CONDITION
Atypical Mycobacterial Infection
Atypical mycobacterial infection describes a group of bacterial infections caused by mycobacteria other than the organisms responsible for tuberculosis. In dogs and cats, these infections most often appear as persistent lumps under the skin, slow-to-heal wounds, or draining tracts that do not respond to ordinary antibiotics. The bacteria involved are environmental organisms found in soil and water, and they tend to establish infection following penetrating injuries or contaminated wounds. Owners typically notice firm nodules that may ulcerate, ooze, or form discharging sinus tracts, often on the limbs or body wall. In cats, a form called panniculitis—inflammation of the fat layer beneath the skin—can produce softer, spreading swellings. These signs may develop weeks to months after an initial wound, or appear without any obvious injury being recalled. The pattern can look puzzling because the tissue does not behave like a simple abscess or infected bite. This page explores what these infections can look like, the organisms and circumstances that underlie them, how samples are collected and cultured to identify the bacteria, and the medical and surgical approaches used to manage them.
Why this matters now
Atypical mycobacterial infections can occur at any age, though the pattern varies with the organism involved. The cutaneous form, caused by rapidly growing species such as Mycobacterium fortuitum or M. smegmatis, tends to develop after wound contamination—following surgery, bite wounds, or other trauma that introduces soil or water-borne bacteria into tissues—and occurs somewhat more often in cats than dogs, with adult male cats that hunt outdoors representing a frequently affected group. Disseminated infections caused by slow-growing species such as M. avium complex are seen more often in dogs, particularly certain breeds including Miniature Schnauzers and Basset Hounds, where inherited differences in immune signalling may increase susceptibility.
The cutaneous form often develops slowly, with nodules or swellings appearing weeks to months after the initial injury, sometimes long after the wound itself has seemed to heal. Lesions may appear to improve with routine treatment for abscesses, only to recur within a few weeks, and new nodules or draining tracts can form adjacent to areas that appeared to be resolving. In disseminated disease, signs may develop more insidiously, with weight loss, intermittent digestive upset, or respiratory changes accumulating over an extended period. The course in any individual animal can be quite variable, influenced by the specific organism, the site of infection, and the individual's immune response.
Signals & patterns
Early signals
Firm swelling under the skin
An owner may notice a lump or thickened area, often on the abdomen, groin, or limbs, that feels different from a typical abscess—firmer, less fluctuant, and without the heat or pain that accompanies many bacterial infections. The swelling may appear weeks after a known injury or surgery, or with no obvious preceding event.
Non-healing wound or surgical site
A wound that was expected to close continues to look inflamed, may develop a small opening that weeps fluid, or begins to improve and then deteriorates again after a course of routine antibiotics finishes. The area around the wound may feel thickened or nodular.
Small area of hair loss with moisture
A patch of skin loses hair and remains persistently moist or crusted, sometimes with a slight discharge. In cats, this can appear on the lower abdomen or near old bite wounds, and the surrounding skin may feel lumpy beneath the surface.
Slow weight loss without obvious cause
In cases involving internal organs—more common in certain dog breeds—an owner may observe gradual thinning, reduced appetite, or a coat that loses condition over weeks to months, without dramatic signs of acute illness.
Later signals
Multiple draining tracts or punctures
What began as a single lump may develop several small openings that ooze thick, sometimes blood-tinged material. New tracts can appear near areas that seemed to be healing, creating a pattern sometimes described as resembling a pepper pot. The skin between these openings often feels indurated and thickened.
Enlarging or coalescing nodules
Individual lumps grow larger or merge into broader areas of firm, ulcerated tissue, particularly over fat-rich areas such as the ventral abdomen or groin. The overlying skin may become fragile, discoloured, and prone to breaking down despite attempts at cleaning and bandaging.
Persistent cough or breathing changes
In disseminated cases affecting the lungs, an owner may notice a cough that does not resolve, increased effort during breathing, or reduced stamina during walks. These signs develop gradually and may be accompanied by ongoing weight loss.
Click to read about the biological mechanisms
How this is usually investigated
Investigation typically begins with the history of the lesion—how long it has been present, whether it has changed, and whether the animal has access to soil or water environments. Physical examination notes the character of any lumps, their texture, whether they are fixed to deeper tissues, and whether discharge is present. Sampling of affected tissue and laboratory culture are usually needed to confirm the identity of the organism and distinguish mycobacterial infection from other causes of chronic nodules or non-healing wounds.
Physical examination
Cytology
Histopathology
Bacterial culture and sensitivity
Radiography
Options & trade-offs
Management of atypical mycobacterial infection often involves a combination of approaches tailored to the extent and location of disease, the species identified, and what is practical for the individual animal and household. Treatment can be prolonged, often lasting many weeks to months, and response is variable. Some animals achieve resolution with medical management alone, whilst others benefit from surgical removal of localised lesions, and some experience recurrence despite intervention.
Antimicrobial therapy
Once culture and sensitivity results are available, combinations of antibiotics are selected based on the susceptibility pattern of the organism. Rapidly growing mycobacteria—such as Mycobacterium fortuitum, M. smegmatis, M. chelonae, and M. porcinum—are often susceptible to agents such as fluoroquinolones, doxycycline, azithromycin, or amikacin, though resistance patterns vary. Treatment is typically continued for weeks to months beyond clinical resolution, as the bacteria can persist within tissues even when lesions appear healed.
Trade-offs: Prolonged courses increase the risk of gastrointestinal upset, cost, and owner fatigue with daily administration. Not all organisms are susceptible to readily available oral antibiotics, and some cases fail to respond despite appropriate therapy.
Surgical excision
Removal of localised nodules or masses can be curative if the infection is confined to a single site and wide margins can be achieved. Surgery is often combined with antimicrobial therapy to address any bacteria that may remain at the margins or in surrounding tissues.
Trade-offs: Excision is less practical when lesions are numerous, poorly demarcated, or located in areas where wide removal would compromise function or healing. Recurrence at the surgical site or development of new lesions elsewhere can occur, particularly if the underlying exposure continues.
Wound management and supportive care
Draining tracts or ulcerated areas may benefit from gentle cleaning, warm compresses, and protection from self-trauma whilst antimicrobial therapy takes effect. Keeping the environment clean and limiting access to soil or standing water can reduce the chance of reinfection or introduction of additional organisms.
Trade-offs: Wound care alone does not address the underlying infection and is generally used alongside other measures. Some lesions continue to discharge or enlarge despite careful attention, and prolonged open wounds carry a risk of secondary bacterial contamination.
Monitoring and adjustment
Because response can be slow and unpredictable, repeat examinations are often scheduled to assess healing, check for new lesions, and decide whether to continue, change, or stop therapy. In cases where initial antibiotics are ineffective, culture may be repeated or alternative agents tried based on updated sensitivity results.
Trade-offs: The need for ongoing monitoring adds to the time and cost of management. Some animals experience partial improvement followed by relapse, and deciding when to stop treatment can be difficult without clear markers of bacterial clearance.
Common misconceptions
"Atypical mycobacterial infections are contagious and pose a risk to other pets or people in the household."
These organisms are environmental—found in soil and water—and infection arises from direct contact with contaminated material entering a wound, not from animal-to-animal or animal-to-person transmission. Routine hygiene is sufficient, and isolation is not required.
"All mycobacterial infections take many weeks to culture and diagnose."
Most cases of cutaneous mycobacterial infection in cats and dogs are caused by rapidly growing species that produce visible colonies in culture within three to seven days, allowing relatively prompt identification. Slowly growing mycobacteria do exist and can take weeks to months to culture, but these are more often associated with disseminated disease in certain dog breeds rather than the typical nodular skin presentation.
"Once the visible lesions have healed, treatment can be stopped."
The bacteria can persist within tissues even after lesions appear resolved, and stopping treatment prematurely often leads to recurrence. Most protocols continue antimicrobial therapy for several weeks beyond clinical healing to reduce the chance of relapse.
Related conditions
Sterile Nodular Panniculitis
Sterile nodular panniculitis can produce firm swellings in the fat layer beneath the skin that may resemble the nodules seen in atypical mycobacterial infection, particularly the panniculitic form seen in cats. The key distinction lies in culture: panniculitis is sterile, whilst mycobacterial infection yields organisms when samples are processed appropriately.
Soft Tissue Sarcomas
Soft tissue sarcomas can present as persistent lumps under the skin that may ulcerate or fail to resolve, a pattern that can overlap with the nodules and draining tracts of atypical mycobacterial infection. Biopsy and culture help distinguish tumour from infection, though both may require surgical intervention.
Cryptococcosis
Cryptococcosis is another environmental infection that can produce persistent nodules, draining wounds, or granulomatous inflammation in skin and other tissues. Both conditions may require prolonged treatment and can present diagnostic challenges, though the organisms involved and the approaches to culture and therapy differ.
Dermatophytosis (Ringworm)
Dermatophytosis can occasionally produce nodular or ulcerated skin lesions that fail to respond to routine antibiotics, particularly in the form known as kerion or pseudomycetoma. Culture is essential to distinguish fungal from mycobacterial infection, as the treatments required are entirely different.
Bartonellosis
Bartonellosis can cause chronic, sometimes granulomatous inflammation in skin and other tissues following penetrating injuries or contact with vectors, a context that shares some overlap with how atypical mycobacteria establish infection. Both conditions may require prolonged antimicrobial therapy and can prove difficult to culture or diagnose definitively.
If an infection has been confirmed and treatment is underway, it can be useful to think about the environments your animal has access to and whether any changes might reduce the chance of future exposure or reinfection. For animals with recurrent or slow-responding lesions, conversations about the balance between ongoing treatment and quality of life can help clarify what feels sustainable over the longer term. The immune and inflammatory health section offers context on how the body's defences interact with persistent infections, which may be relevant as you navigate the course of this condition.