Primary cutaneousnocardiosis in an immunocompetent hostShihuan He 1, Youkun Lin1, *1 Department of Dermatology and Venereology, The first Affiliated Hospital of Guangxi Medical University, Nanning, China* Correspondence: Youkun Lin, Department of Dermatology and Venereology, The first Affiliated Hospital of Guangxi Medical University, Nanning, China (e-mail: [email protected])Key Clinical Message: Nocardiosis is a rare opportunistic infection that is most commonly seen in immunocompromised or lung disease patients, but can also occur in immunocompetent persons. It often involves the lung, central system and skin and can cause disseminated infection in severe cases. The clinical manifestations of cutaneous infection are diverse and easy to be misdiagnosed.KEYWORDS: nocardiosis; cutaneous; immunocompetent; misdiagnosisA 53-year-old male patient presented to the hospital with swelling, nodules, and empyema of the right upper limb. In the previous 6 years, the patient had been treated with antibiotics several times, but no significant improvement was observed. In contrast, the condition gradually worsened. He had no diabetes, tumour, AIDS or other immunodeficiency diseases. After detailed questioning, we learned that he had a history of trauma to the right upper limb before the onset of the disease. At this time, we were highly vigilant for special infections, such as nontuberculous mycobacteria, fungi, and actinomycetes, so we conducted a comprehensive examination.Physical examination revealed a swollen right upper limb, with many purplish red nodules on the surface and some nodules that were purulent and fluctuating, with scattered scabs and scars (Fig. 1). Laboratory investigations revealed a C-reactive protein level of 31.8 mg/L (0-5 mg/L) and negative results for others. Pathology of the skin lesion showed infectious granuloma. Gram-positive bacilli were observed in the purulent secretion under the microscope (Fig. 2). The colony was white with a wrinkled and granular surface on Columbia blood agar (Fig. 3). The skin lesion tissue yielded was attributed to Nocardia brasiliensis , which was confirmed by mNGS, 16S RNA sequencing and matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS). Ultimately, the patient received trimethoprim sulfamethoxazole treatment. Two months later, the nodules in his right upper limb obviously subsided.Nocardiosis is a pyogenic or granulomatous disease caused by infection with Nocardia spp.. The disease is distributed all over the world but is more common in tropical areas such as India, Pakistan, and Iran.1 The incidence rate in males is higher than that in females. Nocardiosis is most commonly seen in immunocompromised people, but in recent years, the incidence of nocardiosis in immunocompetent people has been increasing, which may be related to unrecognized primary immunodeficiency.2Pulmonary nocardiosis is the most common, manifesting as fever, cough, haemoptysis, shortness of breath, night sweats, weight loss, and fatigue. Central nervous system involvement often manifests as brain abscess, fever, headache, vomiting, and altered consciousness. Cutaneous nocardiosis accounts for approximately 20% of cases and occurs mostly in people with normal immunity. It can be divided into primary infection and secondary infection caused by the dissemination of systemic infection. Primary cutaneous nocardiosis is uncommon and often has a history of trauma. Moreover, it is mainly caused by infection withNocardia brasiliensis and manifests as nodules, subcutaneous abscesses or cellulitis.3 The patient had a history of trauma and skin infection. However, the clinical physicians ignored the infection with Nocardia spp., which eventually led to a delay in diagnosis and treatment.Cutaneous nocardiosis is a great imitator and is not easily distinguished from superficial cellulitis, sporotrichosis, nontuberculous mycobacterial infection, leishmaniasis, etc. Its clinical manifestations are nonspecific, and diagnosis is mainly dependent on aetiology. Nocardia spp. are gram-positive, partially acid-fast, and catalase-positive aerobic bacteria. Traditional methods include smear microscopy and culture, which are time-consuming and cannot identify Nocardia species. In recent years, molecular biology techniques have become the gold standard for Nocardia species identification, including 16S rRNA gene sequencing, PCR, MALDI-TOF MS, etc.4 These detection methods have higher accuracy and shorter time, which are helpful for early diagnosis and guiding treatment.The treatment of nocardiosis should follow the principles of standardization, adequate dose and full course of treatment. Sulfamethoxazole-trimethoprim is the first choice for empiric treatment. Other alternative antibiotics include amikacin, ceftriaxone, minocycline, moxifloxacin, linezolid, etc. It is worth noting that drug susceptibility testing should be routinely performed to better guide treatment. Monotherapy is recommended for most patients with primary cutaneous nocardiosis, and combination therapy can be selected for severe infection. The duration of treatment ranged from 2 to 6 months.5 The patient’s condition was significantly improved after 2 months of monotherapy with sulfamethoxazole-trimethoprim. Treatment of nocardiosis varies from person to person and should be based on the patient’s immune status and degree of disease.In summary, primary cutaneous nocardiosis is rare, mostly caused by trauma, and commonly occurs in people with normal immune function. The clinical manifestations and laboratory tests are nonspecific, making it easy to misdiagnose and miss a diagnosis.Support: There is no funding.Conflicts of interest: There are no conflicts of interest.Declaration of patient consent: The patient has given written informed consent to publication of the case.Acknowledgement: We greatly appreciate the patient in this manuscript who has given written informed consent to publication of the case.Data Availability Statement: The datasets are available from the corresponding author on reasonable request.