It seems almost like an oxymoron. Parasites are known as infectious agents, and mental health is traditionally seen as a separate entity from physical health. When parasites end up in the brain, however, mental health can be affected. An idea known as infectious mental illness describes the role of viral, bacterial, and parasitic infections in affecting one’s mental health. Not the typical idea behind the practice of mental health.
It should be noted that while there is a trio notorious for infecting the brain, the final one, Naegleria fowleri (the brain-eating amoeba), rarely leaves survivors and kills quickly, enough so that mental health of such patients is difficult to study. Thus we’ll save that one for a rainy day.
The first, Toxoplasma gondii, is a protozoan parasite that is normally found in cats. The parasite is excreted through the faeces, where animals and humans come into contact and can be infected with the parasite. While the parasite is widespread with little signs of infection, there has been plenty of talk of the protozoan causing changes in mental status. Studies on mice infected with Toxoplasma have found that those mice lose their fear of cats and will actually run towards cats! The reason for this is because the best chance for the parasite to grow and reproduce is inside a cat’s digestive tract, and all the better for the mouse to be eaten for the parasite.
Of course, when such mental changes happen in other mammals when infected, scientists like to look at humans. After all, we are animals too. Despite the fact we like to believe that we are better than drugs, medications, and little parasites, we are all susceptible to them. It’s sometimes hard to swallow that at our basic level, neurotransmitters are what controls our personalities and emotions. That what we feel and who we are is simply a level of neurotransmitters and hormones. And that’s what Toxoplasma affects. This little protozoan affects the production of dopamine, the so-called happiness hormone.
Too much “happiness” cannot be a bad thing, right? Wrong. We balance between too much dopamine causing schizophrenia and too little causing Parkinson’s Disease. In people infected with Toxoplasma, there was found to be a direct “statistical link” between infection and schizophrenia [1]. In addition, toxoplasmosis in women leads to an increase of a chance in having a baby with schizophrenia. Toxoplasma produces the enzyme precursor to the dopamine, tyrosine hydroxylase, and it would make sense that this increase in dopamine precursor could cause schizophrenia.
It's not just the possible link between the protozoan and schizophrenia. Studies have found that those infected with Toxoplasma exhibit more risky behavior, and thus are more likely to be involved with car accidents [2]. Furthermore, a new study done in Denmark describes the association between toxoplasmosis and suicide in women. Women with the Toxoplasma parasite were more likely to attempt suicide and more likely to do so via violent methods (ie. with a gun or knife) [3].
The seemingly harmless parasite is subtle in its changes in our mind and body. Most people do not show symptoms when infected, and it's only when we become immunocompromised that Toxoplasma causes apparent problems. Toxoplasmosis is not as harmless as it seems when it affects our personalities and has a potential to cause mental illness.
The second, Taenia solium, is actually a pig tapeworm that infects our intestines. Normally, the parasite remains in the gut, but it can migrate to the muscles, spinal cord, and brain. The problem arises when the worms die, and the immune system reacts to the carcasses, forming calcified cysts around the dead worms. When the cysts are in the muscles, it is merely called cysticercosis and is generally asymptomatic. The problem arises when these worms die in the spinal cord or brain, when the cysticercosis becomes neurocysticercosis.
Neurocysticercosis causes everything from seizures to confusion to excruciating headaches. The problem comes when diagnosing neurocysticercosis because the symptoms can be similar to meningitis and in California, it isn't considered a health concern. In fact, the opposite is true. Tapeworms are on the rise in the US (or at least California) due to plenty of immigrants coming in from various, many times developing, countries. The problem is that the cysts can only be removed surgically, a very costly and dangerous procedure. The only way to prevent neurocysticercosis is to not be infected with tapeworms and to avoid those infected. Problem is... you don't know who is infected.
The issues don't end there. People with neurocysticercosis also have a possibility to have an altered mental status, including depression or psychosis. It's not surprising, considering these cysts are in the brain and depending on what region they're in, what symptoms will manifest. For example, one patient with the disease lost feeling in her right leg, foot, and hand [4].
For more on neurocysticercosis from a patient's perspective, here is a blog from someone who was infected and the aftermath of his infection.
Both taxoplasmosis and neurocysticercosis are conditions where parasites turn on our most beloved organ, our brain. In one case, the parasite dies and it's our immune system trying to isolate the foreign entity from native tissue that causes the issue. Because of the location, mental health can be affected. In the other case, the parasite is made to alter personality in an attempt to return to its preferred environment, inside a cat. Although this usually works for mice and rats, in humans, the parasite is simply doing what it does best. To the detriment of our health.
The lesson to be learned here is that parasites don't just infect our bodies, but can affect how we feel and how we act. If a parasite takes hold inside you, you never know... It may just be calling the shots.
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