Thursday, 27 July 2017

Trypanosoma cruzi - A Parasite in the Blood Supply

Blood donation is now infamous for transmitting infectious diseases: HIV/AIDS and hepatitis B and C are the best-known offenders and the blood supply is screened for these viruses. American trypanosomiasis, or Chagas disease, is caused by a parasite, and it, too, can be transmitted through organ or blood donation, so we can add Trypanosoma cruzi, the cause of Chagas disease, to our list of concerns.
How do You Get American trypanosomiasis?
Triatoma infestans is
the best known of the
kissing bugs.
Image by Bärbel Stock;
CC BY-SA 3.0
Chagas disease usually starts with an insect bite: the reduviid bug, also known as the triatomid bug, or kissing bug, bites a human and takes a blood meal. As blood goes in at the mouth end, the bug defecates at the opposite end, leaving a fecal droplet full of infectious T. cruzi parasites on the skin. Later, when the bite itches and the victim scratches, the tiny parasites are rubbed into the wound and gain access to the tissues under the surface of the skin.
The parasites enter fat and muscle cells and multiply, eventually being released into the blood and thus distributed throughout the body. In time, they become quite rare in the blood but continue to live and multiply in organs indefinitely. Should another hungry reduviid bug feed on the host, there are enough T. cruzi trypanosomes circulating to pass the parasite to the bug, and subsequently to another human. Parasites present in donor blood and donated organs will also thrive in their new host.​
Symptoms of American Trypanosomiasis
Early symptoms of Chagas disease include a painful swelling at the site of the bite, swollen lymph nodes, high fever, aching muscles, enlarged liver, rash, inflammation of the heart and swelling of the face and extremities. This stage of the disease is often severe and dangerous in small children, while symptoms in victims over five years of age are generally milder and progress to a more prolonged, often asymptomatic though still destructive, form of the disease. 
In chronic Chagas disease, there is organ damage, particularly enlargement of the heart with gradual loss of function, and enlargement of the esophagus (megaesophagus) and colon (megacolon) leading to loss of function in the digestive tract. The severity of disease varies from person to person, and from one geographic area to another – when the heart and/or digestive system damage are severe enough, the patient dies.
Chagas Disease Distribution
The WHO estimates that between 6 and 7 million people are infected with T. cruzi. Most of them are in South and Central America, with about 12,000 deaths attributed to the parasite annually. 

Global distribution of Chagas disease. The red area shows where the disease is frequently transmitted by the kissing bug. In the blue area, occasional transmission occurs. In red, blue, and green areas, the disease is also transmitted from mother to fetus, and through blood or organ donation. Derived from a United Nations map.

With increasing migration of people from endemic areas, incidences of Chagas disease acquired through blood transfusion or organ transplant are increasing, and there is evidence that reduviid bugs in North America are carrying and transmitting the parasite. Thus, Chagas disease has become a matter for concern – an emerging disease – in North America, and both blood and organ donors are now being selectively screened for T.cruzi in both the United States and Canada.

Further reading on Chagas disease

Trypanosoma cruzi may owe its success in humans to the domestication of the guinea pig. I wrote about this in my book Parasites: Tales of Humanity's Most Unwelcome Guests.

PAHO/WHO. “ChagasDisease.” Accessed March 29, 2017. 
Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 8th Ed. New York: McGraw Hill, 2009.