Is your brain secretly infected? No? Are you sure?

20-60% of adults on this planet have been infected by Toxoplasmosis. The infection usually comes out if the person’s immune system is low, as in AIDS, cancer, or treatment for cancer. We used to think that the parasite was dormant or sleeping until the immune system dropped, but at this latent stage, there may be behavioral effects.

This is the parasite that is in cat feces. Rats eat that, it infects their brain, causing them to be attracted to the smell of cat urine in the same way they are attracted to a rat to mate with. They are not cautious and another cat, or the same cat eats them and the parasite completes its life cycle. Other animals, including humans, are exposed too and the parasite goes to their brain.

Toxoplasma have the ability to quickly migrate through the human intestines into the blood to many organs but preferentially will settle in the eye, brain and muscles. Humans ingest Toxoplasma through undercooked meat (mainly lamb and pork) or directly ingesting eggs in water, garden soil, children’s sandboxes or cat litter contaminated by infected cat feces, or it can cross the placenta. The congenital infection can cause severe neurological and eye problems. The eye problems usually become apparent when you are in your 20s or 30s. Antibodies keep the LT in check but it can be transformed to a more active infection when the immunity fails, as in AIDS or during immunosuppression from organ transplant or cancer treatment. It is the most common brain AIDS-related opportunistic infection in developed and developing countries.

In humans, latent toxoplasmosis (LT) has been shown to be associated with higher testosterone levels in men and lower levels in women. LT decreased testosterone levels in both female and male mice in comparison to uninfected controls. Interestingly, Hepatitis B and C viruses also alter sex hormone and increase the number of male babies. LT does this too: Up to 72% of babies born from LT mothers are male. Usually 51% of babies are male. Another virus that is latent in humans, CMV, can cause mothers to produce more daughters. So maybe one should get the second infection to even out the score!

Furthermore, a number of studies have now suggested LT as a risk factor for the development of schizophrenia, depression, and suicide in humans. Interestingly cytomegalovirus infection is also associated with schizophrenia. Are these effects due to direct invasion of tissues, products of the infection, or the immune response? LT reduces gray matter in schizophrenia but not in controls. Altered concentrations of dopamine via two toxoplasmosis genes coding for enzymes may be the mechanisms for these changes. LT damages a specific brain cell, astrocytes, and astrocyte damage has been associated with schizophrenia also. Some medications used to treat schizophrenia inhibit Toxoplasma growth.

LT, leads to longer reaction times in infected human subjects with a 2.65 times greater risk to have been involved in an automobile crash than the general population. In rats there is an increase in activity, less anxiety, decreased reaction speed, deficits in learning capacity and memory, decreased fear of novelty and predator vigilance.

Many studies have been performed to explain the differences in psychological profile of LT infected humans. This infection affects some of the key traits of human personality. Toxoplasma-positive subjects had lower Novelty seeking scores, including Impulsiveness and Disorderliness than the Toxoplasma-negative subjects. The infected subjects had also lower IQ and lower probability of achieving a higher education.

Gender differences have been shown in the psychological profile of LT infected humans also. Men with LT disregarded rules and were more expedient, more suspicious, jealous, and dogmatic; whereas women with LT were more warm-hearted, outgoing, and were less trusting, accepting, and tolerant. The longer the infection, the more pronounced these factors were. An evolutionary reason for these changes might be for the male animal to pass the agent on to the cat by taking risks, and the female stays alive long enough to pass it on to her fetuses (which then become cat prey). The Toxoplasmosis is programmed to go to our brains and it doesn’t know we are not cat food.

Besides Hepatitis B and C and cytomegalovirus influencing behavior change, there are other models of infection causing similar effects to complete their complicated life cycle. Parasites cause an ant’s abdomen to turn bright red to make it look like a tasty berry to a predator or making the tentacles of snails pulsate with light to make them look like caterpillars. Rabies changes animal behavior to promote biting with more viruses in the increased saliva. Certain infected caterpillars, as they get sicker, stay in the trees and die high up and then there are other genes in the virus that then make the caterpillar melt and millions of virus particles drop onto leaves for the same animals to consume.

In addition to a high rate of Toxoplasmosis infections, HIV patients also have a high incidence of low testosterone levels. The normal decrease of testosterone levels with age, confound the picture. Modern antiretroviral therapies and strategies have caused the aging of HIV patients to progress so they have the opportunity to acquire more of the diseases the negatives have.

How much can we blame on LT: car crashes, mental illness, more risk taking, the fact you were born a male? Treatment of LT hasn’t been tried to reverse these changes since it is not clear there will be a benefit. Of course we treat anyone with low CD4s and a brain lesion of Toxoplasmosis, or prevent it with antibiotics when the CD4 is less than 100. We also ask HIV patients with low CD4s and pregnant women to avoid cat litter.

Adelante readers may be at risk. No, there are not Toxoplasmosis parasites in the magazine… that I know of. But I have noticed that there is a higher incidence of LT in my non-Anglo HIV patients; we routinely test for it when a patient first comes to the HIV clinic, so they got it before they got HIV.

You may ask your provider if you have been infected with Toxoplasmosis, but unless you have cancer or HIV, it won’t be necessary since there is nothing we can do about it. Weird, huh?!
Please send your medical questions to Adelante (

Daniel Pearce, D.O., FACOI, AAHIVMS
Associate Clinical Professor of Internal Medicine, Loma Linda University and Western University
HIV Specialist, Riverside County Public Health Department