The impact of migration on the epidemiology of IBD

The estimated number of international migrants has increased over the past five decades. The total number of people living in a country other than that of their birth was estimated to be 281 million in 2020, which was 128 million more than in 1990 and over three times the estimated number in 1970 [3].

When populations move from an area with a low IBD incidence to one with a high IBD incidence, they adopt the incidence of the host country and their offspring have a similarly high incidence compared to non-immigrants [4–6]. Also, a fascinating study by Hammer et al. showed the reverse effect: The Faroe Islands has the highest reported IBD incidence in the world; however, among immigrants from the Faroe Islands to Denmark, the excess Ulcerative Colitis (UC) risk was found to normalise over one generation in men and over two generations in women [7].

These observations have clear implications for disease pathogenesis, highlighting the impact of environmental change as a key driver of disease expression in populations with a genetic predisposition. Diet, early-life microbiota exposure, changing hygiene status, pollution and socioeconomic status are all associated with changes in microbiota which may potentiate the risk of developing IBD [8].

The impact of urbanisation has been demonstrated in the city of Guangzhou, China. Over a 30-year period the incidence of IBD has increased 20-fold, mirroring a rapid rise in urbanisation and a population explosion from 2.5 million people to 15 million [9].

Identifying which environmental factors are most important is difficult and a precise mechanism of action remains to be elucidated.

Not only have changes in disease incidence and prevalence been demonstrated, but a clear difference in disease phenotype has been identified between populations of different ethnicities. For example, a study of 33,157 UC patients participating in the UK biobank showed pancolitis to be significantly more prevalent among South Asians than among Caucasians [10].

If the epidemiology and phenotype differ, how does this correspond to treatment response? More studies are needed to examine different treatment responses in different ethnic groups. Trials should reflect different ethnicities and be more generalisable to different populations in order to reflect the global world in which we live.

References

1. Molodecky NA, Soon IS, Rabi DM, et al. Increasing incidence and prevalence of the inflammatory bowel diseases with time, based on systematic review. Gastroenterology. 2012;142:46–-54.e42; quiz e30. http://www.ncbi.nlm.nih.gov/pubmed/22001864
2. Burisch J, Pedersen N, Cukovic-Cavka S, et al. East-West gradient in the incidence of inflammatory bowel disease in Europe: the ECCO-EpiCom inception cohort. Gut 2013;1–10. http://www.ncbi.nlm.nih.gov/pubmed/23604131
3. International Migration Watch. World Migration Report 2024. https://worldmigrationreport.iom.int/msite/wmr-2024-interactive/
4. Probert CS, Jayanthi V, Pinder D, Wicks AC, Mayberry JF. Epidemiological study of ulcerative proctocolitis in Indian migrants and the indigenous population of Leicestershire. Gut 1992;33:687–93. http://www.ncbi.nlm.nih.gov/pubmed/1307684
5. Probert CS, Jayanthi V, Wicks AC, Carr-Locke DL, Garner P, Mayberry JF. Epidemiological study of abdominal tuberculosis among Indian migrants and the indigenous population of Leicester, 1972-1989. Gut 1992;33:1085–8.
6. Benchimol EI, Manuel DG, To T, et al. Asthma, type 1 and type 2 diabetes mellitus, and inflammatory bowel disease amongst South Asian immigrants to Canada and their children: a population-based cohort study. PLoS One 2015;10:e0123599. http://www.ncbi.nlm.nih.gov/pubmed/25849480
7. Hammer T, Lophaven SN, Nielsen KR, et al. Inflammatory bowel diseases in Faroese-born Danish residents and their offspring: further evidence of the dominant role of environmental factors in IBD development. Aliment Pharmacol Ther 2017;45:1107–14. http://doi.wiley.com/10.1111/apt.13975
8. Zuo T, Kamm MA, Colombel J-F, Ng SC. Urbanization and the gut microbiota in health and inflammatory bowel disease. Nat Rev Gastroenterol Hepatol 2018;15:440–52. http://www.nature.com/articles/s41575-018-0003-z
9. Ng SC, Tang W, Ching JY, et al. Incidence and phenotype of inflammatory bowel disease based on results from the Asia-Pacific Crohn’s and Colitis Epidemiology Study. Gastroenterology 2013;145:158–65. http://dx.doi.org/10.1053/j.gastro.2013.04.007
10. Balarajah. Crossing borders in IBD: Session 3 - IBD care across the world Abstract citation ID : jjad212 . 0007 Consistent IBD treatment approaches across South Asian and White ethnicities despite phenotypic variations : a study of 33,157 patients using the IBD B. JCC 2024;25:2024.