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EQUINE VETERINARY EDUCATION / AE / APRIL 2018


discharge and exercise intolerance (Lyashchenko et al. 2012). Differential diagnoses for pulmonary infection may include neoplasia, pleuropneumonia or Rhodococcus equi infection in foals. Haematogenous dissemination of infection results in advanced disease with the appearance of miliary or nodular tubercles in all the viscera and lymph nodes; this results in rapid deterioration due to multiple organ failure. Significant hepatopathy resulted in signs of hepatic encephalopathy in the case reported here, demonstrating again the variability of clinical presentation. Pulmonary tuberculosis should be considered in the differential diagnosis for horses presenting with respiratory distress, an inflammatory profile and diffuse multinodular pulmonary densities, particularly if there is concurrent immunosuppression.


Zoonotic concerns of equine tuberculosis


Horses with pulmonary disease are most commonly affected by M. bovis; M. tuberculosis infection is rare (Lyashchenko et al. 2012). We are not aware of any reports of other members of the M. tuberculosis complex as a cause of tuberculosis in horses. Direct transmission of either M. bovis or M. tuberculosis infection from horse to handlers has not been confirmed, but post-mortem evaluation of infected animals does carry a significant zoonotic risk as opening the thorax, abdomen or cranium may aerosolise infected droplets. Three veterinary personnel acquired M. tuberculosis following post- mortem examination of a dog with central nervous system infection (Posthaus et al. 2011). Horses suspected to be positive for tuberculosis should be reported to the APHA, and appropriate care taken if undertaking a post-mortem procedure. On confirmation of a horse with tuberculosis, the relative exposure risk should be determined for all personnel and livestock that have been in contact due to the implications of zoonotic infection. An interferon-gamma release assay (IGRA) is the test of choice to run on blood samples from in-contact human subjects: this interferon is increased if peripheral lymphocytes have been sensitised to M. tuberculosis complex antigens (Posthaus et al. 2011). Chest radiographs should be obtained from human patients with a positive test result in order to exclude active TB.


Diagnostic investigation


The positive confirmation of mycobacterial infection in horses ante mortemrequires proactive inclusion of this infection on the differential diagnosis list and a logical approach to comprehensive evaluation. Biochemical and haematological evaluation of equids presenting with tuberculosis disease caused by M. bovis is not specific but is likely to reveal a chronic inflammatory response in both alimentary and respiratory disease, plus hypoalbuminaemia, hyperglobulinaemia and variable increases in hepatic enzymes in the former depending on extent of organ infiltration. Detection of features such as thickening of the wall of the


small or large intestine and mesenteric lymph node enlargement (M€


onki et al. 2016) or granulomatous hepatic/


splenic densities on ultrasound examination of the abdomen should increase the suspicion of mycobacterial involvement. Similarly, detection of multiple areas of pulmonary consolidation on thoracic ultrasound should increase the suspicion of pulmonary bTB or MAC involvement, and thoracic radiographs should be obtained. Detection of either


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Fig 1: Lateral radiograph of the caudodorsal lung of a pony presenting with dyspnoea and pyrexia, in which a diffuse alveolar pattern is present. Histopathological examination of a tru-cut biopsy with Ziehl–Neelsen staining was used to rule out mycobacterial involvement in this case and the pony was treated subsequently for interstitial pneumonia.


an alveolar pattern or focal mass on thoracic radiographs, in combination with the clinicopathological features as described, should further increase the suspicion of mycobacterial involvement (Fig 1). Histopathological examination of a rectal biopsy with


Ziehl–Neelsen staining for acid-fast bacteria may be an efficient means of confirming alimentary bTB, but false negatives are common. Nonetheless, histopathological detection of multinucleated giant cells and lymphocyte and plasma cell aggregates in the lamina propriae mucosae (Sarradell et al. 2015) may be further indicative of bTB involvement. In the report by Charlesworth (2018), hepatic biopsies were collected ante mortem and histopathology revealed pyogranulomatous inflammatory foci containing acid-fast bacilli, thus confirming the pony’s zoonotic potential. In the case of miliary pulmonary TB in equids, acid-fast


bacilli may be detected in a tracheal wash or bronchoalveolar lavage fluid specimen, although sensitivity is likely to be much higher if a tru-cut lung biopsy is submitted for histopathology. The latter approach has the advantage of ruling out other differential diagnoses such as neoplasia or equine multinodular pulmonary fibrosis in the case of nodular lung disease. Tracheal wash specimens (or pleural effusions if present) should also be submitted for mycobacterial culture if characteristic radiographic changes have been noted. The major clinical limitation of mycobacterial culture is that 4–6 weeks may be required for positive results due to the slow doubling time of this organism (Gazi et al. 2015). Concurrent culture of secondary bacterial pathogens may also confuse the interpretation of initial results. Positive ante mortem confirmation of mycobacterial


infection may therefore be difficult in horses. The intradermal tuberculin test is not reliable in this species (Konyha and Kreier 1971) and serological response to infection has not been fully elucidated.


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