Abstract. We evaluated Trypanosoma cruzi infection rates by means of minicircle DNA-based polymerase chain reactions (PCRs) in 70 starved Mepraia gajardoi from northern Chile and 65 M. spinolai from central Chile after feeding. Immediately after collection in the field, 20% of M. gajardoi were found infected; after feeding, 67% of the uninfected were infected. One group of M. spinolai seemed to be completely uninfected, but after the first and second feedings, 62% and 59% were positive, respectively.The protozoan Trypanosoma cruzi (Kinetoplastida Trypanosomatidae), etiologic agent of Chagas disease, is mainly transmitted to mammals by contamination of the skin lesions with triatomine insect feces with parasites.1 In a regularly fed infected triatomine, T. cruzi amplify as epimastigotes, which later, differentiate to trypomastigotes that are eliminated by the feces. Several biological factors modulate T. cruzi-insect vector interactions, including food supply, intestinal components, gut flora, T. cruzi strain, and insect physiology.2 The Triatominae subfamily is composed of 146 species distributed in 18 genera and grouped into six tribes.3,4 Important vectors rapidly defecate on the host. These vectors urinate/defecate during or after feeding from a rapid to a delayed behavior according the triatomine species.5 Several factors, such as the competition of enterobacteria of the flora and T. cruzi with its vector for nutrients, and thereby, feeding affect not only parasite density and insect molting but also, changes in the epimastigote/trypomastigote ratio in the rectum of Triatoma infestans (Hemiptera, Triatominae). 6 The rectal parasite density increases until several weeks post-infection with regular blood meals, reaching maximal values of several million parasites. The parasite density is only strongly influenced by very long starvation up to 20 weeks. [7][8][9] In the laboratory, the feeding status also modulated the olfactory host search behavior of triatomines.10 In Chile, the wild vector of Chagas disease is Mepraia spp. (Hemiptera, Triatominae), which is composed of three different species: M. gajardoi, M. spinolai, and M. parapatrica.11-13 M. gajardoi and M. spinolai are frequently found in corrals of domestic animals and stony hills and rock crevices of arid zones of the northern and semiarid areas of central Chile, respectively. Between 11% and 27% infections with T. cruzi were found in M. gajardoi.14 Infection rates in M. spinolai range from 46% to 54%. 15 In the national surveillance program of triatomine vectors, 11-47% of these species were infected. 16 Thus, the aim of this study is to determine T. cruzi infection in M. gajardoi and M. spinolai by assessing infection on insects under natural conditions (right after collection) and reassessing after feeding in the laboratory.In the laboratory overall, 70 M. gajardoi nymphs stages III-V from Vitor and 35 and 30 M. spinolai nymphs stages III-V from Illapel and Colina, respectively, were studied. The insects were maintained and fed individually inside ...