Parasites, Mood and the Nervous System

Table of Contents

1. What the Presentation Said
2. Toxoplasma: The Parasite at the Centre of This Research
3. Toxoplasma and Behaviour in Rats and Mice
4. Toxoplasma and Dopamine
5. Do Parasites Steal Serotonin?
6. Toxoplasma and Suicidal Behaviour: The Human Studies
7. Schizophrenia, Bipolar Disorder and Depression
8. Have Antiparasitic Drugs Changed Psychiatric Symptoms?
9. Parasites That Live in the Brain
10. Parasite Neurotoxins: Which Ones Exist
11. Intestinal Worms, Children and Thinking
12. How the Claim Compares With the Research
13. What Is Not Known
14. Connections
15. Key Research Papers
16. Featured Videos

What the Presentation Said

The speaker returned to the nervous system several times during the talk. The main points, in the order they were made, were these:

• Parasites compromise the nervous system. The speaker said parasites consume neurotransmitters — the chemical messengers that nerve cells use to signal each other — and that this leads to worsening mental health, memory loss and decline.
• Parasites release neurotoxins. The speaker said these toxins act on the nervous system and impair thinking, clear thought and mood, and that this is why depression, anxiety and suicidal tendencies can be characteristics of a parasite infection.
• Mood, sleep and irritability. Showing an illustration of the effects of parasites on mental and emotional health, the speaker listed suicidal tendency, mood changes, sleep disturbance, anxiety and irritability, and said that parasites can steal neurotransmitters such as serotonin and dopamine.
• Fatigue and brain fog. The speaker said patients with parasites feel fatigue and brain fog, and invited listeners to ask themselves whether their own anxiety might have a parasitic cause.
• Nerve coatings. Early in the talk the speaker also said parasites eat into the protein coating of nerves, the myelin sheath.

As an example the speaker described the case of a 64-year-old woman whose symptoms included memory loss and depression and in whom surgeons found a live roundworm about 8 centimetres long in the brain, a parasite normally found in carpet pythons. That case is examined on the Parasites in the Brain page.

The speaker did not name a particular parasite as the cause of mood changes, did not name a neurotoxin, and did not cite a study for the neurotransmitter claims. This page therefore tests the general claim against the parasite that has been studied most for its effects on mood and behaviour, Toxoplasma gondii, and against the other known cases of parasites affecting the brain and nerves.

Toxoplasma: The Parasite at the Centre of This Research

Toxoplasma gondii is a single-celled parasite (a protozoan). Cats are its final host: it reproduces sexually only in the cat gut and leaves in cat faeces as hardy egg-like cysts called oocysts. Almost any warm-blooded animal, including people, can become an intermediate host by swallowing those oocysts from soil, water or unwashed produce, or by eating undercooked meat that contains tissue cysts. Researchers estimate that roughly one-third of the world's population carries the infection (Amouei and colleagues, 2020).

In most healthy adults the first infection causes mild flu-like symptoms or none at all. The parasite then settles into a dormant, walled-off form — tissue cysts — in muscle and in the brain, where it can stay for life. This is called latent or chronic infection. The danger is well established for two groups: unborn babies infected through the mother, and people with a weakened immune system, in whom the dormant cysts can reawaken and cause brain inflammation (toxoplasmic encephalitis). These are covered on the site's Toxoplasma pages.

Because the cysts sit in the brain for decades in otherwise healthy people, researchers have asked whether latent infection subtly changes behaviour or mood. Almost all of the human research on parasites and mental health comes from this question.

How the human studies measure infection

Nearly every human study below measures antibodies in the blood, not the parasite itself. Immunoglobulin G (IgG) antibodies show that a person was infected at some point in life and usually still carries cysts. Immunoglobulin M (IgM) antibodies suggest a recent infection. "Seropositive" means the antibodies were found; "serointensity" means how high the antibody level was. An antibody test cannot show where the cysts are, how many there are, or whether they are doing anything.

Toxoplasma and Behaviour in Rats and Mice

The strongest evidence that a parasite can change behaviour comes from rodents. A rat or mouse is the parasite's way back into a cat, and infected rodents lose their inborn fear of cat smell.

• In rats, infected animals approached cat odours that uninfected rats avoid. The same study found that infection did not change the rats' response to the odour of a non-cat predator, and did not change their behaviour on general smell, social, fear or anxiety tests. When infected rats were exposed to cat urine, brain regions involved in sexual attraction became more active (House and colleagues, 2011).
• The effect depends on the strain. In mice, two strains of the parasite both increased attraction to cat odour two months after infection, but only one strain still did so at seven months; one strain made mice heavier and more active, the other impaired spatial working memory (Kannan and colleagues, 2010).

These are findings in animals. They show that this one parasite can alter one specific fear response in its natural prey. They do not show that the same happens in people, and a narrow change in fear of cat odour is not the same thing as depression or anxiety. In the rat study above, general anxiety measures were in fact unchanged.

Toxoplasma and Dopamine

Dopamine is a brain chemical involved in motivation, reward, movement and, when it is out of balance, in psychosis. The genome of Toxoplasma contains genes for its own version of tyrosine hydroxylase, the enzyme that controls the first, rate-limiting step in making dopamine. That discovery is the main reason scientists link this parasite to dopamine.

• In cell culture, dopamine-producing cells infected with Toxoplasma released several times more dopamine when stimulated, and the amount rose with the number of infected cells. In mice, brain tissue cysts stained strongly for dopamine and for the parasite's own tyrosine hydroxylase (Prandovszky and colleagues, 2011).
• In rats, a 2025 study engineered parasite lines that over-produce this enzyme. All the genetically modified lines caused weaker behaviour changes than the natural parasite, but within those lines the size of the behaviour change tracked the amount of enzyme. The authors describe the enzyme's role as debated and conclude that it contributes to the behaviour changes (Calvo-Urbano and colleagues, 2025).

Two points matter for the presentation's claim. First, this research describes the parasite adding to or raising dopamine activity around its cysts, not consuming or stealing the host's dopamine. Second, every one of these measurements was made in cells, mice or rats. No study found for this page measured a parasite lowering dopamine in a human brain.

Do Parasites Steal Serotonin?

Serotonin is a chemical messenger in the brain and gut, made from the amino acid tryptophan, which people get from protein in food. It is often linked with mood because several antidepressant drugs act on serotonin signalling.

No study was found that shows a parasite consuming or stealing serotonin from a human host. The closest real finding runs the other way round: it is the host's immune system that breaks down tryptophan, as a weapon against Toxoplasma.

• In cell culture, human cells treated with interferon gamma — an immune signalling protein released during infection — stopped Toxoplasma from growing by rapidly breaking down tryptophan into a compound called kynurenine. The more tryptophan in the culture medium, the more interferon was needed to stop the parasite (Pfefferkorn, 1984). The parasite cannot make its own tryptophan, so the host starves it.

This shows a real link between infection, inflammation and the raw material for serotonin. That study did not measure serotonin, brain chemistry or mood in people, and it describes the body's defence, not theft by the parasite. Whether this immune process changes mood in infected people is a research question, not an established finding.

Toxoplasma and Suicidal Behaviour: The Human Studies

The presentation linked parasites to suicidal tendencies. This is the area where human data are most extensive, and it concerns Toxoplasma specifically.

A prospective cohort in Denmark

The most informative single study followed 45,788 women in Denmark whose Toxoplasma immunoglobulin G antibodies had been measured when they gave birth between 1992 and 1995, and tracked them through national registers until 2006. Because the antibody test came first, this design shows that infection preceded the outcome. Compared with uninfected mothers, infected mothers had a relative risk of self-directed violence of 1.53 (95% confidence interval 1.27 to 1.85), and the risk appeared to rise with higher antibody levels. The relative risk of violent suicide attempts was 1.81 (1.13 to 2.84). For suicide itself the relative risk was 2.05, but the confidence interval (0.78 to 5.20) included no difference, so that result was not statistically clear (Pedersen and colleagues, 2012).

Pooled studies

• A 2019 systematic review and meta-analysis found that people with antibodies had higher odds of a past suicide attempt, with an odds ratio of 1.39 (95% confidence interval (CI) 1.10 to 1.76). The link was clear when attempters were compared with healthy people (odds ratio about 1.9) but disappeared when they were compared with other psychiatric patients (1.06, not significant), and it was absent in studies limited to people with schizophrenia. The authors estimated that if the link were causal about 10% of suicide attempts could be attributed to infection, while stating that it remains uncertain whether infection and outcome are truly causally related (Sutterland and colleagues, 2019).
• A 2020 meta-analysis of 27 studies also reported an association between suicidal behaviour and latent infection, seen with immunoglobulin G (past infection) but not immunoglobulin M (recent infection) antibodies, and noted the limited number of studies (Amouei and colleagues, 2020).

Single studies that differ

• In Korea, immunoglobulin G antibodies were found in 21 of 155 psychiatric patients who had attempted suicide and in 8 of 135 healthy controls; among the attempters, those with antibodies scored higher on depressed mood and on the severity and lethality of attempts (Bak and colleagues, 2018).
• In Turkey, a study of 50 adolescents who had attempted suicide and 50 healthy adolescents found a higher rate of antibodies in the attempters, but the difference was not statistically significant (Sari and Kara, 2019).

What these numbers mean

An odds ratio or relative risk of about 1.4 to 1.8 is a modest increase on top of a baseline risk. Most people who carry Toxoplasma never attempt suicide, and most people who attempt suicide do not carry it. The comparison with psychiatric controls matters: when the comparison group also has mental illness, the link largely disappears, which leaves open whether infection, mental illness, or shared factors (diet, living conditions, contact with soil and animals) explain the pattern. A leading researcher in this field wrote in 2025 that causality has not been fully demonstrated and that there are, so far, no effective and safe treatments for chronic toxoplasmosis with demonstrated mental-health benefits in people with a healthy immune system (Postolache, 2025).

Anyone having thoughts of suicide can reach a crisis line at any hour; in the United States, the 988 Suicide & Crisis Lifeline answers calls and texts at 988.

Schizophrenia, Bipolar Disorder and Depression

The presentation named depression and anxiety. The research has looked hardest at schizophrenia, and the findings differ by condition.

• Schizophrenia. A 2007 meta-analysis of 23 controlled studies from 17 countries found that people with schizophrenia were more likely to have Toxoplasma antibodies, with a combined odds ratio of 2.73 (95% confidence interval 2.10 to 3.60). The authors stated that an association had been shown and that a cause had not yet been proven (Torrey and colleagues, 2007).
• A wider 2015 review of 50 studies compared people with psychiatric disorders against healthy controls. Immunoglobulin G antibodies were linked with schizophrenia (odds ratio 1.81), bipolar disorder (1.52), obsessive-compulsive disorder (3.4) and addiction (1.91). After adjusting for publication bias — the tendency of positive results to be published more often — the schizophrenia odds ratio fell to 1.43 but stayed significant (Sutterland and colleagues, 2015).
• Major depression. The same 2015 review found no significant link with major depression (odds ratio 1.21, p = 0.28). A separate 2019 meta-analysis of 29 studies, covering several thousand people with depression, also found no significant link: pooled odds ratio 1.15 (95% confidence interval 0.95 to 1.39). Its authors concluded that toxoplasmosis is not a risk factor for major depressive disorder, while calling for more work on milder forms of depression (Nayeri Chegeni and colleagues, 2019).

No meta-analysis of Toxoplasma and anxiety disorders was found in the records gathered for this page; the Korean suicide study above reported higher anxiety scores among antibody-positive attempters, but that is one small study within a selected group.

So for the two conditions the speaker named most often, the findings are: depression — pooled studies do not show a significant association; anxiety — very little direct research. The stronger associations are with conditions the speaker did not name, such as schizophrenia.

Have Antiparasitic Drugs Changed Psychiatric Symptoms?

If a parasite were driving a mental illness, treating the parasite might be expected to help. This has been tested in schizophrenia.

A 2017 review identified four published randomised controlled trials in which people with schizophrenia were given an antiparasitic or antibiotic drug with activity against Toxoplasma — azithromycin, trimethoprim, artemisinin or artemether — on top of their usual treatment. None of the trials showed a change in psychiatric symptoms. The reviewer pointed out that the drugs chosen may not reach or kill the dormant brain cysts, that doses may have been too low, and that the trials were small, so the question is not closed (Chorlton, 2017).

No randomised trial was found that treated depression, anxiety or suicidal behaviour with antiparasitic medicines. The Danish and pooled studies above therefore show an association that has not been turned into a tested treatment. Decisions about mental-health treatment and antiparasitic medicines belong with a clinician.

Parasites That Live in the Brain

Some parasites physically grow inside the brain or spinal cord. When they do, changes in mood, memory and thinking are documented, but they arise from a space-taking cyst, swelling or inflammation in a specific place, not from a general theft of brain chemicals.

• Neurocysticercosis is infection of the brain with the larval cysts of the pork tapeworm, Taenia solium. A 2021 review describes it as the most common parasitic disease of the nervous system worldwide, affecting more than 50 million people. Reported psychiatric effects include depression, cognitive problems, dementia and visual hallucinations, and the symptoms depend on where the cysts lie. The review also notes that how often it occurs among psychiatric patients is seldom measured, partly because brain imaging is scarce where the infection is common (El-Kady and colleagues, 2021). The infection itself comes from swallowing tapeworm eggs; see the site's Neurocysticercosis page.
• Reactivated Toxoplasma in people with a weakened immune system can cause toxoplasmic encephalitis, with confusion and neurological deficits; this is a recognised medical emergency rather than a subtle mood effect.
• Rare roundworms, including rat lungworm and the python roundworm in the case the speaker cited, can migrate into the brain and cause inflammation. These are covered on the Parasites in the Brain page.

On the presentation's claim that parasites eat into the myelin coating of nerves: no study was found describing intestinal parasites in general consuming myelin. Brain inflammation from any cause, including the infections above, can damage nerve tissue, but that is a different statement from the one made.

Parasite Neurotoxins: Which Ones Exist

The presentation said that parasites release neurotoxins. Some real nerve toxins do come from parasites, but they are few and specific.

• Tick paralysis. Ticks are external parasites that feed on blood. A 2024 review describes tick paralysis as a potentially fatal condition caused by neurotoxins made in the salivary glands of certain ticks; 75 tick species, mostly hard ticks, have been identified as able to cause it, with cases reported mainly in the United States, Canada and Australia. The paralysis is fully reversible once the tick is removed (Deng and colleagues, 2024). It causes weakness and paralysis, not depression or anxiety.
• Intestinal worms and protozoa. No neurotoxin produced by common intestinal worms or protozoa and shown to cause depression, anxiety or suicidal thinking in people was found in the research gathered for this page. Parasites do release many molecules that dampen or redirect the immune system, and inflammation can affect how people feel, but none of these has been identified as a "mood toxin".

The speaker did not name a toxin, so the specific claim could not be matched to any published compound.

Intestinal Worms, Children and Thinking

The presentation linked parasites with memory loss and brain fog. The best-tested version of that idea is whether treating intestinal worms improves children's thinking and school performance in places where worms are common. If worms were draining the brain, deworming would be expected to help. Large trials have tested this.

• A Cochrane systematic review of 34 randomised trials found some weight gain after a single dose of deworming medicine in some settings, but in six of seven trials with clear data on cognitive tests and school performance, five reported no significant effect. The authors concluded that no effect on cognition or school performance had been demonstrated (Taylor-Robinson and colleagues, 2007).
• A 2017 review in The Lancet Global Health combined 52 studies covering more than 1.1 million children. Mass deworming for soil-transmitted worms led to little or no difference in short-term attention (−0.23 points on a 100-point scale) or school attendance (1% higher, 95% confidence interval −1 to 3), both rated high-certainty evidence (Welch and colleagues, 2017).

These results concern mass treatment of whole school populations, most of whom carry light infections. Heavy infections cause anaemia and poor growth, which are real harms (see the sibling page Do Parasites Steal Your Nutrients?). But the trials do not show that ordinary worm infections measurably dull thinking, or that removing them sharpens it.

How the Claim Compares With the Research

Supported in part

• "Parasites can affect the brain and behaviour." Supported for specific parasites: Toxoplasma changes fear behaviour in rodents, and parasites that grow in the brain, such as the pork tapeworm larva, are documented to cause depression, cognitive problems and other psychiatric symptoms.
• "Suicidal tendencies are linked with parasites." Published studies report an association between past Toxoplasma infection and suicide attempts and self-harm, including a prospective cohort of 45,788 Danish mothers. The increase is modest, weakens or disappears against psychiatric comparison groups, and has not been shown to be causal.

Not supported by the studies found

• "Depression is a characteristic of parasite infection." Two meta-analyses found no significant link between Toxoplasma and major depression.
• "Parasites steal serotonin and dopamine." The research on Toxoplasma describes the parasite raising dopamine activity in animal and cell studies, not consuming it. The tryptophan finding describes the host's immune system breaking down a serotonin building block to starve the parasite. No study was found showing a parasite consuming a human host's neurotransmitters.
• "Parasites cause memory loss and brain fog." Large deworming trials in children did not show improvements in attention, cognition or school performance after treatment.

No research found

• No named or identified parasite neurotoxin that causes depression, anxiety or suicidal thinking in people. The known parasite neurotoxins, from ticks, cause paralysis.
• No study showing that intestinal parasites in general consume myelin.
• No randomised trial of antiparasitic treatment for depression, anxiety or suicidal behaviour. In schizophrenia, four add-on trials found no change in symptoms.

What Is Not Known

• Whether latent Toxoplasma infection causes any part of the higher rate of suicide attempts seen in infected people, or whether shared background factors explain the pattern.
• Whether the parasite's dopamine-making enzyme, shown to matter in rats, has any measurable effect in the human brain.
• Whether the immune breakdown of tryptophan during infection changes serotonin or mood in people.
• Whether a drug able to clear dormant brain cysts would change any psychiatric outcome; no such safe drug has been shown to exist for people with a healthy immune system.
• How often parasites that grow in the brain, such as pork tapeworm cysts, go undiagnosed among psychiatric patients in regions where imaging is scarce.
• Whether anxiety disorders have any link with Toxoplasma; direct pooled research is lacking.

Connections

• Cancer and Parasites
• Parasites in the Brain
• Toxoplasma and Brain Tumors
• How Parasites Evade the Immune System
• Do Parasites Steal Your Nutrients?
• Toxoplasma
• Toxoplasma Prevention: Food and Cat Safety
• Toxoplasmosis
• Neurocysticercosis
• Depression
• Anxiety
• Schizophrenia
• Bipolar Disorder
• Mass Deworming Programs
• Parasites

Key Research Papers

1. House PK, Vyas A, Sapolsky R. Predator cat odors activate sexual arousal pathways in brains of Toxoplasma gondii infected rats. PLoS One. 2011;6(8):e23277. PubMed PMID: 21858053
2. Kannan G, Moldovan K, Xiao JC, Yolken RH, Jones-Brando L, Pletnikov MV. Toxoplasma gondii strain-dependent effects on mouse behaviour. Folia Parasitol (Praha). 2010;57(2):151-5. PubMed PMID: 20608478
3. Prandovszky E, Gaskell E, Martin H, Dubey JP, Webster JP, McConkey GA. The neurotropic parasite Toxoplasma gondii increases dopamine metabolism. PLoS One. 2011;6(9):e23866. PubMed PMID: 21957440
4. Calvo-Urbano B, Kaushik M, Cheng AC, Bristow GC, Prandovszky E, Walker M, Lamberton PHL, McConkey GA, Webster JP. The role of parasite-produced dopamine in Toxoplasma gondii-altered host behaviour. Nat Commun. 2025;16(1):10906. PubMed PMID: 41350261
5. Pfefferkorn ER. Interferon gamma blocks the growth of Toxoplasma gondii in human fibroblasts by inducing the host cells to degrade tryptophan. Proc Natl Acad Sci U S A. 1984;81(3):908-12. PubMed PMID: 6422465
6. Pedersen MG, Mortensen PB, Norgaard-Pedersen B, Postolache TT. Toxoplasma gondii infection and self-directed violence in mothers. Arch Gen Psychiatry. 2012;69(11):1123-30. PubMed PMID: 22752117
7. Sutterland AL, Kuin A, Kuiper B, van Gool T, Leboyer M, Fond G, de Haan L. Driving us mad: the association of Toxoplasma gondii with suicide attempts and traffic accidents - a systematic review and meta-analysis. Psychol Med. 2019;49(10):1608-1623. PubMed PMID: 31010440
8. Amouei A, Moosazadeh M, Nayeri Chegeni T, et al. Evolutionary puzzle of Toxoplasma gondii with suicidal ideation and suicide attempts: An updated systematic review and meta-analysis. Transbound Emerg Dis. 2020;67(5):1847-1860. PubMed PMID: 32198980
9. Bak J, Shim SH, Kwon YJ, Lee HY, Kim JS, Yoon H, Lee YJ. The Association between Suicide Attempts and Toxoplasma gondii Infection. Clin Psychopharmacol Neurosci. 2018;16(1):95-102. PubMed PMID: 29397671
10. Sari SA, Kara A. Association of Suicide Attempt With Seroprevalence of Toxoplasma gondii in Adolescents. J Nerv Ment Dis. 2019;207(12):1025-1030. PubMed PMID: 31688493
11. Postolache TT. Toxoplasma gondii and suicidal behaviour: discovery, cross-diagnostic confirmation and pathway exploration. Folia Parasitol. 2025;72:2025.025. PubMed PMID: 40923495
12. Torrey EF, Bartko JJ, Lun ZR, Yolken RH. Antibodies to Toxoplasma gondii in patients with schizophrenia: a meta-analysis. Schizophr Bull. 2007;33(3):729-36. PubMed PMID: 17085743
13. Sutterland AL, Fond G, Kuin A, Koeter MW, Lutter R, van Gool T, Yolken R, Szoke A, Leboyer M, de Haan L. Beyond the association. Toxoplasma gondii in schizophrenia, bipolar disorder, and addiction: systematic review and meta-analysis. Acta Psychiatr Scand. 2015;132(3):161-79. PubMed PMID: 25877655
14. Nayeri Chegeni T, Sharif M, Sarvi S, et al. Is there any association between Toxoplasma gondii infection and depression? A systematic review and meta-analysis. PLoS One. 2019;14(6):e0218524. PubMed PMID: 31194852
15. Chorlton SD. Toxoplasma gondii and schizophrenia: a review of published RCTs. Parasitol Res. 2017;116(7):1793-1799. PubMed PMID: 28508166
16. El-Kady AM, Allemailem KS, Almatroudi A, Abler B, Elsayed M. Psychiatric Disorders of Neurocysticercosis: Narrative Review. Neuropsychiatr Dis Treat. 2021;17:1599-1610. PubMed PMID: 34079258
17. Deng YP, Fu YT, Elsheikha HM, et al. Comprehensive analysis of the global impact and distribution of tick paralysis, a deadly neurological yet fully reversible condition. Clin Microbiol Rev. 2024;37(4):e0007424. PubMed PMID: 39440956
18. Taylor-Robinson DC, Jones AP, Garner P. Deworming drugs for treating soil-transmitted intestinal worms in children: effects on growth and school performance. Cochrane Database Syst Rev. 2007;(4):CD000371. PubMed PMID: 17943740
19. Welch VA, Ghogomu E, Hossain A, et al. Mass deworming to improve developmental health and wellbeing of children in low-income and middle-income countries: a systematic review and network meta-analysis. Lancet Glob Health. 2017;5(1):e40-e50. PubMed PMID: 27955788

PubMed Topic Searches

1. Toxoplasma gondii and suicide
2. Toxoplasma gondii and schizophrenia (meta-analyses)
3. Toxoplasma gondii and dopamine
4. Neurocysticercosis and psychiatric symptoms
5. Deworming and children's cognition

Other Sources

1. Substance Abuse and Mental Health Services Administration, 988 Suicide & Crisis Lifeline. samhsa.gov

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