Toxoplasma gondii is a tissue parasite in all warm blooded vertebrates. It is an intestinal parasite only in the cat. The difference is that the cat is the definitive host of this parasite – the host in which the sexual stage of the parasite life cycle takes place:
- Usually, cats get toxoplasmosis when they eat small animals and birds that have Toxoplasma gondii parasites encysted in their tissues. The parasites in the tissue cysts, called bradyzoites, enter cells in the cat’s intestinal lining and begin to divide, producing between 2 and 40 merozoites.
- Merozoites burst out of intestinal cells, invade new ones and multiply anew. This asexual reproduction continues for a few days to several weeks, producing huge numbers of parasites in the cat’s intestine.
- About three days after the infection begins, some of the merozoites start to mature into gametes, the male and female stages of T. gondii. Sexual reproduction produces oocysts, which are passed by the millions in the cat’s feces.
- Oocysts passed into the environment mature and are infective in 2 to 5 days. When they are swallowed by birds, mice, and other warm blooded vertebrates, including humans, they invade the tissues of the new host and reproduce asexually. Sexual reproduction begins again only in cats.
Cats with toxoplasmosis often have no symptoms at all. Some have pneumonia-like symptoms and they may suffer from diarrhea. Cats usually pick up the parasite when they are young and most cats recover; however, kittens with overwhelming infection sometimes die. Infected humans usually have mild symptoms or don’t notice that anything is wrong; however, for pregnant women and people with decreased immunity, toxoplasmosis can be a very serious disease. Recent research indicates that the infection can have subtle and long-lasting psychological effects.
T. gondii is present in warm blooded vertebrates worldwide. The high numbers of domestic and feral cats in and around human communities, and the prevalence of the parasite in mice and birds, and other small animals that cats typically hunt, assures that the parasite is ubiquitous in the environment. If your cat ever goes outdoors, it will probably come in contact with T. gondii eventually.
Other articles about Toxoplasma gondii and toxoplasmosis:
Toxoplasmosis – Parasitic Disease
Toxoplasma gondii and Behavior
Other articles about cat health issues:
Caring for Newborn Kittens
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.