Feline leprosy due to Mycobacterium lepraemurium: Further clinical and molecular characterisation of 23 previously reported cases and an additional 42 cases

Abstract

Objectives:

This paper, the second in a series of three on ‘feline leprosy’, provides a detailed description of disease referable to Mycobacterium lepraemurium, the most common cause of feline leprosy worldwide.

Methods:

Cases were sourced retrospectively and prospectively for this observational study, describing clinical, geographical and molecular microbiological data for cats definitively diagnosed with M lepraemurium infection.

Results:

A total of 145 cases of feline leprosy were scrutinised; 114 ‘new’ cases were sourced from the Victorian Infectious Diseases Reference Laboratory records, veterinary pathology laboratories or veterinarians, and 31 cases were derived from six published studies. Sixty-five cats were definitively diagnosed with M lepraemurium infection. Typically, cats were 1–3 years of age when first infected, with a male gender predilection. Affected cats were generally systemically well. All had outdoor access. Lesions tended to consist of one or more cutaneous/subcutaneous nodules, typically located on the head and/or forelimbs, possibly reflecting the most likely locations for a rodent bite as the site of inoculation for organisms. Nodules had the propensity to ulcerate at some stage in the clinical course. The cytological and histological picture varied from tuberculoid, with relatively low bacterial numbers, to lepromatous with moderate to high bacterial numbers. Treatment was varied, although most cats underwent surgical resection of lesions with adjunctive medical therapy, most often using a combination of oral clarithromycin and rifampicin. Prognosis for recovery was generally good, and in two cases there was spontaneous remission without the requirement for medical intervention. Untreated cats continued to enjoy an acceptable quality of life despite persistence of the disease, which extended locally but had no apparent tendency to disseminate to internal organs.

Conclusions and relevance:

M lepraemurium causes high bacterial index (lepromatous) or low bacterial index (tuberculoid) feline leprosy. The infection typically causes nodules of the skin and/or subcutis (which tend towards ulceration) on the head and/or forelimbs. The disease usually has an indolent clinical course and infected cats have a generally favourable response to therapeutic interventions, with rare cases undergoing spontaneous resolution. Genomic analysis may yield clues as to the environmental niche and culture requirements of this elusive organism. Prospective treatment trials and/or additional drug susceptibility testing in specialised systems would further inform treatment recommendations.

Introduction

Initial reports of ‘feline leprosy’ were likely dominated by Mycobacterium lepraemurium infections, and cat-to-cat inoculation studies fulfilled ‘Koch’s postulates’ for disease causation. Lesions had a propensity to be sited on the head and/or forelimbs, and were nodular in nature involving the skin and subcutis, with a tendency to ulcerate as the disease progressed. The histological appearance varied from tuberculoid disease, comprising a mixture of lymphocytes and macrophages, usually associated with relatively small numbers of acid-fast bacilli (AFB), to lepromatous, comprising predominantly epithelioid macrophages or giant cells associated with numerous, generally intracellular, AFB. There was a tendency for tuberculous lesions to develop central caseous necrosis, which grossly was correlated with an ulcerated appearance.

Many different approaches to treatment have been tried over the years, with the anti-leprosy agent clofazamine perhaps having the best efficacy in vivo, while rifampicin has documented efficacy in vitro. M lepraemurium is extremely difficult to grow in the laboratory, although a few specialised laboratories in Japan^1,2 and Belgium ³ have managed to propagate this organism in vitro, permitting limited susceptibility testing and other studies.

This paper, the second in a series of three, concerns patients with skin lesions proven to be caused by M lepraemurium and represents the largest collection of such cases reported to date. Feline leprosy case series published prior to the turn of the century could not determine if a mixture of different aetiological agents, including M lepraemurium, but also other mycobacterial patho gens such as Myco bacterium visibile, Candidatus ‘Myco bacterium tarwinense’ and the novel species Candidatus ‘Myco bac terium lepraefelis’, were involved in these cutaneous feline mycobacterial infections.

Materials and methods

Case recruitment, demographics, data retrieval, cytological and histological assessment, molecular microbiological methods, genetic analysis, mycobacterial culture and statistical analysis were essentially the same as described in the first paper in this series, and interested readers are referred to that publication for information. ⁴ different primers were used to partially sequence the sodA gene of M lepraemurium: ‘AvsodAF’ 5’-ACATCTCGGGGCAGATCA-AC-3’ (forward) and ‘AvsodAR’ 5’-GTAGTCCGCCTTGACGTTCT-3’ (reverse).

Results

Clinical data

A total of 145 cases of feline leprosy were analysed; 114 were sourced from the Victorian Infectious diseases Reference Laboratory (VIdRL) records, veterinary pathology laboratories or veterinarians, with an additional 31 cases derived from six published studies.^5–10 Of this total, 65 had M lepraemurium infection, comprising 42 ‘new’ cases and 23 cases recorded previously in the literature (Table 1).

Table 1.

Detailed case data for 65 cats infected with Mycobacterium lepraemurium

Case	Age at diagnosis (years)	Gender	Breed	Location	Lifestyle	Bacterial index	Clinical details	Retroviral status	Treatment	Outcome	Reference or source
1	2	MN	DSH	Williamstown, VIC, Australia	NR	High	Nodule on right upper lip	NR	Surgical resection, medical treatment (details NR)	NR	This study
2	1.5	MN	NR	NR	NR	NR	NR	NR	NR	NR	This study
3	1	M	DSH	Glenfield, New Zealand	NR	NR	NR	NR	NR	NR	This study
4	2	MN	Burmese	Paddington, NSW, Australia; lived previously in New Zealand	Moved to Sydney from New Zealand within 6 months prior to presentation; had caught rats in New Zealand	High	Lesion on right lateral forelimb	NR	Surgical resection; CLFZ and CLM for 10–11 weeks	Resolved (4 year follow-up)	This study
5	1	MN	DSH	NR	NR	NR	NR	NR	NR	NR	This study
6	NR	MN	NR	Albany, WA, Australia	NR	NR	NR	NR	NR	NR	This study
7	1.5	MN	NR	TAS, Australia	NR	NR	Lesion on commissure of lip	NR	NR	NR	This study
8	0.75	FN	DSH	New Zealand	NR	High	NR	NR	NR	NR	This study
9	NR	NR	NR	Whangarei, New Zealand	NR	NR	NR	NR	NR	NR	This study
10	1	MN	DSH	Auckland, New Zealand	Previously lived in sand dunes, Omaha Beach, New Zealand	NR	Excoriated lesions over rump and in axillae	NR	Surgery	Resolved	This study
11	2	MN	DSH	Auckland, New Zealand	NR	NR	Multiple cutaneous masses on lateral aspect of left tarsus, lateral aspect of nail bed, and deep SC tissues of right shoulder	NR	NR	NR	This study
12	3	MN	DSH	Wellsford, New Zealand	History of cat fights	NR	Two non-healing wounds after fighting – left metacarpal region and left upper lip	NR	Surgery, CLM, ENFLX	Resolved	This study
13	1	FN	DLH	Rotorua, New Zealand	NR	High	Small cystic mass on right forelimb	NR	Surgery	Resolved	This study
14	1.5	FN	DSH	Launceston, TAS, Australia	Farm mouser	NR	Ulcerated lesion on left lip over upper canine; lesion on upper right gum	NR	Surgery	Resolved	This study
15	3.5	MN	DSH	Christchurch, New Zealand	Hunts and lives along a river bank	High	Lesions on right forelimb and dorsal carpus	NR	Surgery, RIF, CLM	Resolved	This study
16	2	FN	DSH	Christchurch, New Zealand	Prolific hunter and lives close to a stream	Low	Ulcerated lesion below left lower eyelid; rim of lesion thickened with large central ulcerated area	NR	Surgery	Resolved	This study
17	4.5	MN	DSH	Tauranga, New Zealand	Indoor/ outdoor rural; no history of fights	High	Large nodular lesion on forelimb	NR	ENFLX and DOXY, 5 months – no change. RIF and CLM, 3 months – no change	Euthanased after 1 year once lesion started to enlarge	This study
18	3	MN	DSH	New Lynn, Auckland, New Zealand	NR	NR	NR	FIV –ve	NR	NR	This study
19	2	MN	NR	Kawerau, New Zealand	NR	NR	NR	NR	NR	NR	This study
20	2.5	MN	DSH	Opotiki, New Zealand	Rural area; indoor/outdoor, hunter	High	Single hard lesion on chin	FIV –ve FeLV –ve	RIF, CLM	LTFU	This study
21	4	FN	DSH	North Shore, Auckland, New Zealand	Suburban	High	Smooth domed lesion on face	FIV –ve FeLV –ve	Surgery, DOXY before and after	Resolved	This study
22	0.75	M	Manx cross	Glenfield, New	NR	NR	Multiple raised nodules on both forelimbs	NR	NR	NR	This study
23	2	MN	DSH	Wanganui, New Zealand	NR	Low	Periorbital lesion	NR	No treatment	Spontaneously resolved within a few weeks	This study
24	3	MN	DSH	Wanganui, New Zealand	NR	High	Multiple lesions on distal limb	NR	No treatment	Spontaneously resolved within a few weeks	This study
25	0.75	M	DSH	Milford, New Zealand	NR	NR	Ulcerated skin mass on left flank	NR	NR	NR	This study
26	1.3	FN	DSH	New Plymouth, New Zealand	Indoor/ outdoor	NR	Skin nodule on hindlimb; enlarged draining popliteal lymph node	NR	Surgical removal and 2 weeks of ENFLX	Resolved	This study
27	14	MN	DSH	Launceston, TAS, Australia	NR	Low	Multiple disseminated nodules	FIV +ve	RIF and CLM, 2–3 months	Resolved	This study
28	14	NR	Ragdoll	New Zealand	NR	NR	Keratitis	NR	NR	NR	This study
29	1.5	FN	DSH	Whangarei, Northland, New Zealand	Farm cat, hunts and fights	NR	Anterior aspect of forelimb; inguinal lymph node	NR	Surgery, DOXY, 1 month	Resolved (2 year follow-up)	This study
30	2	FN	DSH	Havelock North, New Zealand	Rural	Low	Ulcerated mass on right carpus; eight smaller masses on left side of body	FIV –ve FeLV –ve	Surgery, DOXY, 1 month	NR	This study
31	1.5	MN	DSH	New Norfolk, TAS, Australia	Rural	High	(Prior to skin lesions, received corticosteroids for eosinophilic granuloma.) Disseminated cutaneous nodules	NR	RIF and CFZ	Resolved	This study
32	3.5	MN	Persian	Gladesville, NSW, Australia	NR	High	Ulcerated lesions on right forelimb digit pad and dew claw region; lesions on cranial antebrachium and lateral shoulder. Prescapular lymph node enlarged	NR	RIF and CLM	NR	This study
33	10	MN	DSH	Snug, TAS, Australia	Mainly outdoor	High	NR	NR	CLM, 1 month	NR	This study
34	1.5	MN	DSH	Vineyard, NSW, Australia	Outdoor, hunter	Low	Ulcerated lesions on lip and chin	FIV –ve FeLV –ve	RIF, CLM, 6 months	Resolved	This study
35	3	MN	Maine Coon	Old Beach, TAS, Australia	Hunter	High	Ulcerated lesion on right upper lip	NR	CLM, RIF, 6 months	Resolved	This study
36	1.5	MN	DSH	Dural, NSW, Australia	Outdoor, hunter	NR	Two lesions on right forelimb and one on lateral upper right eyelid	NR	No response to cefovecin, or DOXY. CLM and surgical resection, followed by RIF	NR	This study
37	NR	FN	DSH	Kingston, TAS, Australia	Semi-rural	NR	Oral cavity, face and limbs covered in multiple raised, alopecic, erythematous, well demarcated dermal masses (approximately 30 lesions)	FIV –ve FeLV –ve	RIF, CLM	Apparent cure, but lesions returned after diagnosis of renal disease	This study
38	5	FN	British Shorthair	Balmoral, New Zealand	NR	NR	NR	NR	NR	NR	This study
39	2	FN	Ragdoll	Auckland, New Zealand	Suburban; hunts in adjacent bush reserve	NR	Five nodular lesions, 3–6 mm in diameter. Four were ulcerated. One was medial to the left pinna. Two lesions were on the caudal mandibular area. One lesion was axillary, another was inguinal	NR	DOXY, MOXIFLX	Resolving. Still undergoing treatment at time of writing	This study
40	0.6	FN	DSH	Auckland, New Zealand	Previously feral	NR	Lesions on face	FIV –ve	DOXY, MOXIFLX	Resolving. Still undergoing treatment at time of writing	This study
41	0.6	FN	DSH	Auckland, New Zealand	Previously feral	NR	Lesions on face	FIV –ve	DOXY, MOXIFLX	Resolving. Still undergoing treatment at time of writing	This study
42	2	MN		Bellerive, TAS, Australia		NR	Nodules range from 3–15 mm in diameter, affecting head, feet, legs and trunk	NR	CLM, RIF	Still undergoing treatment at time of writing	This study
43	1.5	ME	DSH	Narraweena, NSW, Australia	Suburban	Low	Left and right axillae and antebrachium, progressing to generalised skin with ulceration	FIV –ve	Surgery, CLFZ, CLM, RIF	No response, euthanased	Case 1 5
44	3	MN	DSH	Winston Hills, NSW, Australia	Suburban	Low	‘Localised’	NR	Extensive surgery, DOXY	LTFU	Case 3 5
45	3	M?	DSH	New Zealand	NR	High	‘Tumour’	NR	NR	NR	Case 5 5 Case 3 11
46	8	F?	NR	New Zealand	NR	High	Domed forehead	NR	NR	NR	Case 6 5 Case 4 9
47	1	F?	DSH	New Zealand	NR	Low	Lesion on left shoulder	NR	NR	NR	Case 7 5 Case 5 9
48	2	M?	DSH	New Zealand	NR	Low	Generalised skin lesions	NR	NR	NR	Case 8 5 Case 7 9
49	2.5	MN	DSH	British Columbia, Canada	NR	Low*	Ulcerated mass on scrotum	NR	NR	NR	Case 14 6
50	2	MN	Siamese	British Columbia, Canada	NR	Low*	Two nodules on skin	NR	NR	NR	Case 16 6
51	NR	ME	DSH	New Zealand	NR	Low*	Three masses – flank, hindlimb, chest wall	NR	NR	NR	Case 12 6
52	1.5	MN	DSH	British Columbia, Canada	NR	Low*	Lesion on right carpus	NR	NR	NR	Case 1A 6
53	1	FN	NR	British Columbia, Canada	NR	Low*	Chronic ulcerated mass on leg, second small mass	NR	NR	NR	Case 7A 6
54	7	MN	DLH	British Columbia, Canada	NR	High*	Lesion on lower eyelid	NR	NR	NR	Case 18 6
55	3	NR	NR	British Columbia, Canada	NR	High*	Two masses (location not specified)	NR	NR	NR	Case 20 6
56	3	FN	NR	British Columbia, Canada	NR	High*	Large mass on forelimb, several other nodules	NR	NR	NR	Case 23 6
57	2	FN	DSH	Kythira, Greece	Hunter	High	Ulcerated nodules on commissures of mouth; enlarged lymph nodes	FIV –ve FeLV –ve	2 months MARBFLX, no response. Surgery plus 14 weeks of CLFZ and CLM	Resolved	Courtin et al 7
58	3	ME	DSH	Nimes, France	Outdoors	High	Four lesions on back and abdomen	NR	Marginal excision	Resolved	Case 2 8
59	2	ME	DSH	Marseille, France	Outdoors	High	Five lesions on forelimb, hindlimb, back, popliteal lymph node	NR	NR	NR	Case 3 8
60	2	ME	DSH	Nimes, France	Outdoors	High	Multiple lesions on face, forelimb, paw pads, scrotum	NR	RIF, ENFLX, DOXY	Resolved	Case 4 8
61	1	FE	DSH	Nimes, France	Outdoors	Low*	Lesion on face	NR	Marginal excision, DOXY	Resolved	Case 5 8
62	1	MN	DSH	Nimes, France	Outdoors	High	Lesion on head	NR	Marginal excision, DOXY	Resolved	Case 7 8
63	2	MN	DSH	Aix-en- Provence region, France	Outdoors	Low*	Lesion on forelimb	NR	Marginal excision, amoxicillin, cefalexin	Resolved	Case 8 8
64	3	M	DSH	New Caledonia	Outdoors	Low*	Multiple lesions on forelimb, lymph nodes	NR	CLFZ, cefovecin	Resolved	Case 9 8
65	2	MN	DSH	Marseille, France	Outdoors	Low	Lesion on caudoventral abdominal skin	NR	Marginal excision, cephalexin	Resolved	Case 10 8

F = entire female; FN = female spayed; M = entire male; MN = male castrated; NR = not recorded; ? = neuter status unknown; DSH = domestic shorthair; DMH = domestic mediumhair; DLH = domestic longhair; VIC = Victoria; QLD = Queensland; NSW = New South Wales; TAS = Tasmania; WA = Western Australia; BI = bacterial index; FIV = feline immunodeficiency virus; FeLV = feline leukaemia virus; +ve = positive; –ve = negative; CLFZ = clofazamine; RIF = rifampicin; CLM = clarithromycin; DOXY = doxycycline; ENFLX = enrofloxacin; MARBFLX = marbofloxacin; MOXIFLX = moxifloxacin; PRDFLX = pradofloxacin; SC = subcutaneous

^*Although not directly stated, bacterial numbers were presumed from the pathological description of ‘tuberculoid’ as likely being low bacterial index (BI), with ‘lepromatous’ likely being high BI by definition

Cats were generally young, with a median age of 2 years, although older cats (up to 14 years) were occasionally infected. The preponderance of young cats was striking, with the great majority of affected cats being less than 4 years of age. Where gender was recorded, 43/62 (69%) M lepraemurium infections were in male cats, which is substantially more than would be expected based on the age/gender pyramid for Australian cats. ¹² Where breed was recorded, 44/54 were domestic shorthair, 2/54 were domestic longhair, two were Ragdoll, and there was one each of Burmese, Manx cross, Maine Coon, Siamese, Persian and British Shorthair; the crossbreed to pedigree ratio was similar to that of the Australian feline population. ¹²

Geographical location data were recorded for 63 cats; 16 from Australia, 31 from New Zealand (Figure 1), seven from Canada, seven Zealand (Figure 1), seven from Canada, seven from France, one from the Greek Island of Kythira and one from New Caledonia. M lepraemurium infection was documented to have a wider geographical distribution than any of the other species responsible for feline leprosy, and was the only causative species of this condition in the Australian island state of Tasmania. Although they could be encountered in almost any geographical location, M lepraemurium infections appeared to be overrepresented in coastal cities and rural areas.

Figure 1.

Geographical distribution of feline infections caused by M lepraemurium in the Australasian region, where accurate location data was known. Note the prominence of cases in coastal areas

Where lifestyle was recorded, all cats had unsupervised outdoor access and many were known hunters and/or fighters. Feline immunodeficiency virus (FIV) status was only recorded in nine cats, with just one cat being antibody-positive. None of the nine cats tested for feline leukaemia virus were seropositive.

Where clinical features and clinical course were well documented, 34/49 (69%) cats had a few lesions, with only 4/49 cases having generalised nodular skin disease; the remainder had multiple lesions at disparate anatomical sites. Lesions were mainly on the head (21/49 [43%] cases) and forelimbs (23/49 [47%]) (Figures 2 and 3). No cats with this aetiology underwent necropsy examination. Of the 40 cases where cytology or histology was available for review, a high bacterial index (BI) was observed in 23/40 cases (58%), while the remainder had a low BI. In many cases where a low BI was recorded, the lesions were ulcerated; however, ulceration was also occasionally observed as a clinical feature of high BI lesions.

Figure 2.

(a–c) Representative photographs of M lepraemurium infection in cats, chosen to illustrate the occurrence of lesions on the head

Figure 3.

(a,b) Representative photographs of M lepraemurium infection in cats, chosen to illustrate the occurrence of lesions on the forelimbs

Treatment and outcome data

Treatment protocols used in cats in this study varied widely and often were influenced by owner finances and commitment, and also the patient’s temperament.

Where it was possible to implement medical therapy, protocols included several months of treatment with one or more of the following drugs: rifampicin, clarithromycin, clofazimine, a fluoroquinolone (enrofloxacin, marbofloxacin, moxifloxacin or pradofloxacin) and doxycycline. A combination of rifampicin and clarithromycin was used most commonly, mainly due to favourable cost and drug availability. Many cats were treated with surgical resection alone (often with incomplete margins due to the anatomical location of the lesions; eg, eyelids, distal limbs) or, more commonly, a combination of surgical resection and adjunctive medical therapy.

The outcome was generally favourable for cats with M lepraemurium infections, with resolution in 24/27 cases (89%) where sufficient follow-up was available to make an assessment, even with multiple or generalised lesions. disease burden did not appear to influence outcome for this infection, nor did the BI of the lesions. Two cats were euthanased due to progressive or persistent cutaneous lesions, rather than evidence of systemic disease (cases 17 and 43). Spontaneous resolution was observed in two cats infected with M lepraemurium (cases 23 and 24).

A summary of data concerning age, gender, anatomical distribution of lesions and outcome for cats infected by M lepraemurium is provided in Table 2.

Table 2.

Summary data concerning age, gender, anatomical distribution of lesions and outcome for cats infected by M lepraemurium

			Anatomical location of lesions
Median age at diagnosis (years)	Male	High BI	Head	Forelimbs	Hindlimbs	Body	Tail/perineum	Disseminated skin	Subjective outcome – resolved
2 (n = 61)	69% (43/62)	58% (23/40)	43% (21/49)	47% (23/49)	14% (7/49)	12% (6/49)	4% (2/49)	8% (4/49)	89% (24/27)

BI = bacterial index

Microbiological and molecular data

Fresh tissue, and fixed cytological and paraffin-embedded samples from 37 cases underwent dNA extraction, PCR amplification and sequencing (see supplementary material). Known acquired mutations in the rpoB gene conferring rifampicin resistance in other mycobacterial species^13–15 were not found in any of the isolates in this study (data not shown).

Mycobacterial dNA derived from fresh biopsy tissue from case 36 underwent draft genome sequencing during the course of the study. The methods and results of these investigations will be published separately (C o’Brien et al, manuscript in preparation).

Discussion

This study greatly extends the body of knowledge concerning M lepraemurium infections in cats.

Anatomical predilection

This organism has a predilection to involve structures about the head and/or forelimbs, but generally without involvement of ocular tissues or periocular skin. This would be consistent with the theory that this organism is likely a pathogen inoculated into a feline predator via the bite of a rodent suffering from systemic mycobacteriosis. ¹⁶ This is in contrast to Candidatus ‘M tarwinense’ infections, where lesions often involve the ocular tissues (conjunctiva, cornea, eyelids, nictitating membrane), and nasal and periocular skin and subcutis as a result, perhaps, of inoculation via inter-cat aggression or selfinoculation during grooming.

Signalment

The age range of affected cats is wide (<1 to 14 years); however, the vast majority of cases occurred in young adult cats (<4 years old), presumably because cats of this age group are more likely to roam, explore and exploit territory for hunting. Additionally, older cats may acquire resistance to the organism after asymptomatic exposure at a younger age. Similar factors may also influence the male predilection, as this group, even if castrated, generally roam over a larger area and are more aggressive than female cats. The breakdown of domestic crossbreeds vs pedigree cats is in accord with their ratio in the overall Australian cat population, without any suggestion of immunocompromise in certain pedigree cats playing a role in predisposing to infection.

There were too few cats subjected to retroviral testing to draw any firm conclusions, but with only 1/9 cats tested being FIV-positive, it appears unlikely that acquired immunodeficiency is an underlying predisposition, especially considering the young age and apparent good health of the majority of cats. Indeed, the epidemiology suggests a localised disease of a low-grade pathogen in an immunocompetent patient, with a mycobacterial species somewhat adapted to mammalian hosts, but much more likely to produce systemic disease in rodents than in Felidae.

Clinical course

The clinical course of M lepraemurium in the majority of cats could be perhaps described as indolent, with no suggestion of internal organ involvement in any case based on clinical criteria. Occasional cases have widespread, generalised cutaneous nodules, suggestive of haematogenous spread of the organism. The authors have also observed a limited number of cases in which there has been what appears to be sporotrichoid spread (ie, via the lymphatics from the primary focus of infection).

Organism biology

This biological behaviour is consistent with an environmental saprophyte adapted to a rodent host. The pathogen is less adept at living in a feline patient, and infected cats thus represent an incidental ‘dead end’ host.

Analysis of five gene targets in M lepraemurium demonstrated that all clinical isolates were remarkably clonal at the five loci examined. If this homogeneity was carried over the entire genome, the variation in clinical severity from cat to cat would most likely reflect the number of organisms inoculated and/or variability in the innate and acquired immunological response of the feline host. The apparent genetic homogeneity of M lepraemurium is reminiscent of two other fastidious mycobacterial species – the unnamed agent of the canine leproid granuloma syndrome, which appears to be clonal over an extensive worldwide geographical range, and Candidatus ‘M tarwinense’, which appears to be clonal, but over a far more restricted region. ¹⁷

Treatment and outcome

As is typical of an observational study, treatment regimens were not consistent. However, cats tended to respond favourably, especially to surgical excision and long-term use of a combination of two or more antimicrobials usually effective against slow-growing mycobacteria (see box above).

Risk of contagion

To date, there has been no instance of direct spread from cat to cat. Although it is theoretically possible to effect this using infectious material in a contrived experimental setting, in the authors’ view the risk of contagion from cats is slight. It is fascinating that, to date, M lepraemurium infections have not been recorded in dogs, ferrets or native wildlife.

Key Points

• M lepraemurium, a fastidious mycobacterium related to M avium, is capable of causing both high BI (lepromatous) or low BI (tuberculoid) ‘feline leprosy’. It is likely that infections follow an altercation with rodents that inoculate infectious propagules into the dermis or subcutis of the feline patient.

• Lesions generally consist of nodules (which tend towards ulceration) present on the head and/or forelimbs, with or without local lymph node involvement.

• Cases tend to have an indolent clinical course and a generally favourable response to therapeutic interventions.

• The best management approach consists of complete surgical resection of easily accessible lesions, and/or debulking of lesions in difficult anatomical locations, with adjunctive medical therapy using two or more antimicrobial agents with known activity against slow-growing mycobacteria.

• The inability of the organism to grow using both traditional and unconventional mycobacteriological media remains a mystery, although it is hoped that detailed examination of the draft genome will yield some clues.