A new study of people with schizophrenia reported that participants were infected with Toxoplasma gondii showed worse verbal memory, learning, and social cognition than participants not infected with this parasite. However, cognitive training exercises had similar effects on both groups, and no differences were found in the severity of schizophrenia symptoms. The study was published in Schizophrenia.

Toxoplasma gondii it is a single-celled parasite that invades neural tissue. It is known to infect all warm-blooded mammals. Toxoplasma gondii it 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 that increase the risk of schizophrenia.

Schizophrenia is a complex neuropsychiatric disorder affecting approximately 1% of the world’s population. Symptoms of schizophrenia include delusions, hallucinations, and disorganized thinking and speech. These are the so-called positive symptoms.

People suffering from this disorder can also develop negative symptoms. These include 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 at discomfort with people and others; and a range of cognitive impairments.

While antipsychotic medications can be quite effective at combating the positive symptoms of schizophrenia, they generally aren’t effective for the negative symptoms, especially cognitive impairments.

Studies have shown that people with schizophrenia are more likely to become infected Toxoplasma gondii compared to healthy individuals. Children whose mothers were infected with this parasite during pregnancy are more likely to develop cognitive impairment and schizophrenia.

Noting all of this, study author Anna Luiza Guimares and her colleagues wanted to explore whether infection with Toxoplasma gondii it is associated with poorer cognitive performance and increased symptoms of schizophrenia. They also wanted to know if this infection can lead to a different response to digital cognition training.

Participants were 60 individuals with schizophrenia or schizoaffective disorder who participated in a clinical trial of a 40-hour neuroscience-based cognitive training. The researchers measured the concentration of antibodies for Toxoplasma gondii in their blood. The results showed that 25 study participants were infected with the parasite, while 35 were not. 18 participants were women. The participants suffered from schizophrenia for an average of 14.5 years.

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.

As part of the study, participants were asked to practice the neuroscience-based cognitive training exercises for 1 hour per day, 3 to 5 times per week, until 40 hours of training were completed. These exercises aimed to train processing speed, attention, memory, working memory, executive function and social cognition, all of which are negatively affected by the negative symptoms of schizophrenia.

The results showed that the participants were infected with Toxoplasma gondii had worse attention, social and global cognition at study entry than participants without this infection. After the researchers controlled for age effects, the difference in attention disappeared, but the infected participants showed poorer verbal memory.

There were no differences between these two groups on processing speed, 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 the infected showed lower scores in attention than the uninfected group. The extent of post-training changes was similar in the two groups. There were no differences between groups in symptom severity after the training was completed. Furthermore, the parasite-infected group showed greater adherence to the training program: 1 participant in this group dropped out, compared with 9 in the uninfected group.

The main findings were that the TOXO+ [participants infected with Toxoplasma gondii] had worse global cognition, with impairments in social cognition and verbal learning. Furthermore, we found that TOXO+ subjects showed greater adherence to digital cognitive training, although changes in cognition and symptoms after training were similar between groups,” concluded the study authors.

The study sheds light on the consequences of infection with Toxoplasma gondii. However, it should be noted that the study sample was relatively small and that the 20% dropout rate is relatively high.

The study, Effects of Toxoplasma gondii infection on cognition, symptoms, and response to digital cognitive training in schizophrenia, was written by Anna Luiza Guimares, David Coelho, Linda Scoriels, Juliana Mambrini, Lis Antonelli, Priscilla Henriques, Andra Teixeira-Carvalho , Olindo Martins-Filho, Jos Mineo, Lilian Bahia-Oliveira and Rogrio Panizzutti.