original September 28, 2025 September 28, 2025

Cyclical and seasonal variations in the incidence of type 1 diabetes mellitus between 1985 and 2016 in Bauru, São Paulo, Brazil

DOI: 10.20945/2359-4292-2025-0156

ABSTRACT

Objective:

To evaluate cyclical and seasonal variation in the incidence of type 1 diabetes mellitus (T1DM) from 1985 to 2016 in Bauru, São Paulo, Brazil.

Subjects and methods:

This was a retrospective longitudinal study. Clinical data were collected for individuals known to have T1DM, who aged from 0-14 years, residing in Bauru, São Paulo State, and followed at a local endocrinology clinic from 1985 to 2016. Incidence rates were calculated annually and grouped into quadrennial intervals. Trends were analyzed using Joinpoint Regression to estimate annual percentage changes. Poisson regression models assessed cyclical and seasonal patterns over various periods (3- to 7.5-year cycles). Seasonal variation was evaluated using the Akaike Information Criterion and chi-square likelihood ratios to assess model fit.

Results:

Among the 298 included patients, the mean annual incidence was 12.1 per 100,000 person-years (95% CI: 10.7-13.4), with an average annual increase of 2.77% (95% CI: 1.3-4.3%). A significant cyclical variation of 18% every 7.5 years was observed, with girls exhibiting a 22.9% variation every 5 years. No cyclical pattern was identified for boys. Seasonal analysis revealed higher amplitudes among girls (±26.4%) and in the 5-9.99-year age group (±26.2%), predominantly during colder months.

Conclusion:

T1DM cyclical variations with a 7.5-year cycle were observed, with girls showing a pronounced variation and a distinct 5-year cycle. Seasonal variations were found among girls, particularly in the 5-9.99-year age group. Outbreaks of H1N1 and dengue, along with the lowest temperatures, coincided with higher incidence rates, aligning with the 7.5-year cycles. Targeted health policies are needed to mitigate the impact of these factors, supporting surveillance, early diagnosis, and preventive strategies for T1DM.