A 29-year-old primiparous Malagasy woman came for a consultation to the dermatological clinic of the Nord Franche-Comte Hospital in June 2020 for painful nodules on her legs typical of erythema nodosum. These lesions developed a month after a swelling of the right breast undergoing etiological investigation. She was not pregnant or lactating. She had last breastfed her child two years ago. She did not use oral contraceptives or any other medication. The patient had no history of tuberculosis infection or autoimmune disease or any family history of breast cancer. A contact with SARS-CoV-2 was noted in April 2020 for which she had not presented any symptoms.
On clinical examination, she was not febrile. Voluminous painful tender mass (10x6 cm diameter) was located in upper inner of the right breast with retraction of the nipple (Figure 1). The contralateral breast was unremarkable. The right axillary lymphadenopathy was palpable. The dermatological examination revealed a typical erythema nodosum lesions characterized by multiple erythematous, painful, infiltrated nodules of varying size on the lower limbs (Figure 2). The remaining examination was unremarkable.  
Biological investigations for an underlying cause such as inflammatory tests (full blood count, erythrocyte sedimentation rate, C-reactive protein), autoimmune markers (antinuclear antibody, anti-DNA, anti-neutrophil cytoplasm antibody) and infective serologies (HIV, hepatitis B and C) were negative or within normal range. Serology SARS-CoV-2 IgG testing as part of a systematic work-up came back positive at over 100.
An ultrasound examination, performed twice, showed a hypoechoic mass on the right side, frankly hyper vascularized in favor of mastitis. Mammography did not reveal any suspicious mass or microcalcification. A purulent nipple discharge occurred during the mammography.
Breast biopsies showed a lobulocentric subacute mastitis with a non-caseating granulomatous inflammation and multinucleated giant cells (Figure 3a and b). There was no evidence of malignancy. Immunohistochemistry was normal. No organisms were identified on Grocott methanamine silver (GMS), Gram, Periodic acid-Schiff (PAS) and Ziehl-Neelsen stainings. Bacterial, fungal and mycobacterium cultures of the aspiration specimen were all negative. 
TB quantiferon, polymerase chain reaction test and acid-fast bacilli staining were negative. On the other hand, blood calcium and angiotensin-converting enzyme level were within normal range. The chest X-ray did not show any hilar adenopathy or pulmonary parenchymal infiltrate. Either tuberculosis or sarcoidosis was excluded with these results.
Initially, an infectious origin was suggested. She was empirically treated with antibiotics: pristinamycin (7 days), amoxicillin-clavulanic acid (10 days), and trimethoprim-sulfamethoxazole (2 months). Then, a treatment with colchicine 1mg/day was administered as the disease worsened with discharge and fistulation of the swelling at the biopsy sites. Spectrometry of a pus sample identified a germ called Corynebacterium kroppenstedtii. The other two bacteriological control samples were aseptic.
The diagnosis of IGM associated with erythema nodosum was evocated. Daily prednisone 60 mg was started with a gradual taper 5 months. The evolution was rapidly favorable with complete regression of the breast swelling and erythema nodosum. No recurrence was reported within two years of follow-up.