Chagas' Disease: Vectors

The bugs are also known locally as “vinchucas” or “barbeiros”. The latter common name refers to the blood-sucking (since in the olden days the barber carried out blood letting as well as shaving). Of the approximately 120 vector species only about 7 are important. Each species has its own region of distribution:

• Central America and northern South America: Triatoma dimidiata and Rhodnius prolixus

• South America (south of 5 S): T. infestans, T. braziliensis, T. sordida, Panstrongylus megistus

The bugs mentioned here are the main vectors. Other bugs also play a part (Triatoma barberi [Mexico], Rhodnius pallescens [Panama and Colombia], T. phyllosoma group [Mexico] and T. protracta. Twelve species of triatomines are known to occur in the United States, the most important being Triatoma sanguisuga in the Eastern United States, Triatoma gerstaeckeri in the region of Texas and New Mexico, and Triatoma rubida and Triatoma protracta in Arizona and California. The bugs each have their own preferred biotopes. T. dimidiata, for example, is often found inside houses on the floor or the lower 150 cm of the walls or immediately outside in dung heaps, hollow trees, etc. In contrast, R. prolixus prefers to live in palm leaves either in the roof of the house or in the tree itself. In and around the house the bugs can feed on animals (e.g. dogs are important because they sleep at night, when the bugs are active). The vectors often live in chicken runs, but chickens themselves are not infected (they do eat bugs). During the day the insects hide in all kinds of cracks and crannies (importance of earthen or adobe walls) and in the roofing (straw, wood, etc). It can be seen immediately that the key word in Chagas’ disease is "poverty". These are insects which reproduce slowly and whose geographical spread is slow. Migration of bugs, by migrating birds for example, still needs to be studied. In view of these characteristics and the fact that the important vectors live around houses, they can easily be reached by eradication campaigns.

The adult insects measure 2-3 cm. The front wings (hemi-elytra or hemelytra: "half wing sheath"[Gr. elytron = cover]), consist of a hardened foremost part (divided into corium and clavus) and a rearmost membranous part. This order is named "Hemi-ptera": "half-wings"[Gr. pteron = wing]) after their wing construction. The name Heteroptera is used for the suborder which includes the bugs, while the other Hemiptera are classified in the suborder Homoptera. Each species has a typical morphology and colour pattern which includes connexival markings. The latter are markings (red, orange, yellow) of the lateral edges of the abdominal tergites [Lat. “tergum” = back; tergite = sclerosed dorsal segment]. They are visible because the wings are folded over each other so that the back is not fully covered. Naturally the identification of bugs is very important in the evaluation of vector control programmes. Differentiation between re-infestation from neighbouring areas or insufficient local control can be made by genetic analysis of the bugs.

A fertilised female lays several hundred eggs in her lifetime. From the egg comes a nymph which always needs a blood meal for its subsequent development stages (both sexes suck blood). The last instar will develop into an adult insect. During a blood meal they suck more than their own weight in blood. This takes 10-25 minutes. The insects may live for up to 2 years (5 years for T. barberi). Rhodnius prolixus has a relatively short generation time (3-5 months), while for T. dimidiata this time is quite long (1 year or longer). Long generation times make the development of resistance to insecticides difficult.

Category: Medicine Notes