Complementary Paths to Chagas Disease Elimination: The Impact of Combining Vector Control With Etiological Treatment

Abstract

Background: The World Health Organization's 2020 goals for Chagas disease are (1) interrupting vector-borne intradomiciliary transmission and (2) having all infected people under care in endemic countries. Insecticide spraying has proved efficacious for reaching the first goal, but active transmission remains in several regions. For the second, treatment has mostly been restricted to recently infected patients, who comprise only a small proportion of all infected individuals.

Methods: We extended our previous dynamic transmission model to simulate a domestic Chagas disease transmission cycle and examined the effects of both vector control and etiological treatment on achieving the operational criterion proposed by the Pan American Health Organization for intradomiciliary, vectorial transmission interruption (ie, <2% seroprevalence in children <5 years of age).

Results: Depending on endemicity, an antivectorial intervention that decreases vector density by 90% annually would achieve the transmission interruption criterion in 2-3 years (low endemicity) to >30 years (high endemicity). When this strategy is combined with annual etiological treatment in 10% of the infected human population, the seroprevalence criterion would be achieved, respectively, in 1 and 11 years.

Conclusions: Combining highly effective vector control with etiological (trypanocidal) treatment in humans would substantially reduce time to transmission interruption as well as infection incidence and prevalence. However, the success of vector control may depend on prevailing vector species. It will be crucial to improve the coverage of screening programs, the performance of diagnostic tests, the proportion of people treated, and the efficacy of trypanocidal drugs. While screening and access can be incremented as part of strengthening the health systems response, improving diagnostics performance and drug efficacy will require further research.

Figure 1.

Combined impact of vector control and effective parasite clearance (measured as proportion of parasitological cure [PPC]) on years to reduce seroprevalence in children under 5 to <2%. Annual vector control defines the proportion by which vector density is reduced (0–100%); annual PPC defines the proportion of humans effectively treated, ie, the percentage of the infected human population achieving parasitological cure (0–40%). The color scale corresponds to number of years to achieve the serological criterion. The panels represent: A: low; B: moderate; C: high; D: very high endemicity levels (see Supplementary Materials).

Figure 2.

Infection prevalence in humans (red lines), nonhuman mammal hosts (green lines), and domiciliated Trypanosoma cruzi vectors (blue lines) over 30 years following the implementation of sustained and continuous control strategies beginning at year 1. A, B, and C present, respectively, prevalence trends following vector control on its own that leads to 10%, 50%, and 90% reductions in vector density. D–F depict prevalence trends following implementation of vector control (same reductions in vector density as above) in combination with an annual 10% proportion of parasitological cure in the population through treatment of the T. cruzi–infected human population.

Figure 3.

Age-specific prevalence profiles of Trypanosoma cruzi infection in the human population in a highly endemic setting, following over 30 years the implementation of sustained control strategies. A, B, and C present, respectively, age prevalence profiles corresponding to vector control and status quo treatment (1%), with annual reductions in vector density of 10%, 50%, and 90%. D–F depict human T. cruzi infection prevalence profiles following the same reductions in vector density as above in combination with etiological treatment that effects a 10% population parasite cure among the infected population annually. Horizontal blue dashed lines indicate the 2% seroprevalence threshold in children under 5. The color scale represents time, with blue representing the beginning of the intervention and red representing 30 years of sustained intervention.

Figure 4.

Probability of achieving 10% proportion of parasite clearance (PPC; A) or 20% PPC (B) in a T. cruzi–infected human population based on the combined probability of being diagnosed and treated for Chagas disase. The horizontal axis represents the combined contribution of diagnosis as a product of the proportion of infected people who are tested (pT) and the proportion of those tested with a positive test result, that is, the sensitivity of the test (pP). The vertical axis represents the combined contribution of treatment, as the product of the proportion of those testing positive who are treated with currently available drugs (pD) and respond to treatment by clearing parasites according to efficacy (pE). Colored lines represent the proportion (p) of infected people who would have to be reached by a test and treat program (90% [blue], 50% [red], 33% [green], and 20% [orange]) to achieve the desired level of effective PPC.