Trypanosoma cruzi enzootic cycle: general aspects, domestic and synanthropic hosts and reservoirs
A.M. Jansen, A.L.R. Roque and S.C.C. Xavier Oswaldo Cruz Institute, Rio de Janeiro, Brazil
The complex Trypanosoma cruzi transmission cycle 265 What are the Trypanosoma cruzi reservoirs? 266
Importance of wild and synanthropic mammals in public health—Brazil 269 Domestic mammalian species 271 Domestic nonmammalian species 273
Importance of infected domestic mammals on public health in Brazil 274 Mixed infection 275 References 278
The complex Trypanosoma cruzi transmission cycle
T. cruzi is a successful parasite that is transmitted among more than 100 mammalian species dispersed in 7 different orders and dozens of species of insect vectors, bugs of the Reduviidae family.1,2 Actually, birds and cold-blooded vertebrates are refractory to the parasite but may act as a feeding source for triatomine enhancing their survival chances, contributing in this way, indirectly, to the maintenance of the parasite. T. cruzi transmission cycles occur in all phytogeographical regions in the Americas, from northern Argentina to southern United States. The diverse environments (habitats and niches) where the parasite can be found consist of numerous microfoci of transmission that display peculiar epidemiological profiles. T. cruzi is able to colonize almost all tissues in its many mammalian hosts, including unconventional sites, such as the scent glands of Didelphis spp.3 and Lutreolina crassicaudata,4 as well as the cornea of Thrichomys apereoides.5 The multiplicity of host—parasite interactions in the wild, constitute transmission cycles that can be characterized as complex and multivariable systems.2
Triatomines transmit the parasite only if infected, which occurs when they feed on an infected mammalian host and ingest, with their blood, the trypomastigote forms of the parasite. In the digestive tract of triatomines, the parasites differentiate into epimastigotes (multiplicative form) and then to metacyclic trypomastigotes in
American Trypanosomiasis Chagas Disease. DOI: http://dx.doi.org/10.1016/B978-0-12-801029-7.00012-5
Copyright © 2017 Elsevier Inc. All rights reserved.
the final portion of the intestine. Infection of mammals occurs when they come into contact with the infective metacyclic forms of the parasite that are eliminated with the feces of triatomines after feeding. This contact occurs through the mucosa or through injury, preexistent or resulting from the bite of the bug. The oral route, i.e., ingestion of infective forms of the parasite, occurs when the animal scratches with his mouth the place of its body where the bugs’ feces were deposited, eats food contaminated with the parasite, or preys on bugs or other infected mammals. Very probably the oral route is the main infection route of wild free ranging mammals. Actually, the contaminative infection route is quite unlikely6 mainly if we consider the dense fur of the animals that certainly acts as a barrier to the infective metacyclic forms. The oral route has proven to be highly efficient for the establishment of T. cruzi infection.7,8 In fact, the chances of animals to acquire the infection orally, by predation of infected triatomine bugs was estimated9 as superior to that of humans acquiring the infection by the contaminative route.6
Even having already been studied for over a century, there are still epidemiological challenges that need to be faced: (1) the epidemiological importance of congenital transmission for reservoirs and humans; (2) prophylactic measures to prevent the recurrent outbreaks of Chagas disease due to the oral route; and (3) the ecology of the distinct T. cruzi discrete typing units (DTUs). Mother to child T. cruzi transmission, a rather rare event10 represents a major public health problem in Bolivia, a highly endemic country. Additionally, due to migration of people from endemic areas, congenital Chagas disease is also becoming a problem in nonendemic countries.11 Among the still open questions are the role played by the distinct DTUs in the establishment of congenital infections as well as the mother’s immune response, genetic background, and even microbiome.10 This issue is discussed in a specific chapter in this book (see Chapter 23, Maternal—fetal transmission of Trypanosoma cruzi). Regarding the congenital transmission in free ranging animals, almost nothing is known. Thus, a great effort is presently needed to clarify what variables (host- and parasite-related) determine vertical transmission.