Avian Flu Diary: Toxoplasmosis: Some Intriguing Para-Cites

Credit CDC

# 6535

If you gauge a pathogen’s success by how many people it infects, then Toxoplasmosis gondii must surely rank as a superstar among parasites. Somewhere between 1/3rd and 1/2 of all humans on the planet are incidental hosts to this remarkable parasite – most without apparent signs of illness.

While Toxoplasmosis, the disease caused by T. gondii , is usually comprised of minor and short-lived flu-like symptoms, that is not to say that infection is always trivial.

There are serious health risks with T. gondii, particularly to fetuses exposed from freshly infected mothers during the first trimester, and to those with compromised immune systems (including organ transplant recipients and persons with HIV).

The CDC lists Toxoplasmosis as one of five Neglected Parasitic Infections (others include Chagas Disease, Cysticercosis, Toxocariasis, & Trichomoniasis) needing more more attention by public health officials.

And there’s even a fascinating, albeit controversial, debate over whether T. gondii infection can subtly influence an individual’s (or an entire society’s) psychological leanings.

But more on that later.

First, the Advisory Committee on the Microbiological Safety of Food (ACMSF) has provided the UK’s Food Standards Agency with a 70-page report on T. gondii in the food supply, and states that too little is known about the risks and recommends further study of the issue.

While hard numbers are difficult to derive, they estimate roughly 350,000 people become infected with T. gondii each year in the UK, of which 80%-90% will be asymptomatic.

The FSA says they will review this report, and issue a response in the near future. First a link to the report, followed by the FSA press release, after which I’ll return with more.

Further studies on toxoplasma needed

A risk profile on toxoplasma in the food chain by the Advisory Committee on the Microbiological Safety of Food (ACMSF) has recommended that further studies are needed to establish the importance of foodborne routes of infection and identify the highest risks. The suggested studies include work to assess the prevalence of toxoplasma in UK livestock and in food.

(Continue . . . )

While still considered, for the most part, a relatively benign infection in humans, there is a growing body evidence that suggests T. gondii may exert a greater influence on our lives and health than previously thought.

So today, a bit of a review of this ever-so-clever pathogen.

While many mammals (including man) can harbor T. gondii – the definitive host is the feline – which is only animal that can promote the sexual reproductive part of the parasite’s life cycle.

The resultant oocyst is shed in cat feces and can infect the soil -from where it can go on to infect a wide variety of animals – but outside of feline hosts is unable to complete its life cycle.

T. gondii, however, has developed unique methods of getting itself back into cats, where it can reproduce.

Mice, traditionally a mainstay of feline cuisine, lose their fear of the scent of cats after the parasite invades their brains. Absent their normal survival instincts, they are more likely to become dinner, and thereby return the parasite to its primary host.

While cats, mice, and birds make up the primary circle of infection for T. gondii, humans and other mammals are often infected as well. The CDC lists four main methods of transmission.

Foodborne
Animal-to-human (zoonotic)
Mother-to-child (congenital)
Rare instances (transplants, blood transfusions)

As far as its prevalence is concerned, the CDC states:

In the United States it is estimated that 22.5% of the population 12 years and older have been infected with Toxoplasma. In various places throughout the world, it has been shown that up to 95% of some populations have been infected with Toxoplasma. Infection is often highest in areas of the world that have hot, humid climates and lower altitudes.

Before you make plans to exile your cat to a new home, Cornell University’s College of Veterinary Medicine advises:

Can I "catch" toxoplasmosis from my cat?

Because cats only shed the organism for a few days in their entire life, the chance of human exposure is small. Owning a cat does not mean you will be infected with the disease. It is unlikely that you would be exposed to the parasite by touching an infected cat, because cats usually do not carry the parasite on their fur. It is also unlikely that you can become infected through cat bites or scratches. In addition, cats kept indoors that do not hunt prey or are not fed raw meat are not likely to be infected with T. gondii.

In the United States, people are much more likely to become infected through eating raw meat and unwashed fruits and vegetables than from handling cat feces.

The aberrant psychological behavior in infected mice is pretty well documented, and that has led some researchers to wonder whether toxoplasmosis infections can affect human personality traits as well.

In recent years toxoplasmosis has been tenuously linked to increased rates of suicide, some cases of schizophrenia, and even the rate of traffic accidents.

The caveat being, that (repeat after me): correlation does not automatically imply causation.

A few cites:

Toxoplasma gondii seropositivity and suicide rates in women.

Ling VJ, Lester D, Mortensen PB, Langenberg PW, Postolache TT.

Toxoplasma Infection Increases Risk of Schizophrenia, Study Suggests

Release Date: 01/16/2008

Findings from what is believed to be the largest comparison of blood samples collected from healthy individuals and people with schizophrenia suggest that infection with the common Toxoplasma gondii parasite, carried by cats and farm animals, may increase the risk of schizophrenia.

Increased risk of traffic accidents in subjects with latent toxoplasmosis: a retrospective case-control study

Jaroslav Flegr, Jan Havlícek, Petr Kodym, Marek Malý, and Zbyněk Smahel

Conclusion

The subjects with latent toxoplasmosis have significantly increased risk of traffic accidents than the noninfected subjects. Relative risk of traffic accidents decreases with the duration of infection. These results suggest that 'asymptomatic' acquired toxoplasmosis might in fact represent a serious and highly underestimated public health problem, as well as an economic problem.

An Internet search on Toxoplasmosis will return a great many research articles linking T. gondii to a variety of psychological issues, although causality is tough to prove.

We don’t know, for instance, whether T. gondii causes schizophrenia or whether schizophrenics are just more susceptible to T. gondii parasite.

On a more `macro scale’ the cultural personality of entire nations, it has been hypothesized, may hinge on the prevalence of T. gondii infection among their population.

In 2006, USGS ecologist Kevin D. Lafferty published an article in the Proceedings of the Royal Society: Biological Sciences that looked at that very possibility.

Can the common brain parasite, Toxoplasma gondii, influence human culture?

Kevin D Lafferty

It’s a complex question, and his article raises some intriguing possibilities. First, Lafferty noted a couple of things:

The rate of T. gondii infection varies widely around the world, from a low of just over 4% in South Korea, to over 45% in France.
Subtle psychological changes have been observed in people with T. gandii infection, including guilt-proneness in both men and women.

Infected women tend to be more dutiful, rule conscious, and conforming.
Infected men were more rigid, frugal, reactive emotionally and easily upset.

If one assumes a nation’s culture is, at least to some degree, dependent upon the sum of the individual personality traits of its citizens, then toxoplasmosis might be a cultural driving force.

Lafferty looked for a correlation between reported levels of neuroticism – the tendency to experience negative emotional states (anxiety, anger, guilt & depression) – in various countries and the seroprevalence of T. gondii in its people.

And in many instances, he believes found that correlation. He wrote:

The associations between prevalence and cultural dimensions are consistent with the prediction that T. gondii can influence human culture. Just as individuals infected with T. gondii score themselves higher in the neurotic factor guilt-proneness, nations with high T. gondii prevalence had a higher aggregate neuroticism score.

It is, admittedly, a bit disconcerting to think that our individual cultures, and perhaps – even the course of human history - may have been subtly influenced by this common parasite over the years.

On the other hand, as a long-time cat owner, if I’m ever involved in car accident I fully intend to cite these research articles as part of my defense.

For the 77% of Americans who are not infected – and would like try to stay that way – the CDC has a webpage on Toxoplasmosis Prevention & Control. As you might expect, the primary focus is on safe food handling and preparation.

And finally, for a truly fascinating (albeit sometimes technical) discussion of Toxoplasmosis, I can heartily recommend the 80 minute podcast: