Rhodnius prolixus

Rhodnius prolixus, parasitic species of South American assassin bug that is one of the primary vectors of the protozoan parasite Trypanosoma cruzi, which causes Chagas disease in humans, dogs, and other mammals. The insect, like other members of the kissing bug subfamily Triatominae, feeds on vertebrate blood and tends to bite the thin skin near their hosts’ mouths. The World Health Organization estimates that more than 7 million people worldwide are infected with T. cruzi (which can be acquired from several related insect vectors as well as from contaminated food and other sources), and more than 10,000 people die of Chagas disease annually.

Rhodnius prolixus is native to northern South America, specifically Colombia and Venezuela, where it predominantly occupies areas with diverse palm species, including tropical savannas, rainforests, and scrub forests. In 1915 the insect was reported in Central America after specimens given to a scientist in El Salvador escaped and began colonizing local homes. From El Salvador, R. prolixus spread to Guatemala, Honduras, Nicaragua, Costa Rica, and southern Mexico, likely by travelers unintentionally, and soon became a major vector for Chagas in the region. Eradication efforts were undertaken, and in 2008 Guatemala became the first Central American country to be formally certified as free of Chagas disease transmission from R. prolixus. Since then the insect has been eradicated from Mexico and the other infested Central American countries, with no reports of R. prolixus since June 2010. The countries do, however, still have endemic vectors of Chagas disease, particularly the kissing bug Triatoma dimidiata.

Physical description

As an insect, R. prolixus has a body formed of a head, thorax, and abdomen, and six thin legs. An adult R. prolixus has an average length of 17 mm (0.7 inch) and can grow up to 34 mm (about 1 inch). Females are generally slightly larger than males. The species is yellowish-brown or red-brown with dark brown markings.

It has a flattened abdomen and a triangular thorax that tapers to a long, narrow, conical head. Its head features a pair of four-segmented antennae and black and red compound eyes. Retractable sucking mouthparts characteristically tuck into a groove between the front legs, beneath the head and thorax, when not in use. Like most other true bugs (suborder Heteroptera), R. prolixus has two pairs of wings. The unusual forewings (called hemelytra) are stiff and have an oblique line that abruptly separates the leathery basal half from the membranous apical half. The hind wings are thin, delicate membranes and are folded under the protective forewings when at rest.

Life cycle

R. prolixus is an obligate, hematophagous parasite, meaning it cannot survive without the blood of its hosts. A nocturnal insect, it emerges at night to feed on sleeping animals, particularly mammals (including humans) and birds. Its bite is painless, and its saliva contains anticoagulants, allowing it to feed on blood undetected for several minutes. It can hide in crevices and cracks in human dwellings and is sometimes brought inside on firewood or construction materials.

Its life cycle can last three months to two years, depending on temperature and host availability. The species undergoes incomplete (hemimetabolous) metamorphosis with three main stages: egg, nymph, and adult. The nymph, or immature insect, resembles the adult in form and eating habits, differing in size, body proportions, and color pattern. Rudimentary wings are visible and develop externally. Development is gradual through five molts (periodic shedding of the exoskeleton), each of which is fueled by at least one blood meal from a host. The adult form emerges from the final molt to reproduce. Females mate more than once and can lay hundreds of pinkish-red eggs before dying.

Chagas disease transmission

R. prolixus receives disease-causing T. cruzi when it feeds on the blood of an infected animal. The trypanosome passes part of its life cycle in the insect and again becomes infective to humans and other animal hosts. Instead of injecting the parasite into a new victim (as is the pathway for mosquito- and tick-borne diseases), R. prolixus deposits T. cruzi in its excrement on the skin of a potential victim. The trypanosome can enter the bloodstream only through mucous membranes or breaks in the skin, as typically occurs when a victim scratches at the site of the bite. T. cruzi can also be transmitted through blood transfusions and some organ transplants, during pregnancy or childbirth, or by the consumption of contaminated food.

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About a week after inoculation, the first local signs of Chagas disease may appear, including edema (swelling) and lymph node enlargement; the patient may have fever and flulike symptoms for several weeks. The antiparasitic agents nifurtimox and benznidazole are the drugs used to treat acute T. cruzi infections. If left untreated, Chagas disease may enter a chronic stage, characterized chiefly by heart symptoms, mainly disturbances of the rhythm; heart failure sometimes occurs because of the development of the parasite in the muscle fibers of the heart and because of the accompanying inflammatory reaction. The infection may end in death, especially in children, or the course may be mild. There is no effective treatment for chronic infections. Prevention is centered on control of the insect carrier and screening of the blood supply to avoid transfusion of infected blood.

Research

Because R. prolixus is a primary vector for Chagas disease, the insect has been a vital research tool in helping scientists understand disease transmission and develop control strategies. Its unique characteristics established it as a model organism for insect physiological and behavioral studies, largely through research conducted by British entomologist Sir Vincent Wigglesworth in the 1930s. Wigglesworth’s groundbreaking studies demonstrated the importance of the prothoracicotropic hormone (PTTH) and the juvenile hormone (JH) in regulating metamorphosis. He showed that selective activation of hormone signals, particularly from the insect’s brain and corpus allatum (a small gland behind the brain), controls molting and growth.

The insect’s genome was sequenced in 2010, the first genome sequencing completed for any hemimetabolous species.