Domestic cats overly criticised for their role as hosts of Toxoplasma gondii

by Michael Broad · June 13, 2022

The life-cycle of Toxoplasma gondii is complicated. People tend to oversimplify it. They see the domestic cat infecting humans with this protozoan parasite which is about the extent of general knowledge. It isn’t that simple; far from it. This article sets out to redress the balance and to mention an interesting study on a very sensitive subject which speculates that toxoplasmosis can be sexually transmitted by humans (hetero and homosexual) with serious clinical consequences. Critics of the domestic cat consistently use the fact that the species is the definitive host of Toxoplasma gondii and use this as a weapon to demand changes in cat ownership. I don’t think that there is any species which is criticised more for the simple fact that they are unfortunately the definitive hosts of a parasite.

Toxoplasma gondii oocyst. Source: CDC and Science Direct.

Definitive hosts – what does it mean?

But domestic cats are not the only hosts. They are the “definitive hosts” but humans and other species of animal are hosts as well. The reason why domestic cats are described as ‘definitive hosts’ is because Toxoplasma gondii can sexually reproduce only within cats. In all other hosts asexual reproduction can occur and therefore they are described as “intermediate hosts”. Cats get the disease by eating intermediate hosts such a mice infected with it.

Eating undercooked meat

It might be useful to remind ourselves that evidence strongly suggests that cats and people can also get the disease from eating vegetables and raw undercooked pork, beef, and other meats or unpasteurised dairy products which contain Toxoplasma organisms.

Ingesting oocysts

When a human ingests Toxoplasma gondii sporozoites in Toxoplasma gondii oocysts, they transform into tachyzoites after ingestion. These tachyzoites end up in neural and muscle tissue and develop into tissue cyst bradyzoites.

Sexual intercourse

Toxoplasma gondii infects about 30% of the human population. I mention the way humans can contract the disease above but sometimes it is not possible to identify the source of infection. A study published in 2015 ¹ hypothesised that toxoplasma gondii can be transmitted between humans engaging in sex as Toxoplasma tachyzoites are present in seminal fluid and tissue of the testes of various animals including humans. They state that “up to two-thirds of Toxoplasma infections in pregnant women cannot be explained by the known risk factors”.

They further state that in women “the probability of being Toxoplasma-infected correlates positively with the amount of unprotected sex with the child’s father before the conception”.

If their hypothesis is true “then the sexual route of transmission, even if rare, could be responsible for a large part of cases of congenital toxoplasmosis”.

Map showing where T. gondii is most prevalent in wild animals

CLICK ON THE IMAGE FOR A LARGER MORE READABLE VERSION. Figure 1. Distribution of study sites included in the global analysis of Toxoplasma gondii prevalence data for free-ranging wild mammal populations. For source the study please click link.

Oral sex

In a separate study, “Oral sex, a new, and possibly the most dangerous route of toxoplasmosis transmission, Medical Hypotheses (2020)” – see link below, the researchers refer to the above-mentioned study and extend that work to hypothesize that Toxoplasma gondii is transmitted between gay men engaging in oral sex in which male ejaculate is transmitted from one to another. They state that 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.”

Link to above-mentioned study: Study on oral sex as a transmitter of Toxoplasma gondii from human to human

They also state that “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”.

Toxoplasma tachyzoites present in sperm can be transmitted when “an infected person swallows ejaculate containing toxoplasma tissue cysts”.

They also state that “in both women and men who report practising fellatio with men, there is a higher prevalence of toxoplasmosis than in corresponding controls”.

They speculate that if their hypothesis is correct a public health campaign “with emphasis on early sexual education about the risks of unprotected oral sex will be necessary”. I don’t recall anything happening in this regard.

These two studies inform us that humans play a significant role in the transmission of this highly prevalent disease of which the domestic cat is the definitive host. I think that it is unfair if we only focus on the role of domestic cats and conveniently fail to mention the important role that humans play in spreading this disease. This article attempts to redress the balance somewhat. It provides an example of how the disease can be spread through human behaviour in a sexual context.

Toxoplasma gondii is a zoonotic disease and it is an endoparasite (internal).

1. Study referred to: Toxoplasmosis can be a sexually transmitted infection with serious clinical consequences. Not all routes of infection are created equal.

Below are some more articles on Toxoplasmosis. COMMENTS ARE WELCOME ?.