Higher Seroprevalence of Trypanosoma cruzi Infection in Dogs Than in Humans in an Urban Area of Campeche,Mexico

T he reservoir competence of dogs and cats for Trypanosoma cruzi infection and their epidemiological importance has been identified in different geographical regions of Latin-America and recently also recognized in the rural north western Argentina, where dogs were three more epidemiologically important than cats (Gürtler et al. 2007). Few studies have been done on this subject. In Mexico, some of these include the pioneering work in the 1940s, indicating the role of different mammals as reservoirs for T. cruzi infection (Aguirre 1947, Beltrán 1949) and recent work where dogs have been identified as important reservoirs in some areas of Mexico (Sosa-Jurado et al. 2004, Estrada-Franco et al. 2006, Jimenez-Coello et al. 2008). The transmission of T. cruzi to humans is more common in the domestic environment where triatomine bugs and animal reservoirs such as dogs are involved (Cohen and Gürtler 2001). The city of Campeche, which is located in the Yucatan Peninsula, is inhabited by ∼96,000 dogs and ∼168,000 humans (Department of Zoonosis, Health Ministry, and INEGI-2005); and the only vector recognized in this region, involved in sylvatic and peridomestic transmission cycles, is Triatoma dimidiata (Dumonteil et al. 2002). In the present work, we sampled in the city of Campeche 262 dogs (148 stray dogs and 114 pet dogs) and 2800 young people (range 15–20 years old) for the presence of T. cruzi antibodies. Enzyme-linked immunosorbent assay was the primary serological screening test followed by secondary confirmatory Western blotting and Indirect Immunoflourescence tests (Mazariego-Arana et al. 2001). The detection of anti-T. cruzi antibodies by two serologic tests was the criterion used to characterize an infected individual.

We found a positive serology for T. cruzi in 14 out of 148 (9.5%) stray dogs and in 6 out of 114 pet dogs (5.3%), the total seroprevalence in dogs was 20 out of 262 (7.6%). These data suggest that stray dogs may be more vulnerable, because they are more exposed to peridomestic transmission; whereas pet dogs that live inside houses are more protected from infection. If we compare the 7.6% of T. cruzi positive dog population in Campeche with that recently reported of 14% in Yucatán (Jimenez-Coello et al. 2008), it represents a little more than half of infection in Campeche, in spite of the fact that the vector is the same in both regions. One possible explanation could be that the infection rate of T. dimidiata in Yucatán can reach up to 46% as reported (Guzman-Tapia et al. 2005); whereas for Campeche, the infection rate in T. dimidiata is around 20% (unpublished data). This may suggest that T. dimidiata in Campeche has lower vector competence.

According to published data in Mexico, the seroprevalence of T. cruzi infection in dogs is around 10%; whereas for endemic areas of Central and South America, the infection rate is higher than 27% (Montenegro et al. 2002, Gürtler et al. 2007). In our work, the observed seroprevalence was much lower, which may be due to various reasons. One hypothesis is that dog is not the preferred source of meals for T. dimidiata in the peridomestic cycle of transmission in Campeche or that T. dimidiata is a poor vector. However, taking into account the high population density of dogs in the city, they could be unquestionably an important reservoir of the disease in this context.

On the other hand, of the young people sampled, only 3 out of 2800 (0.1%) tested positive for T. cruzi infection, which was 76 times lower than the infection rate found in dogs (Table 1). These findings may suggest that trypanosomiasis in Campeche is mainly a zoonosis, where T. dimidiata shows a synantropic behaviour with poor domiciliation. Consequently, the main cycle for T. cruzi transmission is primarily sylvatic and peridomestic with low involvement of humans in the cycle. In addition, peridomestic reservoir animals (such as dogs) may function more like a natural barrier blocking the transmission to humans. This hypothesis deserves further investigation with further studies searching for blood meal sources in triatomine vectors.

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Table 1.

Comparative Results of Seroprevalence in Dogs and Humans in Campeche, Mexico

Origin of samples	Seroprevalence (%)
Stray dogs	14/148 (9.5%)
Pet dogs	6/114 (5.3%)
Total dogs	20/262 (7.6%)
Human subjects	3/2800 (0.1%)

Finally, we observed in Western blotting that dogs' antibodies recognized three immunodominant antigens of 15, 75, and 80 KDa; whereas the human antibodies strongly recognized 21, 30, 52, 99, and 116 KDa proteins as immunodominant antigens. These findings tell us that the humoral immune responses between humans and dogs to the same antigen challenge induce a different clonal repertoire, producing a distinctive antigen pattern. In general, these findings are to be expected, as immune response diversity relies on major histocompatibility complex. Thus, the induction and observation of different patterns of humoral immune response between species could be a used as antigenic finger printing.

Although dogs have been used as an animal model for Chagas disease, it has been observed that experimental infection usually requires a high concentration of parasites to model the disease (Guedas et al. 2002), which suggests that their immune response against T. cruzi infection could be different enough to infer a natural resistance to infection and the outcome of the disease.

In conclusion, T. cruzi infection in Campeche, Mexico, represents a low potential risk to humans but deserves vigilance.