New cycles establish in the United States
Although Chagas disease in humans is generally considered to be present in Latin America from 38°S up to the southern part of Mexico (25°N), T. cruzi is present from Argentina and Chile (43° S) up to the southern part of the United States (42°N), as high as in Missouri, occasionally in humans, and regularly in domestic animals, particularly dogs and cats,62 but also in a large range of wild animals, such as opossums, wood rats, racoons, striped skunks, armadillos, antelope squirrels, gray foxes, and coyotes,2,63 with some high prevalences such as 33% in opossum and 63% in raccoons.4
T. cruzi is known to be present in the following states: California, Arizona, Texas, Oklahoma, Tennessee, Missouri, Louisiana, Florida, Georgia, Virginia, South Carolina, and North Carolina.53,63-68 In humans in Texas, blood donor screenings have shown prevalence of antibodies to T. cruzi as high as 1/6500, and a strong link was drawn with poverty,69 however, the rate of possibly autochtonous cases was not determined.
Several vectors have been reported in the United States: Triatoma gersaecheri in Texas; Triatoma sanguisuga and T. lecticularia in South Carolina and Georgia, etc.66 The very few cases of native Chagas disease were recorded in California, Texas, and Tennessee, most often by means of postmortem PCR.70,71 Many wild and domestic animals were found to be infected, sometimes with high prevalence, such as in armadillos, badgers, and coyotes, as well as cattle and sheep which were found to be carriers of antibodies in Texas and Louisiana; in South Carolina and Georgia, nearly 50% of raccoons (Procyon lotor) were serologically positive as well as some opossums (Didelphis virginiana).66
The isolation of T. cruzi in dogs in Virginia72 showed that a new epidemiological pattern is being established by the spread of the parasite; in this case, the mother Walker hound and seven of its eight pups were found to be infected. This pattern relies on a wild host/bug/carnivore cycle in which wild host or rodents probably act as reservoir (direct transmission from rodent to rodent is also possible by biting) whereas dogs are thought to be an epidemiological cul-de-sac.
In Tenessee, five autochtonous human cases were reported so far; in the last case, a child was infected without significant clinical signs and would not have been detected if the bugs had not been noticed by the mother; in this case Triatoma sanguisuga were caught and found to be infected as well as three raccoons which were trapped in the vicinity of the house.71
Another autochtonous case was reported from California73 and the strain isolated could develop in two species of triatominae native to California: Triatoma protracta and T. rubida.
The way of infection of autochtonous human cases have not been identified yet. Several hypotheses remain, including vectorial transmission, transconjunctival or peroral infection by bug feces or contaminated food, or another unidentified way.
These cases where the origin and way of infection are not clearly elucidated do not preclude the possible cyclical transmission of the parasite by a vicariant vector that the parasite might be found on its way toward the North.
One must remember that cyclical transmission of T. cruzi has been described in opossums (D. marsupialis).28 Hence, in the United States, T. cruzi has a huge wild and domestic reservoir together with two cyclical vectors (bugs and opossum) and other potential vectors (louse). Infections in dogs are seen more and more often. It is presumed that the main cycle occurs in bugs and wild animals (raccoons and opossums). Domestic animals are infected by ingesting bugs and/or, in the case of carnivores, by ingesting infected wild prey. Although infected bugs may be found in the vicinity of human habitat, there are very few reports of human contamination and this is thought to be attributable to the long interval that elapses in the United States between the time when the vectors take their meals and the time when they defecate, making it unlikely for a bite wound to be contaminated, i.e., the most common mode of human contamination in South America. Furthermore, living and hygiene conditions in the United States are far less likely to foster contact between humans and vectors than those that prevail in Latin America. However, dogs and cats may be a potential link between wild reservoir and humans, since they attract and/or maintain bugs in the vicinity of human habitat, thus making possible the contamination of human’s food by bug’s feces.
Progression in the distribution and establishment of T. cruzi should be taken very seriously. T. cruzi is already capable of transmitting cyclically in the United States, by ingestion of food contaminated by opossum or bugs excreta and by entry of the metatrypanosomes present in the feces of bugs (or opossum) through bite wounds or mucosal membranes. Thus, infection focus in humans can already occur in the United States by a similar way as in Brazil where numerous people have been infected when drinking fruit juice, further to the contamination of the fruit stock or utensils by bugs or opossum’s feces. Eloquent is the recent case detected in a Texan horse, indicating that horse, and possibly other domestic animals such as livestock, may already act potential links between wild fauna and humans.47 The geographical extension of T. cruzi in wild and peridomestic fauna is currently spreading. Furthermore, T. cruzi may be able to find vicarious vectors in the course of its progression, possibly establishing a new epidemiological link to humans.