Triatominae, kissing bugs
The Triatominae (commonly known as kissing bugs) are defined as subfamily of Reduviidae (commonly known as assassin bugs) that suck vertebrate blood (strictly hematophagous) and are mainly restricted to the New World, in contrast to the other 30 or so reduviid subfamilies (over 6,000 described species) that prey on invertebrates and are distributed worldwide. Within the subfamily, genera Triatoma, Rhodnius, and Panstrongylus contain species of bugs that are especially important vectors of Trypanosoma cruzi, the agent of Chagas disease in humans. At least 40 triatomines can harbor the parasite and are all potential transmitters of the infection. There is strong evidence to suggest that the Triatominae subfamily is polyphyletic (has more than one common ancestor).
Trypanosoma cruzi, causative agent of Chagas disease: facts, life cycle, structures, references at MetaPathogen
Distribution and ecology of T. cruzi vectors
Species of Triatominae became adapted to diverse terrestrial ecosystems limited by parallels 42º in the Northern United States and roughly 42º to 46º in Argentina. Specialist niches include broad-leaf, humid tropical forests of South America, rocky habitats and the major dry ecosystems cerrado and savanna. Ttrypanosomes and vertebrate hosts vary depending on the habitat: in the palm niches dwell marsupials harboring trypanosomes defined as zymodeme 1 (Z1/DTU I), whereas in the tree cavities, ground burrows, and rocky outcrops are rodents, armadillos, and anteaters harboring trypanosomes of zymodemes Z2 and Z3 (DTU II subgroups a to e). Bugs that feed primarily on birds are considered of secondary importance because birds are refractory (not susceptible) to T. cruzi and do not carry the parasites in their blood.
Role in transmission of T. cruzi
Parasite and vectors are associated and throughout geographical distribution of triatomines, T. cruzi is a common parasite of small mammals, especially nest-building species of rodents and opossums which are commonly associated with sylvatic species of Triatominae. Most transmissions of T. cruzi to humans occur when feces of infected bug contaminate bite wound, other breaks in the skin or mucosa of eye, nose or mouth, across which the parasite can pass readily. Transmission to small mammals would seem most likely to occur when the mammal eats an infected bug, or licks triatomine fecal deposits while grooming its coat.
Prerequisites for hematophagy
Exploitation of vertebrate blood requires important behavioral changes, especially in terms of avoiding predation by the host (cryptic behavior and feeding when the host is inactive) and physiological adaptations (capability of ingesting and digesting vertebrate blood, painless biting to avoid undue host disturbance when feeding). Hematophagy also requires a rapid compensation of the enormous amount of blood that triatomines ingest. The insect therefore excretes great amounts of water and salts immediately to reduce its weight.
Parasite and vector co-evolution
T. cruzi is a relatively ancient parasite, which earliest forms probably have been associated with marsupials such as opossums at the time of separation of South America from Gondwanaland about 40 million years ago.
The Reduviidae is also an ancient family, with the fossil record suggesting that the earliest predatory forms may have derived from phytophagous (plant-eating) insects some 230 million years ago after first veterbrates appeared.
The ancient predatory traits are reflected in many characteristics of modern Triatominae:
They are poorly differentiated from predatory reduviids in appearance and anatomy.
They can attack other insects and even exhibit cannibalistic behavior: when triatomine nymphs cannot reach the vertebrate host, they will penetrate feeding nymphs and take blood through them (cleptohemodeipnonism).
The bite of many blood-sucking Triatominae is still very painful. For example, bites of Panstrongylus geniculatus on pigs and humans in the Amazon region leave painful lesions and there is at least one record of a person dying from anaphylactic shock after being bitten by T. rubrofasciata.
In conclusion, Triatominae remain imperfectly adapted for feeding on vertebrates which indicates that their rise as vertebrate blood-suckers is relatively recent and is still continuing.
Probably, in ancient times, about 65 million years ago, when hematophagy in reduviids did not yet developed, parasitic trypanosomatids were mostly associated with opossums and were transmitted directly between animals via their anal gland secretions and/or urine. About 2-5 million years ago, when opossums became common throughout the continent, predatory reduviids started to invade their nests and commenced their evolution toward hemophagy.
Medical importance and vectorial predisposition
There are several factors, which may predispose triatomine species to be potential vectors:
geographical distribution;
frequency of invading peridomestic ecotopes;
capacity for establishing peri- and domestic colonies;
adaptability to different habitats and hosts;
life history traits and physiological characteristics, such as longevity, efficiency of feeding, time of defecation;
ability to facilitate multiplication and maturation of ingested parasites.
The most important species of Chagas' disease vectors are Triatoma infestans, T. dimidiata, T. brasiliensis, T. maculata, T. sordida, Rhodnius prolixus, R. neglectus, R. pallescens and Panstrongylus megistus. Species that maintain sylvatic colonies like Triatoma dimidiata are most difficult to control because of threat of re-colonization of treated houses. Highly domiciliated species such as Triatoma infestans can infest houses in great quantities but are easier to control because sylvatic colonies are virtually non-existent.
The Southern Cone Initiative
The Southern Cone Initiative (Iniciativa de Salud del Cono Sur, INCOSUR) that was launched in 1991, aimed at elimination of the main vector, Triatoma infestans, and elimination of transfusional transmission of T. cruzi in Argentina, Bolivia, Brazil, Chile, Paraguay, and Uruguay (PAHO, 1993). The initiative successfully eliminated domestic T. infestans over large areas and because transmission by triatomine bugs accounts for over 80% of Chagas disease transmission, this large-scale regional cooperation has significantly reduced disease transmission.
