Abstract
Summary: Chagas' disease is caused by the protozoan parasite Trypanosoma cruzi and causes potentially life-threatening disease of the heart and gastrointestinal tract. The southern half of the United States contains enzootic cycles of T. cruzi, involving 11 recognized triatomine vector species. The greatest vector diversity and density occur in the western United States, where woodrats are the most common reservoir; other rodents, raccoons, skunks, and coyotes are also infected with T. cruzi. In the eastern United States, the prevalence of T. cruzi is highest in raccoons, opossums, armadillos, and skunks. A total of 7 autochthonous vector-borne human infections have been reported in Texas, California, Tennessee, and Louisiana; many others are thought to go unrecognized. Nevertheless, most T. cruzi-infected individuals in the United States are immigrants from areas of endemicity in Latin America. Seven transfusion-associated and 6 organ donor-derived T. cruzi infections have been documented in the United States and Canada. As improved control of vector- and blood-borne T. cruzi transmission decreases the burden in countries where the disease is historically endemic and imported Chagas' disease is increasingly recognized outside Latin America, the United States can play an important role in addressing the altered epidemiology of Chagas' disease in the 21st century.
Chagas' disease is caused by the protozoan parasite Trypanosoma cruzi (234). World Health Organization disease burden estimates place Chagas' disease first among parasitic diseases in the Americas, accounting for nearly 5 times as many disability-adjusted life years lost as malaria (343). An estimated 8 million people are currently infected, and 20 to 30% of these will develop symptomatic, potentially life-threatening Chagas' disease (Table 1) (214). T. cruzi is carried in the guts of hematophagous triatomine bugs; transmission occurs when infected bug feces contaminate the bite site or intact mucous membranes. T. cruzi can also be transmitted through transfusion, through transplant, and congenitally
Historically, transmission and morbidity were concentrated in rural areas of Latin America where poor housing conditions favor vector infestation. However, in the last several decades, successful vector control programs have substantially decreased transmission in rural areas, and migration has brought infected individuals to cities both within and outside Latin America (87, 111, 196). Since 1991, several subregional initiatives have made major advances in decreasing vector infestation in human dwellings and extending screening of the blood supply for T. cruzi (87, 269). In 2007, control efforts in Latin America were formally joined by an initiative to address the “globalization” of Chagas' disease, recognizing the increasing presence of imported cases in Europe, North America, and Japan and the potential for local transmission through nonvectorial routes (344). The United States occupies an ambiguous position in this new initiative. While the United States has never participated in Latin American Chagas' disease control programs, it cannot be classified as an area where the disease is “not endemic” in the same sense as Europe or Japan. The southern tier of states from Georgia to California contains established enzootic cycles of T. cruzi, involving several triatomine vector species and mammalian hosts such as raccoons, opossums, and domestic dogs (26, 151, 345). Nevertheless, most T. cruzi-infected individuals in the United States are immigrants from areas of endemicity in Latin America (29).
This article will present an overview of clinical and epidemiological aspects of Chagas' disease, with a focus on data and issues specific to T. cruzi and Chagas' disease in the United States. Topics to be covered include vector biology and ecology, animal reservoirs, T. cruzi strain typing, human Chagas' disease, and future research needed for control of Chagas' disease in the United States.
Life Cycle Nearly all the salient features of the T. cruzi life cycle were described by Carlos Chagas, the scientist who discovered the organism, in 1909 (62). T. cruzi is a kinetoplastid protozoan which infects vertebrate and invertebrate hosts during defined stages in its life cycle (234, 292). The triatomine vector ingests circulating trypomastigotes when it takes a blood meal from an infected mammalian host. In the midgut of the vector, trypomastigotes transform through an intermediate form sometimes called a spheromastigote to epimastigotes, the main replicating stage in the invertebrate host. Epimastigotes migrate to the hindgut and differentiate into infective metacyclic trypomastigotes, which are excreted with the feces of the vector. Metacyclic trypomastigotes enter through the bite wound or intact mucous membrane of the mammalian host and invade many types of nucleated cells through a lysosome-mediated mechanism (50). In the cytoplasm, trypomastigotes differentiate into the intracellular amastigote form, which replicates with a doubling time of about 12 h over a period of 4 to 5 days. At the end of this period, the amastigotes transform into trypomastigotes, the host cell ruptures, and the trypomastigotes are released into the circulation. The circulating parasites can then invade new cells and initiate new replicative cycles, and they are available to infect vectors that feed on the host. In the absence of successful antitrypanosomal treatment, the infection lasts for the lifetime of the mammalian host.
Transmission Routes Vector-borne transmission. The vector-borne transmission route, occurring exclusively in the Americas, is still the predominant mechanism for new human infections. The feces of infected bugs contain metacyclic trypomastigotes that can enter the human body through the bite wound or through intact conjunctiva or other mucous membranes.
Congenital transmission. Between 1 and 10% of infants of T. cruzi-infected mothers are born with congenital Chagas' disease (14, 24, 289). Congenital transmission can occur from women themselves infected congenitally, perpetuating the disease in the absence of the vector (263). Factors reported to increase risk include higher maternal parasitemia level, less robust anti-T. cruzi immune responses, younger maternal age, HIV and, in an animal model, parasite strain (9, 32, 34, 107, 289).
Blood-borne transmission. Transfusional T. cruzi transmission was postulated in 1936 and first documented in 1952 (109, 307). The risk of T. cruzi transmission per infected unit transfused is estimated to be 10 to 25%; platelet transfusions are thought to pose a higher risk than other components such as packed red cells (31, 308). In 1991, the prevalence of T. cruzi infection in donated blood units ranged from 1 to 60% in Latin American cities (268). Since then, blood donation screening has become accepted as an important pillar of the Chagas' disease control initiatives (220, 269). Serological screening of blood components for T. cruzi is now compulsory in all but one of the countries in Latin America where the disease is endemic, and the prevalence of infection in screened donors has decreased substantially (196, 269). Nevertheless, Chagas' disease screening coverage by country was estimated to vary from 25% to 100% in 2002, and the risk of transmission, though much decreased, has not been eliminated (269). The residual risk in Latin America where screening has been implemented is estimated to be 1:200,000 units (269, 308).
Organ-derived transmission. Uninfected recipients who receive an organ from a T. cruzi-infected donor may develop acute T. cruzi infection. However, transmission is not universal; in a series of 16 uninfected recipients of kidneys from infected donors, only 3 (19%) acquired T. cruzi infection (238). Nineteen instances of transmission by organ transplantation have been documented in the literature (13 kidney, 1 kidney and pancreas, 3 liver, and 2 heart transplants) (16, 61, 66, 79, 99, 101, 157, 238, 279). The risk from heart transplantation is thought to be higher than that from kidney or liver transplantation (65). One case of transmission through unrelated cord blood transplantation has been reported (104).
Oral transmission. Recently, increasing attention has focused on the oral route of T. cruzi transmission; several outbreaks attributed to contaminated fruit or sugar cane juice have been reported from Brazil and Venezuela (28, 82, 208). Most outbreaks are small, often affecting family groups in the Amazon region, where the palm fruit açaí is a dietary staple that appears to be particularly vulnerable to contamination, perhaps from infected vectors living in the trees themselves (74, 208). The largest reported outbreak to date led to more than 100 infections among students and staff at a school in Caracas; locally prepared guava juice was implicated
TRIATOMINE VECTOR BIOLOGY AND ECOLOGY
Background The epidemiology of vector-borne T. cruzi is closely linked to the biological and ecological characteristics of local vectors and mammalian reservoir hosts. Triatomines of both sexes must take blood meals to develop through their nymphal stages to adults, and females require a blood meal to lay eggs. Thus, nymphs and adults of either sex may be infected with T. cruzi, but infection rates increase with increasing vector stage and age. Most domestic triatomine species feed nocturnally and are able to complete their blood meal without waking the host (169). The major Latin American vectors defecate during or immediately after taking a blood meal. T. cruzi infection is transmitted to wild mammals by sylvatic triatomine species; these bugs often colonize the nests of rodent or marsupial reservoir hosts (169, 311). Sylvatic triatomine adults may fly into human dwellings because of attraction by light and cause sporadic human infections (74). Domestic transmission cycles occur where vectors have become adapted to living in human dwellings and nearby animal enclosures; domestic mammals such as dogs, cats, and guinea pigs play important roles as triatomine blood meal sources and T. cruzi reservoir hosts (69, 124, 131). Some triatomine species can infest both domestic and sylvatic sites and may play a bridging role (192). There are more than 130 triatomine species in the Americas, many of which can be infected by and transmit T. cruzi (169, 311). However, a small number of highly domiciliated vectors are of disproportionate importance in the human epidemiology of disease (Table 2) (311). The domestic environment provides abundant blood meal sources, and poor quality housing with adobe or unfinished brick walls provides crevices and other diurnal hiding places for triatomines (170, 201). Thatch roofs provide an attractive habitat for some species (117). In communities where the disease is endemic, 25 to 100% of houses may be infested, and a house and its immediate surroundings may support large colonies of juvenile and adult bugs
In areas of the Amazon where deforestation and human immigration have occurred, tree-dwelling sylvatic triatomine populations have survived and rebounded by adapting to new vertebrate host species (2). These opportunistic vertebrates (opossums and rodents) are competent Chagas' disease reservoirs and are acclimated to living in close proximity to humans where remnant vegetation is located. The concentration of triatomines and vertebrate reservoirs in the peridomestic realm has lead to increased interactions between sylvatic triatomine species and humans in deforested areas of the Amazon and Panama and to an apparent increase in the incidence of Chagas' disease in humans (4, 244).
Triatomine Distribution in the United States Eleven species of triatomine bugs have been reported from the United States: Triatoma gerstaeckeri, T. incrassata, T. indictiva, T. lecticularia, T. neotomae, T. protracta, T. recurva, T. rubida, T. rubrofasciata, T. sanguisuga, and Paratriatoma hirsuta (Fig. 1 and Table 3). Triatomines are present across the southern half of the country, distributed from the Pacific to Atlantic coasts (Fig. 2). One species (T. rubrofasciata) is found in Hawaii. A high degree of polymorphism has been noted in several species across their geographic ranges, particularly T. protracta, T. rubida, and T. sanguisuga, resulting in proposed subspecies classifications (249, 251, 254, 296). However, due to the recognition of morphological intermediates across some subspecies groups and the absence of supporting data (e.g., paired molecular and morphological studies), these subspecies have not been universally accepted as valid taxonomic groups
All U.S. species except T. rubrofasciata and T. sanguisuga have been collected in Mexico; the distribution of T. sanguisuga likely extends into northeastern Mexico as well (255). A review of the published literature from 1939 to 2010 resulted in reports of wild-caught triatomine bugs from 262 counties in 28 states. The greatest species diversity occurs in the southwest, particularly Texas, Arizona, and New Mexico. More specifically, high species diversity is concentrated in south-central Arizona and southwestern Texas, where up to five species have been recorded in a single county (Fig. 2). T. cruzi-infected specimens have been reported from 10 states, predominantly from counties in the Southwest (Fig. 3A). All species except T. incrassata and P. hirsuta have been found naturally infected with T. cruzi
County-level maps (Fig. 2 and and3)3) reflect in part where collection efforts have been focused over the past 70 years. There is no evidence of a temporal or spatial trend in the published reports to suggest any recent migration of species into or within the United States. The county maps do not necessarily reflect triatomine population densities or provide a complete representation of their distributions. Rather, the maps more likely provide an indication of where the bugs have been considered a pest to humans or animals and where field efforts were concentrated as a consequence or where specimens were collected coincidentally by researchers studying other animal systems (i.e., reports based on museum specimens). Collection records are more comprehensive in the southwestern states and Florida, with sparse records in the southeastern states. Early discovery of the association of U.S. triatomine bugs with Neotoma species of woodrats may have aided field research in the southwestern states, because woodrat species in this region build easily identifiable, above-ground dens. The absence of records in some areas of the southeastern United States may reflect a paucity of field studies or published records in those locations rather than being an indication of true absence of the bug. The detection of T. cruzi-infected wild mammals in many of these areas suggests the presence of the vectors. Additionally, recent efforts to model the geographic distribution of U.S. species based on the land cover, climate, and host composition of known collection sites indicate favorable habitat suitability in many of these unsurveyed or underreported regions (26, 137, 158, 259). Characteristics of each species are summarized in Table 3 and described in detail in the sections that follow. Description of U.S. Triatomine Species Triatoma gerstaeckeri (Stål). T. gerstaeckeri is one of the most frequently collected and tested species in the United States; 57.7% (1,038/1,800) of tested specimens were found to harbor T. cruzi. T. cruzi-infected specimens have been found in both Texas and New Mexico and in the majority of the counties where testing has been reported (Fig. 3B). Published reports from the 1930s to 1960s describe T. gerstaeckeri as a pest species of humans and livestock; the adult bugs were frequent invaders of rural houses in Texas, and reports of humans being bitten were common (217, 330, 332). Human encounters have been less frequently reported in recent decades (49, 151). Infected T. gerstaeckeri specimens were recently recovered from the residence of a child with acute Chagas' disease in southern Texas (151). In northeastern Mexico, this species is considered an important Chagas' disease vector due to its close association with human dwellings (184, 288). U.S. T. gerstaeckeri data derive predominantly from Texas, where the bug has been found in a wide variety of habitats. The species was collected from a rock squirrel burrow in a cave in the southeastern corner of New Mexico (341). Triatoma incrassata Usinger. T. incrassata is somewhat similar to T. protracta in size and general appearance of legs and head, but it has a distinctive abdominal margin which is largely yellow on the dorsal surface and entirely yellow on the ventral surface. It has been collected at lights in the two southern Arizona counties of Santa Cruz and Pima (Fig. 3C) (169, 255). The major mammalian hosts and T. cruzi infection prevalence for this species are unknown. Triatoma indictiva Neiva. T. indictiva was considered a subspecies of T. sanguisuga in the past but is currently accorded full species status (110, 169, 296). This species is very similar in appearance to T. sanguisuga, with the exception of the uniformly black pronotum and narrower horizontal markings on the abdominal edge. The distributions of the two species overlap in the central regions of Texas, with T. indictiva continuing further west to Arizona and T. sanguisuga continuing east to the Atlantic coast (Fig. 3D and K). Reported collection of T. indictiva is much less frequent than that of T. sanguisuga. Additional collection sites for T. indictiva in New Mexico and Arizona were provided in a map by Lent and Wygodzinsky in 1979, but specific location designations were not given (169). Specimens were collected from woodrat nests in New Mexico and at lights in Texas (229, 332). T. indictiva has been found naturally infected with T. cruzi in specimens from Texas (151, 229). Triatoma lecticularia (Stål). T. lecticularia has a geographic distribution similar to that of T. sanguisuga, from the south-central United States east to the Atlantic coast (Fig. 3E). Its range probably includes Oklahoma, Arkansas, Louisiana, Mississippi, and Alabama based on similarities in ecological characteristics between these states and adjacent areas where it has been reported. Specimens of T. lecticularia from New Mexico have been reported, but specific location information was not provided (254, 296). T. lecticularia had been variously classified as a subspecies of as well as synonymized with T. sanguisuga prior to Usinger's 1944 reclassification (296). Therefore, early reports of T. lecticularia and T. sanguisuga may be difficult to confirm without reviewing the actual specimens. Ryckman in 1984 contended that reports of T. lecticularia from Arizona and California are erroneous, presumably based on earlier taxonomic confusion and contemporary knowledge of the species distribution (254). T. lecticularia can be distinguished from T. sanguisuga and T. indictiva based on its shorter, domed head and uniform covering of all body surfaces with dark hairs. T. lecticularia has been collected from houses, dog kennels, woodrat nests, and rock squirrel burrows in hollow logs in Texas, from houses in South Carolina, and at lights in Missouri (151, 195, 256, 312, 345). In early reports, this species was described as a nuisance species, commonly found in well-constructed homes of central Texas (218). In 1940, Packchanian conducted experimental inoculation of the gut contents of a T. cruzi-infected T. lecticularia bug into the eye of a human subject in order to demonstrate the infectivity of a T. cruzi strain from Texas (216). Localized symptoms, fever, lymphadenopathy, and trypomastigotes visualized on blood films confirmed infection in this individual. The high T. cruzi infection prevalence (144/282; 51%) in T. lecticularia was derived primarily from specimens collected from woodrat nests in Texas (282, 332). Triatoma neotomae Neiva. In the United States, T. neotomae is known only from Texas, primarily the southern tip (Fig. 3F). The inclusion of other states in its range by some authors is most likely an error, as published records of T. neotomae outside Texas or northeastern Mexico could not be found. This species is similar in size to T. protracta but with distinctive yellow markings around the abdominal margin and basal half of wings, a glossy body surface, and a ventrally flattened abdomen. Also like T. protracta, this species is closely associated with Neotoma spp. of woodrats, for which it was named. It has been found almost exclusively in woodrat nests throughout its range, with a single report from a dog kennel in Cameron County, TX (151). The small sample size limits interpretation of this species' high cumulative T. cruzi infection prevalence (40/53; 76%); however, this is likely related to the high infection levels reported among woodrats in this region (49, 93, 219). Triatoma protracta (Uhler). T. cruzi was first reported in the United States from a T. protracta specimen collected in 1916 in a woodrat nest in San Diego County, CA (155). T. cruzi testing data are most abundant for this species, with an overall prevalence of 17.5% (723/4,124). Infected specimens have been reported from four of seven states across its range: California, Arizona, New Mexico, and Texas (Fig. 3G). T. protracta is closely associated with western woodrat species and is commonly found in nests throughout the bug's geographic distribution. Large aggregations of T. protracta were reported from roadbeds in southern California in an area where woodrat nests were removed as a consequence of highway construction (340). Attracted by lights, the displaced bugs frequently entered houses in the area and became a source of annoyance for residents. T. protracta has also been reported as frequently entering houses in other areas of California, New Mexico, and Arizona (187, 273, 304, 332, 336). First reported as a pest of humans in Yosemite Valley, CA, in the 1860s, T. protracta continues to be an important cause of severe allergic reactions in humans who are bitten (152, 198). This species was implicated in a human case of Chagas' disease in north-central California (203). Triatoma recurva (Stål). T. recurva naturally infected with T. cruzi has been found in the southern half of Arizona (Fig. 3H). A single report of T. recurva collected in western Texas has not been confirmed or replicated (138, 151). Early reports describe T. recurva as a pest of humans, primarily in the Alvardo Mine area of Yavapai County, AZ, where it was a common invader of houses and tents of mining employees (332, 336). Recent reports describe home invasions and hypersensitivity reactions due to bites that occurred in and around houses in Pima and Cochise Counties, AZ (152, 237). Although the species has been collected occasionally from woodrat nests, the woodrat is not considered the primary host of T. recurva (96, 255, 321). The preferred host for this species is unknown, but it has been observed in association with rodents, particularly rock squirrels, and feeds on reptiles and guinea pigs in laboratory settings (96, 255, 324, 334, 336). T. recurva is the largest of the U.S. species (average length, 29 mm) and has relatively hairless body surfaces, including the first two segments of the mouthparts. It is brown to black in appearance, with slender, long legs and head and an orange-yellow abdominal margin. Its body size, head and leg characteristics, and uniformly colored pronotum distinguish this species from others in its range. Triatoma rubida (Uhler). In the United States, T. rubida has been found from western Texas to southern California; T. cruzi-positive specimens have been reported from Arizona and Texas (Fig. 3I). The cumulative infection prevalence in the published literature is low (96/1,340; 7.2%). However, in a recent study, the gut contents of 65 (41%) of 158 T. rubida specimens collected in and around houses in Pima County, AZ, yielded positive results by T. cruzi PCR (237). Despite the presence of nymphal stages inside houses in this study, the authors remarked that the numbers were too low to conclude that colonization was established. In contrast, a study from Sonora, Mexico, reported that 68% of houses were colonized by T. rubida, suggesting that this species was domesticated in that region (221). Both the U.S. and Mexican study areas had experienced disruption of previously undisturbed environments considered suitable habitats for both triatomine and T. cruzi vertebrate hosts. Human bite encounters, including hypersensitivity reactions due to T. rubida, continue to be a public health issue in Arizona (152, 226, 237). This species has been frequently collected from woodrat nests throughout its range (96, 256, 321, 332, 336). It can be distinguished morphologically from other species in its range by the first antennal segment, which reaches or surpasses the tip of the head. Triatoma rubrofasciata (DeGeer). Described in 1733, T. rubrofasciata was the first species classified in the Triatominae subfamily and is the current type species for the Triatoma genus (270). It is the only triatomine species found in both the Eastern and Western Hemispheres and is frequently found in port cities in close association with the roof rat (Rattus rattus) (255). Molecular and morphometric data support the hypothesis that Old World triatomine species derive from T. rubrofasciata carried from North America with rats on sailing ships during the colonial period (136, 223, 270). In the United States, this species has been collected from houses in Florida and Hawaii and in chicken and pigeon coops and cat houses in Hawaii. Specimens have been reported from Jacksonville, FL, and Honolulu, HI (Fig. 3J) (296, 337). Wood (in 1946) reported 2 specimens collected from Honolulu to be infected with T. cruzi based on morphological and motility characteristics (337). Allergic reactions to T. rubrofasciata bites have been reported in humans from Hawaii (12). Triatoma sanguisuga (Leconte). T. sanguisuga is one of the most widely distributed species in the United States, with its range spanning from Texas and Oklahoma eastward to the Atlantic coast (Fig. 3K). This species has been reported in Pennsylvania, New Jersey, Maryland, and Kentucky, but without specific location data (169, 254, 296). Although published records are lacking, its range probably includes West Virginia. Reports of T. sanguisuga from states west of Texas were likely mistaken due to taxonomic reclassification (see “Triatoma indictiva Neiva” above). In every state where testing has been conducted, T. cruzi-infected T. sanguisuga has been found, including Texas, Oklahoma, Louisiana, Alabama, Tennessee, Georgia, and Florida. It has been collected from diverse natural settings across its range, in association with many different vertebrate hosts, including woodrats, cottonrats, armadillos, raccoons, opossums, frogs, dogs, chickens, horses, and humans (120, 150, 212, 215, 332, 348). Human annoyance and allergic reactions to T. sanguisuga bites were reported as early as the mid-1800s in Georgia, Kansas, Oklahoma, and Florida and recently in Louisiana (116, 147, 152, 161, 215). This species was found inside the residences of human Chagas' disease patients in Tennessee and Louisiana and in the vicinity of the home of a T. cruzi-seropositive blood donor in Mississippi (54, 90, 134). Paratriatoma hirsuta Barber. P. hirsuta is known from the western United States, collected from arid regions of California, Nevada, and Arizona (Fig. 3L). Although it has been demonstrated to be a competent vector of T. cruzi in experimental settings, a naturally infected specimen has yet to be reported (321). It has been most frequently collected from woodrat nests in its range but has also been found in houses and other human dwellings in Yavapai County, AZ, and Riverside County, CA, and at lights in Palm Springs, CA (251, 296, 336). Ryckman (in 1981) described this species as having important public health significance due to allergic reactions caused by its bite (252). This is one of the smallest U.S. triatomine species (average length, 13 mm) and can be distinguished from T. protracta, which is similar in size and geographic distribution, by a pervasive covering of dark hairs on all body surfaces. Human-Vector Interactions and T. cruzi Transmission Potential in the United States Eight of the 11 species have been associated with human bites, and seven have been implicated in allergic reactions (Table 3). Allergic reactions occur in response to antigens delivered in the vector saliva during blood feeding and are unrelated to the T. cruzi infection status of the bug. Most allergic reactions are localized at the bite site, characterized by a large welt and intense itching (315). Severe reactions are generally systemic and may involve angiodema, urticaria, difficulty breathing, nausea, diarrhea, and/or anaphylaxis (152, 226). Although allergic reactions to triatomine bites have been reported from states throughout the southern United States, the incidence is highest in the southwestern states, with T. protracta and T. rubida most frequently implicated (106, 152, 204, 226, 237). The most common scenario involves invasion of an adult bug into a human dwelling, where it bites a sleeping individual. Contemporary encounters between humans and triatomine bugs in the United States are often associated with destruction or invasion of vertebrate host habitats, compromised housing structures, or both. Disruption of host burrows (as described above for T. protracta) provokes the bugs to seek new refuges, and their innate attraction to lights often leads them to nearby human dwellings. Most triatomine species show flexibility in host and habitat requirements, which allows them to adapt to changing environments. A host preference for some species has been difficult to establish due to association with multiple vertebrate habitats and the ability of the insects to mature and reproduce successfully on multiple host species in laboratory settings. Although mammals are the only vertebrate reservoirs for T. cruzi, many triatomine species utilize other animal groups as blood hosts, including reptiles and amphibians (T. gerstaeckeri, T. protracta, T. recurva, T. rubida, and T. sanguisuga) and birds (T. gerstaeckeri and T. sanguisuga) (169, 228, 253, 338). A recent blood meal analysis study of Texas field specimens provides evidence of a broad host range for T. gerstaeckeri and T. sanguisuga. The DNAs from nine vertebrate species (woodrat, dog, cat, cow, pig, raccoon, skunk, armadillo, and human) were detected in T. gerstaeckeri gut specimens, and DNAs from three species (dog, avian, and human) were detected in T. sanguisuga gut specimens (149). Because vector colonization of houses in the United States is rare, the risk of vector-borne transmission to humans is considered to be quite low. With the exception of the 2006 Louisiana case in which the residence was found to harbor triatomine colonies, vector-borne transmission to humans in the United States has been attributed to adult bugs invading houses (90, 134, 203). Expansion of human settlements into environments that support an active sylvatic disease cycle could result in an increase in adult invaders and, potentially, colonization events. Colonization of houses by triatomines is an important factor in vector-borne transmission because it increases the probability of encounters between humans and potentially infected vectors. In addition to adaptability to domestic structures, triatomine feeding and defecation behaviors are important risk factors for vector-borne transmission and vary across species. The timing and placement of defecation after feeding greatly influence the risk of transmission via fecal contamination of the host bite site or other exposed tissues. A small number of studies have reported on these characteristics in U.S. species. In 1951 Wood reported the following average postfeeding defecation times (minutes) for the adults of four U.S. species: T. protracta, 30.6 (n = 10); T. rubida, 1.6 (n = 5); T. recurva, 75.7 (n = 3); and P. hirusta, 35.0 (n = 2) (327). In a similar study in 2007 using both nymphs and adults of three Mexican species (also present in the United States), Martinez-Ibarra et al. reported the following results: T. protracta, 6.7 (n = 475); T. lecticularia, 8.3 (n = 368); and T. gerstaeckeri, 11.5 (n = 733) (183). Likewise, Zeledon et al. (1970) reported the following results for nymphs and adults of three Latin American species: R. prolixus, 3.2 (n = 210); T. infestans, 3.5 (n = 210); and T. dimidiata, 11.3 (n = 210) (352). In 1970 Pippin reported the proportion of bugs defecating within 2 min postfeeding for adults of two U.S. and one Latin American species: R. prolixus, 74.6% (n = 169); T. gerstaeckeri 19.4% (n = 160); and T. sanguisuga 16.9% (n = 136) (228). Similarly, in 2009 Klotz et al. reported the proportion of bugs defecating before or directly after feeding for adults of two U.S. species: T. rubida, 45% (n = 40); and T. protracta, 19.4% (n = 31) (153). In that study, it was noted that none of the bugs of either species defecated on the host during the experiment. Although direct comparisons across studies is problematic due to variation in methods and conditions (e.g., temperature, blood host, and feeding apparatus), it appears that U.S. species in general exhibit greater postfeeding defecation delays than important Latin American vector species. Delayed defecation and a low frequency of domestic colonization contribute to a low probability of autochthonous U.S. human infection due to vector-borne transmission, which is the primary route of infection in areas of hyperendemicity in Latin America.