Disseminated Mycobacterium avium infection in young cats: overrepresentation of Abyssinian cats

Abstract

Disseminated Mycobacterium avium-intracellulare complex (MAC) infection was diagnosed in 10 young cats (1-5 years of age) from Australia or North America between 1995 and 2004. A further two cats with disseminated mycobacteriosis (precise agent not identified) were recognised during this period. Of the 12, 10 were Abyssinian cats, one was a Somali cat and one was a domestic shorthair cat. None of the cats tested positive for either FeLV antigen or FIV antibody. The clinical course of these infections was indolent, with cats typically presenting for weight loss, initially in the face of polyphagia, with a chronicity of up to several months. Additional clinical features included lower respiratory tract signs and peripheral lymphadenomegaly. A marked diffuse interstitial pattern was evident in thoracic radiographs, even in cats without overt respiratory involvement. Hair clipped to perform diagnostic procedures tended to regrow slowly, if at all. Diagnosis was generally made by obtaining representative tissue specimens from mesenteric lymph nodes, liver or kidney at laparotomy, or from a popliteal lymph node. The primary antecedent event was most likely colonisation of either the alimentary or respiratory tract, followed by local invasion and eventual lymphatic and haematogenous dissemination. Nine cases were treated using combination therapy with agents effective for MAC infection in human patients. Two cats are still undergoing initial therapy and have responded. Of the remaining seven, all responded during long courses (5-14 months) of clarithromycin combined with either clofazimine or rifampicin, and a fluoroquinolone or doxycycline. Of these, three cats remain well (with durations between 2 months and 2 years following therapy); two developed recurrent disease (at 3 months and 2 years, respectively, following therapy) and have restarted therapy. The remaining two cats improved 1 year and 5 months, respectively, after diagnosis but ultimately succumbed. The two cats in which therapy was restarted have improved dramatically. Certain lines of Abyssinian and Somali cats likely suffer from a familial immunodeficiency that predisposes them to infection with slow-growing mycobacteria such as MAC.

Fig 1

Lateral thoracic radiographs of cases 2, 3 and 8 (a, b and c, respectively), demonstrating a severe diffuse interstitial pattern which was remarkably similar in each patient. This pattern was characteristic of lung involvement in Abyssinian cats with disseminated M avium infections and presumably reflected haematogenous spread from a primary focus to the pulmonary parenchyma.

Fig 2

Case 9 photographed 5 weeks after an exploratory laparotomy but before combination antimicrobial therapy. Note the poor regrowth of hair that had been clipped for surgery.

Fig 3

Low power photomicrograph (a) of the enlarged popliteal lymph node of case 1. The normal lymph node architecture has been completely effaced by sheets of epithelioid macrophages, with only scattered lymphocytes remaining. Haematoxylin and eosin (H & E) stain; scale bar=100 μm. A high power photomicrograph (b) of the same lymph node, stained using the Ziehl-Neelsen method, showing the less common scenario of enormous numbers of intracellular acid-fast bacilli (staining pink with the carbol fuchsin) in the majority of macrophages. Such multibacillary lesions were less common in our series than paucibacillary lesion. Note that the acid-fast bacilli are short, averaging 3 μm in length. Scale bar=5 μm.

Fig 4

Case 2, 2 years after completing therapy. Note the excellent body condition and the normal hair coat.

Fig 5

Low power photomicrograph (a) of an enlarged lymph node from case 8. Lymph node architecture has been partially effaced by sheets of epithelioid macrophages. Only a remnant of the normal cortical histological architecture remains. H & E stain; scale bar=100 μm. Higher power Ziehl-Neelsen stained section (b) of the same lymph node showing the more common paucibacillary reaction pattern; only one acid-fast bacillus (stained red, just left of centre) can be seen; scale, bar=10 μm.