Chagas Disease, also known as American trypanosomiasis, is a tropical disease caused by the blood-borne parasitic protozoa, Trypanosoma cruzi. It was discovered by, and named after, Dr Carlos Chagas who was investigating a malaise affecting Brazilian workers during the building of the railway in 1909.
T.Cruzi infects mainly small mammals across the Americas, but where the conditions are right, such as in poor areas of Central and South America, humans are also highly susceptible. It is thought that about 17 million people carry T. cruzi, of whom 50,000 will die every year.
The symptoms of Chagas disease are variable. Some people carry the infection for years, never showing any symptoms, while others suffer from intestinal disorders (usually resulting in constipation, difficulty swallowing and choking ), irregular heartbeat and severe fatigue. Around 2% of those infected have an acute reaction, including fever, diarrhea, vomiting, anorexia, enlarged liver and acute infection of the heart, sometimes resulting in death, especially in young children.
T. cruzi is a trypanosome, a single celled organism that passes through three stages of development. It lives for part of its lifecycle replicating in the gut of a blood-sucking triatomine insect (vinchuca, assassin bug or kissing bug).
It is then passed out in the faeces and enters the blood stream of its mammalian host via a scratch, the mucous membranes or through the eye. In the mammal it finds its way to muscle or neurone cells, replicates again and is released back into the blood stream where it is picked up by more biting vinchuca.
The first sign of infection is a hard lump or chagoma which develops at the site of entry, about a week after infection.
The parasites infect the nerves of the digestive system and muscles (including the heart muscle), affecting peristalsis and muscle contractions. Acute symptoms persist for about 4 to 12 weeks, and then disappear, although the infection often remains for life. The infection also elicits a strong immune response which exacerbates the problem, and often causes more damage to the host than the parasite itself.
The disease may then become latent, with no parasitaemia, for 10 to 30 years. Up to a third of patients die years later from heart disease as a direct result of infection with T. cruzi.
Interestingly, it is thought that Charles Darwin may have contracted Chagas disease. He is known to have suffered from all the symptoms after returning from his travels aboard the Beagle, and he eventually died from chronic heart disease aged 71. Using modern techniques it should be possible to test his remains to see if this was indeed the case.
It used to be that the only way to test for Chagas disease was to allow uninfected vinchuca to bite and feed on the blood of the person in question, and then be dissected a few weeks later to see if the parasite was present.
Microscopic examination of blood smears sometimes shows the presence of the trypanosome, but this does not necessarily mean that it is T.cruzi.
Modern technology has meant that there are now a number of serological tests available and a simple blood sample is all that is required.
One such test is an ELISA test which checks for antibodies to T. cruzi. This isn't 100% reliable as there are cross matches with similar species of trypanosome and antibodies are still present even if the infection has been wiped out
The most reliable test is called the PCR (polymerase chain reaction) assay. This is a DNA hybridization test specific for T. cruzi and as such will also be of interest to archaeologists and historians since DNA persists in mummified remains.
The most effective way of preventing Chagas disease in humans is to interfere with the life cycle by killing the insect host.
It is common in many rural areas of
Latin America for people to share their dwellings with their animals. Since the animals are often infected with T cruzi, keeping them some distance away from the house lessens the chances of the insect vector picking the parasite up from the blood of the animal and passing it on to the human.
Vinchucas like to live in palm fronds (often used for roofing in rural areas), cracks in walls, in dark corners, and under mattresses and bedding. Spraying these areas with insecticide and sleeping under mosquito nets to prevent contact with the bugs or their faeces can really make a difference.
Chagas disease is beginning to make an appearance in more affluent parts of the world. This is not only due to an increasingly mobile population, but also to spread of T cruzi through blood transfusion. In many parts of South America screening of blood banks is now compulsory by law, but this isn't the case everywhere.
Drug treatment, using nifurtimox, benznidazole or allopurinol, is only possible during the acute stage of the disease. Once the disease has become chronic the best that can be achieved is management of the symptoms. There is no long term cure, and as yet no vaccine, despite years of reasearch in laboratories around the world.
Refs:
http://www.thirdworldtraveler.com/Disease/Chagas.html
http://www.uta.edu/chagas/html/chagBiol.html
For a nice 3D image: http://www.lifehouseproductions.com/bvessel_anim.html
For a pic of the vinchuca bug: http://www.uta.edu/chagas/html/biolTinf.html
Thanks to wertperch for suggestions