. The same situation persists today in the wild, where the disease maintains enzootic epidemiological character. The presence of T. cruzi does not seem to affect triato- mines significantly, nor does it impact the mammals that have been naturally infected, suggesting that a balance exists between species as a result of long periods of adaptation and coevolution.23

Although in general the Hemiptera represent an ancient order, with fossilized remains dating from the Permian, nearly 232—280 Mya, it is possible that the triato- mines evolved later starting at different times and from diverse ancestral forms.

The Hemiptera comprise a large order with over 80,000 species widely distributed in all tropical and temperate areas. Ancestral predatory habits among the triatomines can be inferred from the fact that some species occupy a relatively wide spectrum of ecotopes and are able to exploit different species of hosts, while others occupy restricted habitats and hosts.27 The vectorial transmission of T. cruzi is restricted to the New World.

 
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