Natural infections in domestic animals and livestock
The role of animals in the epidemiology of Chagas disease is variable, depending on the type of the 200 species found infected.4 Thus we can globally divide them into three categories which may be involved in the three mammalian cycles (domestic, peridomestic, and wild).
Domestic species are dogs, cats, guinea pigs, hamsters, and rabbits. T. cruzi is markedly pathogenic in dogs (and to a lesser extend in cats) in which it produces cardiac signs with a potentially fatal outcome. It is speculated that dogs and cat can get the infection by the bug’s bite, but also by eating infected bugs, or, by eating fresh infected preys (especially mice for cats), and may have an important role in the epidemiology of the human disease in some circumstances, like in Argentina40 or in newly settled human populations of Amazon.41 The prevalence of infection in domestic carnivores is around 10% in Chile, 9—24% in Mexico,42 20% in Brazil, 37% in dogs and cats in Paraguay,7 and 42% in Argentina,40 and up to 50% in dogs in some areas of Venezuela. An expert report published by WHO on Chagas disease3 mentions that the studies conducted in Argentina, Brazil, Chile, Bolivia, and Venezuela yield highly variable rates of infection by T. cruzi that range, in humans, from 0.5% to 2% in large cities, to between 20% and 63% in highly endemic areas; in dogs, from 4.5% to 100% and, in cats, from 0.5% to 60.9%. In Chile, serological studies in the provinces of Elqui, Limari, and Choapa revealed the presence of antibodies for 12—24% of dogs, 0—15% of cats, and 4—26% of rabbits.6 In guinea pigs in Bolivia, prevalence ranges from 10% to 60%.43 In the United States, dogs are sometimes found infected and are most often related to human cases44; sporting and working breeds account for the majority of the cases, presumably due to greater exposure to infected vectors and mammalian tissues.45
Domestic but living in the peridomestic area are the livestock species such as cattle, buffaloes, horses, sheep, goats, and pigs. None of them is considered to be highly susceptible to the infection by T. cruzi. In Chile, serological studies in the provinces of Elqui, Limari, and Choapa revealed the presence of antibodies in 5— 12% of goats and 4.8% of sheep.6 In another study in Chile, the seroprevalence by IFA and ELISA in goats ranged from 6.5% to 38.3%.46 In Paraguay, a survey demonstrated that antibodies raised against T. cruzi were found in 8% of cattle and 10% of pigs.7 Ox and pigs were found to be naturally infected in Mexico.5 In the other countries, very little, if any, information is available. In some instances, T. cruzi could be isolated from these animals, but in serological surveys conducted in areas endemic for other Trypanosoma species, the seroprevalence observed cannot be attributed to T. cruzi with certainty. Because appropriate diagnostic tools are lacking, the prevalences in livestock cannot really be determined. Putting aside Trypanosoma theileri, which does not induce serological cross-reaction with other Trypanosoma species, there is a reciprocal interference of all trypanosomes potentially present in livestock: T cruzi, T. equiperdum, and T. evansi in equids, and T. cruzi, T. evansi, and T. vivax in others. For example, in 1995 although there were no clinical signs, positive antibody serologies (CFT) for T. equiperdum were found in the state of Chihuahua (Mexico) on horses and mules intended for export to the United States. The same samples tested for T. cruzi with hemagglutination inhibition also turned out positive. The positive animals were slaughtered but infection was never demonstrated. Furthermore, investigations conducted on 3000 equidae in that state were never able to isolate the pathogen or discover clinical signs of dourine. Interference from T. cruzi or T. evansi may be the cause of serological cross-reactions. T. cruzi has been found in circulation in Mexico even in urban areas; a recent serological survey has shown that 8.8—24.2% of dogs are infected.42 Another example is in Argentina, where diagnosis in horses cannot distinguish T. evansi from T. cruzi infection, both of them being equally probable.34—36 Hence, it would seem that the incidence of T. cruzi infection of livestock is by no means negligible. Little research has been conducted either in the field, where no specific diagnostic tools are available, or under experimental conditions owing to the limited risk of human infection and/or a lack of interest in this work because its pathogenicity is presumed to be low and because of the short economic life expectancy of farm animals. However the recent case of a 10-year-old Texan Quarter horse found to be infected brings a new light on the potential for horse infection by T. cruzi41; this animal exhibited ataxia and lameness in the hind limbs for 6 months and was finally euthanized after its condition worsened. Amastigote trypanosomes morphologically similar to T. cruzi, were detected within segments of the thoracic spinal cord, with mild lymphoplasmacytic inflammation. Identification was confirmed by T. cruzi satellite DNA polymerase chain reaction (PCR) and sequencing. Such a case might easily remain undetected if the authors were not be so cautious. T. cruzi should now be considered as a differential diagnosis in horses with similar neurologic clinical signs and lesions.47 Horses and other livestock infections might occur and remain undetected if accurate diagnosis procedures are not employed.
Synanthropic species, which may be peridomestic or sylvatic, are mice, rats, armadillos, raccoons, coyotes, and marsupials; the latter are considered to be the oldest and most important reservoir of the parasite.48 In a review made in Mexico in 1997, 12 peridomestic mice, rats, squirrel, armadillo, bat, and marsupials were found to be infected. A huge number of species have been found to be infected but few studies indicate the prevalence of the infection; it ranges from 13% in octodon (Octodon degus) to 28% in rats (Rattus rattus) in Chile,49 and from 6% in P. opossum to 43% in D. marsupialis in French Guiana.8 Complex epidemiological features may include marsupials, armadillo, monkeys, and raccoons as reservoirs, bugs as active or passive vectors toward same or other host species, including mice and rats, which may be responsible for the infection of cats and dogs. The epidemiological studies carried out in diverse circumstances tend to show how complex and unique the situations can be, therefore no generalization or prediction should be made and each ecotope should be considered as an unique system.48 In a sylvatic area Brazil, collared peccary (Tayassu tajacu), white-lipped peccary (Tayassu pecari), and feral pig (Sus scrofa) are maintenance hosts for Trypanosoma cruzi and Trypanosoma evansi.50 In wild rabbits, prevalence reaches 38% in Chile.51 In Bolivia, wild populations of Triatoma infestans were shown to create a link between humans and domestic and peridomestic hosts, including cat, donkey, and wild rodents.52 In the United States, T. cruzi is frequently found in raccoons; in Tennessee, a higher seroprevalence is observed in rural habitats (35%) than in suburban habitats (23%) with an average of 29%,53 but it can reach 63% in Oklahoma54; however autochtonous human cases seem to be related to both raccoons and dogs.44