A new study on people suffering from schizophrenia reported that participants infected with Toxoplasma gondii showed worse verbal memory, learning and social cognition, compared to participants not infected with this parasite. However, cognitive training exercises had similar effects on both groups and there were no differences in severity of schizophrenia symptoms. The study was published in Schizophrenia.
Toxoplasma gondii is a single-celled parasite that invades neural tissue. It is known to infect all warm-blooded mammals. Toxoplasma gondii is found in raw and undercooked meat, unwashed fruits and vegetables, contaminated water, dust, soil, dirty cat-litter boxes, and outdoor places where cat feces can be found. Infection with this parasite is listed as one of the factors increasing risk of schizophrenia.
Schizophrenia is a complex neuropsychiatric disorder affecting approximately 1% of the world population. Symptoms of schizophrenia include delusions, hallucinations, disorganized thinking and speech. These are the so-called positive symptoms.
Persons suffering from this disorder can also develop negative symptoms. These include the loss of interest and motivation in life and activities, including relationships and sex, lack of concentration, not wanting to leave the house, changes in sleep patterns, being less likely to initiate conversations, feeling uncomfortable with people and others and a range of cognitive impairments.
While antipsychotic medication can be quite effective at combating the positive symptoms of schizophrenia, it is generally not efficient for the negative symptoms, particularly the cognitive impairments.
Studies have shown that people suffering from schizophrenia are more likely to be infected with Toxoplasma gondii than healthy individuals. Children whose mothers were infected with this parasite during pregnancy were found to be more likely to develop cognitive impairment and schizophrenia.
Noting all this, study author Anna Luiza Guimarães and her colleagues wanted to explore whether infection with Toxoplasma gondii is associated with poorer cognitive performance and increased symptoms of schizophrenia. They also wanted to know whether this infection can lead to a different response to digital cognitive training.
Participants were 60 individuals suffering from schizophrenia or schizoaffective disorder who participated in a clinical trial of a 40-hour neuroscience-informed cognitive training. Researchers measured the concentration of antibodies for Toxoplasma gondii in their blood. Results showed that 25 study participants were infected with the parasite, while 35 were not. 18 participants were female. Participants suffered from schizophrenia for 14.5 years on average.
Participants completed assessments of 7 cognitive domains known to be impaired by schizophrenia (the MATRICS Consensus Cognitive Battery and the Cambridge Neuropsychological Test Automated Battery), schizophrenia symptoms (the Positive and Negative Syndrome Scale) and socioeconomic status.
In the scope of the study, participants were required to practice the neuroscience-informed cognitive training exercises for 1 hour daily, 3 to 5 times a week, until they completed 40 hours of training. These exercises aimed at training speed processing, attention, memory, working memory, executive function and social cognition, which are all functions adversely affected by negative symptoms of schizophrenia.
Results showed that participants infected with Toxoplasma gondii had worse attention, social and global cognition at the start of the study compared to participants without this infection. After researchers controlled for the effects of age, the difference in attention disappeared, but participants with infection showed poorer verbal memory.
There were no differences between these two groups on the speed of processing, working memory, spatial memory or reasoning and problem solving. The two groups did not differ in the severity of schizophrenia symptoms.
After completing the training, the group with the Toxoplasma gondii infection showed lower scores in attention compared to the group without infection. The magnitude of changes after training was similar in the two groups. There were no differences between groups in the severity of symptoms after the training was completed. Additionally, the group infected with the parasite showed higher adherence to the training program – 1 participant from this group dropped out, compared to 9 in the group without the infection.
“The main findings were that TOXO+ subjects [participants infected with Toxoplasma gondii] presented worse global cognition, with impairments in social cognition and verbal learning. Additionally, we found that TOXO + subjects showed higher adherence to the digital cognitive training, although changes in cognition and symptoms after training were similar between groups,” the study authors concluded.
The study sheds light on consequences of infection with Toxoplasma gondii. However, it should be noted that the study sample was relatively small and that dropout rate of 20% is relatively high.
The study, “Effects of Toxoplasma gondii infection on cognition, symptoms, and response to digital cognitive training in schizophrenia”, was authored by Anna Luiza Guimarães, David Coelho, Linda Scoriels, Juliana Mambrini, Lis Antonelli, Priscilla Henriques, Andréa Teixeira-Carvalho, Olindo Martins-Filho, José Mineo, Lilian Bahia-Oliveira, and Rogério Panizzutti.
