Parental saliva contact during infancy leads to a reduction in the risk of allergies — Large epidemiological survey by researchers from Wakayama Medical University and collaborators shows significant association

Postdoctoral Fellow Yoshimi Kubo of the Department of Dermatology, School of Medicine and Specially Appointed Professor Yasushi Kuraishi of the Industry-Government-Academia Collaboration Promotion Headquarters at Wakayama Medical University, Professor Nobuo Kanazawa, Head of the Department of Dermatology at Hyogo Medical University, Professor Shigemi Yoshihara, Head of the Department of Pediatrics at Dokkyo Medical University, and Head of the Center for Dermatology Fukumi Furukawa at Takatsuki Red Cross Hospital conducted a large multicenter cross-sectional epidemiological survey involving Japanese school-age children and their parents in two prefectures (Ishikawa and Tochigi) and demonstrated a significant association between saliva contact during infancy and reduced risk of allergy development in school-age children. Parent-to-child saliva contact during infancy significantly correlated with a reduced risk of developing eczema and allergic rhinitis during school age. The findings are expected to contribute towards clarifying the mechanism of reduced risk of allergy development and were published in the April 25 edition of the Journal of Allergy and Clinical Immunology: Global.

In recent years, allergic diseases such as atopic dermatitis (eczema), allergic rhinitis, food allergy, and asthma have been increasing in developed countries including Japan.

The development of allergic tolerance in children is affected by multiple factors, including the diversity of gut microbiota, immune stimulation by microbes during infancy and early childhood, and microbe acquisition from the mother. Insufficient immune stimulation by microbes during infancy and early childhood may lead to hypersensitivity in barrier tissues such as the skin and an increase in type 2 immune responses (allergic disease). The oral cavity has the second richest microbiota after the gastrointestinal tract, and the results of animal and human studies suggest that oral microbes alter gut microbiota.

In 2015, Kubo and Visiting Scientist Kazuko Yoshizawa of the Harvard T.H. Chan School of Public Health (Assistant Professor at the Graduate School of Human Health Science of the University of Nagasaki at the time of research) showed that premastication (chewing the child's food to soften it before feeding) during infancy may reduce the risk of allergies during school age, especially the risk of eczema (atopic dermatitis).

The study reported the possibility of the involvement of immune stimulation by the transmission of oral microbes from caregivers to infants via saliva.

In the current study, the research group investigated whether parent-child saliva contact during infancy (younger than 12 months) reduces the risk of developing allergies among children in Japan. An anonymous self-administered questionnaire survey was conducted among children in grades 1 to 9 in elementary and junior high schools in two regions (Kaga City, Ishikawa and Tochigi City, Tochigi). The survey in Ishikawa was conducted in 2016 (1718 children and their parents), and in Tochigi in 2017 (1852 children and their parents). The questionnaire was distributed at schools, and the school age children and their parents answered the survey at home.

The average valid response rate was 94.7%, without marked differences between the sexes and between the grades.

The questionnaire was composed of two parts with 91 items in total: 41 including questions from the International Study of Asthma and Allergies in Childhood (ISAAC) and original food allergy and oral allergy questions; and questions regarding the Mother's Pregnancy and Children's Infancy Lifestyle or Environment.

The response data were analyzed using descriptive statistics, chi-squared or Fisher's exact tests, and multiple logistic regression. Independent associations between the development of eczema, allergic rhinitis, or asthma in school-age children and saliva contact during infancy via the sharing of eating utensils or via pacifiers cleaned by parental sucking were examined.

From the obtained data, 48.5% of participants answered yes to having a maternal history of allergy, 11.4% reported maternal smoking during pregnancy, 50.1% reported maternal second-hand smoke exposure during pregnancy, 9.9% shared eating utensils, 39.6% used pacifiers during infancy, and 2.2% reported pacifier cleaning by parental sucking.

The analysis of factors associated with eczema, allergic rhinitis, and asthma symptoms revealed the following significant correlations: (1) between maternal history of allergy and the three allergies, (2) between smoking during pregnancy and asthma symptoms, and (3) between having oral infection knowledge (knowledge about dental caries and periodontal disease infection), the sharing of eating utensils, and pacifier cleaning by parental sucking and allergic rhinitis.

A more detailed analysis of the associations between children's allergic symptoms and the sharing of eating utensils or pacifier cleaning by parental sucking was conducted using logistic regression.

The factors highly associated with allergic symptoms (maternal history of allergy, smoking during pregnancy, having oral infection knowledge) were adjusted.

Sharing of eating utensils significantly correlated with a reduced risk of developing eczema at school age (odds ratio: 0.52, 95% Confidence Interval (CI): 0.32−0.84). Pacifier cleaning by parental sucking was significantly associated with a reduced risk of developing eczema (odds ratio: 0.35, 95% CI: 0.13−0.91) and allergic rhinitis (odds ratio: 0.33, 95% CI: 0.14−0.73) at school age. A potential association between pacifier cleaning by parental sucking and a reduced risk of developing asthma was suggested, although the association only showed borderline significance (odds ratio: 0.17, 95% CI: 0.02−1.31).

The initial transmission of Streptococcus mutans by saliva contact reportedly occurs mostly at 19−31 months after birth (average 26 months). Saliva contact at a younger age may result in a reduced risk of allergy development in a safer manner.

According to Kubo, "It can be presumed that pacifiers are used before the sharing of eating utensils and that earlier saliva contact may reduce the risk of allergy development. However, because our study was based on an epidemiological survey, further research is needed. In the future, we would like to clarify the mechanism of reduced risk of allergy development in children by focusing on oral/intestinal microbiota in parents and children during pregnancy and infancy, and to lead to safe and effective allergy preventive methods in children."

Journal Information
Publication: Journal of Allergy and Clinical Immunology: Global
Title: Saliva contact during infancy and allergy development in school-age children
DOI: 10.1016/j.jacig.2023.100108