Introduction

T. gondii is a very common germ that can make people sick, but it’s also super fascinating to study.

Toxoplasma gondii is a tiny parasite that infects many different animals, including humans, and can make them sick with a disease called toxoplasmosis. Some people call it the “mind-control parasite” because it can cause changes in behavior and thinking in infected animals. However, most people who get infected don’t show any signs of being sick.

This germ is one of the most widespread parasites globally and might stay inactive in nearly half of the world’s population and many different warm-blooded animals. Yet, there’s still a lot we’re unsure about regarding this incredibly strange germ.

From its strange fondness for cats to its connections with schizophrenia, here are 10 unexpected facts about T. gondii.

There are lots of ways you can get infected with T. gondii

People mostly get infected with T. gondii by accidentally swallowing the parasite’s eggs, which come out only in cat poop. This can occur when folks drink water that’s been contaminated, clean up after cats, or forget to wash their hands after gardening or eating unwashed veggies. That’s what the Centers for Disease Control and Prevention (CDC) says.

People can also get infected by eating meat from other animals that are infected, like pigs, sheep, and shellfish. These animals can develop small, infectious cysts called bradyzoites after eating the parasite’s eggs from the environment, says the CDC.

T. gondii can also pass from mothers to babies before birth, and it’s possible during organ transplants and blood transfusions, but these cases are much less common, according to the CDC.

Most people don’t show any symptoms at all

The majority of people who get infected with T. gondii don’t even know it because they don’t show any symptoms. Some might feel a bit like they have the flu for a few weeks as their body fights off the infection, but usually, there are no lasting issues, says the CDC.

However, pregnant women, babies, and folks with weaker immune systems can get really sick with toxoplasmosis, which can cause serious damage to the brain, eyes, or other organs, according to the CDC. Sometimes, T. gondii stays quiet in cells for years after infection before toxoplasmosis shows up.

If you think you might have toxoplasmosis, you can ask your doctor for a simple blood test.

There’s currently no cure for T. gondii

Doctors can treat the symptoms of toxoplasmosis with a mix of drugs like pyrimethamine with folinic acid or sulfadiazine, says the CDC. However, unless someone has a severe infection or is at high risk, doctors typically don’t give medication to infected patients.

Once someone gets infected with T. gondii, the parasite can stay asleep in their body for years, maybe even forever. Right now, doctors can’t get rid of the parasite, so getting infected again is possible, meaning there’s no permanent cure for toxoplasmosis. However, it’s uncommon for the parasite to wake up and make someone sick later unless their immune system gets weaker.

Right now, there isn’t a vaccine for T. gondii in humans. However, in the UK, farmers can protect their sheep from the parasite for life by using the Toxovax vaccine, as stated by the manufacturer MSD Animal Health Hub.

Around half of all humans may be infected with T. gondii

Because it can easily pass to humans through various means and often goes unnoticed, T. gondii is one of the most common parasites in people.

A study from 2014, published in the journal PLOS One, suggested that between 30% and 50% of people worldwide might have been infected with T. gondii at some point. However, infection rates likely differ greatly in different places.

For instance, the CDC estimates that about 40 million Americans, or roughly 12% of the population, could have a T. gondii infection. Yet, a study from 2020, published in Scientific Reports, discovered that around 64% of pregnant women in Ethiopia have had a T. gondii infection at some stage in their lives.

T. gondii has been connected to schizophrenia and other brain-related disorders

One of the parasite’s most scary – and debated – possible effects is its influence on the mind. In rats and other animals, it can lead to changes in behavior, and it has also been connected to various neurological disorders in humans.

Two significant conditions linked to T. gondii are schizophrenia and bipolar disorder.

In 2006, a study in the journal Biological Psychiatry proposed that babies exposed to T. gondii in the womb had higher rates of schizophrenia later in life compared to those not exposed prenatally.

Additionally, a 2014 study in The Journal of Nervous and Mental Disease found that individuals with schizophrenia and bipolar disorder were more likely to have T. gondii antibodies in their system, indicating a prior infection.

T. gondii has also been associated with alterations in human behavior, some of which could be dangerous. In 2015, a study in the Journal of Psychiatric Research proposed that T. gondii infections might lead to increased aggression and impulsivity in people, potentially raising the risk of suicide.

The connection between T. gondii and the brain is still not fully understood

While several studies have connected T. gondii to neurological disorders, it’s premature to conclude that the parasite is directly or indirectly accountable for these conditions.

Moreover, other studies have cast doubt on these associations. For instance, a 2016 study in the journal PLOS One, examining over 800 individuals born with T. gondii antibodies, found “little evidence” linking T. gondii to an elevated risk of psychiatric disorder, poor impulse control, personality deviations, or neurocognitive impairment.

Nearly all warm-blooded animals can be infected by T. gondii

Scientists have discovered evidence of T. gondii infection in various warm-blooded animals, including major livestock species.

In 2005, a study in the International Journal of Parasitology showed that T. gondii contributed to the decline in the population of sea otters in California, with up to 38% of deceased otters being infected.

Researchers suspect that agricultural run-off from contaminated soils may have introduced T. gondii oocysts to the otters’ main food source, sea kelp.

T. gondii also poses a significant threat to penguins. In 2019, a study in the journal Veterinary Parasitology found that about 42% of Magellanic penguins on Magdalena Island, Chile, had been infected by T. gondii, despite the absence of cats on the island, suggesting that infections likely came from humans.

T. gondii can only reproduce inside cats

Although T. gondii is found in various animals, it’s only ever been observed reproducing in species from the cat family, Felidae, which includes domestic cats and their wild cousins like lions, cheetahs, and tigers. Domestic cats are thought to be the parasite’s preferred host.

It’s still a mystery why T. gondii can’t reproduce in other infected animals, but this means that cat poop is the only way the parasite can enter the environment.

As of 2018, there are approximately 373 million pet cats worldwide, according to Statistica, along with possibly hundreds of millions of unregistered stray cats.

Cats can only release the infectious eggs for about one to three weeks after they get infected, and after that, they can’t spread the parasites anymore.

Birds and rodents serve as intermediate hosts for T. gondii

Although T. gondii can only reproduce inside cats, it’s commonly found in most rodents and several bird species.

These animals serve as intermediate hosts, meaning they act as a temporary stop between two different feline hosts. For instance, a bird might get infected by T. gondii after consuming seeds on soil contaminated by cat feces.

The bird could then develop an infectious cyst in its body before being captured and eaten by a cat, which then becomes infected.

Therefore, rodents and birds play a crucial role in the life cycle of T. gondii because they are the primary means through which cats become infected.

Infected rodents may exhibit increased boldness or fearlessness

Rodents infected with T. gondii appear to lose their usual fear of cats, particularly their fear of cat urine.

A study in PLOS ONE in 2011 suggested that infected rats start to exhibit a sort of “sexual attraction” to the smell of cat urine instead of their typical defensive reaction to the scent. If accurate, this would make infected rats more inclined to live near cats, increasing the likelihood of them being preyed upon.

A subsequent study published in PLOS Neglected Tropical Diseases in 2011 replicated the experiment in humans. While infected men showed a slight increase in attraction to the scent of cat urine, women did not.

A study of mice published in the journal Cell Reports in 2020 also demonstrated that T. gondii can reduce general anxiety and enhance exploratory behavior in infected mice.