Clinical symptoms of patients with Kawasaki disease include smoldering fever.

Background: Persistent low-grade fever has been observed in some patients during intravenous immunoglobulin (IVIG) therapy for Kawasaki disease (KD); however, smoldering fever (SF) has not previously been reported in patients with KD. This study aimed to clarify the clinical characteristics of SF in patients with KD.

Methods: A single-center retrospective cohort study, which included a total of 621 patients who received IVIG therapy, was conducted. Patients with a fever of 37.5-38°C lasting ≥3 days after 2 days of the initial-IVIG were defined as the SF group. Patients were divided into four groups according to the fever course: SF (n = 14), biphasic fever (BF, n = 78), non-fever after initial-IVIG (NF, n = 384), and persistent fever (PF, n = 145). The clinical features of SF were described and compared between the groups.

Results: The median duration of fever in the SF group was 16 days, which was longer than that in any other group. The neutrophil fraction after IVIG therapy in the SF group was higher than that in the BF and NF groups but similar to that in the PF group. Repeated IVIG administration in the SF group resulted in increased IgG levels but decreased serum albumin levels. In the SF group, 29% of the patients had coronary artery lesions at 4 weeks.

Conclusions: The frequency of SF in KD was 2.3%. Patients with SF continued to have moderate inflammatory responses. Repeated administration of IVIG doses was not effective in treating SF, and acute coronary artery lesions were occasionally observed. Active therapeutic intervention was needed.