Can Parasites Cause Schizophrenia? The Scientific Link

Infection with the parasite Toxoplasma gondii roughly doubles the odds of a schizophrenia diagnosis, according to multiple meta-analyses spanning decades of research. That does not mean the parasite causes schizophrenia in any simple sense, but the statistical link is stronger than most people realize, and the biological pathways connecting the two are plausible enough that researchers continue to investigate them seriously. The story involves dopamine, brain inflammation, cats, the gut microbiome, and an odd coincidence involving antipsychotic medications.

What the Numbers Actually Show

Two large meta-analyses anchor most discussions of this topic. A 2007 analysis combining 23 studies found that people with schizophrenia were about 2.7 times more likely to carry antibodies against T. gondii than people without the disorder. The authors noted that while this odds ratio is modest in absolute terms, it exceeded the effect size of any single genetic or environmental risk factor identified at the time.¹ A more recent 2022 systematic review and meta-analysis, which incorporated newer studies and stricter inclusion criteria, found a somewhat lower but still significant odds ratio of about 1.9.²

Those numbers deserve context. An odds ratio of 2 means that people with schizophrenia are about twice as likely to test positive for past Toxoplasma infection compared with matched controls. It does not mean that half of infected people develop schizophrenia. Given that roughly a third of the world’s population carries the parasite and the lifetime prevalence of schizophrenia sits around 1%, the vast majority of infected people never develop psychosis. The association is real but clearly involves other factors, likely including genetic vulnerability, timing of infection, and the strain of parasite involved.

How the Parasite Gets Into the Brain

One reason researchers take this association seriously is that T. gondii does not just linger in the bloodstream. It actively reaches and persists in brain tissue, forming dormant cysts that can remain for the host’s lifetime. The parasite appears to cross the blood-brain barrier partly through a “Trojan horse” strategy: it infects immune cells in the bloodstream, which then carry it across the barrier. Lab studies have shown that T. gondii infection causes brain endothelial cells to ramp up adhesion molecules and inflammatory signals, essentially opening the door for infected immune cells to migrate through.³ Separate in vitro experiments using human brain endothelial cells found that T. gondii exposure disrupted barrier integrity directly, interfering with cell cycles, damaging tight junctions, and reducing cell viability.⁴

Once inside, the parasite does not just sit quietly. Several biological disruptions have been documented that overlap with abnormalities seen in schizophrenia. The most studied is dopamine. Researchers found that T. gondii tissue cysts in the brain contain an enzyme that directly drives dopamine production, encoded by the parasite’s own genome rather than borrowed from the host.⁵ That finding attracted wide attention because excessive dopamine signaling in certain brain circuits is one of the leading neurochemical models for schizophrenia symptoms and is the target of virtually every antipsychotic drug on the market.

The Kynurenine Pathway and Brain Inflammation

Dopamine is not the only neurotransmitter story. Infection with T. gondii also kicks the kynurenine pathway into overdrive. In infected mice, brain levels of tryptophan dropped early after infection while levels of several downstream metabolites, including kynurenic acid and quinolinic acid, rose significantly.⁶ This matters because kynurenic acid blocks a receptor (NMDA) that is itself implicated in schizophrenia. Some researchers have argued that reduced NMDA receptor signaling may underlie the cognitive symptoms of the disorder, and elevated kynurenic acid levels have been found in the cerebrospinal fluid of people with schizophrenia independently of any parasitic infection. The fact that T. gondii infection can reproduce this same neurochemical shift in animals strengthens the plausibility of a biological link.

On top of the neurotransmitter changes, infection triggers chronic low-grade neuroinflammation. The parasite activates brain immune cells called microglia and prompts elevated levels of inflammatory signaling molecules. A 2025 review paper summarized the converging evidence: T. gondii infection has been tied to increased kynurenic acid, elevated dopamine, heightened inflammatory cytokines, disrupted NMDA receptor signaling, compromised blood-brain barrier integrity, and reduced gray matter volume, all features that align with the neuropathology observed in schizophrenia.²

What Brain Scans Reveal

Neuroimaging studies offer another line of evidence. A voxel-based morphometry study compared brain scans of schizophrenia patients who tested positive for T. gondii antibodies with those who tested negative. The infected group had significantly less gray matter in several regions, including the caudate nucleus, thalamus, cingulate cortex, and cerebellum. Crucially, this pattern did not appear when the same comparison was done in healthy controls who tested positive for the parasite, suggesting that T. gondii may interact with schizophrenia-related vulnerability to produce structural brain changes rather than damaging everyone’s brain in the same way.⁷

Infection Before Birth

Some of the most compelling evidence involves timing. Two studies using stored maternal blood samples examined whether a mother’s infection during pregnancy affected her child’s later risk. In one, offspring of mothers with high Toxoplasma antibody levels had about 2.6 times the odds of developing schizophrenia-spectrum disorders in adulthood. Moderate maternal antibody levels showed no association, hinting at a dose-response relationship where higher parasite loads carry more risk.⁸

A second study went further, examining whether the strain of Toxoplasma mattered. The offspring of mothers whose blood showed a pattern consistent with Type I infection were at nearly twice the risk of developing psychosis. The risk was even more pronounced for affective psychoses, with about a fivefold increase. Other parasite genotypes showed no association at all.⁹ The strain-specific finding is important because it suggests that not all Toxoplasma infections carry equal psychiatric risk and may partly explain why so many infected people never develop symptoms.

The Cat Ownership Question

Cats are the only animals in which T. gondii can complete its sexual reproduction cycle, and they shed the parasite’s infectious form in their feces. This has led to decades of interest in whether growing up around cats raises the risk of later psychosis. A 2024 systematic review and meta-analysis pooled data from studies on cat ownership and schizophrenia-related disorders and found a significant association, with an adjusted pooled odds ratio of about 2.4.¹⁰

Not all cats carry the same risk, though. A study of over 2,200 adults in Montreal found that growing up with rodent-hunting cats was associated with a measurably higher frequency of psychotic-like experiences in adulthood among men, while non-hunting cats showed no such link. The strongest effect appeared in a narrow subgroup: people who had experienced head trauma, moved homes more than once in childhood, and owned rodent-hunting cats. Hunting cats are more likely to become infected with T. gondii by eating prey, which may explain the pattern.¹¹ These conditional findings reinforce the point that parasitic exposure alone is probably not enough, but rather it interacts with other risk factors.

Why Toxoplasma Manipulates Behavior at All

T. gondii has an evolutionary incentive to alter its host’s brain. The parasite can infect virtually any warm-blooded animal, but it can only reproduce sexually inside a cat’s gut. For the parasite’s life cycle to continue, an infected rodent ideally needs to be eaten by a cat. Research has shown that infected rats and mice lose their innate aversion to cat urine and may actually become attracted to the scent. This behavioral flip is remarkably specific: infected rodents show no reduction in learned fear, general anxiety, or sense of smell, only the hardwired avoidance of cat odor changes. Parasite cysts tend to cluster more densely in the amygdala, the brain region governing fear and threat assessment, which may explain the targeted nature of this manipulation.¹²

In humans, there is no evolutionary pressure for the parasite to alter behavior (humans are not regularly preyed on by cats), so any psychiatric effects may be unintended collateral damage from the same neurochemical machinery the parasite uses to manipulate rodent hosts. The dopamine-boosting enzyme encoded in the parasite’s genome presumably evolved to affect rodent behavior, but it does not discriminate between rodent and human brain tissue.

A Strange Coincidence With Antipsychotic Drugs

One of the more provocative findings in this field is that several drugs used to treat schizophrenia also happen to inhibit T. gondii replication in laboratory settings. A study testing 12 neuroleptic compounds found that haloperidol, a widely used antipsychotic, and valproic acid, a mood stabilizer, were the most effective at suppressing parasite growth in cell cultures. Valproic acid worked at concentrations lower than what is normally present in treated patients’ blood and spinal fluid, and it showed synergistic activity when combined with haloperidol or with trimethoprim, an antibiotic already used to treat toxoplasmosis.¹³ A follow-up study confirmed that some antipsychotics, including fluphenazine and zuclopenthixol, have notable anti-Toxoplasma activity, though the effect varies widely between drugs and does not appear to be a uniform class property.¹⁴

This does not prove that antipsychotics work by fighting parasites. They almost certainly work primarily through dopamine receptor blockade. But the overlap has fueled speculation that early clinicians may have unknowingly selected for drugs with dual action, or that at least some of the clinical benefit in infected patients comes from reducing parasite burden. Unfortunately, a direct test of the antiparasitic theory did not pan out: a randomized trial adding trimethoprim to standard antipsychotic treatment in patients with chronic schizophrenia found no benefit over placebo.¹⁵ That trial did not stratify patients by T. gondii infection status, however, so the question of whether antiparasitic treatment helps infected patients specifically remains open.

Effects in Healthy People

The psychiatric effects of T. gondii are not limited to people who develop schizophrenia. A 2021 systematic review and meta-analysis examined cognitive function in otherwise healthy people who tested positive for the parasite. It found a small but significant decrease in processing speed among infected individuals, measured across reaction-time and trail-making tests.¹⁶ The effect was subtle enough that no individual would likely notice it, but it suggests the parasite’s neurological influence operates on a spectrum rather than as an all-or-nothing switch flipped only in vulnerable individuals.

Other Parasites Under Scrutiny

Toxoplasma is not the only parasite that has been investigated in relation to schizophrenia. Toxocara, a roundworm commonly carried by dogs and cats, has drawn increasing attention. A 2021 meta-analysis found that people with schizophrenia had a substantially higher rate of Toxocara antibodies compared with controls, with a pooled odds ratio of about 4.¹⁷ A larger 2025 meta-analysis covering 34 studies of Toxocara and various neuropsychiatric conditions echoed this, reporting a pooled risk ratio for schizophrenia of about 3.8, though the overall association with psychiatric disorders as a group fell just short of statistical significance.¹⁸

Counterbalancing that, a 2024 case-control study comparing Toxocara antibody rates between schizophrenia patients and healthy controls found no significant difference at all, with seroprevalence nearly equal between groups.¹⁹ The Toxocara evidence is considerably thinner and more mixed than the Toxoplasma literature. Proposed mechanisms are similar in broad strokes, involving neuroinflammation and neurotransmitter disruption, but far less work has been done to trace the specific pathways.

The Gut Microbiome Connection

A newer line of research explores whether parasitic infections influence the brain partly through the gut. Mouse studies have shown that T. gondii infection dramatically reshapes the intestinal microbiome, reducing microbial diversity, depleting beneficial bacteria like certain Lactobacillus species, and expanding potentially harmful groups.²⁰ This matters because the gut microbiome communicates with the brain through immune signaling, metabolite production, and the vagus nerve.

In a particularly elegant experiment, researchers found that chronically infected mice developed anxiety-like behavior, and that wiping out the gut bacteria with antibiotics actually reduced that anxiety. Even more telling, transplanting fecal matter from infected mice into uninfected, antibiotic-treated mice was enough to reproduce the anxiety and trigger gene-expression changes in the amygdala, the same brain region where T. gondii cysts tend to accumulate.²¹ Separate work showed that supplementing infected mice with specific Lactobacillus strains reduced parasite burden in the intestine, liver, and brain while improving intestinal barrier damage and reducing neuronal inflammation.²² This is still early-stage animal work, but it opens the possibility that gut-targeted interventions, potentially even certain probiotics, could one day complement traditional psychiatric treatment in infected patients.

Why This Has Not Changed Clinical Practice Yet

With all this evidence, you might wonder why psychiatrists do not routinely test schizophrenia patients for T. gondii. Several obstacles stand in the way. The association is statistical, not individual: most infected people never develop psychosis, and most people with schizophrenia are probably not infected. The one randomized trial of antiparasitic treatment added to standard care showed no benefit, though its design had limitations. And the precise causal pathway remains unproven. It could be that T. gondii genuinely triggers schizophrenia in a subset of genetically vulnerable people, or it could be that people who develop schizophrenia are more likely to have lived in conditions that expose them to the parasite, with the infection itself being a marker rather than a cause.

Animal experiments and neuroimaging data make the “just a marker” explanation harder to sustain, because the parasite demonstrably alters dopamine metabolism, triggers neuroinflammation, and reduces gray matter volume in infected patients but not in infected healthy controls. Still, a definitive clinical trial, one that tests antiparasitic treatment specifically in T. gondii-positive schizophrenia patients and measures psychiatric outcomes, has not yet been completed. Until it is, the field remains in a frustrating but familiar position: strong association, plausible mechanism, uncertain causation.