Ankylosing spondylitis may affect about 1 in 1,000 people in the United States. It can affect anyone but is more common in white people and those assigned male at birth. There‘s a strong genetic component, but environmental factors also play a role.

Ankylosing spondylitis (AS) is a type of arthritis that affects the spine. The joints and tissues of the spine become inflamed and cause stiffness. In severe cases, the vertebrae of your spine may fuse together.

Many people with AS have a certain gene, but many don‘t have this genetic marker. You can also have the gene and not develop AS. If you have a relative with AS, you may be more likely to have it. But factors besides genetics play a role.

AS is the most common form of spondyloarthritis. Many people with the condition do not have a diagnosis.

Statistics vary by source regarding how common AS is in the United States. A 2021 study found that doctors diagnosed AS in 3.60 people per 10,000 in the Medicare database in 2014 and 1.42 people per 10,000 in the IBM MarketScan database in the same year. Both figures were increases in the diagnosis rates compared to 2006.

A 2016 study found 1.07 people per 1,000 (10.7 in 10,000) in the Kaiser Permanente Northern California system had an AS diagnosis. This study examined Kaiser records from 1996 to 2009.

The 2021 study noted that the Kaiser database might show higher diagnosis rates because those in the Kaiser network may have better access to rheumatology care than those in the Medicare system. Therefore, people in this group may have been more likely to get a diagnosis.

People of all genders and ethnicities get AS. The condition is three times as common in people assigned male at birth than in people assigned female. But the American College of Rheumatology notes that women may be underdiagnosed.

AS is also three times more common in white people in the U.S. than in Black people.

About 90% of white people with AS have the HLA-B27 allele, a specific gene. But a person may have HLA-B27 and not develop AS.

Although less common among Black people, a 2017 study found that Black people may have more severe AS disease than white or Latino people. They were also less likely to carry the HLA-B27 allele. The study authors stated they could not say whether the reasons for the differences in severity were genetic or socioeconomic.

Although there appears to be a genetic component to AS, environmental factors are also important. So, even if you have a close relative with the condition, you may not develop AS.

Only about 50% of pairs of identical twins, with identical genetic material, both have AS.

Still, you may be more likely to develop AS if you have a first-degree relative with AS who also has the HLA-B27 allele. A small 2016 study found 33% of such relatives met the criteria for spondyloarthritis, a group of conditions that includes AS.

Other sources say AS is 10 to 20 times more common in first-degree relatives of people with AS than in those whose close relatives do not have AS.

AS can develop at any age, but symptoms usually start before age 45. Most commonly, the signs of AS appear between the ages of 20 to 40.

AS can start to affect people even earlier in life, such as in their teens or early adulthood.

Spondyloarthritis includes several conditions besides AS.

According to the Arthritis Society of Canada, spondyloarthritis falls into two broad categories:

  • axial spondyloarthritis, which affects the spine
  • peripheral spondyloarthritis, which affects other parts of the body

AS affects the spine and is a form of axial spondyloarthritis.

The results of a large international study published in 2021 revealed the most common diagnoses among 4,465 people with spondyloarthritis from 24 countries:

The American College of Rheumatology says that AS is the most common type of spondyloarthritis.

About 1 in 1,000 people in the U.S. may have AS, according to one source. But statistics vary by database.

AS is the most common form of spondyloarthritis. If you have a close relative with AS, you may be more likely to develop it. But people without a genetic marker also get the condition.