Introduction Neoehrlichia mikurensis is a tick-borne bacterium that primarily causes disease in immunocompromised patients. The bacterium has been detected in ticks throughout Europe, with a 0%–25% prevalence. N. mikurensis infection presents unspecific symptoms, which can easily be mistaken for inflammatory disease activity. We aimed to determine the prevalence of N. mikurensis in rheumatological patients receiving tumour necrosis factor inhibitors (TNFi) and a cohort of healthy individuals.

Materials and methods This retrospective cohort study included 400 rheumatological patients treated with TNFi and 400 healthy blood donors. Plasma samples were retrieved from the Danish Rheumatological Biobank and the Danish Blood Donor Study between 2015 and 2022. Age, sex, diagnosis and duration of TNFi treatment were recovered from the Danish Rheumatological Database, DANBIO. Data on age and sex were available for the blood donors. One plasma sample per individual was tested for N. mikurensis DNA-specific real-time PCR targeting the groEL gene.

Results In the rheumatological patients, the median age was 61 years (IQR 55–68 years), 62% were women, and 44% had a diagnosis of seropositive rheumatoid arthritis. In total, 54% of the patients were treated with infliximab. The median time from TNFi initiation to blood sampling was 20 months (IQR, 5–60 months). N. mikurensis DNA was not detected in any samples from patients or blood donors.

Conclusion N. mikurensis infection does not appear to represent a prevalent risk in Danish rheumatological patients receiving TNFi or in blood donors.