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The epidemiological understanding of inflammatory bowel disease has been limited by the referral bases of most inflammatory bowel disease studies. The Colitis-Ileitis Study Group of Rochester, N.Y., developed a community-wide, computerized cumulative registry of all inflammatory bowel disease patients hospitalized at the 8 community hospitals for 1973-86. Clinical data were abstracted from each of the 1651 identified hospital charts. All of these patients resided in Monroe County (city and suburbs) and the 5 contiguous counties (Genesee/Finger Lakes Region, population 1,030,640). Of the 1651 hospital patients identified in the study, 1358 resided in Monroe County (Rochester and its immediate suburbs, population 702,238). Incidence, defined as time of onset of symptoms of inflammatory bowel disease, rose from baseline rates in the 1930s to peak in 1980 (Crohn's disease = 50.29/10(5) per decade, ulcerative colitis = 35.12/10(5) per decade) and declined through 1986. For Crohn's disease, the age-specific incidence rates peaked in the 20-29-yr-old group in each of the 5 decades studied. Ulcerative colitis seems to occur at all ages and may have a bimodal distribution. There was a period effect, with the 1970s having the highest incidence of Crohn's disease and ulcerative colitis for each age group. However, the age-specific incidence rate for Crohn's disease showed a 40% decrease in the 1980s compared with the 1970s in the 10-39-yr-old group (p less than 0.001). The age-specific incidence rate for ulcerative colitis showed a 50% decrease in the 1980s compared with the 1970s in the 10-49-yr-old group (p less than 0.001).

From January 1979 through December 1980, a total of 664 Indochinese refugees were screened for tuberculosis in Monroe County, New York; 307 (46%) had positive 5-tuberculin unit PPD Mantoux tests. When 217 refugees with initially negative tests were retested at 60 days, 94 (43%) had converted from a negative to a positive test. Although none of the converters had evidence of active tuberculosis disease, 90 (96%) were given isoniazid preventive therapy. This conversion phenomenon has continued over time, discounting an effect of local, national, or international policy changes. The conversions do not seem to be a function of antigen, tester, or reader; criteria for testing, retesting, or reading; home country, camp country, or length of time in camp; or history of prior BCG vaccination, tuberculosis, or immunization exposure, current illness, or testing intervals since arrival in the United States and subsequent testing. Further studies to evaluate boosting or anergy are in progress to assess this potential public health problem.

Indochinese refugees entering the United States have a high rate of tuberculosis and tuberculin reactivity. In addition, several investigators have noted that a large number of refugees with initial tuberculin tests that are "not significant" change to "significant" reactions when retested within 8 wk. This "conversion" phenomenon has been reported in 21 to 43% of refugees and has been unexplained by antigen, testing, demographic, or exposure risk factors. A prospective evaluation of 218 refugees, conducted to assess the role of anergy and boosting, confirmed earlier findings, with 52% of 118 persons with initial tuberculin reactions that were "not significant" developing "significant" reactions on subsequent testing. Anergy, as measured by nonreactivity to mumps and candida skin tests, was not found to be a contributing factor, as few refugees were anergic and as rates of anergy did not differ significantly among refugees with different responses to tuberculin. Boosting, however, played a major role in explaining the "conversions," as 59% of persons who changed to "significant" tuberculin tests did so when retested with tuberculin at 1 to 3 wk. "Delayed" boosting rather than incubating disease or anergy appeared to be the most likely explanation for the remaining "conversions" that occurred on a third PPD test conducted at approximately 8 wk. If the "conversion" phenomenon is due to boosting, it remains to be seen whether the boosting is a result of previous exposure to Mycobacterium tuberculosis or to other, nontuberculous mycobacteria.