Toxoplasma gondii shares the taxonomic class Coccidia with the parasites that cause malaria and cryptosporidiosis. T. gondii is initially a cat parasite and cats are the only hosts in which the parasite can complete all stages of its complicated life cycle. T. gondii is well adapted to other hosts, however, and it can infect any warm blooded vertebrate causing various degrees of illness. The infection is called toxoplasmosis. Humans acquire Toxoplasma gondii by swallowing the infective stage of the parasite, either in undercooked meat, or when hands, food, or water are contaminated with feces from an infected cat:
- Invasive microscopic parasitic cells are set free in the intestine. They penetrate the intestinal wall and move through the tissues. Eventually, these cells enter host cells and begin to multiply. This multiplication is asexual: the parasites, called tachyzoites at this stage, duplicate their nuclear material and divide without the need for male and female individuals.
- Infected cells full of tachyzoites eventually burst, releasing parasites that then invade other cells and continue to multiply. Liver cells and connective tissue in the abdominal cavity are invaded first, but parasites will also move to other areas such as the heart, brain and skeletal muscles.
- As the human immune system produces antibodies to the infection, multiplication slows down and the parasites form tissue cysts, particularly in muscle and brain tissue. The cysts have strong outer walls to protect the parasites, now called bradyzoites, from attacks by the immune system. Inside the cysts, bradyzoites live for years.
- For most people, this is the end of the story – infection results in lifelong immunity to T. gondii and the parasites remain trapped in their tissue cysts forever. However, in certain relatively unusual situations, there may be an epilogue:
- If the immune system loses its hold on T. gondii, perhaps due to immunosuppressive drugs or a disease such as AIDS that affects immunity, multiplication can begin again, this time without constraint because the immune system is not functioning properly. In these circumstances, toxoplasmosis can be fatal.
- If the infected individual dies and his or her organs are donated for transplant, tissues infected with T. gondii may be transplanted into a recipient who has never had toxoplasmosis and therefore has no immunity. Because organ recipients have drug induced immunosuppression to prevent organ rejection, this infection is serious and can be fatal.
- Most unusual in humans, if an infected person is eaten by a warm blooded vertebrate, T. gondii passes to another host in the natural way. Bradyzoites emerge in the new host’s intestine, invade host cells and begin multiplying. If the new host is a cat, the parasite can proceed to the sexual stage of its life cycle.
Toxoplasmosis - Parasitic Disease
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Sources:
Garcia, Lynn S. and David A. Bruckner. Diagnostic Medical Parasitology 3rd ed. Washington: ASM Press, 1997.
Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.
