Journal of the New Zealand Medical Association, 11-March-2005, Vol 118 No 1211
Mycobacterium fortuitum infection caused by a cat bite
Natalie Ngan, Arthur Morris, Tristan de Chalain
In recent years, infections caused by mycobacteria other than Mycobacterium tuberculosis have been increasingly reported. Mycobacterium fortuitum is a rapidly growing mycobacterium and can be found in soil and natural water supplies. While it is one of the most common nontuberculous mycobacteria (NTM) associated with nosocomial disease, it has mainly been associated with wound infections, particularly after trauma.1,2
Infections caused by bites from animals are not uncommon and infections with M. fortuitum after a dog bite have been described.3 However no case following a cat bite has been reported.
A 44-year-old, previously fit and well woman presented with a 6 months’ history of a slowly enlarging lesion on the volar aspect of her left forearm. She had initially developed a small red nodule following a cat bite at that site. The painful, shiny, raised, erythematous area did not settle in the months following the bite. After 2–3 months, she presented to her general practitioner (GP) because the lesion was enlarging distally and had begun to discharge serous fluid.
The lesion had remained localised and there was no regional lymphadenopathy. The nodule was lanced in the GP’s rooms. Only serous fluid was encountered; no pus and no foreign bodies were discharged. Nothing was sent for microscopy or culture. She was commenced on a course of oral amoxycillin-clavulanate but without improvement.
A referral was made to a plastic surgeon who prescribed a further course of oral antibiotics. The lesion, however, continued to enlarge and was now occasionally discharging pus. A second plastic surgical consult was sought, from a different surgeon, now 6 months following the cat bite. An atypical infection was suspected and an excisional biopsy of the lesion was performed under general anaesthesia. The lesion was excised en bloc and then divided into three parts; two fragments were submitted for culture and one portion for histology.
Both portions sent for culture were processed for bacterial and mycobacterial culture. On Gram stain, neither polymorphonuclear leukocytes nor organisms were seen and cultures were sterile after prolonged (14 days) incubation. Mycobacterial cultures were performed on solid medium, at 27 and 36°C, and liquid medium at 36°C. Mycobacterium fortuitum was recovered from the liquid culture after 7 days’ incubation at 36°C. The isolate was susceptible to amikacin, ciprofloxacin, imipenem, cefoxitin, sulphonamide, doxycycline, and clarithromycin.
Histopathology revealed a central localised area of lipogranulomatous inflammation. The inflammatory cells included epithelioid histiocytes, multinucleated giant cells, lymphocytes, and occasional neutrophils. The overlying epidermis was intact and showed no significant epidermal hyperplasia. Special stains for bacteria, acid-fast bacilli, and fungal elements were negative.
The surgical wound healed well and no recurrences were observed at 1-year follow up.
Clinical disease caused by rapidly growing mycobacteria usually follows accidental trauma or surgery in a variety of clinical settings.1,2 M. fortuitum is one of the NTM species that most commonly causes localised infections of the skin and subcutaneous tissue.3,4 Diagnosis is made by culture of the specific pathogen from drainage material or tissue.
The incubation period between the time of injury and the onset of symptoms is an important diagnostic feature in infections caused by mycobacteria. It averages 1 month and can be as long as 6 months.3 Pain, local swelling, and mild serous drainage are typically present. Systemic symptoms are rare. The aetiology of the infection can be suspected by the history of trauma, the relatively long incubation period, the absence of serious or systemic symptoms, and the nonpurulent serous nature of the drainage. Definitive diagnosis depends on culture of the organism.3,5
M. fortuitum infections caused by animals are not uncommon. Usually the infective organism is a part of the bacterial flora of the animal, but in some cases the animal may have become contaminated or infected with an organism from the surrounding environment and so is able to transmit it.6
M. fortuitum, like other rapidly growing mycobacteria, are environmental micro-organisms isolated from diverse habitats, most commonly water and soil.7 It is also present in the saliva and sputum of asymptomatic people.8 Infection, at least in humans, is usually associated with an underlying immunosuppressive condition such as cancer, corticosteroid administration, trauma (surgical or accidental), or chronic renal failure.9 They are usually considered to be non-pathogenic unless introduced deep into the body.10
Ip and Chow reported five cases of Mycobacterium fortuitum infections in the hand.10 Trauma was the main precipitating factor in all cases; three of the five followed local steroid injections from a single practitioner. The authors concluded that a high index of suspicion was important to obtain the correct diagnosis. NTM infection should be suspected when infection follows trauma and responds poorly to standard antibiotics as in our patient. Examination of the tissue to confirm the diagnosis is recommended followed by a combined approach of early radical debridement and appropriate antibiotic therapy to give the best chance of controlling the infection.10
Surgery is generally indicated with extensive disease, abscess formation, or where drug therapy is difficult. Removal of local foreign bodies (e.g. breast implants and percutaneous catheters) is important, or even essential, for recovery.11–13
Because of differences in susceptibilities among species of rapidly growing mycobacteria (and even within species), susceptibility testing should be performed on all clinically significant isolates as well as isolates that have been recovered after treatment failure or relapse.11,13
Treatment of M. fortuitum infections can be extremely challenging because of its resistance to traditional anti-tuberculous drugs and commonly used antibiotics.4,14 Resistance often develops with monotherapy, necessitating a multi-drug combination. Recommended duration of therapy varies from 3–4 months depending upon clinical resolution.15 There have been estimates that approximately 10 to 20% of infections will resolve within a few months (either spontaneously or following surgical debridement,4 as in our patient).
Our case illustrates that cats can be added to the list of animals that can inoculate people with NTM. An appropriate history, physical examination, and failure to respond to commonly used antibiotics might suggest an infection with an NTM organism. In our case, resolution of the infection was achieved with surgical excision alone.
Author information: Natalie C Ngan, Plastic Surgical Registrar; Arthur J Morris, Clinical Microbiologist; Tristan B de Chalain, Consultant Plastic Surgeon, Department of Plastic and Reconstructive Surgery, Middlemore Hospital and Diagnostic Medlab, Auckland
Correspondence: Dr Natalie Ngan, Department of Plastic and Reconstructive Surgery, Middlemore Hospital, Private Bag, Auckland. Email: firstname.lastname@example.org
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