Candida yeast infection was found to be more common in women with schizophrenia or bipolar disorder who had memory problems.
The study, led by Johns Hopkins University School of Medicine in Baltimore, MD, is published in the journal npj Schizophrenia.
The Johns Hopkins researchers are part of a group that is investigating whether pathogens - such as fungi, bacteria, or viruses - trigger or contribute to certain mental illnesses.
They caution that their findings do not show that Candida yeast infections cause mental illness or memory impairment, or vice versa, merely that they appear to be strongly related.
A more detailed investigation of lifestyle, immune system factors, and gut-brain connections could shed more light on the underlying cause-and-effect mechanisms, they note.
However, Emily Severance, first author and assistant professor of pediatrics, says, nevertheless, "clinicians should make it a point to look out for these infections in their patients with mental illness."
She notes that most Candida infections are treatable in the early stages. They can also be avoided by reducing sugar intake and other changes to diet, improving hygiene, and avoiding unnecessary antibiotics.
C. albicans - a yeast-like fungus - is found in small amounts in the healthy gut of humans and animals, but it can get out of balance and cause infection. In infants and those with weakened immune systems, it can lead to rashes in the throat or mouth (thrush) and it can cause sexually transmittable genital yeast infections in men and women.
The infection can become severe if it enters the bloodstream. In most people, however, healthy levels of bacteria and a strong immune system can stop the fungus getting out of balance.
Different patterns of mental illness linked to Candida in men, women
For their study, the team tested blood samples from 808 people aged 18-65 for antibody evidence of a previous or current infection with Candida. The group included 261 individuals with schizophrenia, 270 people with bipolar disorder, and 277 without a history of mental disorder (the controls).
The researchers also included data from another group of 139 people with first-episode schizophrenia, 78 of whom had not yet been treated with medication.
After taking into account factors that could skew the results - such as age, race, medication, and socioeconomic status - the researchers looked for links between mental illness and yeast infection.
Overall, there was no link between presence of mental illness and current or previous yeast infection. It was when the team drilled down to look at men and women separately that the patterns emerged.
For example, in the men, they found 26 percent of those with schizophrenia had a current or previous Candida infection, compared with only 14 percent of the male controls.
However, among women, while a greater proportion had a current or previous yeast infection, the difference between those with schizophrenia and the controls was much smaller (31.3 percent versus 29.4 percent, respectively).
Men with bipolar disorder showed a similar higher rate of Candida infection, compared with controls (26.4 percent versus 14 percent).
However, when they looked more closely, the researchers found that the link between bipolar disorder and yeast infection in the men was most likely due to homelessness. No such explanation could be found in the men with schizophrenia, however.
Candida infection linked to worse memory for women with mental illness
The participants also took part in tests of mental ability, such as short-term memory, delayed memory, attention, language skills, and visual-spatial skills.
The mental skills tests showed no measurable differences in control men and women with and without a current or previous Candida infection. However, women with schizophrenia or bipolar disorder with current or previous Candida infection had lower scores on the memory tests, compared with women without infection.
The researchers note that while they could not show a direct link between Candida yeast infection and brain processes, the results show something linked to infection appears to affect memory performance in women with schizophrenia and bipolar disorder and warrants further investigation.
They point out that one drawback of their study design is the fact they could not tell which part of the body was affected by yeast infection, and whether or not participants had a current or past infection. Also, it was not possible to account for every lifestyle variable that might affect the results.
"Because Candida is a natural component of the human body microbiome, yeast overgrowth or infection in the digestive tract, for example, may disrupt the gut-brain axis. This disruption in conjunction with an abnormally functioning immune system could collectively disturb those brain processes that are important for memory."
Prof. Emily Severance
Prof. Severance says she and her colleagues now plan to look at the gut-brain connection in mice to find out whether there is a cause-and-effect mechanism between Candida and memory deficits.