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| Journal of the New Zealand Medical Association,
11-March-2005, Vol 118 No 1211
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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.
Case reportA 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.
DiscussionClinical 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).
ConclusionOur 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: nngan@paradise.net.nz
References:
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