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Chagas disease is a serious human parasitic disease in the Americas that is caused by the flagellate protozoan Trypanosoma cruzi and transmitted by blood-sucking insects of the subfamily Triatominae (Hemiptera: Reduviidae).1 Because of low parasitemia in different hosts with chronic infections, the diagnosis of this disease is made mainly by serologic methods. However, low sensitivity of currently used parasite detection techniques such as xenodiagnosis limit their use in the diagnosis of Chagas disease. To avoid this problem, efforts have been made to detect T. cruzi DNA in blood samples by polymerase chain reaction (PCR) assays. Specific amplification of kinetoplast minicircle DNA from animals and humans has been reported.2,3 Detection of T. cruzi minicircle DNA by PCR also has been used in naturally infected triatomines4,5 and insect vectors fed on blood from infected patients.6,7
The main vector of Chagas disease in Chile is the predominantly domestic bug Triatoma infestans, which has been successfully controlled in this country.8 The wild vector Mepraia spinolai is often found in rocky sylvatic ecotopes associated with small rodents and rabbits and also in peridomestic ecotopes such as henhouses and goat corrals.9 Mepraia spinolai is endemic in Chile and distributed between the 22°S and 33°S.10 Unlike other triatomines, this species is usually a diurnal bug, but it is active at night.11
More than 150 species of sylvatic mammals have been found naturally infected with T. cruzi, and some of these reservoirs have an important role in the maintenance and interaction of domestic, peridomestic, and sylvatic cycles of Chagas disease.12 One study in Chile with the native rodent Octodon degus and microscopic blood examination and xenodiagnosis with Triatoma infestans reported an infection rate of 8.9%.13 Other studies detected variable frequencies of infection ranging from 2% to 21% by xenodiagnosis with Triatoma infestans and hemoaglutination, respectively.14,15 Octodon degus inhabits stone quarries throughout the year, which makes it an ideal host for M. spinolai. In this study, we analyzed detection of T. cruzi by PCR in O. degus blood samples and intestinal contents of both insect vectors, Triatoma infestans and M. spinolai fed on peripheral blood (xenodiagnosis) of O. degus.
The study site was located in the Las Chinchillas National Reserve (31°30'03''S, 71°06'20''W), which is approximately 300 km north of Santiago, Chile and 60 km east of the Pacific coast. The reserve is in a mountainous terrain (area = 4,229 hectares) with a semiarid climate and sporadic precipitation concentrated during winter months.16
In this study, we captured 35 O. degus with traps (H. B. Sherman Traps, Tallahassee, FL) in diurnal and nocturnal periods. Captured animals were weighed and anesthetized with isofurane at a dose of 13 mg/kg of body weight. A small area of abdominal skin was cleaned for anesthetic injection. Once the animal was completely anesthetized, 0.5–1.0 mL of blood was withdrawn by cardiac venipuncture using heparinized syringes. After blood samples were obtained, O. degus were transported to the laboratory and kept in a breeding ground (Faculty of Medicine, University of Chile, Santiago, Chile) at 25°C and a relative humidity of 60% with a 12:12 hour light:dark photoperiod, individually housed in plastic cages (31.0 x 37.0 cm), and given water and alfalfa pellets ad libitum.
Extraction of genomic DNA from 0.2 mL of whole blood from O. degus was performed with the E.Z.N.A. kit (Omega Bioteck, Inc., Doraville, GA). DNA was concentrated by ethanol precipitation and resuspended in 50 µL of deionized, sterile water. The PCR was performed as previously reported using primers 121 and 122 to amplify the variable region of minicircle kinetoplast DNA.17,18 The sensitivity of detection of minicircle DNA by PCR with the primers and conditions used is 0.4 log parasite genome equivalents/mL.19 Each experiment included positive and negative controls. Samples were tested in triplicate, and an animal was considered positive when at least two of the three assays showed positive results. Doubtful samples (i.e., when one of three PCR assays showed positive results) were retested four times by PCR. Animals with four or more positive PCR results were considered positive but with low parasitemias. The PCR products were analyzed by electrophoresis on 2% agarose gels and visualized by staining with ethidium bromide. A 330-basepair product indicated a positive result.
For xenodiagnosis purposes, O. degus specimens were intramuscularly anesthetized with 50 mg/kg of ketamine and placed in a tray with 10–20 uninfected first and second instar nymphs of each vector species. Triatoma infestans and M. spinolai nymphs were obtained from established insect colonies at the Faculty of Medicine and Faculty of Science of the University of Chile (Santiago, Chile), respectively. The amount of blood extracted by each pool of bugs from O. degus did not differ between the two vector species (t test; F1,14 = 0.155, P = 0.699). Thirty and sixty days post-feeding, a pool of feces from all bugs fed with each O. degus was obtained by abdominal extrusion. Five microliters of fresh fecal material was compressed between a glass slide and an 18 mm x 18 mm coverslip20 and examined with a light microscope (Diaphot-FXA; Nikon, Tokyo, Japan). Motile parasites in 50 microscopic fields were observed at a magnification of 400x. A pool of total intestinal contents 60 days post-feeding was mixed with 200 µL of phosphate-buffered saline, centrifuged at 10,000 x g, and frozen at –20°C for PCR assay. A 1–5-µL sample of this extract was boiled for 10 minutes and used as a DNA template. Since intestinal contents were free of fresh blood, no DNA extraction was required.5 After xenodiagnosis, all animals were returned to the study site. Experiments were reviewed and approved by the Ethical Committee of the Faculty of Medicine, University of Chile.
Our results indicated that 22.9% of the O. degus at the study site were naturally infected with T. cruzi. However, these results varied with the biologic sample used for the PCR and the vector species used for the xenodiagnosis. Specifically, PCR analysis of blood samples did not detect T. cruzi DNA in the O. degus specimens. Conversely, results of the PCR of intestinal contents indicated that both vectors were positive for T. cruzi. The PCR performed with M. spinolai intestinal contents detected four times more T. cruzi-positive O. degus than the PCR with Triatoma infestans intestinal contents (22.9% and 5.7%, respectively). These differences in detection sensitivity were statistically significant (
2 = 4.20, degrees of freedom = 1, P = 0.040). Table 1
summarizes detection of T. cruzi by each vector species using blood and intestinal content samples for PCR analysis. Direct microscopic observation of fecal samples from both vector species showed that two O. degus were infected with T. cruzi (5.7%). These rodents were also positive for T. cruzi by PCR for intestinal contents of both vector species.
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Our results of higher parasite detection sensitivities by xenodiagnosis combined with intestinal content PCR assays when compared with blood sample PCRs should be further analyzed. Although the volume of blood used to extract DNA and the volume to feed insect vectors was equivalent, parasitemia was observed only with insect intestinal samples. This finding can be explained by the high rate of parasite amplification of epimastigote forms in the intestines of the bugs. Parasitemias in mammals during the chronic phase of the disease may be too low to be detected by the PCR. Parasitemia varies among O. degus, and rodents that were positive by PCR with both insects showed higher parasitemias than those detected by PCR and only M. spinolai samples. This fact is confirmed by the higher amounts of DNA amplicons in four rodents. Conversely, four O. degus were detected only with M. spinolai and seven PCR assays.
Lower infection rates in O. degus were observed in the present study when compared with results of previous surveys. The infection rate detected in this study (22.9%) is less than half the rate reported for this rodent species in Las Chinchillas National Reserve five years ago.2 Both studies used PCR assays on O. degus blood samples. However, our results with blood did not detect T. cruzi-infected rodents. This fluctuation in the percentage of T. cruzi-infected O. degus may have resulted from natural oscillations in parasite-vector-host systems.
In summary, this study reports the improvement of T. cruzi detection in sylvatic animals by a combination of PCR and xenodiagnosis using sylvatic vectors from the same area as the mammals analyzed. We showed that using M. spinolai, the wild vector of Chagas disease in Chile, it would be possible to carry out more accurate epidemiologic studies in disease-endemic areas of with low T. cruzi parasitemias.
Received May 30, 2006. Accepted for publication September 28, 2006.
Financial support: This study was supported by FONDECYT grants 1040762 to Aldo Solari, 1040711 to Pedro E. Cattan, and 3050033 to Carezza Botto-Mahan.
* Address correspondence to Aldo Solari, Program of Cellular and Molecular Biology, Faculty of Medicine, University of Chile, Casilla 70086, Santiago 7, Chile. E-mail: asolari{at}machi.med.uchile.cl 
Authors’ addresses: Ricardo Campos, Sylvia Ortiz, and Aldo Solari, Program of Cellular and Molecular Biology, Faculty of Medicine, University of Chile, Casilla 70086, Santiago 7, Chile, Telephone: 56-2-978-6062, Fax: 56-2-735-5580, E-mail: asolari{at}machi.med.uchile.cl. Carezza Botto-Mahan, Department of Ecological Sciences, Faculty of Sciences, University of Chile, Casilla 653, Santiago, Chile, Telephone: 56-2-978-7232, Fax: 56-2-272-7363, E-mail: cbotto{at}uchile.cl. Mariana Acuña and Pedro E. Cattan, Department of Biological Sciences, Faculty of Veterinary Sciences, University of Chile, Casilla 2 Correo 15, Santiago, Chile, Telephone: 56-2-978-5629, Fax: 56-2-978-5526, E-mail: pcattan{at}uchile.cl.
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