A 62-year-old woman was admitted with a 1-month-old non-healing wound on the right leg and persistent erythematous nodules around the wound. One month prior to admission, she was bitten by a cat on her right leg. The patient immediately disinfected the wound with iodophor (0.5%) after being bitten but did not take other measures for dealing with the wound. Subsequently, skin redness, swelling, nodules and pain gradually appeared around the bite. She received a 7-day treatment regimen of cefazolin (1 g b.i.d. intravenous infusion) and amoxicillin (0.5 g t.i.d. oral) in another hospital, but it did not work. Three days prior to admission, the patient complained that after walking for a long time, the redness and swelling increased and the bite wound had a purulent secretion. Pressing around the wound caused her obvious pain. During the development of the disease, she complained of pain around the bite but did not have fever, nausea, vomiting, headache, abdominal pain or arthralgia. On admission, we performed a physical examination of the patient. The superficial lymph nodes of the whole body were not palpable. Edematous erythema of about 7 cm x 10 cm was seen on the inside of the right calf (Figure 1A), and two puncture wounds caused by two teeth of the cat could be seen in the centre of the erythema. Local skin temperature was high, induration could be felt, and tenderness was obvious. Pus secretion could be seen at the bite wound after pressing the local area. Combined with the patient's medical history, we made a presumptive diagnosis of bacterial infection. After admission, a routine intravenous infusion of amoxicillin-clavulanate potassium (1.2 g, q 8 hr) for 5 days was selected to treat her lesions, but it was ineffective.
On admission, laboratory testing showed completely normal humoral and cellular immune function. The blood test revealed that white blood cells 10.63 x 109/L (reference range 3.5 to 9.5 x 109/L), serum biochemistry and C-reactive protein were normal. On admission, we performed a skin biopsy of the erythematous area of the patient. Histopathological examination of erythema showed that the epidermis was generally normal, with neutrophils, histiocytes, lymphocytes and giant cells in the middle and deep dermis (Figure 2). After proper disinfection, pus was collected by pressing the wound and sent for culture. However, 2 days later, the laboratory reported that the bacterial culture was negative. We recommended that the laboratory extend the incubation time for identification of potential fastidious and/or slowly growing microorganisms. In order to eliminate colonized bacteria on the skin surface and environmental pollutants, we obtained the pus directly from the wound and sent it for culture for fungi and bacteria. The pus was cultured on a blood agar plate and placed in a carbon dioxide (5%) incubator for culture. The culture temperature was controlled at 35 C. After 4 days, these bacterial cultures were positive (Figure 3; white colonies on culture medium shown in panel a; positive acid-fast staining is shown in panel b). Matrix-assisted laser desorption/ionization (MALDI) identified M. chelonae with a confidence of 99.9%. At the same time, fungal cultures were negative. Partial sequences of the 16S-23S rRNA internal transcribed spacer (ITS) region and hsp65 geneTb11 were amplified from the strain, and amplicons were sequenced. Further molecular identification confirmed M. chelonae as the cause of the cat wound infection. The patient started taking levofloxacin 500 mg q.d. and clarithromycin 500 mg b.i.d. orally, based on susceptibility results. The redness and swelling subsided after 3 months of treatment (Figure 1B). After 6 months' treatment, the wound healed and brown pigmentation was left locally (Figure 1C). During follow-up visits for the next 6 months, the patient remained stable.