Kaňková Š., Hlaváčová J., Flegr J. 2020: Oral sex: A new, and possibly the most dangerous, route of toxoplasmosis transmission. Medical Hypotheses, 141: 109725. DOI: 10.1016/j.mehy.2020.109725
Toxoplasmosis is a parasitic disease widespread in the temperate zone. The definitive hosts of Toxoplasma gondii, which causes the disease, are cats. All warm-blooded vertebrates, including humans, can be intermediate hosts. A person is usually infected by ingesting oocysts, e.g. by consuming along with vegetables some contaminated soil, by drinking contaminated water, or by ingesting tissue cysts contained, for instance, in poorly cooked meat. Less common is congenital transmission or transmission via organ transplant from an infected donor. Recently, it has been suggested that toxoplasmosis could also be transmitted sexually from infected men to uninfected women. In this article, we discuss and present evidence for an alternative hypothesis, which suggests that toxoplasmosis could be transmitted by oral sex (via fellatio) from an infected man to an uninfected person (male or female), especially if the uninfected individual swallows the infected ejaculate. This hypothesis finds support in the following facts and findings: (1) Toxoplasma has been found in male ejaculate. (2) In several animal species, presence of the parasite in the seminal fluid of infected males can lead to infection of uninfected females during mating. (3) A higher prevalence of toxoplasmosis has been reported in both homosexuals and promiscuous individuals, i.e. in populations which practice a broader spectrum of sexual activities, including oral sex. (4) In heterosexual couples, a partner’s infection seems to be a risk factor for infection in women but not in men. (5) A higher prevalence of toxoplasmosis in females compared to males has been observed in adolescents aged 10 to 14, where oral sex, including fellatio, is highly prevalent among those who have not yet engaged in penetrative intercourse. (6) On a theoretical level, one could expect that when an uninfected person swallows ejaculate containing Toxoplasma tissue cysts, this results in a similar infection pattern to ingestion of Toxoplasma-contaminated undercooked meat. (7) Approximately two-thirds of Toxoplasma infections in pregnant women cannot be explained by any of the known risk factors. (8) In both women and men who report practicing fellatio with men, there is a higher prevalence of toxoplasmosis than in corresponding controls. If our hypothesis is correct, an effective public health campaign with emphasis on early sexual education about the risks of unprotected oral sex will be necessary, especially in pregnant women and HIV-positive people. This route of toxoplasmosis transmission could be experimentally verified by force-feeding laboratory mice with the ejaculate of infected men.