Epidemiology

Introduction

Inflammatory bowel disease (IBD) consists of two closely related, and possibly overlapping, disease processes: ulcerative colitis (UC) and Crohn’s disease (CD). The hallmark of IBD is chronic uncontrolled inflammation of the intestinal mucosa. In contrast to CD, which can affect any part of the gastrointestinal tract, the inflammation of UC is limited to the colon. The segment of colon involved with UC varies; however, the inflammation usually begins in the rectum and extends proximally. Disease can be limited to the rectum (ulcerative proctitis) or involve more proximal regions, possibly involving the entire colon (pancolitis). Typically presenting in the second or third decade of life, UC usually presents with bloody diarrhea, urgency and abdominal cramps.

The clinical course of UC is most commonly characterized by periods of flares and remission, occurring either spontaneously or in response to treatment. Although chronic inflammation can occur in the healthy gastrointestinal tract, such as in response to potential pathogens, the healthy gastrointestinal tract eventually returns to a normal uninflamed state. In patients with IBD, there is an inability to downregulate these inflammatory responses, so the mucosal immune system remains activated, thus the intestine remains chronically inflamed. The exact cause of aberrant inflammation in patients with UC is unclear, but genetic, dietary and environmental risk factors are all proposed to play important roles.

Incidence and Prevalence in North America

A recent study that analyzed the 2015 National Health Interview Survey (NHIS) estimated that 3.1 million US adults (1.3%) have received a diagnosis of IBD. The study found that IBD was more prevalent among certain population subgroups, including adults aged ≥45 years, Hispanics, non-Hispanic whites and adults with less than a high school level of education, not currently employed, born in the US, living in poverty, or living in suburban areas. The prevalence of IBD did not differ by sex, current marital status, health insurance coverage type, or region of residence. The study did not specify the proportion of adults with UC or CD. In more developed nations, the prevalence of UC and CD is often similar, in contrast to what has been observed in less developed nations, where UC is generally more common than CD.

In North America, the incidence of IBD increased during the latter half of the 20th century. Since the 1990s, most studies find that the incidence of both UC and CD has stabilized or even decreased. A 2017 systematic review of population-based studies found the incidence and prevalence of IBD to vary greatly by geographic region. In North America, the lowest incidence estimate for UC was 8.8 per 100,000 person-years (in Olmsted County, USA) and the highest was 23.14 per 100,000 person-years (in Nova Scotia, Canada). The lowest prevalence estimate for UC was 139.8 per 100,000 persons (in Quebec, Canada) and the highest was 286.3 per 100,000 persons (in Olmsted County, USA). The same study provided similar estimates for other global regions. A study of nine million American health insurance claims found a lower prevalence of UC in the South compared with the Northeast, Midwest and West. This finding is consistent with global patterns, where the risk of UC is increased in persons living at higher latitudes.

A 2019 study estimated the lifetime total and incremental costs by age at diagnosis for patients with CD or UC compared with matched control subjects using a large health care claims database. For UC, the lifetime incremental cost was $369,955 among patients who received their diagnosis at 0-11 years, and $132,396 for individuals ≥70 years, averaging $230,102 for a diagnosis at any age. Lifetime total cost was $405,496, consisting of outpatient ($163,670), inpatient ($123,190), pharmacy ($105,142) and ER ($13,493) costs. Overall, the population of patients with UC in the United States in 2016 was expected to incur lifetime total costs of $377 billion.

Incidence and Prevalence Worldwide

The incidence and prevalence of IBD are greatest in more industrialized regions, such as North America, the United Kingdom, Northern Europe, Australia and New Zealand (Figure 1-1 and 1-2). In these regions, the incidence rates for UC and CD have plateaued or decreased in recent decades. In developing countries, such as those in Latin America, Asia and Eastern Europe, incidence rates vary greatly between nations, ranging from 0.5 to 31.5 cases per 100,000 person-years, depending on the study population. As developing nations become more industrialized, the incidence of IBD increases, a pattern mirroring the experience in the west more than 50 years ago. The cause of this trend is unknown but is likely in part due to environmental factors, such as microbial exposure and diet. In addition to an industrialization gradient, the incidence of UC increases with increasing latitude, with higher incidence rates in Northern countries and lower rates in Southern countries.

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Morbidity and Mortality

UC is a chronic condition that commonly requires a lifetime of care and can cause significant morbidity. In the United States, IBD constitutes about 0.1% of all emergency department visits. Between the years of 2006 and 2014, the rate of IBD emergency department visits per 100,000 persons rose significantly, from 75.7 per 100,000 to 100.1 per 100,000, a 32.2% increase. Patients with UC accounted for less than half as many emergency department visits as those with CD. The rate of IBD-related in-patient admissions from the emergency room was 52.6% in 2014. Compared to UC patients, those with CD are significantly less likely to be admitted from the emergency department. Other factors associated with a higher IBD in-patient admission rate include older age, current or former smoking and having private insurance or high income.

The risk of surgery in IBD patients has decreased over the past several decades. Current estimates for the cumulative-risk of surgery among UC patients are 4.9%, 11.6% and 15.6% after 1, 5 and 10 years following diagnosis, respectively. This compares to a higher cumulative-risk in CD patients: 16.3%, 33.3% and 46.6% over the same time periods.

In the 1980s, the mortality rate of patients with UC was higher than the general population. However, current studies find that patients with UC have mortality rates similar to, or only slightly higher than, the general population when properly managed, possibly due to increased use of immune modulator and biologic therapy and the refinement of surgical techniques.

Certain populations, including newly diagnosed patients and patients with more extensive colitis, are at greater risk of mortality. A Danish study of 36,080 patients with UC found that the overall risk of dying of any cause was higher compared with the general population in the first year following diagnosis (HR = 2.43), but then rapidly declined to approximately 1.1 after 2 years. The long-term risk of death from certain causes remained significantly higher in patients with UC, including infectious diseases (HR=1.64), gastrointestinal disorders other than colorectal cancer (HR=1.47), gastrointestinal disorders other than UC (HR=1.26) and cardiovascular disease (HR=1.11).

Age

In North America, older estimates for the mean age of diagnosis for UC ranged from 40 to 45 years, compared with an earlier diagnosis for CD of 33 to 45 years. More recent estimates place the peak age of onset of UC between the ages of 30 and 40 years. According to a population-based study in Olmsted County, Minnesota, the median age of UC diagnosis was 34.9 years (Figure 1-3), compared with 29.5 years for CD.

UC is less common in children and adolescents, but studies have found that its incidence is increasing in these age groups. In a recent systematic review of pediatric IBD studies, of the 63 studies focused on UC, 29 (46%) reported a significant increase, 29 (46 %) reported no significant changes and 5 (8%) reported a significant decreasing trend in UC. Like in the adult population, the incidence of UC in children and adolescents appears to have reached a plateau in the Western world, but is increasing in other parts of the world, most prominently in Southern Europe, Eastern Europe, Oceania.

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Racial and Ethnic Disparity

IBD affects people of all races and ethnic groups. Although UC appears to be more common in whites and people of Ashkenazi Jewish origin, racial and ethnic gaps are narrowing, with an increasing incidence in African Americans and second-generation Asians who have migrated to more industrialized countries. Some older studies concluded that IBD was more aggressive and had earlier onset in African American patients; however, several subsequent studies found such disparities in disease severity to be due to economic inequalities, rather than genetic factors.

Gender

Although the mechanism is not understood, the age of onset of IBD varies with gender. In a recent pooled analysis of 112,004 incident cases of UC among over 478 million people in the Western world, the rates of UC incidence were found to be 22% higher for females in early childhood (5-9 years of age), similar for males and females between the ages of 10 and 44 and subsequently 13% to 32% lower for females until the age of 70-74. This pattern contrasts with the incidence of CD, which according to the same analysis is less common in females until the age of 10-14, but is more common in all subsequent age groups.