Cats and Toxoplasmosis: The Parasite of Parasites Among Us.


               By Dr. Michael W. Fox

                 Executive Summary

Toxoplasmosis is a disease infecting 11% of the U.S. population and some 60% worldwide. The proliferating reservoir of this parasite are domestic cats who spread millions of its eggs in their feces that can infect humans; pregnant women birthing brain damaged and blind children and adults experiencing neurobehavioral problems including increased risk-taking and impulsive behaviors as documented below. Gardeners, and children in playgrounds where infected cats have defecated, can become infected.

Farmed animals are infected by cats shedding the eggs of Toxoplasma gondii in their feed and where they graze. They get into their meat as cysts, leading to human infection just as mice and other wildlife become infected and in turn, infect cats who have not yet been infected by Toxoplasma.

Most state public health and veterinary associations may be aware of this parasite but have yet to initiate effective legislation and ordinances to prevent owned cats, who can spread this disease, from being allowed to roam free outdoors: and from reducing free-roaming/”feral” cat populations by more effective means than practiced by many animal shelters and humane societies, namely TNR-trap, neuter and release. This has not been shown to effectively reduce the outdoor cat population, a major factor in the decline of song birds and other wildlife. TNR is inhumane, virtually guaranteeing a slow death for many cats from injuries and diseases since these cats are not provided veterinary care as needed.

Horses in stables and zoo animals are also at risk of Toxoplasmosis when cats are around their facilities, ostensibly to control rodents that continue the cycle of infection. All animal feed and grains on farms should be made rodent-proof and all cats re-homed. Every owned cat should be secured and never roam free.

There are many more other diseases that cats can transmit, more so than rats, to people. It is therefore a public duty of all health authorities, state veterinary associations, animal control agencies and those involved in municipal legislation to address the free-roaming cat issue and responsible cat care. Imagine a parasite evolving over millennia to be able to influence the behavior of whatever animal it infects so that its continuation by transmission to other animals is insured. Domestic cats are the primary host for this super-parasite which they shed in their feces and the eggs then infect other animals, especially small rodents. Inside the rodents they alter the brains of their new hosts, making them attracted to the urine and other odors of cats and reducing their instinctual fear so they are more likely to be killed and eaten by cats who then become infected.

Toxoplasmosis in humans is summarized by the CDC as follows: ( The tissue form of the parasite (a microscopic cyst consisting of bradyzoites) can be transmitted to humans by food. People become infected by:

• Eating undercooked, contaminated meat (especially pork, lamb, and venison) or shellfish (like oysters, clams, and mussels);

• Accidentally ingesting undercooked, contaminated meat or shellfish after handling it and not washing hands thoroughly (Toxoplasma cannot be absorbed through intact skin); and

• Eating food that was contaminated by knives, utensils, cutting boards or other foods that had contact with raw, contaminated meat or shellfish.

• Drinking unpasteurized goat’s milk (tachyzoites)

The CDC estimates that more than 30 million people in the United States may be infected with the Toxoplasma parasite. Lifecyle of Toxoplasma gondii. ( From James B. McAule, Congenital Toxoplasmosis J Pediatric Infect Dis Soc. 2014 Sep; 3(Suppl 1): S30–S35. doi: 10.1093/jpids/piu077

(1) Unsporulated oocysts are shed in the cat’s feces, although oocysts are usually only shed for 1–2 weeks, large numbers may be shed.

(2) Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water, or plant material contaminated with oocysts.

(3) Oocysts transform into tachyzoites that localize in neural and muscle tissue and develop into tissue cyst bradyzoites.

(4) Cats become infected after consuming intermediate hosts harboring tissue cysts or by ingestion of sporulated oocysts.

(5) Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment.

Dr. McAule emphasizes: “Cats that are allowed to roam outside are much more likely to become infected than domestic cats that are confined indoors.” Pregnant women can pass this parasite on to the developing fetus. He reports that “Congenital toxoplasmosis has a wide spectrum of clinical manifestations, but it is subclinical in approximately 75% of infected newborns….. When clinically apparent, it may mimic other diseases of the newborn. In a proportion of cases, spontaneous abortion, prematurity, or stillbirth may result.

Involvement of the CNS is a hallmark of congenital Toxoplasma infection. The presence of chorioretinitis, intracranial calcifications, and hydrocephalus is considered the classic triad of congenital toxoplasmosis. Fever, hydrocephalus or microcephaly, hepatosplenomegaly, jaundice, convulsions, chorioretinitis (often bilateral), cerebral calcifications, and abnormal cerebrospinal fluid are the classic features of severe congenital toxoplasmosis. Other occasional findings included rash (maculopapular, petechial, or both), myocarditis, pneumonitis and respiratory distress, hearing defects, an erythroblastosis-like picture, thrombocytopenia, lymphocytosis, monocytosis, and nephrotic syndrome”.

Some public health officials, when confronted by citizens concerned about the public health risks and risks to farmed animals and wildlife of infection by this parasite carried by domestic cats, seem either uninformed or indifferent. This must change for many reasons including the probable influence of this parasite on human behavior, increasing risk-taking, impulsivity and aggression, influencing sexual behavior and elevating testosterone levels. Toxoplasmosis has also been linked to suicidal behavior, psychosis, intermittent explosive disorder, schizophrenia and may contribute to Alzheimer disease. Correlations, causality and hypotheses aside, the plethora of studies exploring how this parasite affects human health and behavior is impressive. ( See Coccaro, E. F. et al. Toxoplasma gondii infection: Relationship with aggression in psychiatric subjects. J. Clin. Psychiatry 77, 334–341 (2016). Flegr J. Does Toxoplasma infection increase sexual masochism and submissiveness? Yes and no. Commun Integr Biol. 2017 Nov 28;10(5-6). Zouei, N.,et al The association of latent toxoplasmosis and level of serum testosterone in humans. BMC Res Notes 11, 365 (2018).. Kusbeci O.Y., et al. Could Toxoplasma gondii have any role in Alzheimer disease? Alzheimer Dis Assoc Disord. 2011;25:1–3. Ling V.J., et al. Toxoplasma gondii seropositivity and suicide rates in women. J Nerv Ment Dis. 2011;199:440–4. Sorlozano A,I. et al. Infectious agents associated with schizophrenia: a meta-analysis. Schizophr Res. 2012;136:128–36.).

Flegr introduces his review article with this statement: “The parasite Toxoplasma needs to get from its intermediate hosts, e.g. rodents, to its definitive hosts, cats, by predation. To increase the probability of this occurrence, Toxoplasma manipulates the behavior of its hosts, for example, by the demethylation of promoters of certain genes in the host’s amygdala. After this modification, the stimuli that normally activate fear-related circuits, e.g., the smell of a cat in mice, or smell of leopards in chimpanzees, start to additionally co-activate sexual arousal-related circuits in the infected animals”.

Researchers have revealed for the first time that males infected with the Toxoplasma parasite can impact their offspring’s brain health and behavior. Studying mice infected Toxoplasma, researchers discovered that sperm of infected fathers carried an altered ‘epigenetic’ signature which impacted the brains of resulting offspring. Molecules in the sperm called ‘small RNA’ appeared to influence the offspring’s brain development and behavior. (Shiraz Tyebji, Anthony J. Hannan, Christopher J. Tonkin. Pathogenic Infection in Male Mice Changes Sperm Small RNA Profiles and Transgenerationally Alters Offspring Behavior. Cell Reports, 2020; 31 (4):

Wolves infected with Toxoplasmosis are more likely to become leaders of the pack and strike out on their own. In a data set spanning 27 years, wolves in Yellowstone National Park that were infected with Toxoplasma gondii were 46 times more likely to become pack leaders and 11 times more likely to start a new pack than were uninfected animals. “Parasites might have a much larger role than anyone generally gives them credit for,” says wildlife ecologist and study co-author Connor Meyer. (Meyer, C.J., Cassidy, K.A., Stahler, E.E. et al. Parasitic infection increases risk-taking in a social, intermediate host carnivore. Commun Biol 5, 1180 (2022).

Scientists learn how T. gondii infects the nervous system Toxoplasma gondii protozoa spread throughout the body, including the central nervous system, by latching on to macrophages, which protect the body from harmful cells, and injecting a protein that makes them behave like migratory dendritic cells, according to a study by Arne L. ten Hoeve et al. The Toxoplasma effector GRA28 promotes parasite dissemination by inducing dendritic cell-like migratory properties in infected macrophages, Cell Host & Microbe, 30: Issue 11, p 1570-1588. 2022).

Scientists think T. gondii alters brain function by forming cysts in regions that process fear and decision-making. The cysts may also affect behavior by ramping up levels of dopamine, a neurotransmitter involved in reward and risk-taking. There’s some evidence that T. gondii can rewire the brain permanently, making mice unafraid of cats even long after the parasite has been cleared. T. gondii also forms cysts inside human neurons. In people with HIV or other immune-weakening conditions, the cysts can grow and replicate, causing deadly brain inflammation, dementia, and psychosis. Although scientists have long assumed the cysts are benign in healthy people, a growing body of data suggests T. gondii infection can alter personality and increase the chance of developing schizophrenia and other mental illness. Even without directly infecting the brain, a chronic T. gondii infection can ramp up inflammation, and inflammation has been linked to mental disorders such as schizophrenia, autism, and Alzheimer’s disease. —Reality check: Can cat poop cause mental illness? Science breaks down the evidence on the link between Toxoplasma gondii and mental illness by Emily Underwood, Science, Feb 15th 2019.

According to a research review of this ubiquitous parasite Drs.D.E.Hill and J.P.Dubey (Toxoplasma gondii prevalence in farm animals in the United States. Int J Parasitol. 2013 Feb;43(2):107-13) “Toxoplasmosis, caused by Toxoplasma gondii, is one of the most common parasitic infections of humans and other warm blooded animals. It has been found worldwide and nearly one-third of humans have been exposed to the parasite. Congenital infection occurs when a woman becomes infected during pregnancy and transmits the parasite to the foetus. Besides congenital infection, humans become infected by ingesting food or water contaminated with sporulated oocysts from infected cat faeces or through ingestion of tissue cysts in undercooked or uncooked meat. Food animals (pigs, chickens, lambs and goats) become infected by the same routes, resulting in meat products containing tissue cysts, which can then infect consumers. Toxoplasma infection is common in food animals in the United States. Implementation of management factors such as biosecure confinement housing are important in reducing the levels of infection in animals destined for human consumption.”

I would add that vegans are also at risk when their kitchen gardens and farmers’ crop soils are contaminated with cat feces laden with sporulated oocysts (infective eggs) of Toxoplasma gondii. This is why it is an urgent public health issue to mandate owned cats be enclosed and not allowed to roam free, community cat TNR ( trap-neuter-release) programs prohibited and all feral cats to be trapped and either kept in humane community holding facilities or be euthanized if not re-homed. Surely, in a more enlightened time, engaging in TNR and opposing legislation mandating the neutering of cats and prohibiting owned cats from roaming free would be seen as crimes against Nature and humanity—along with condoning the wholesale consumption of animals, wild and domesticated, for food.