Is toxoplasmosis a serious problem for humans?

Toxoplasma gondii oocyst
Toxoplasma gondii oocyst. Source: CDC and Science Direct.
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There is a disconnect between what we read about the seriousness of toxoplasmosis as stated on the Internet and what we experience in real life. Nobody is screaming and shouting about the damaging effect of toxoplasmosis on the human population in any area of the world. But if you believe what you read on the Internet about this zoonotic disease you would think that humankind was surviving a new pandemic.


Despite toxoplasmosis being one of the most common parasitic diseases in the world, and despite the fact that an estimated one third of the world’s population are infected, the most common form of the disease is latent i.e. asymptomatic. My research indicates that up to 90% of people who have the disease have no symptoms. So, the disease is there but no one knows it’s there and I guess nobody is being tested for it because they don’t know they’ve got it.

And if there are no symptoms there is no effect upon the person and therefore it is as if they have not got the disease. The doomsayers discuss the disease on a theoretical basis which is distanced from reality but please read on.

Toxoplasma gondii cyst in mouse brain.
Toxoplasma gondii cyst in mouse brain. Image in public domain.

Climatic conditions

It’s worth stating that in places where there are greater fluctuations in average daily temperatures the disease is more prevalent because this kind of climate is associated with greater Toxoplasma gondii oocysts shedding by domestic cats. Conversely, high ambient temperatures during dry seasons are linked with a reduced shedding by wild cats.

High level of human infection

CDC state that in the United States an estimated 11% of the human population of six years and older have been infected with toxoplasmosis. In other places, around 60% have been infected. In general, humid climates and low altitudes encourage the disease to spread.

One study (link) states that the diseases carried by 6.5% of the general cat population in Bangkok, Thailand. It is higher in semi-domesticated cats at 11.5% and lower in full domestic cats at 1.5%.

Shedding

It must also be said that cats only shed toxoplasma gondii oocysts on a one-off basis for 7-10 days during their entire life after an acute infection. This puts into perspective the reality of the role that the domestic cat plays in transmitting the disease to people.

Mental health

I’ve got to answer the question in the title. The answer must be no; it is not a serious problem for humans because there are nearly always no symptoms. However, one study (link) states that it is “unsafe for human, but also may play various roles in the etiology of various mental disorders”. The word “etiology” (aetiology) means “the course, set of causes, or manner of causation of a disease or condition”.

A few years ago, there was a lot on the Internet about toxoplasmosis causing schizophrenia. On the Science website they state that current research indicates that there is a low chance of developing schizophrenia from toxoplasmosis. How low? The author states that they don’t know but it is likely to be as low as the incidence of schizophrenia occurring for other reasons.

RELATED: Health tip: preventing a toxoplasmosis infection from cat faeces

Schizophrenia affects one in 300 people (0.32%) worldwide. That’s according to the World Health Organisation. Schizophrenia can be caused by stressful life events, drug and alcohol use, genetic inheritance and ultimately differences in brain chemistry.

The vast majority of people don’t fear becoming schizophrenic. The same goes for getting toxoplasmosis.

The potential seriousness of the disease toxoplasmosis in humans is its connection with schizophrenia but we don’t know how prevalent that connection is.

RELATED: Infographic on toxoplasmosis and its zoonotic ability

Pregnancy

And lastly, we have to look at the potential danger of toxoplasmosis to pregnant women. What are the risks of toxoplasmosis during pregnancy? This is a big issue for women who live with a domestic cat. Some of them give up their cat as a consequence.

The NHS in the UK states that, “Toxoplasmosis is a common infection but is usually harmless”. But they add that if you get it for the first time when pregnant or a few months before conceiving “there is a small risk the infection could cause either a miscarriage or a stillbirth or birth defects or problems after the baby is born although this is very rare. A pregnant woman will not develop obvious symptoms herself.

Conclusion

To the huge majority of people, toxoplasmosis is not a serious issue or an issue at all. It is completely ignored. That’s the honest truth about the disease but people should be aware of the rare potential of mental health conditions being caused by it as stated and the need for pregnant women to take precautions which I’ve discussed on another page (click here to read it). There is no need to abandon a cat.

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