Analysis of environmental risk factors, prevalence, and associated symptoms of Giardia intestinalis infection across age groups in Poland: a survey-based study.

Giardia intestinalis is one of the most common intestinal protozoa worldwide, yet its true occurrence is challenging to estimate due to frequent asymptomatic or nonspecific presentations and limited diagnostic access. Epidemiological data from high-income Central European countries remain scarce. This study aimed to assess the proportion of G. intestinalis infections and to identify environmental, behavioral, anthropometric, and clinical correlates across age groups in Poland. A cross-sectional, survey-based study was conducted among 518 individuals presenting for parasitological stool examination at a certified diagnostic laboratory. Participants provided stool samples and completed standardized epidemiological and clinical questionnaires. Associations between exposures, anthropometric variables, and symptoms and infection status were analyzed using χ² tests and logistic regression, stratified by age (children, adolescents, younger adults, older adults). G. intestinalis infection was identified in 24.7% of participants in this clinically referred, self-selected cohort. Due to the sampling method, this value cannot be generalized to the general population. In the total cohort, self-reported bloating, supplement intake, and having children in preschool or primary school were associated with lower infection odds, whereas constipation increased risk; however, these associations likely reflect cohort-specific behavioral and clinical characteristics rather than causal effects. Age-specific patterns were observed: in younger adults (18–40 years), previous giardiasis and pet ownership were significant risk factors, while dietary changes were protective. In adults > 40 years, infection correlated with anthropometric measures and supplement intake, the latter showing an opposite direction of association relative to the overall cohort. Symptom profiles varied by age, with gastroesophageal complaints in children, oligosymptomatic presentation in younger adults, and constipation with systemic symptoms in older adults. A high proportion of G. intestinalis infections was observed among individuals undergoing parasitological diagnostics in Poland. These findings reflect patterns within a clinically referred population and do not provide estimates of national prevalence. The observed age-dependent symptom profiles and associations with behavioral and host-related factors highlight the need for improved diagnostic vigilance in clinical settings. Population-based studies using representative sampling and standardized diagnostics are required to determine prevalence in the general population and clarify causal pathways.