Parasite needs chemical (lipid/nutrient) in cat intestines for sex

Toxoplasma can live asexually in any mammal, including humans. But it forms gametes (sexual cells) only in cats, a restriction that has long been recognized, but whose reason was not understood. The authors suspected something was missing when reproducing the infections in vitro and not observing sexual development. It is known that fungi require linoleic acid for sexual development. To test Toxoplasma's requirement for the same lipid, they generated cat intestinal organoids -- three-dimensional "test tube" models that share several essential properties with actual intestines -- and showed that linoleic acid was required for sexual reproduction of the parasite. Cats lack an enzyme called delta-6-desaturase, which catalyzes the conversion of linoleic acid to arachidonic acid, accounting for the peculiarly high levels of linoleic acid in the cat intestine, but not in other mammals. When the authors supplemented the diet of mice with linoleic acid, and added a specific inhibitor of the enzyme, Toxoplasma could complete the sexual phase of its life cycle in the mouse intestine.

Toxoplasma is the leading cause of foodborne illness in the US, according to CDC. The most common route of infection for human is by consumption of contaminated raw or undercooked meat. Cat litter, after 24-48 of being cleaned, can also a source of Toxoplasma infection. However, cats can only shed oocysts once in their life time. Pregnant women are urged to avoid eating raw or undercooked meat, as also avoiding cleaning the litterbox after 48 hours, to prevent Toxoplasma infection. Congenital toxoplasmosis can have potentially serious consequences for the unborn child. An improved understanding of the parasite life cycle stemming from this study may lead to production of vaccines that could inhibit Toxoplasma's sexual reproduction or the transmission of Toxoplasma to livestock.