Agent-based mathematical modeling as a tool for estimating Trypanosoma cruzi vector-host contact rates

Modeling the impact of non-human host predation on the transmission of Chagas disease

In addition to the traditional transmission route via the biting-and-defecating process, non-human host predation of triatomines is recognized as another significant avenue for Chagas disease transmission. In this paper, we develop an eco-epidemiological model to investigate the impact of predation on the disease's spread. Two critical thresholds, Rvp (the basic reproduction number of triatomines) and R0p (the basic reproduction number of the Chagas parasite), are derived to delineate the model's dynamics. Through the construction of appropriate Lyapunov functions and the application of the Bendixson-Dulac theorem, the global asymptotic stabilities of the equilibria are fully established. The vector-free equilibrium E0 is globally stable when Rvp<1. E1, the disease-free equilibrium, is globally stable when Rvp>1 and R0p<1, while the endemic equilibrium E∗ is globally stable when both Rvp>1 and R0p>1. Numerical simulations highlight that the degree of host predation on triatomines, influenced by non-human hosts activities, can variably increase or decrease the Chagas disease transmission risk. Specifically, low or high levels of host predation can reduce R0p to below unity, while intermediate levels may increase the infected host populations, albeit with a reduction in R0p. These findings highlight the role played by non-human hosts and offer crucial insights for the prevention and control of Chagas disease.

Effectiveness of fluralaner treatment regimens for the control of canine Chagas disease: A mathematical modeling study

BACKGROUND

Canine Chagas disease is caused by the protozoan parasite Trypanosoma cruzi and transmitted by insect triatomine vectors known as kissing bugs. The agent can cause cardiac damage and long-term heart disease and death in humans, dogs, and other mammals. In laboratory settings, treatment of dogs with systemic insecticides has been shown to be highly efficacious at killing triatomines that feed on treated dogs.

METHOD

We developed compartmental vector-host models of T. cruzi transmission between the triatomine and dog population accounting for the impact of seasonality and triatomine migration on disease transmission dynamics. We considered a single vector-host model without seasonality, and model with seasonality, and a spatially coupled model. We used the models to evaluate the effectiveness of the insecticide fluralaner with different durations of treatment regimens for reducing T. cruzi infection in different transmission settings.

RESULTS

In low and medium transmission settings, our model showed a marginal difference between the 3-month and 6-month regimens for reducing T. cruzi infection among dogs. The difference increases in the presence of seasonality and triatomine migration from a sylvatic transmission setting. In high transmission settings, the 3-month regimen was substantially more effective in reducing T. cruzi infections in dogs than the other regimens. Our model showed that increased migration rate reduces fluralaner effectiveness in all treatment regimens, but the relative reduction in effectiveness is minimal during the first years of treatment. However, if an additional 10% or more of triatomines killed by dog treatment were eaten by dogs, treatment could increase T. cruzi infections in the dog population at least during the first year of treatment.

CONCLUSION

Our analysis shows that treating all peridomestic dogs every three to six months for at least five years could be an effective measure to reduce T. cruzi infections in dogs and triatomines in peridomestic transmission settings. However, further studies at the local scale are needed to better understand the potential impact of routine use of fluralaner treatment on increasing dogs' consumption of dead triatomines.

Identification of Triatomines and Their Habitats in a Highly Developed Urban Environment

Eleven triatomine species, the vector for Chagas disease, are endemic in the southern U.S. While traditionally thought to only occur in rural habitats and sylvatic transmission cycles, recent studies provide compounding evidence that triatomines could exist in urban habitats and domestic transmission cycles in Texas. We conducted a study of active and passive surveillance techniques over 3 years (2016-2018) in the City of Houston, Harris County, Texas to determine the presence of triatomines in this metroplex. Active surveillance methods uncovered Triatoma sanguisuga nymphs from two locations in downtown Houston city parks. We also documented the first Trypanosoma cruzi positive kissing bug collected in an urban environment of Harris County, Texas. Our findings provide evidence that triatomines can be found in heavily populated U.S. urban environments, and warrant public health support for expanded triatomine and Chagas disease surveillance in city settings.

Approximation methods for analyzing multiscale stochastic vector-borne epidemic models

Stochastic epidemic models, generally more realistic than deterministic counterparts, have often been seen too complex for rigorous mathematical analysis because of level of details it requires to comprehensively capture the dynamics of diseases. This problem further becomes intense when complexity of diseases increases as in the case of vector-borne diseases (VBD). The VBDs are human illnesses caused by pathogens transmitted among humans by intermediate species, which are primarily arthropods. In this study, a stochastic VBD model is developed and novel mathematical methods are described and evaluated to systematically analyze the model and understand its complex dynamics. The VBD model incorporates some relevant features of the VBD transmission process including demographical, ecological and social mechanisms, and different host and vector dynamic scales. The analysis is based on dimensional reductions and model simplifications via scaling limit theorems. The results suggest that the dynamics of the stochastic VBD depends on a threshold quantity R₀, the initial size of infectives, and the type of scaling in terms of host population size. The quantity R₀ for deterministic counterpart of the model is interpreted as a threshold condition for infection persistence as is mentioned in the literature for many infectious disease models. Different scalings yield different approximations of the model, and in particular, if vectors have much faster dynamics, the effect of the vector dynamics on the host population averages out, which largely reduces the dimension of the model. Specific scenarios are also studied using simulations for some fixed sets of parameters to draw conclusions on dynamics.

Daily activity patterns of movement and refuge use in Triatoma gerstaeckeri and Rhodnius prolixus (Hemiptera: Reduviidae), vectors of the Chagas disease parasite

Transmission risk for vector borne disease is greatest during periods of heightened vector activity. Triatomine bugs (Hemiptera: Reduviidae), which transmit Trypanosoma cruzi, the causative agent of Chagas disease, are generally considered nocturnal, but their patterns of activity in controlled settings have rarely been studied. We quantified activity patterns across a 24-hour period in nymphs of two triatomine species: (1) Triatoma gerstaeckeri Stål, a North American triatomine which is closely associated with sylvatic and peridomestic hosts, and (2) Rhodnius prolixus Stål, a Central and South American triatomine that thrives in the domestic environment. T. gerstaeckeri showed activity throughout the night, with peaks around midnight, 3:00, and again around dawn. Refuge use was highest in the pre-dawn hours then again after the dawn activity peak. We hypothesize that the dawn activity peak may represent a period of host seeking for this sylvatic species that targets nocturnal hosts returning to their home at dawn. In contrast, R. prolixus displayed high activity near 23:00 and again around noon. This study provides a baseline understanding of diel activity for future behavioral experiments and identifies periods of time that may represent the greatest risk of host exposure to triatomines and T. cruzi.

Invasion speeds of Triatoma dimidiata, vector of Chagas disease: An application of orthogonal polynomials method

Demographic processes and spatial dispersal of Triatoma dimidiata, a triatomine species vector of Chagas disease, are modeled by integrodifference equations to estimate invasion capacity of this species under different ecological conditions. The application of the theory of orthogonal polynomials and the steepest descent method applied to these equations, allow a good approximation of the abundance of the adult female population and the invasion speed. We show that: (1) under the same mean conditions of demography and dispersal, periodic spatial dispersal results in an invasion speed 2.5 times larger than the invasion speed when spatial dispersal is continuous; (2) when the invasion speed of periodic spatial dispersal is correlated to adverse demographic conditions, it is 34.7% higher as compared to a periodic dispersal that is correlated to good demographic conditions. From our results we conclude, in terms of triatomine population control, that the invasive success of T. dimidiata may be most sensitive to the probability of transition from juvenile to adult stage. We discuss our main theoretical predictions in the light of observed data in different triatomines species found in the literature.