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Chagas and Chickens: The Impact of Incompetent Hosts on the Incidence of Disease

By Lina Sorg

Chagas disease—also known as American trypanosomiasis—is a tropical illness caused by the Trypanosoma cruzi protozoan parasite and spread via the triatominae insect (the kissing bug). The disease occurs exclusively in the Americas, and vector-borne transmission is primarily confined to rural, poverty-ridden regions of Latin America. However, large-scale population movements into more urban areas and across borders have increased its geographic distribution. As of 2018, at least six million people were infected with Trypanosoma cruzi, and another 70 million were at risk. Because less than one percent of patients have access to diagnosis and treatment, 14,000 people die annually from this disease.

Chagas disease is endemic to Central and South America, with the highest number of cases in rural Latin America. Public domain image.
Community biodiversity can reduce the risk of many vector-borne zoonotic diseases—including Chagas—via the dilution effect. Yet predicting the exact influence of biodiversity on the spread of these illnesses is challenging and contingent upon a number of factors. For instance, an increase in species’ richness does not always lessen disease susceptibility; sometimes improved diversity correlates with a heightened risk of infection. Additionally, the strength of dilution depends on the abundance of so-called dilution hosts relative to focal hosts. And the vectors’ preference for host organisms dictates whether the host population’s robustness amplifies or dilutes disease prevalence.

According to the World Health Organization, vector control is the most useful means of preventing Chagas infection. One such method is called the decoy effect, and involves the addition of a second host to distract the vector insects from their focal host. Humans are the focal hosts for Chagas, so introducing an additional incompetent host that cannot be infected would hypothetically reduce the interaction between vectors and humans. In other cases, bringing in an alternative host whose rate of infection is lower than that of humans can also ultimately reduce the presence of parasites in vulnerable populations.

In rural areas of certain Latin American countries, farmers commonly keep chickens in their houses to protect them from predators. Fortunately, chickens are incompetent decoy hosts for the Trypanosoma cruzi parasite but the preferred food source for the insect vectors. In 2014, a community-based intervention promoted chicken management as the primary method for reducing Chagas infection. During a minisymposium presentation at the 2019 SIAM Conference on the Applications of Dynamical Systems, currently taking place in Snowbird, Utah, Mondal Hasan Zahid of the University of Texas at Arlington analyzed the effectiveness of this technique. Zahid’s study aimed to identify the conditions for which the presence of chickens reduces vector-human interaction and ultimately decreases human risk for Chagas infection.

Chagas has two different phases of infection: the acute phase and the chronic phase. The acute phase lasts for the first few weeks or months following initial contagion. During this time, the patient is either asymptomatic or exhibits mild symptoms—headaches, dizziness, diarrhea, etc.—that mirror those of other illnesses. For this reason, many patients do not know that they are infected. If left untreated, Chagas moves into a chronic phase that lasts anywhere from a few decades to one’s entire lifetime. “It can be inside your body for your whole life,” Zahid said of the parasite. 20 to 30 percent of these patients develop an enlarged heart and/or irregular heartbeat, which ultimately result in heart failure or sudden death from cardiac arrest.

The triatominae insect, also known as the kissing bug, spreads the parasite that causes Chagas disease. Public domain image.
Zahid considers three cases involving different proximities between the incompetent decoy host (chickens) and the focal host (humans), based on the ways in which Latin American farmers typically keep chickens. The proximities are as follows: far (chickens are away from the home in hen houses or other separate enclosed buildings), intermediate (chickens are probably in the house but far from the bedroom), and close (chickens are in the bedroom to avoid predators). He uses this information to create a deterministic model. “We are not interested in a stochastic model because we are not interested in the variance,” he said. “We are interested in how the presence of chickens can strengthen or weaken the force of the infection.” He also accounts for other types of competent domestic hosts, such as dogs and cats, by converting them to a transmission-equivalent number of humans based on the vector insect’s known feeding preferences.

When chickens live in isolated hen houses and are far from the focal hosts, Zahid found that migration rates of vectors are independent of the host density because the vector insects cannot fly. However, migration rates of vectors for the intermediate distance are dependent on host density, as the insects can sense the presence of both chickens and humans due to their close proximity. Upon reaching a certain point, the number of chickens keeps increasing and the number of human cases begins decreasing. And when both hosts are in the same room, the vectors do not need to migrate. “They’re biting the humans, they’re biting the chickens, they don’t need to migrate anywhere else because they can switch whenever they want,” Zahid said. He finished by conducting sensitivity analysis on the model’s parameters, and found that the most influential parameters are those with which researchers are already familiar.

In conclusion, Zahid proved that the presence of chickens as incompetent decoy hosts reduces the number of human cases of Chagas only when farmers keep the chickens at an intermediate distance from the focal hosts. It is also important that the vector rate associated with the chickens falls below a given threshold value. These findings introduce a new method for reducing the incidence of Chagas disease amongst the Latin American population. In the future, biologists and ecologists can estimate the true values of Zahid’s parameters to visualize the practical implications of his results. 

Lina Sorg is the associate editor of SIAM News
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