Toxoplasmosis is an infection caused by the parasite Toxoplasma gondii. This parasite is found in cat feces and undercooked meat, especially pork and lamb. Toxoplasmosis infects nearly one-third of the global population, making it one of the most common parasitic infections worldwide. But how exactly does toxoplasmosis affect human behavior when it infects the brain?
How does toxoplasmosis infect humans?
Toxoplasma gondii has a complex life cycle that allows it to infect most warm-blooded animals. Cats are the definitive host, meaning the parasite reproduces sexually in the cat’s gut. The infected cat then sheds parasite eggs called oocysts in its feces.
Humans can become infected by:
- Ingesting food, water or soil contaminated with infected cat feces
- Eating undercooked infected meat containing tissue cysts
- Receiving an infected organ transplant or blood transfusion
- Transmission from mother to fetus during pregnancy
Once ingested, the parasites convert into tachyzoites that spread throughout the body via the bloodstream. They cross the blood-brain barrier and invade the central nervous system, including the brain. Here they form cysts containing bradyzoites. The cysts remain in brain tissue for life.
What happens when toxoplasmosis infects the brain?
The lifelong presence of Toxoplasma cysts in the brain was long thought to be asymptomatic. But newer research shows this latent infection can alter brain function and behavior in humans and other intermediate hosts (any host except cats).
In the brain, toxoplasmosis primarily targets astrocytes, microglia and neurons in the amygdala and nucleus accumbens. These are areas involved in processing emotional reactions and reward-seeking behavior.
Toxoplasma manipulates its host’s brain through the following mechanisms:
- Secretes effector proteins that alter neurotransmitter production in the host brain
- Forms cysts that directly impair neuronal function
- Triggers sustained inflammation and altered gene expression in the brain
The result is subtle changes in personality, cognition, and behavior in the infected host. The parasite enhances its own transmission chances by making the host more attracted to cats.
How does toxoplasmosis change human personality?
Numerous studies show infected humans have different personality traits compared to uninfected controls. Meta-analyses found those with toxoplasmosis exhibit:
- Reduced conscientiousness
- Increased risk-seeking behavior
- Higher neuroticism scores
- More signs of introversion
Infected men appear more jealous and dogmatic, while infected women are more warm-hearted, moralistic and outgoing. The reasons for gender differences are not yet clear.
Some patterns emerge – those with latent toxoplasmosis tend to be more self-doubting and anxious, yet also more impulsive and drawn to risky behaviors. But more research is needed to fully understand toxoplasmosis’ effects on human personality.
How does toxoplasmosis alter human cognition?
Toxoplasmosis also impacts cognitive function in several ways:
- Reflexes – Infected individuals have slower auditory and visual reflex response times in tests.
- Concentration – Those with toxoplasmosis perform worse on tests of focus, attention and short-term memory.
- Intelligence – Latent infection is associated with slightly lower IQ scores.
However, these cognition changes are quite subtle. Overt neurological symptoms are primarily seen only in congenital toxoplasmosis or patients with weakened immunity.
How does toxoplasmosis affect mental illness?
The parasite’s manipulation of neurotransmitters may also increase risk for certain psychiatric disorders. Studies found those with latent toxoplasmosis are:
- Twice as likely to develop schizophrenia
- More prone to self-harm and suicidal behavior
- At higher risk for depression and anxiety
Again, toxoplasmosis alone does not cause mental illness. But it may be one contributing factor among those genetically predisposed. More research is needed to clarify the correlation between toxoplasmosis and mental health.
How does toxoplasmosis increase impulsivity?
Infected humans consistently score higher on impulsivity in behavioral assessments. And brain imaging confirms physiological changes that promote impulsiveness.
Studies link toxoplasmosis to:
- Risky financial investments
- Higher rates of traffic accidents
- Increased drug use
- Violent and antisocial tendencies
Rodent studies also found changes in fear, risk-taking and novelty-seeking after infection. This impulsivity likely reflects the parasite’s evolutionary drive to keep itself moving through different hosts.
Does toxoplasmosis increase suicide risk?
Several epidemiological studies reveal a positive correlation between T. gondii infection and suicidal behavior. A meta-analysis of over 50,000 subjects found:
- Infected individuals are nearly 3 times more likely to attempt suicide
- Risk is higher in men compared to women
- Association is strongest in younger age groups
It’s theorized the parasite’s neurological effects may enhance depression and suicidal thoughts in predisposed people. More clinical studies are underway to confirm if toxoplasmosis is truly a risk factor for suicide.
How does toxoplasmosis affect sex drive and behavior?
Studies consistently find infected men have higher testosterone levels while infected women have higher estradiol levels. Toxoplasma may manipulate hormones to increase transmission chances.
Other behavioral effects include:
- Infected men view themselves as more attractive
- Infected women have more sexual partners
- Both genders express greater commitment to their partners
Rodents also exhibit similar changes, becoming more sexually active and attracted to cat urine odor after infection.
Does toxoplasmosis cause cultural differences?
Population studies reveal interesting correlations between infection rates and cultural practices across different countries. Regions with higher T. gondii prevalence have:
- More acceptance of authoritarian rulers
- Stronger gender stereotypes and roles
- Lower socioeconomic status
- Higher military spending and political instability
While correlation does not equal causation, toxoplasmosis may be one factor shaping broad cultural patterns and norms.
| Country | Toxoplasmosis Prevalence | Collectivism vs Individualism |
|---|---|---|
| Japan | 10% | Collectivist |
| South Korea | 4-7% | Collectivist |
| USA | 22.5% | Individualist |
| Brazil | 50-80% | Collectivist |
This table shows a potential link between higher toxoplasmosis rates and more collectivist cultures.
Does toxoplasmosis increase attraction to cats?
Perhaps the most well-known effect – latent toxoplasmosis seems to subtly increase human attraction toward cats. How does it accomplish this?
- Increases responsiveness to cat odors
- Makes cat urine odors more pleasant
- Improves reaction times when viewing cat images
Studies found infected subjects are also more likely to feed stray cats and be self-reported “cat lovers.” This may represent the parasite’s strategy to get itself back into cats so it can sexually reproduce.
Cat attraction summary
| Study Group | Cat Attraction Behaviors |
|---|---|
| Infected women | More likely to own cats, pay attention to cats, and find cat urine odor pleasant |
| Infected men | Slower reaction times to detect angry cat faces; report no difference in direct cat attraction behaviors |
This table summarizes key research findings on how toxoplasmosis affects cat attraction in humans.
Conclusion
In summary, the common Toxoplasma gondii parasite has complex effects on the human brain and behavior. Latent toxoplasmosis generally increases impulsivity, risk-taking, and attraction to cats. It also subtly alters personality traits like neuroticism, extroversion, and conscientiousness. Effects likely represent the parasite’s evolutionary drive to enhance transmission between hosts.
While influence on individual behaviors is small, toxoplasmosis may also contribute to larger cultural patterns. Some limitations include difficulty proving causation from correlations. Further clinical studies are needed to better understand this fascinating parasite-host relationship.
But current evidence confirms toxoplasmosis is not as asymptomatic as once thought. This ubiquitous parasite may be secretly influencing human psychology and behavior more than we realize.