Abstract Background The incidence of inflammatory bowel disease (IBD) continues to rise globally, with Denmark among the countries with the highest rates.1 As genetic factors explain only a small fraction of the risk,2 early-life environmental exposures are thought to influence immune development and subsequent disease susceptibility.3 In line with the hygiene hypothesis, contact with pets or farm animals may influence immune maturation and modulate IBD risk. However, evidence is inconsistent and largely based on small retrospective studies. We aimed to prospectively examine this association in a large, population-based Danish birth cohort with detailed exposure assessment and long-term registry follow-up. Methods We conducted a population-based cohort study using data from the Danish National Birth Cohort. Prenatal exposure to pets and farm animals was reported by mothers at gestational week 12, and postnatal exposure was assessed at 18 months postpartum, capturing animals in the household since birth. Incident IBD, Crohn’s disease (CD), and ulcerative colitis (UC) were identified through the Danish National Patient Registry. Associations were estimated using Cox proportional hazards models to obtain adjusted hazard ratios (aHR) with 95% confidence intervals (CI), adjusting for sex, calendar year of birth, parental IBD, number of children in the household, municipality at birth, and parental education. Follow-up extended from birth until IBD diagnosis, death, emigration, or 1 December 2024. Results Among 90,306 offspring, 41,499 (46%) were prenatally exposed to pets and 6,249 (7%) to farm animals. Prenatal exposure to pets (aHR 1.05; 95% CI 0.89–1.24) or farm animals (aHR 1.00; 95% CI 0.73–1.37) was not associated with IBD risk (Figure 1). Sub-analysis for CD and UC yielded similar results. Results were consistent across animal types and unchanged when stratified by maternal IBD, sex, or paediatric-onset IBD (18 years). In sub-analyses restricted to 63,958 offspring with maternal reports of exposure in infancy, contact with animals since birth was not associated with IBD (aHR 0.93; 95% CI 0.77–1.13). Categorising timing of exposure as pregnancy only, infancy only, or both did not alter the association (Figure 2). Conclusion In this large, prospective cohort, early-life contact with pets or farm animals was not associated with risk of IBD, CD, or UC. These findings remained unchanged when evaluating prenatal and postnatal exposure separately and were consistent across animal species, family history of IBD, sex, and timing of exposure. Overall, our results indicate that early-life animal contact is unlikely to be a major determinant of IBD development. References: 1. Agrawal M, Christensen HS, Bøgsted M, Colombel JF, Jess T, Allin KH. The Rising Burden of Inflammatory Bowel Disease in Denmark Over Two Decades: A Nationwide Cohort Study. Gastroenterology. Dec 2022;163(6):1547–1554.e5. doi:10.1053/j.gastro.2022.07.062 2. Mahadevan U, Seow CH, Barnes EL, et al. Global Consensus Statement on the Management of Pregnancy in Inflammatory Bowel Disease. Journal of Crohn’s and Colitis. 2025;19(8)doi:10.1093/ecco-jcc/jjaf129 3. Gluckman PD, Hanson MA, Cooper C, Thornburg KL. Effect of in utero and early-life conditions on adult health and disease. N Engl J Med. Jul 3 2008;359(1):61–73. doi:10.1056/NEJMra0708473 Conflict of interest: Ms. Balmer, Christina Lykke: CLB has received financial support from Celltrion to attend the ECCO 2026 conference. Agrawal, Manasi: None Vinkel Hansen, Anne: Anne Vinkel Hansen discloses no conflicts of interest Tinoco da Silva Torres, Joana: Grant: Abbvie, Janssen Personal Fees: Pfizer, Janssen, Abbvie, Sandoz, Lilly, Sanofi, Takeda Non-financial Support: Janssen, Abbvie Jess, Tine: Personal Fees: Consultancy for Ferring, Pfizer, Johnson&Johnson Julsgaard, Mette: Grant: Research grants for investigator-driven studies from Takeda and Novo Nordisk Foundation (grant no. NNF23OC0081717) Personal Fees: Consultation fee: Ferring, Orion Pharma, and Takeda. Speaker’s fees: Tillotts Pharma, Eli Lilly, MSD, Ferring, and Takeda. Advisory board: PharmaCosmos, Eli Lilly, AbbVie and Tillotts Pharma.
Balmer et al. (2026) studied this question.