The reason why cats are definitive hosts for Toxoplasma gondii

Toxoplasma gondii lifecycle
Toxoplasma gondii lifecycle
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In the life cycle of the Toxoplasma gondii protozoan, the only known definitive hosts are domestic cats and their relatives. This singular fact has worked against the cat because it opens the door to criticism by people who dislike seeing cats – domestic, stray and feral – outdoors. They say that the disease caused by this parasitic protozoan, toxoplasmosis, can make people mad. They say the oocysts from cat feces find their way to the oceans where they harm marine life such as Beluga whales and otters. It also means that in research to find a cure for toxoplasmosis the cat is used as a test subject. This happened over a long time at USDA laboratories which caused uproar in high political places because of its inherent cruelty (many cats were killed in this research).

That’s the background in brief. It is therefore interesting to read a research study which tells us for the first time why cats are the hosts.

The research is published on and is titled: Intestinal delta-6-desaturase activity determines host range for Toxoplasma sexual reproduction.

As usual the language used in the summary (the abstract) is very complicated. However, the key section is:

“Here, we defined the mechanism of species specificity for T. gondii sexual development and break the species barrier to allow the sexual cycle to occur in mice. We determined that T. gondii sexual development occurs when cultured feline intestinal epithelial cells are supplemented with linoleic acid. Felines are the only mammals that lack delta-6-desaturase activity in their intestines, which is required for linoleic acid metabolism, resulting in systemic excess of linoleic acid2, 3.”

In short, and as I understand it, cats lack an enzyme in their stomach (delta-6-desaturase) which would normally breakdown linoleic acid. As the enzyme is missing there is an excess of linoleic acid, which is a medium in which toxoplasma gondii thrives. That’s my interpretation of the text.

“We found that inhibition of murine delta-6-desaturase and supplementation of their diet with linoleic acid allowed T. gondii sexual development in mice.”

The discovery means that scientists can create the same state of affairs in mice which in turn means that they can test for toxoplasmosis using mice and not cats which should please cat lovers. It does beg the question as to why mice have less value than cats.


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