author and copyright: Mihailo Alic (alic@rocketmail.com),
April 1999
 
The last half of the century has seen a rising incidence of inflammatory bowel disease in developed countries, with Crohn's disease (CD) contributing most of this increase. This study investigated the relation between workforce participation of affected sex and race groups (an indicator of economic status) and incidence of CD. Both temporal trends and geographic differences were pursued.

Sonnenberg in his analysis of 23 countries (1) has found a significant positive correlation between
Crohn's mortality and its female to male (F/M) ratio, and between mortality and disease incidence. Indeed, countries with lower incidence (Spain, Italy and Greece) historically had male predominance while higher overall incidence in northern countries was associated with F/M incidence ratio greater than one. Research of the workforce composition (2) has shown increasing female presence in the
observation period and geographic gradient (Fig. 1). Although not well recognized, temporal
increase in F/M incidence ratio has been distinctly reported in studies covering a defined geographical area over several time periods for the U.S. (3, 4), France (5), Scotland (6) and Iceland (7, 8). Regional differences were observed in Spain and Italy with a trend for higher F/M incidence ratio in later studies and from more industrialized regions.

European Collaborative Study (EC-IBD), avoiding bias due to changing diagnostic criteria, was
particularly helpful in establishing geographic differences (9). It has reported female incidence rates in southern countries to be two times lower than rates in northern countries while for males this was only 1.5 times lower. After adjustment for smoking and tertiary education, this difference became even greater: north – south ratio was 2.25 for female and only 1.15 for male incidence. Marked differences in female workforce participation between north and south European countries from Fig. 1 may explain this incidence trend.

Fig. 1 - Female participation as a percentage of the whole workforce has shown marked differences between north and south European countries and temporal increase, corresponded to geographic gradient and temporal change in the female to male incidence ratio of Crohn's disease.

To further investigate possible influence of workforce participation, we looked at one of the
exceptions Sonnenberg observed: high F/M Crohn's mortality ratio for nonwhite U.S. population associated with low incidence rates. Two other studies confirmed this trend: higher F/M incidence ratio for nonwhites than whites (3) and higher hospitalization rates for white males compared to black males and inverse for females (10). Workforce participation (11) for race groups (Fig. 2) was able to explain this – higher workforce participation of nonwhite females compared to white females and lower between males yielded F/M labor force participation in nonwhites higher than in whites,
corresponding to observed higher F/M Crohn’s incidence ratio. Low incidence may be attributed to
a known lower socioeconomic status of nonwhite groups in the 1970's and 1980's. Previously
thought of as a disease of whites only, Crohn’s epidemiology is finding race differences in incidence getting smaller, and so are differences in F/M workforce ratios from Fig. 2.

Fig. 2 - Higher workforce participation of nonwhite compared to white females and inverse for males (age 20 and older shown) has yielded F/M labor force participation in nonwhites higher than in whites and corresponded to higher Crohn’s F/M incidence ratio in nonwhites.

These observations may help to explain the rise in incidence of CD in recent decades, and further
our knowledge of risk factors. A factor associated with being employed may itself be a risk for
developing CD, such as being in contact with more people or having a higher standard of living.
Indeed, previous epidemiological studies have already established higher prevalence in occupations associated with higher socioeconomic status (12).
 

1. Sonnenberg A. Geographic variation in the incidence of and mortality from inflammatory bowel
disease. Dis Colon Rectum 1986; 29: 854-61.

2. Office of Productivity and Technology, Bureau of Labor Statistics, U.S. Department of Labor
Comparative civilian labor force statistics, ten countries 1959-1997. 1998 Mar.

3. Calkins BM, Lilienfeld AM, Garland CF, et al. Trends in incidence rates of ulcerative colitis and
Crohn's disease. Dig Dis Sci 1984; 29: 913-920.

4. Loftus EV Jr, Silverstein MD, Sandborn WJ, et al.  Crohn’s disease in Olmsted county,
Minnesota, 1940-1993: incidence, prevalence, and survival. Gastroenterology 1998; 114: 1161-8.

5. Thélot B, Beaugerie L, Delhommeau A, et al. Epidemiology of Crohn disease in public
assistance-hospitals in Paris. Data of hospitalizations 1981-1992. Gastroenterol Clin Biol 1996; 20:
237-42.

6. Morris DL, Kyle J, Montgomery SM, et al. A true rise in incidence of Crohn's disease in north-east Scotland. Gastroenterology 1999;116 (AGA abstracts):G3372.

7. Björnsson S, Johannsson JH, Oddsson E. Inflammatory bowel disease in Iceland, 1980-89. A
retrospective nationwide epidemiologic study. Scand J Gastroenterol 1998; 33: 71-7.

8. Björnsson S. Inflammatory bowel disease in Iceland during a 30-year period, 1950-1979. Scand J Gastroenterol Suppl 1989;170:47-9.

9. Shivananda S, Lennard Jones J, Logan R, et al. Incidence of inflammatory bowel disease across
Europe: is there a difference between north and south? Results of the European collaborative study on inflammatory bowel disease (EC-IBD). Gut 1996; 39: 690-7.

10. Kurata JH, Kantor-Fish S, Frankl H, et al. Crohn's disease among ethnic groups in a large health maintenance organization. Gastroenterology 1992; 102: 1940-1948.

11. Bureau of Labor Statistics, U.S. Department of Labor. Labor force statistics derived from the
current population survey, 1948-87. 1988 Aug, Bulletin 2307.

12. Sonnenberg A. Occupational mortality of inflammatory bowel disease. Digestion 1990; 46:
10-18.