Development of Toxoplasma gondii vaccine: A global challenge.

Toxoplasmosis is caused by the protozoan parasite T. gondii. Humans and other warm-blooded animals are its hosts. The infection has a worldwide distribution; one-third of the world's population has been exposed to this parasite. There are three primary ways of transmission: ingesting uncooked meat containing tissue cysts, ingesting food and water contaminated with oocysts from infected cat feces and congenitally. Those particularly at risk of developing clinical illness include pregnant women, given that the parasite can pose a serious threat to the unborn child if the mother becomes infected while pregnant, and immunosuppressed individuals such as tissue transplant subjects, AIDS subjects, those with certain types of cancer and those undergoing certain forms of cancer therapy. Maternal infections early in pregnancy are less likely to be transmitted to the fetus than infections later in pregnancy, but early fetal infections are more likely to be severe than later infections. In the absence of an effective human vaccine, prevention of zoonotic transmission might be the best way to approach the problem of toxoplasmosis and must be done by limiting exposure to oocysts or tissue cysts. Vaccine development to prevent feline oocyst shedding is ongoing, mostly with live vaccines. The S48 strain Toxovax is a live vaccine originally developed for use in sheep, but when used in cats inhibits sexual development of T. gondii. This vaccine is used in sheep to reduce tissue cyst development. The T-263 strain of T. gondii is a live mutant strain designed to reduce or prevent oocyst shedding by cats by developing only partial infection in the feline intestinal tract.