Spondylitis (also called spondyloarthritis or axSpA) is an autoimmune disease in which the immune system mistakenly attacks the joints and other tissues in the same way it would normally fight viruses or bacteria.
The specific cause of spondylitis is unknown, although it likely involves both hereditary and environmental factors.
People under age 45 are more likely to develop spondylitis than those older. Onset is most often in the 20s or 30s.
Risk factors for spondylitis
It is important to note that while science is good at finding correlations, or apparent relationships, between factors and disease, correlation does not prove that the risk factor causes the disease. Many risk factors for spondylitis have been identified and are being studied, but none have been pinpointed as the cause of spondylitis.
Hereditary risk factors
If you have a parent or sibling with spondylitis, your risk for developing spondylitis is three times higher than for someone with no relatives who have spondylitis. The risk for the general population to develop spondylitis is only 1 percent, so the risk of someone with a close family member with spondylitis is still only 3 percent. Amongst identical twins, if one twin has spondylitis, the other twin has approximately a 50 percent chance of developing the disease. For this reason, most researchers agree that genetic and environmental factors both contribute to the cause of spondylitis.
More than 60 genes have been identified as playing a role in spondylitis risk. One gene, HLA-B27, is present in many people who are diagnosed with spondylitis. HLA-B27 is a more influential risk factor for spondylitis in people of some ethnicities. HLA-B27 is found in 95 percent of those of European descent who are diagnosed with spondylitis; 80 percent of those of Mediterranean descent with the disease; and only 50 percent of those of African descent who have spondylitis. Testing positive for HLA-B27 does not mean that you will definitely develop spondylitis. Only about 2 percent of those with the gene are diagnosed with axSpA. Read more about spondylitis diagnosis.
Men are twice as likely as women to develop axSpa. Women with spondylitis tend to have a milder disease course than men.
Environmental risk factors
Most researchers do not believe that genetics alone determine who gets spondylitis. However, research has not yet identified which environmental factors play a role in causing the disease. Some evidence indicates that bacterial infections, especially those affecting the gastrointestinal system, may trigger an inflammatory reaction that develops into spondylitis. A similar theory suggests that spondylitis develops when there is an imbalance in the bacteria colonizing the intestines. Smoking is also theorized to increase the risk for developing spondylitis.