Why is this study being done? Borrelia arthritis is a joint infection caused by Borrelia bacteria transmitted by tick bites. It often affects the knee and can cause prolonged pain and swelling if not diagnosed and treated in time. While well recognized in North America, Borrelia arthritis is considered rare in Europe. Recent findings from the investigators in Denmark suggest it may be more common than previously thought and frequently diagnosed late. The aim of this study is to improve knowledge about how often Borrelia arthritis occurs in Denmark, how it presents, how long diagnosis takes, and how patients recover. This may help ensure faster diagnosis and better treatment for future patients. Who can take part? Patients referred to an including rheumatology department with inflammation in one or a few joints (mono- or oligoarthritis) may be invited to participate. What does participation involve? If a patient choose to participate: * A blood sample will be taken to test for antibodies against Borrelia. * Fluid from the affected joint(s) will be tested for Borrelia DNA. * Blood and joint fluid samples will be stored in a secure research biobank for future analyses related to this study. * The patient will complete a questionnaire about symptoms, health history, possible tick exposure, and the diagnostic process. * Relevant information from the patients medical record (tests, treatments, and outcomes) will be collected. All procedures are part of standard clinical care or involve minimal additional testing. How will the information be used? The study will investigate: * How common Borrelia arthritis is among patients with joint inflammation in Denmark. * Differences in symptoms, test results, diagnostic delays, treatment, and outcomes between patients with Borrelia arthritis and other forms of arthritis. Risks and benefits Risks are minimal and mainly related to routine blood sampling and joint aspiration. The patient may not benefit directly, but the results may improve care for future patients. Confidentiality and voluntary participation All personal data will be handled confidentially according to data protection regulations. Participation is voluntary, and patients may withdraw at any time without affecting their medical care. Study period The study runs from January 1, 2026, to December 31, 2027 and includes patients from eight hospitals in Denmark.
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Sub-study 1: 1-year cure rate Borrelia arthritis vs. control group
Timeframe: 1 year after inclusion
Sub-study 2: Serum-Borrelia IgG antibodies
Timeframe: At study inclusion