Variation in reporting of heatstroke mortality: evidence from a multi-country study

Background: Heatstroke represents the most severe manifestation of heat exposure. Heatstroke is rare and under-reported, resulting in limited empirical data on its global incidence and burden. This study aimed to examine geographical variations and temporal trends in reported heatstroke mortality across multiple countries. Methods: We collected annual heatstroke mortality data from 34 countries participating in the Multi-Country Multi-City Collaborative Research Network between 2000 and 2022, using the ICD-10 code X30. Country-specific mortality rates were estimated using Poisson regression, alongside analyses of annual trends and associations with mean warm-season temperature. We also assessed the proportion of heatstroke deaths relative to both overall heat and extreme heat-attributable all-cause mortality. Findings: Heatstroke mortality rates varied widely across countries, with Japan reporting the highest rate (5·81 per 1 million population; 95% CI 4·43-7·62), followed by Cyprus (2·51; 1·36-4·61), and China (2·42; 1·21-4·85). By contrast, most countries in Europe, South America, and southeast Asia reported rates of less than one death per 1 million population. Heatstroke mortality increased over time in several countries and was associated with warm-season temperatures in most regions. The proportion of heatstroke deaths relative to overall heat-attributable mortality ranged from less than 1% in many countries to as close to 24% in Japan. When analyses were restricted to deaths attributable to extreme heat, the proportion of heatstroke deaths increased substantially. Interpretation: Our broad international assessment of heatstroke mortality highlights its distinct patterns compared with overall heat-attributable mortality. The observed variability likely reflects differences in recognition, reporting, and diagnostic practices, while climate exposure and health system capacity influence whether heat-related deaths are identified and recorded as heatstroke. Funding: The Japan Society for the Promotion of Science and the Environment Research and Technology Development Fund of the Environmental Restoration and Conservation Agency provided by the Japanese Ministry of the Environment.