For 10 years, major fumigation campaigns in the villages have eliminated the fleas that transmit Chagas' disease in many Latin American countries. But, in several regions, these insects, including those that are the main vector species called Triatoma infestans, persist or are reappearing in human habitat. How can this phenomenon be explained? IRD researchers and their partners have just shown that populations of this species that normally live in nature are recolonizing peridomestic areas.
Widespread fleas
The study conducted in Bolivia, the country where the infestation rate and prevalence were highest, showed that these wild populations are much more abundant in the environment than previously thought. The researchers jointly reviewed the endemic area historically affected by the disease. They discovered more than 40 infested sites that were previously unidentified, and sometimes where they did not expect to discover wild insects: near villages, and even in dwellings, as well as in the lowlands of the Gran Chaco region. Even recently, they thought that the natural populations of Triatoma infestans were restricted to the Cochabamba valley in the centre of the country. Therefore, the wild fleas occupy a much larger ecological niche than initially described.
A risk of re-emergence in humans
A DNA analysis of the wild individuals collected compared with that of domestic insects shows that the two types of populations are genetically very similar. This is the result of an exchange of genes between them due to the movement of the insects from one environment to the other. Thus, the wild fleas seem capable of adapting to human habitat like their domestic counterparts. Moreover, the analysis of their stomach contents showed, surprisingly, that they feed in part on human blood in their natural habitat. In fact, while the fleas found near villages primarily bite small rodents, approximately 20% of their meals come from humans.
These studies also showed an extremely high infection rate in the insects. The researchers analysed their digestive tube to detect the DNA for the parasite Trypanosoma cruzi, which causes the disease. The result: one out of two fleas carry the pathogen. This allows for a significant risk of re-emergence of the disease in humans.
The study lifts the veil on the failure of the actions to control Chagas' disease in some regions of Latin America. The factors driving the wild fleas to migrate to villages still need to be understood to define measures that health authorities and residents can take to protect themselves from any contact with the vectors.
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