High risk of misclassification of acute Parvovirus B19 infection into a systemic rheumatic disease.

Objectives: Parvovirus B19 most frequently causes epidemics of erythema infectiosum in children but also affects adults often leading to rheumatologic manifestations. While the serum profile allows the diagnosis, manifestations may mimic autoimmune conditions. The aim was to evaluate the proportion of patients with acute Parvovirus B19 infection fulfilling classification criteria for rheumatic diseases (RA and SLE). Methods: We evaluated the clinical and serological features of 54 patients diagnosed with acute Parvovirus B19 infection seeking rheumatological attention between March and June 2024. Results: The majority of patients were females (78%), with a mean (s.d.) age of 45 (13) years and 54% could not recall any known exposure. Fifty-one/54 (94%) had arthralgia, 27 (50%) arthritis (oligoarthritis in 67% of them), 24 (44%) fever, 19 (35%) skin rash and 7 (13%) purpura. Symptoms resolution generally occurred within 6 weeks. Complement levels were low in 14/33 (42%) tested patients, while the presence of serum ANA, anti-dsDNA, anti-phospholipids and rheumatoid factor was detected in 21/38 (55%), 10/26 (38%), 6/12 (50%) and 5/37 (13%) patients, respectively. Classification criteria for SLE were fulfilled in 93% of ANA-positive patients and RA criteria in 38% of patients with arthritis. Conclusions: Parvovirus B19 infection manifestations may vary and nearly all patients with positive serum ANA fulfil the classification criteria for SLE. The risk of misclassification in patients with viral infection should not be overlooked.