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(Innes 1949; O’Reilly and Daborn 1995) but the incidence has significantly decreased with the introduction of eradication programmes in other species and the most common mycobacterial disease currently seen in horses is due to M. avium subsp. avium (Baker 1973; Mair et al. 1986; Buergelt et al. 1988; Lofstedt and Jakowski 1989; Gunnes et al. 1995; Pavlik et al. 2004). Mycobacterial infections are less frequently reported in donkeys and mules than in horses. Francis (1958) refers to Galtier (1900) who stated that cases of spontaneous tuberculosis have been observed in donkeys, but these cases were not characterised. Tuberculosis was suspected in a mule
Fig 3: Prominent interstitial fibrosis and granulomatous inflammation within the lung. H&E 3 40, scale bar 250 lm.
imported into Ireland in 1917 based on the gross post-mortem findings of miliary pulmonary lesions and caseous lesions in the bronchial and mediastinal lymph nodes (Kesten 1949). A case of granulomatous enteritis and lymphadenitis in which acid-fast bacilli presumed to be M. avium ssp. paratuberculosis were identified has been reported in a donkey, although the diagnosis was not confirmed by culture
(Dierckins et al. 1990). To the authors’ knowledge, this is the first confirmed case of M. bovis causing disseminated tuberculosis in a donkey. Clinical signs of M. bovis infection in horses are varied,
Fig 4: Pulmonary granuloma characterised by fibrous tissue surrounding aggregates of histiocytes and lymphocytes. H&E, 3100 scale bar 100 lm. Inset Ziehl-Neelsen, 3600 acid fast bacilli within lung tissue.
Growth was observed on Lowenstein-Jensen and Stonebrink’s media after 8 days which showed colony morphology typical for M. bovis. After subculturing, mycobacterial isolates were identified as M. bovis using PCR and reverse hybridisation (GenoType MTBC assay)5. To obtain more information about the molecular type of the isolated M. bovis, spoligotyping was carried out by standard methods (Kamerbeek et al. 1997). Results characterised the isolate as SB0140. Spoligotypes were named according to an agreed international convention (
www.mbovis.org).
Discussion
Infections caused by M. bovis are rare in equines and it has been suggested that an inherent resistance to infection may exist based on the observation that equine disease has remained uncommon in areas where the disease is prevalent in other species (Luke 1958). Historically, M. bovis was the most frequent cause of mycobacterial disease in horses
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depending on the distribution and severity of lesions (Pavlik et al. 2004), and nonspecific signs are often seen including chronic weight loss, lethargy, weakness, anorexia and pyrexia (Innes 1949; Luke 1958; Cousins 2001; Monreal et al. 2001). Other reported clinical signs include neck stiffness, lameness, nasal discharge, dyspnoea and coughing (Luke 1958; Monreal et al. 2001; Keck et al. 2010). In the current case, chronic disease may have been indicated by the history of recent weight loss; however, clinical signs of respiratory disease were acute in onset despite the presence of extensive chronic lung pathology. Compared with horses, donkeys are reported to have a reduced tendency to cough and this, in conjunction with their sedentary nature, means that lower respiratory tract infections of various aetiologies are often advanced by the time they present clinically (Thiemann 2012). This may account for the acute onset of respiratory signs and rapid deterioration in the current case. The post-mortem findings in the current case show many
similarities to the disease in horses. Mycobacterium bovis causes chronic disease in horses and lesions are often disseminated involving multiple organs including the lungs, spleen, liver, kidneys, intestine and lymph nodes (Innes 1949; Luke 1958; Cousins 2001; Monreal et al. 2001). Compared with lesions of M. bovis in other animals, chronic lesions of tuberculosis in the horse are frequently described as having a neoplastic appearance with little caseation or mineralisation and abundant fibrosis (Luke 1958; Caswell and Williams 2007) and this appears to be similar in the donkey. Extensive pulmonary fibrosis was a prominent histopathological feature in the current case and may be similar to the marked fibrotic reaction seen in other animals with increased resistance to M. bovis infection, such as horses (Caswell and Williams 2007). However, chronic fibrosis of the subpleural and interstital lung tissues is a common finding in donkeys at post-mortem examination (Thiemann 2012) and it must be considered that the pulmonary findings in the current case could be caused by concurrent idiopathic pulmonary fibrosis and M. bovis infection. Identification of the aetiological agent in the current case
was based on the presence of acid-fast bacilli within pulmonary lesions and culture of M. bovis from lung tissue
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