Analysis of Trypanosoma cruzi transmission

What is it about?

Calculating epidemiological measures of infection by Trypanosoma cruzi, the causative agent of Chagas disease, is complex, because it involves several species, different stages of infection in humans and multiple transmission routes. Using the next-generation matrix method, we analyzed a model which considers the three stages of human infection, triatomines and dogs (the main domestic reservoirs of T. cruzi when triatomines are present) and the main transmission routes. We derived R0 and typereproduction numbers T. We deduced formulas for the number of new infections generated through each transmission route by each infected individual. We applied our findings in Argentine Gran Chaco. The expressions achieved allowed quantifying the high infectivity of dogs and emphasizing the epidemiological importance of the long and asymptomatic chronic indeterminate stage in humans in the spread of the infection. According to the model, it is expected that one infected human infects 27 triatomines, that 100 infected triatomines are necessary to infect one human and 25 to infect a dog, and that each dog infects on average one triatomine per day. Our results may allow quantifying the effect of control measures on infected humans, triatomines and dogs (or other highly infected vertebrate) or on a specific route of transmission, in other scenarios.

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Why is it important?

We analyzed a model of Trypanosoma cruzi infection in the human population, considering three clinical infected stages corresponding to the evolution of the disease and three main transmission routes (vectorial, congenital and transfusional). The model involves the populations of vectors (triatomines) and dogs (the main domestic reservoirs of T. cruzi when triatomines are present). We analyzed the model using the next generation matrix method giving a detailed biological and epidemiological significance at each entry of the matrix. From this matrix, we showed how to estimate R0, but also derived explicit formulas of the type reproduction numbers T, which are more specific thresholds: expected number of infected individuals of one specific type generated by one individual of that type in a totally susceptible population taking into account the fact that the infection will pass through other types of individuals of the same species or another species. Numbers T allow estimating the control effort required to eliminate an infectious disease when it applied to a specific subpopulation. Moreover, for the first time, we deduced expressions that allowed quantifying the number of new infections in each species generated through each infected individual and through each transmission route. We applied our findings in the Argentine Gran Chaco because the model represents the situation in that region and there is available information about the values of the parameters. The derived expressions can be used under various scenarios and parameter sets. We hope that the expressions obtained can be useful not only to better understand the infection dynamics, that is, to estimate the number of cases generated through the different routes of transmission by infected triatomines and dogs (or any other highly infected mammal) or by any infected human, but also to search for better strategies of epidemiological control.

Perspectives