24 Mar 2020
Francesco Cian discusses the case of an outdoor eight-year-old domestic shorthair cat, seen by a referring veterinarian for a well-circumscribed, non-ulcerated, hairless, cutaneous lesion.
Francesco Cian
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Figure 1.
An outdoor eight-year-old domestic shorthair cat was seen by the referring veterinarian for a well-circumscribed, non-ulcerated, hairless, cutaneous lesion – 1.5cm in diameter – that had been present on the cat’s prescapular area for a few months.
The cat was, otherwise, clinically well.
A fine needle aspiration of the mass was performed as part of the diagnostic process. The aspirate harvested moderate numbers of inflammatory cells – mostly segmented neutrophils and macrophages – on a basophilic background, with small numbers of red blood cells and a few bare nuclei (Figure 1).
Macrophages appeared as large mononuclear cells with moderate to abundant cytoplasm, sometimes vacuolated with an oval and eccentric nucleus. Most of these cells contained numerous small, rod-shaped, negatively stained structures (Figure 1; Wright-Giemsa, 100×, red arrows).
Similar structures were also found free in the background of the slide. Their morphology and appearance were considered highly suggestive of Mycobacterium species and confirmed by acid‑fast staining, which was positive with bacteria retaining the primary stain and appearing red/pink in colour (Figure 2; Kinyoun stain, 100×).
PCR testing was also performed and identified Mycobacterium lepraemurium – one of the main causative agents of feline leprosy syndrome (FLS).
The diagnosis was FLS caused by M lepraemurium.
Mycobacterium species infection in cats can be divided into three main groups:
Tuberculosis complex (TB): caused in cats by Mycobacterium bovis, Mycobacterium microti and Mycobacterium tuberculosis. Depending on the route of infection, affected cats present with localised cutaneous lesions (for example, the face, extremities and tail base) or, less commonly, systemic signs with involvement of internal organs. TB is potentially zoonotic – it occurs worldwide, including the UK, and particularly in areas of cattle, badger and rodents carrying mycobacterial infections.
FLS: caused by a number of mycobacteria, including M lepraemurium. It often causes single or multiple, cutaneous or SC solid lesions, often on the head and/or limbs. Sources of infection can include rodent bites and environmental contamination of wounds. Local lymphadenopathy can occur in later stages of the disease, but systemic involvement is rare as the disease usually has an indolent clinical course. It occurs worldwide – particularly in temperate maritime climates, including the UK. Zoonotic potential is negligible.
Non-TB or atypical mycobacteria: caused by multiple mycobacterial species, particularly M avium. The latter typically causes disease in birds, but can also infect dogs, cats and humans – particularly if immunosuppressed. It is very pathogenic and often presents as cutaneous/SC lesions or, less commonly, disseminated disease. This form can mimic cutaneous TB or FLS and has zoonotic potential.
Clinically, about a third of feline mycobacteriosis cases in the UK are caused by M tuberculosis complex pathogens, with M microti being cultured from 19% of all cases of feline mycobacteriosis and a further 15% caused by M bovis.
A recent outbreak of M bovis in pet cats across the UK has been reported in the literature and “results provided compelling, if circumstantial, evidence of an association between the commercial raw diet of these cats and their M bovis infections”.
Cytopathology and histopathology are only able to provide a generic diagnosis of mycobacterial infection, and, as aforementioned, each form of mycobacterial infection is different in terms of clinical progression, treatment, prognosis and zoonotic potential. Therefore, further investigations are needed and, until the organism is identified, it should be considered a potential zoonosis.
Specific tests include bacteriological culture and PCR. Culture testing requires fresh/frozen tissue, which can be placed in a sterile container and submitted to a reference laboratory. Unfortunately, these organisms can take a long time to culture (months) and some species may fail to grow. Therefore, molecular methods are often preferred to identify the causative species. However, PCR may lack in sensitivity – especially if few mycobacteria are present.
Other diagnostic tests – for example, the interferon gamma release assay – are also available. Treatment options depend on the type of mycobacteria involved, and the extent and severity of the infection.
Treatment of localised cutaneous forms of FLS – as seen in this case – often requires surgical removal of the lesion and specific, long-term antibiotic therapy. Rare cases undergoing spontaneous resolution have been described.
Cytology is often able to provide a presumptive diagnosis of mycobacterial infection when negatively stained intracellular rods are present within the macrophages. However, further diagnostic investigations are required, as different mycobacteria have different zoonotic risks, different treatment options and dissimilar prognoses.
Animal health legislation requires that vets who examine the carcase of a pet mammal suspected of being affected with TB must notify the APHA without delay. The suspicion of TB in a living pet is not currently notifiable.
