Toxo-Talk: Studies on toxoplasmosis are inconclusive

If toxoplasmosis was such a threat to human mental health in the US why haven’t state and federal governments done something about it?

The polarisation of views on T. gondii infections is staggering. For cat haters it is a dire threat to humanity and the reason why the domestic cat should be shot and confined (I subscribe to high standard confinement). Cat lovers see hype, tons of hype denigrating the domestic cat and studies which are profoundly confusing in their conclusions. Even scientists have admitted there is too much hype over toxoplasmosis and I wonder how many scientists have axes to grind and agendas to follow in their so called objective studies.

This is why governments have not taken steps to protect citizens from a T. gondii infection; the studies do not point to a firm conclusion that this parasite negatively impacts mental health. Also some studies conclude that the rate of infection is decreasing in the US.

T. gondii puzzle
T. gondii based puzzle. Photo: Pinterest.
Two useful tags. Click either to see the articles: Toxic to cats | Dangers to cats


America’s CDC state (and they take the lead on this in the US):

“More than 30 million men, women, and children in the U.S. carry the Toxoplasma parasite, but very few have symptoms because the immune system usually keeps the parasite from causing illness.”

Effectively, they say that a T. gondii infection is common and almost always asymptomatic (without symptoms). This conclusion is far removed from the cataclysmic disaster described by cat hating shooters and ornithologists. Note: The USDA have been using cats in toxoplasmosis testing for decades and immorally killing (incinerating) the cats afterwards. Why haven’t they come to some conclusion? They have had long enough.

Studies supporting T. gondii causing personality issues in humans (examples)

In the study: Toxoplasmosis-Associated Difference in Intelligence and Personality in Men Depends on Their Rhesus Blood Group but Not ABO Blood Group it begins:

“The parasite Toxoplasma gondii influences the behaviour of infected animals and probably also personality of infected humans.”

Not much room for doubt as to the findings of this study. In another study: Effects of Toxoplasma on Human Behavior the abstract states:

“…it has been generally assumed that, except for congenital transmission, it is asymptomatic…When infected human adults were compared with uninfected adults on personality questionnaires or on a panel of behavioral tests, several differences were found. Other studies have demonstrated reduced psychomotor performance in affected individuals.”

Studies finding little evidence of an association between T. gondii and personality issues in humans

But a new PLoS ONE paper from Duke University researchers Karen Sugden et al. concludes that there is no evidence that T. gondii infection affects personality and brain function. Study: Is Toxoplasma Gondii Infection Related to Brain and Behavior Impairments in Humans? Evidence from a Population-Representative Birth Cohort [link to page]

The conclusion:

“On the whole, there was little evidence that T. gondii was related to increased risk of psychiatric disorder, poor impulse control, personality aberrations or neurocognitive impairment.”

This is another quote from the study:

“Our results suggest that a positive test for T. gondii antibodies does not result in increased susceptibility to neuropsychiatric disorders, poor impulse control or impaired neurocognitive ability… this is, to our knowledge, the most comprehensive assessment of the possible link between T. gondii infection and a variety of impairments in a single cohort.

Sugden tested the blood samples for antibodies against T. gondii of 837 people taken at aged 38 and born in New Zealand in 1972-73. Twenty-eight percent tested positive which indicates that they were infected with the parasite. The infection was not associated with any personality traits nor with rates of schizophrenia or depression. The study concluded that there was no link between toxoplasmosis and poor impulse control. Suicide attempts were more common but ‘this difference was only of trend significance’ (discover magazine – this phrase may not be scientifically acceptable).

Why did Sugden’s study report no association when others have? They seem suggest that hype is getting in the way of objective science. Another study with a sample size of 7740 produced extensive negative results as well.

In this American study: The relationship between Toxoplasma gondii infection and mood disorders in the third National Health and Nutrition Survey.

The results stated:

“No statistically significant associations were found between T. gondii seroprevalence and a history of major depression (n = 574; adjusted odds ratio [OR]: .8; 95% confidence interval [CI]: .5-1.2), severe major depression (n = 515; adjusted OR: .8; 95% CI: .6-1.2), dysthymia (n = 548; adjusted OR: 1.1; 95% CI: .7-1.8), or dysthymia with comorbid major depression (n = 242, adjusted OR: 1.2; 95% CI: .6-2.4), all p values were > .05, including analysis stratified by gender. However, there was a significant relationship between T. gondii seroprevalence and bipolar disorder type I for respondents in which both manic and major depression symptoms were reported (n = 41; adjusted OR: 2.4; 95% CI: 1.2-4.8; p < .05)."

and they state:

“A substantial literature suggests that schizophrenia is associated with increased seroprevalence of T. gondii, but a possible link of the parasite with mood disorders has not been as thoroughly investigated…”

That perpetual issue from studies

A commenter on the internet stated:

“So do depressed people have toxoplasmosis, or do depressed (suicidal) people gravitate toward becoming cat owners? Perhaps we’ll never know….”

That is an obvious weakness of the toxo-tests.

Decline in infections of T. gondii

And there are reports of a decline in the rate of infection of T. gondii:

“Serologic surveys over past decades indicate that rates of infection with T. gondii have decreased in the United States. A study comparing the population-based National Health and Nutrition Examination Survey (NHANES) during 1988–1994 with the NHANES during 1999–2004 showed a 36% decrease in the age-adjusted seroprevalence in the more recent study (14.1% to 9.0% in persons 12–49 years of age).” [ref:]


“Since the 1960s, rates of infection with Toxoplasma in the United States appear to be declining. In the 1960s, a study of U.S. military recruits indicated that the overall seroprevalence of Toxoplasma was 14% (5). In 1989, a second study of military recruits indicated a seroprevalence of 9.6% (6). Similar downward trends have been observed in France and Sweden (7,8).”[ref:]


“Although T. gondii infects many persons in the U.S., the prevalence has declined in the past decade.”


Hype surrounds toxoplasmosis and it even affects scientists some of whom are biased. The hype is generated by cat haters. More authoritative studies are required. At present we have to conclude as has the CDC that a T. gondii infection is almost always asymptomatic and not a treat to the mental health of Americans. On that basis I’d ask trolls and cat haters to desist from spouting their vitriol on this website and stop attacking me at the same time.

Some old posts on toxoplasmosis

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Useful tag. Click to see the articles: Cat behavior

7 thoughts on “Toxo-Talk: Studies on toxoplasmosis are inconclusive”

  1. Harvey Harrison

    The most important vector for T Gondii is the ground, soil, irrespective of if cats frequent that area. You can look it up. Gardening is the more realistic source of toxoplasmosis infections as well as improperly washed salad vegetables. Infections from cats is limited to only a short period after initial infection.

  2. They actually have done, and do, something about it. Any farm, ranch, or mom & pop roadside produce-stand can be fined and shut-down for up to 4.5 YEARS (how long T. gondii oocysts remain infectious in the environment after being shed by just 1 cat). If any of their produce meant for human consumption has proved to be infected (animals for meats, eggs, unprocessed dairy products (milk, cheese, yogurts, etc); and any vegetables that have been found dusted with T. gondii oocysts from the top-soils); they can lose their license to do business and be shut-down. Many have sadly lost their homes over this fun consequence from their public-health-hazard behaviors. Oh, and all their cats are destroyed too, by law.

    But that’s okay. Keep making up your lies and excuses and not-even-half truths. It’s just quicker to kill your cats for you than argue with and try to educate you. Only about 500,000 more cats will be shot and poisoned to death just today alone because of you and every other person just like you. Makes you all warm and proud about what you are believing and doing, doesn’t it.

    1. What they’ve done is very minor compared to what you want done and you fail to address the point of the article: that the studies are inconclusive. So all your rants are misrepresentations of the truth which is what I have been saving for years. You criticise me for being unscientific and when I produce science you ignore it. Pathetic.

      1. They don’t do this for the inconclusive studies that you like to cite, they do this for the concluded studies. You know, all those fun ones, like toxo caused blindness, prenatal birth defects and miscarriages and stillbirths, or new ones like prostate cancers, IED, etc. All those concluded studies that you don’t want to address (or misrepresent on ad-infinauseum). Most illnesses caused by T. gondii have been known about for nearly a century now. Why is it that you only want to address those still under new study? Evasion, deflection, and misdirection, that’s why. There is no ambiguity about the low numbers of how many lives are killed or maimed by cats carrying T. gondii parasites. There is only ambiguity about the highest number and what other deaths and illnesses are directly caused by this cats’ parasite. Why do you misrepresent the facts? Just to troll for attention of anyone that is compelled to correct your incessant falsehoods and psychotically biased quarter-truth deflections?

        How is a pregnant woman losing her child to T. gondii any different than a woman losing her child to the Zika virus? The former happens far more often, but they wipe-out any swarms of mosquitoes that are carrying the Zika virus. They are now doing this to cats that infect our foods. Why do you think the numbers of T. gondii infections are declining? From destroying your cats near food-sources and stopping anyone providing human foods that are infecting those foods with their cats’ parasites.

        1. Poor you. It is not a question of ‘inconclusive studies’ but “the studies are inconclusive”. You amaze me that you can’t tell the difference.

      2. It would appear the troll is suffering from it’s own brain infection. Sorry Micheal. Hoarding is a mental disorder. Cats are not destroyed for having toxoplasmosis they are treated with antibiotics and become immune. Cats have been an important part of farm life for centuries curtailing vermin infestations. If food is contaminated there are far more serious things to worry about. I would say e coli has killed more people who eat romaine lettuce than any imagined toxoplasmosis contamination in any food. We are all exposed daily to germs, virus and parasites most of which our bodies fight off with no ill effects and we are totally unaware of. This is simply another case for cat haters to latch onto to justify their hate.

        1. Thanks ME. The purpose of the article was to shut up trolls who say studies have found a clear link between T gondii and mental health issues in people. I think I have proved that the studies are inconclusive. They can’t rely on studies to support their case for killing cats.

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