Medicine in the Light of Evolution

The cost of being a killer's accomplice: Trypanosoma cruzi impairs the fitness of kissing bugs

Relatively little is known about the fitness effects and life history trade-offs in medically important parasites and their insect vectors. One such case is the triatomine bugs and the parasite Trypanosoma cruzi, the key actors in Chagas disease. Previous studies have revealed some costs but have not simultaneously examined traits related to development, reproduction, and survival or their possible trade-offs. In addition, these studies have not compared the effects of genetically different T. cruzi strains that differ in their weakening effects in their vertebrate hosts. We compared the body size of the bugs after infection, the number of eggs laid, hatching/non-hatching rate, hatching success, survival, and the resulting number of parasites in Meccus (Triatoma) pallidipennis bugs that were experimentally infected with two strains of T. cruzi (Chilpancingo [CH], the most debilitating in vertebrates; and Morelos [MO], the least debilitating) (both belonging to TcI group). Our results showed that infection affects size (MO < CH; MO and CH = control), number of eggs laid (MO and CH < control) hatching/non-hatching rate (MO < control < CH), hatching success (control < MO, CH = control = MO), and survival (Chilpancingo < Morelos < control). In addition, the CH strain produced more parasites than the MO strain. These results suggest that (a) infection costs depend on the parasite's origin, (b) the more debilitating effects of the CH strain are due to its increased proliferation in the host, and (c) differences in pathogenicity among T. cruzi strains can be maintained through their different effects on hosts' life history traits. Probably, the vectorial capacity mediated by a more aggressive strain could be reduced due to its costs on the triatomine, leading to a lower risk of vertebrate and invertebrate infection in natural populations.