Discussion

According to the National System of Epidemiological Surveillance, Chagas disease is not classified in the 20 causes of death in the country; however, it is a disease where the incidence is mainly located in the most vulnerable states of the country, and it requires attention for its epidemiological surveillance. In this work, it was demonstrated that Chagas disease continues to spread throughout the Mexican Republic, and it continues to be a national concern due to the increase in incidence in the last 10 years (Fig. 1). Annual increases in reported case rates may be due to the longitudinal improvements in surveillance and diagnostic capacities. As previously mentioned, the states located in the Pacific Coast (Gulf of Tehuantepec) and the Gulf Coast of Mexico were the most affected by the infection of this parasite (Fig. 2a). The analysis of the temporal trends of the incidence rates of the five groups in which the states of Mexico were divided shows a slight increase in the incidence rates with regards to the time (groups A, B, C and D), and incidences fluctuating with the incidence rates of group E (Fig. 2b). Previous studies have demonstrated that the southern states of the country have been a focal point due to the high incidence in the open population and in blood donors [Reference Segura and Escobar-Mesa8–Reference Hernández-Romano11]. Furthermore, there are studies that indicate that there is no evidence of an excess risk of infection related to gender [Reference Mott12, Reference Prata13]; however, in the present work, we identified that the most vulnerable groups of infection by T. cruzi were male subjects between 25–44 and 45–49 years old (Fig. 3). It is possible that the acquisition of the disease is strongly related to occupational activities since, in the same age groups in the female gender, the incidences were significantly different (P = 0.05). It is known that agriculture is one of the main occupational activities in the population of the endemic southern states; these activities could play an important role in the incidents identified, suggesting that the possible way of infection of T. cruzi in these regions was vector transmission in the working areas. Even though these data reflect the possible acquisition of the infection through occupational activities, it is necessary to have additional demographic information to prove this hypothesis. It is important to mention that the incidence in the population group under 1 year old (0.5) was significantly higher compared with the other age groups (up to 15–19 years old), this could be related to the increase of congenital transmission in recent years [Reference Jackson14, Reference Oliveira15]. Blood transfusion and blood components can be a transmission route of Chagas disease [Reference Blumental16, Reference Ries17], which is why, serological screening of blood donors has become very significant in the last decades. Mexico, as an endemic country, recognises serological screening of blood and blood components as compulsory for T. cruzi according to the Official Mexican Standard (NOM-253-SSA1-2012) [18], ‘For the provision of human blood and its components for therapeutic purposes’. Since a large number of seropositive cases has been identified in blood banks, studies about the incidence of Chagas disease in blood donors have demonstrated that the true extent of this infection is reflected in these blood collection sites, because the incidences of seropositive donations at the national level, as well as in some specific populations and in field studies, are significantly different from the cases reported outside of these transfusion centres [Reference Mazariego-Arana19, Reference Sánchez-Guillén20]. The latter clearly shows that blood banks could be opportunity areas to detect new cases, and thus, to estimate values close to the actual incidence in the general population. Regarding the analysis of geographical and seasonal distribution of the disease, an increase from March to August is observed, reaching a maximum peak in June, months of high temperature and heavy rains in the central and south parts of the country. A sharp decrease during the cold and dry months (September–December) was observed. These results reflect a direct relationship between the environmental temperature increase by season and the number of new cases of Chagas disease. Nowadays, little is known about the influence of temperature and altitude on the incidence of Chagas disease in our country. A previous study has reported that altitude could be an important factor in the development of the rectosigmoid length in Andean patients with Chagas disease [Reference Lopes21], which suggests a direct relation between altitude and the virulence of the strain. Moreover, De Fuentes-Vicente et al. [Reference De Fuentes-Vicente22] measured in mouse models, the activity of phenoloxidase, key enzyme of defense response of triatomines, the level of parasitaemia and the number of amastigotes. In this study, it was reported that the evaluated parameters were significantly higher for infectious processes of T. cruzi from 700 m a.s.l., suggesting that this species could be more virulent than others. The influence of temperature, rain and altitude was reflected with greater incidences in the coastal states of Veracruz, Chiapas, Quintana Roo, Oaxaca and Yucatán, and in states of the central part of the country, such as Morelos which is located at 720 m a.s.l. in the southern part of the state; while, in the states with dry weather and short rainy seasons such as Durango, Chihuahua and Coahuila, incidences were lower. The National System of Epidemiological Surveillance suggests housing regulation and the use of residual insecticides, such as synthetic pyrethroids, to reduce risk and to control domestic infestation by vectors. This activity must be carried out before the rainy season, between February and May, since it is reported that, at this time of the year, the number of triatomine bites is higher, these data are consistent with the one reported in the present work. Torres-Montero et al. [Reference Torres-Montero23] have shown fluctuations in intradomiciliary infestation, with a maximum peak of presence of adult triatomines in June, month in which a greater number of cases was identified. Although, in this work, socioeconomic factors were not considered, in previous studies, these endemic areas have been identified with a high percentage of intradomiciliary and peridomiciliary infestation. This phenomenon is associated with precarious housing conditions with animal pens near the house [Reference Carbajal de la-Fuente24–Reference Ramsey26]. Low incidences were reported in Mexico City and the urban area, this could be related to the habitat decrease of triatomines and better socioeconomic environments, compared with the endemic areas in the south part of the country. It has been suggested that the cases of Chagas disease in urban areas is related to migration from rural to urban areas [Reference Ramsey27, Reference Escamilla-Guerrero28]. Results reported here can help to enrich the ‘Program of Specific Action to Prevent and Control Chagas Disease 2013–2018’ carried out by the Federal Government and the development of local and state situational diagnoses to assess risks of transmission and to implement a comprehensive prevention and control programme. Among the limitations shown in the present work, there could be: limited coverage of health services in the detection of Chagas disease, cases not reported by health institutions, cases not registered due to lack of assistance from the population for diagnosis and treatment, possible detection of false positive and negative cases in the diagnosis, among others. The analysis of the Chagas disease data during a decade shows that the situation of the disease in Mexico is understated, compared with the data reported in different research articles about seroprevalence of Chagas disease. It is important to mention that a large number of patients are chronic and without symptoms, without visible development of the disease and for which, clinical, epidemiological, laboratory and cabinet follow-up should be improved. The present analysis showed that the rate of Chagas disease increased, possibly because state programmes expanded their research for new cases; presumably the disease is associated with gender occupation activities. However, it is important to mention that the GDE does not consider the cases identified in blood transfusion centres; therefore, it is suspected that the incidences reported in the present work are higher. These results summarise and recall how important it is to implement the monitoring of Chagas disease cases in all instances of health where the detection is carried out in order to determine the current situation of this disease. Therefore, blood banks are opportunity areas to establish flowcharts of confirmatory studies and references to take care of confirmed cases.