Does Toxoplasma Gondii Increase Suicide Risk in Women?

First identified in 1908, the protozoan parasite, Toxoplasma gondii  (T. gondii) is currently estimated to affect up to one-third of the world's population.    Although T. gondii infections (also known as toxoplasmosis) can occur in all mammals, cats are the primary host for the parasites and acts as the chief vector for passing T. gondii on to other animals.   As a result, humans can become infected through contact with cat feces (such as through a cat's litter box).   Typically regarded as relatively minor in adults except in very extreme cases, pregnant women are particularly at risk for toxoplasmosis due to potentially serious complications in unborn children including miscarriages and birth defects.  

Along with the more well-known physical effects of toxoplasmosis, researchers have identified an unusual behavioural component in rodents affected with the parasite.    Once infected, rats and mice become attracted to the smell of cat urine which increases their likelihood of  being eaten by cats.  This in turn allows the parasites to infect cats and resume their normal life cycle.    More controversially, toxoplasmosis has been linked to human behavioural changes including increased reckless behaviour (one study reports a six-fold increase in automobile accidents in adult males infected with toxoplasmosis).    But how reliable is the toxoplasmosis-suicide connection in humans?  

According to a new study published in Archives of General Psychiatry,   the link between toxoplasmosis and later suicidal behaviour appears to be very real.   Conducted by a team of researchers from the United States, Denmark, Germany and Sweden, the study involved 45,788 women in Denmark who gave birth between the years 1992 and 1995.   Since Denmark is one of the few countries in the world to provide mandatory postnatal screening for T. gondii antibodies, the researchers examined Danish health registries for information on toxoplasmosis levels and later suicidal behaviour in mothers.     They also cross-checked the Danish Psychiatric Central Register to rule out pre-existing psychiatric issues that might account for later suicide attempts.   Based on their research findings,  the risk of suicidal behaviour in infected women is one and a half times that of uninfected women and rises with increasing levels of the T. gondii antibodies in their system.  Having a prior history of psychiatric problems does not appear to affect that relationship significantly.   The relative risk appeared to be even higher for women who resorted to violent methods in their suicide attempts although the actual number was far smaller than for the total number of suicide attempts. 

According to senior author, Teodor Postolache of the University of Maryland,  "We can't say with certainty that T. gondii caused the women to try to kill themselves, but we did find a predictive association between the infection and suicide attempts later in life that warrants additional studies.  We plan to continue our research into this possible connection."   While the researchers were able to examine the possible influences of other psychiatric disorders (a problem with other studies examining T. gondii and suicide attempts),  Dr. Postolache also acknowledged that the current research project had significant limitations such as the inability to identify specific causes for the suicidal behaviour.   While Denmark's health registries carry an amazing amount of information about patients, not all suicide attempts become recorded and it is not always possible to determine whether a self-inflicted injury was an actual suicide attempt.   Also, the study focused on solely on women who had given birth  so men and childless women were not included.    As Dr. Postalache added, the study design does not allow for actual causal assumptions to be made.   "T. gondii infection is likely not a random event and it is conceivable that the results could be alternatively explained by people with psychiatric disturbances having a higher risk of becoming T. gondii infected prior to contact with the health system," he reported. 

Whatever the exact nature of the relationship betwen T. gondii and suicide attempts, Dr. Postalache and his associates will be continuing their research to answer the intriguing questions that it raises.  As he points out, "Is the suicide attempt a direct effect of the parasite on the function of the brain or an exaggerated immune response induced by the parasite affecting the brain? We do not know. In fact, we have not excluded reverse causality as there might be risk factors for suicidal behavior that also make people more susceptible to infection with T. gondii.  If we can identify a causal relationship, we may be able to predict those at increased risk for attempting suicide and find ways to intervene and offer treatment."

For more information.

 

Comments

[Zoe Langley]

Is the link between parasite infection and suicide valid, and am I understanding the study's data correctly?

The researchers did not study women who had toxoplasma and who killed themselves. The reviewed data collected in a voluntary neonatal screening program between 1992-95. The authors wrote, "In the cohort of 45,788 women, only 18 committed suicide."

From this they state their estimates of "relative risk of suicide" is derived from 8[of the 18] women thought to be positive for the infection. The intensity of a woman's infection was estimated from the data on the level of antibodies in a woman's infant when screened as a newborn.

The current understanding is that toxoplasmosis usually causes no symptoms once established and needs no treatment. There is no doubt the parasite can damage the brain. However, if the majority of women in this study who committed suicide had no toxoplasmosis symptoms how can one predict that their suicides were linked to being infected? How can the conclusion be made from this data that any woman on the planet who has toxoplasmosis has a greater risk of suicide?

If another group of researchers reinterpret the data to look for a link between blue eyes, toxoplasmosis, and suicide, can a case be made linking blue eyes, toxoplasmosis, and suicide risk?

[Romeo Vitelli]

Based on my understanding of the study, the researchers examined incidence of both successful and attempted suicides. You would need to read the original study or contact one of the researchers regarding the questions that you raised. The authors admit that their research is preliminary in nature and their study was restricted to women who were included in the neonatal screening database.