Borrelia arthritis - a hidden cause of arthritis in Danish patients?

Borrelia arthritis is, despite being a painful and debilitating disease, barely studied in Europe. Our research group has discovered an alarming number of cases in the last few years, and we believe Borrelia arthritis is underdiagnosed in Denmark. We therefore aim to systematically investigate the prevalence and trajectory of Borrelia arthritis in Denmark, in order to enhance the diagnostic capabilities, to improve outcomes for future patients, as well as reduce the costs for the Danish health care system.

Borrelia arthritis is a painful and disabling joint disease caused by infection with Borrelia bacteria transmitted through tick bites. The disease often affects the knee and usually appears several months after the bite. While it can be treated with antibiotics, some patients experience long-lasting joint pain and inflammation that require more advanced treatment.

In the U.S., up to 30% of Lyme disease cases are linked to Borrelia arthritis, but in Europe – and especially in Denmark – it is thought to be rare. However, in the last couple of years we have discovered several cases, especially near Esbjerg, and suspects the disease is far more common than previously believed.

The project aims to:
1) Map how many Danes are affected by Borrelia arthritis.
2) Study how the disease appears in Danish patients.
3) Identify which Borrelia types are responsible – and whether they resemble the ones seen in the U.S.

The project includes three sub-studies:
Study 1: All patients with new joint swelling (in one or a few joints) will be tested for Borrelia at seven hospitals in Denmark in 2026 and 2027.

Study 2: Patients will answer questionnaires, and researchers will compare their disease course and symptoms with those of other arthritis patients.

Study 3: Ticks will be collected in nature from areas with many cases and from control areas. These will be tested to see which Borrelia types they carry and if they match the types found in patients.

Why Is This Important?
Early diagnosis and proper treatment of Borrelia arthritis can prevent long-term illness and reduce the impact on quality of life. This project will help doctors detect the disease earlier and treat it more effectively, and the findings may support better prevention and public awareness of tick-borne diseases in Denmark.