Hiding Inside - Toxoplasma gondii

As a kid, instead of being amused by princesses and unicorns, I was a huge fan of any movie or series that featured weird creatures, like V or Aliens, the SciFi horror movie.

Soon enough I realized that unfortunately, aliens do not exist (as far as we know), but I learned that I did not need to go to space to find equally fascinating (and deadly) creatures. These organisms have proven to be selfish, cause no good to whoever gives them shelter, and some of them show a remarkable taste for our nervous system. They are parasites.

There is a long list of parasites that can potentially camp out in our brain and/or spinal cord, causing considerable damage if untreated. Some of those are considered by the CDC (Centers for Disease Control) to be responsible for “Neglected Parasite Infections” in the US, which have known effects on the human nervous system.

Toxoplasma gondii (T. gondii) is one of those parasites. According to the CDC, 60 million people, in the US alone, may be infected with this parasite. A person with a healthy immune system may not even notice the infection, however, pregnant or immunosuppressed patients need to be especially careful as the effects of T. gondii can be very serious.

The manipulator

T. gondii, is an intracellular parasite responsible for toxoplasmosis. This parasite is capable of manipulating the host’s behavior. For instance, rats are normally repelled by cat urine as a biological mechanism of defense; however, rats infected by T. gondii seem to turn suicidal and are actually attracted by the cat urine scent. This makes the infected rats wander into a cat’s territory, putting themselves at risk of being devoured by their predator. This manipulation of the behavior of the rat, an intermediate host, is a strategy by T. gondii in order to get to its definitive host, the cat. It is there, inside the cat, where T. gondii can complete its life cycle.

Rats can be infected by T. gondii which will alter their behavior. It will turn them "suicidal" in order to reach their ultimate host, the cat.

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But how is T. gondii capable of this sort of manipulation? Apparently, this microscopic parasite produces a protein that synthesizes the precursor of dopamine, thereby, increasing the levels of this neurotransmitter in the brain and impairing normal circuits that would keep the rat away from cats.

T. gondii has been related to personality changes in humans, as some studies show that people infected were more outgoing to the point of recklessness; they even seemed to get into more traffic accidents. However, the ways that T. gondii affect their hosts seem to be dependent on the length of the infection (the longer the parasites lives in its human host, the more acute those personality changes are) and the gender of the host (women and men displayed almost opposite personality changes when infected by the parasite).

T. gondii infection has been related to schizophrenia, a neurological disorder with multiple contributing factors: genetic, environmental, and infectious. Although much research has been done, it is still unclear whether T. gondii increases the risk of suffering from schizophrenia, or if schizophrenia patients are more prone to get infected by T. gondii. Either way, it is very likely that T. gondii is not the sole key to solve the mechanisms behind schizophrenia.

However, the theory that T. gondii can manipulate human behavior appears to be under some controversy, as some studies have questioned whether this parasite is capable of influencing human behavior like it does on rodents.

A very recent study has also shown how T. gondii can induce two hallmark signs of Alzheimer's disease in rodents: hyperphosphorylation of the protein Tau, and accumulation of the protein beta-Amyloid. This potentially predisposes the host to suffer from this disease. These findings are also controversial, as several other research groups found no correlation between the infection and a higher incidence of Alzheimer's.

Not all is about behavior

T. gondii will infect humans who eat anything contaminated with the eggs of T. gondii, or simply getting in touch with the feces of rodents or cats that are infected. It does not represent a threat to people with healthy immune systems, besides the above-mentioned possible effects on behavior. However, the consequences of T. gondii infection in immunosuppressed patients (e.g. suffers from AIDS or going under chemotherapy) or pregnant women can be catastrophic.

Cats are the ultimate hosts for T. gondii. Their feces contain parasite eggs that will infect rodents, or any other warm blood animal - like humans - that will get in touch with it.

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After entering the host’s body (known as the acute phase of the infection), if your immune system is on point, the parasite is rapidly eradicated. If the parasite reaches the central nervous system before it can be destroyed, it forms cysts in neurons.

If the host is immunosuppressed, their immune system is incapable of destroying the parasite and the rupture of the cyst will cause reactivation of the infection and subsequent tissue damage. It can cause microcephaly, a collapse of the brain due to leakage of cerebrospinal fluid, cortical atrophy (dysfunction of the outer layer of the brain), and amorphous calcification (deposits of calcium in the brain, forming “brain stones”).

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Infection in a pregnant woman could end in abortion, stillbirth or prematurity. If the baby makes it, the list of conditions goes from microcephaly to respiratory, renal and hearing defects, plus many others that would affect the heart, the nervous system, the skin, the eyes, and even the blood.

In conclusion, behavioral changes or not, infection by T. gondii can be serious, so be tidy next time you change your cat litter.