Epidemiology is the study of health and disease patterns. What causes a specific disease? How does it spread? How many people are newly diagnosed in a year? How many people are managing the disease on an ongoing basis?
Two of the statistics that epidemiologists give us are incidence and prevalence. This topic would bore me to tears if I had to produce these statistics for a living, but when I’m researching a disease, these are numbers that I want to know. Incidence tells us about how many new diagnoses are made per year (or, more technically, per any given time period). Prevalence tells us how many people are being treated on an ongoing basis during that same time period.
Incidence – In studying rheumatoid arthritis, epidemiologists have learned that for every 100,000 adults, there are an average of 41 new cases of RA diagnosed annually. The number of people newly diagnosed increases with age: 8.7 per 100,000 for ages 16-34, versus 54 per 100,000 for people over age 80.
Prevalence – For every 100,000 adults, there are 500-920 people dealing with RA on an ongoing basis (statistics vary depending on which diagnostic criteria are used, which country is being considered, and a few other variables).
The incidence and prevalence of other types of autoimmune arthritis varies widely1:
As you can see, the data on PsA is sub-optimal. AS is twice as common as RA, yet isn’t nearly as well-known. MCTD and AOSD have such a low prevalence that they are considered rare diseases by the Office of Rare Diseases of the National Institute of Health, and by Orphanet. JIA is also listed in the rare diseases database.
Age – Age of onset varies considerably depending on the type of arthritis. By definition, disease onset before age 16 means that it’s a subtype of Juvenile Idiopathic Arthritis. Children who receive a diagnosis of one of the subtypes of JIA do not receive a different diagnosis as they age; the diagnosis remains JIA, even if the person lives to be 80 years old.
These are averages, not rules. Immune systems don’t read textbooks, thus don’t know when the books say they might malfunction.
1Statistics on the incidence and prevalence of Sjogren’s Syndrome and UCTD are difficult to find.