Should We Be Concerned About the Transmission of Psychiatric Diseases?

Mental disorders are often considered a type of non-infectious disease. Non-infectious/communicable diseases include cardiovascular diseases, diabetes, cancer, chronic respiratory diseases, as well as mental and neurological disorders. Most of these illnesses have a genetic background that manifests when environmental risk factors coexist, such as obesity, tobacco use, or alcohol consumption.

But could mental health disorders be, in a sense, contagious? An interesting study in Finland recently found that adolescents who have classmates with mental illness have a higher chance of developing mental disorders in adulthood. The study was performed between 1985 and 1997, involving a total of 713,809 cohort members with a median age of 16.1 years.

More than 47,000 adolescents involved in the study had already received a mental disorder diagnosis by the ninth grade. The remaining members were followed up until a diagnosis of mental disorder, emigration, or death up to 2019. The exposure variable was defined as having one or more classmates diagnosed with a mental disorder in the same school class in the ninth grade.

The main outcome was the occurrence of mental disorders during follow-up. The results showed that having classmates with a mental disorder diagnosis in the ninth grade was associated with a later risk of being diagnosed with a mental disorder [1]. While more research is needed, the authors conclude that the findings suggest that mental disorders may be transmitted within peer networks.

A paper published in 2018 presented a series of evidence suggesting that psychiatric illnesses could be considered "contagious" in three major ways:

1. Social transmission. Based on this pathway, mental disorders could potentially be transmitted through social interactions, such as shared experiences, social learning, and emotional contagion that can occur within families, peer groups, and communities.
2. Ecological, familial, and sociocultural communicability.
3. Infectiousness factors [2].

Some infectious pathogens can directly harm the brain and impair its functions. Treponema pallidum causes syphilis, for example, and Bartonella may be associated with schizophrenia. Cytomegalovirus, which is linked to bipolar disorder, is also another example of a pathogen associated with mental disorders.

Emerging evidence, however, also suggests that microbiome—the community of bacteria, fungi, and other microorganisms that reside in our bodies, especially in the gut, skin, and mouth—may play a significant role in transmitting psychiatric diseases. Initial research using mice that had their gut microbes removed showed that these models exhibited mental alterations, such as anxiety and stress, in comparison to wild-type counterparts.

Next, mounting evidence reveals that the type of microbiome and alterations in the microbiome population, known as dysbiosis, influence brain activity and health. The gut microbiota affects the immune system and its hosts' inflammatory response, influencing brain health through the gut-brain axis. Moreover, the gut microbiome produces chemical products such as butyrate and some neurotransmitters that directly affect the central nervous system by transferring through the blood-brain barrier. They also produce some vitamins that influence metabolism and overall body health [3].

Recent studies have shown that the microbiome patterns can become similar in individuals who live closely together. These studies have shown that microbiomes are transmitted through social interactions, such as living near others, sharing a household, or having kinship relations. Examples of microbiome transmission include transfer from mother to child during birth, between unrelated individuals, and even between pets and their close human contacts.

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The oral microbiome is transmitted more frequently than the gut microbiome, with saliva serving as a direct vehicle for transmission. Other routes of transmission include face-to-face contact and touching the skin. The transmission of microbiome strains among individuals in long-lasting close contact is a significant factor in shaping the personal genetic composition of the microbiome, which can have implications for health and disease [4].

Overall, it seems that the microbiome, with a significant influence on brain function and psychiatric disease, may play a role in the transmission of certain types of mental illness. Microbiota are easily transmitted between closely related individuals, and the microbiome signature of each individual is significantly associated with mental health. This knowledge presents the gut microbiota as a new target for the treatment of psychiatric disorders.

Moreover, these data suggest that the transmission of these microbes between individuals may be a possible risk factor for the spread of contagious types of mental illnesses. However, new research is needed to elucidate the precise role of microbiome transmission in the communicability of mental diseases.