Answer to June 2019 Photo Quiz

Answer: Epidural abscess with osteomyelitis of the frontal bone due to Mycobacterium bovis. The organism was identified from the direct sample as a Mycobacterium tuberculosis complex organism susceptible to rifampin by Xpert MTB/RIF assay (Cepheid, Sunnyvale, CA, USA). The patient was started on a four-drug antituberculous regimen of isoniazid, rifampin, pyrazinamide, and ethambutol.

Seven days later, the organism grew in Bactec mycobacterial growth indicator tube (MGIT) liquid medium (BD, Franklin Lakes, NJ, USA). The isolate was identified as an M. tuberculosis complex organism by means of a lateral-flow immunochromatographic assay based on the detection of the MPT64 antigen (BD MGIT TBc identification test). Testing for susceptibility to first-line drugs was performed using the Bactec method (BD Bactec MGIT 960 SIRE kit). The strain was resistant to pyrazinamide and susceptible to all other first-line antituberculous drugs. When the results of the susceptibility studies were reported, the regimen was changed to isoniazid, rifampin, linezolid, and moxifloxacin until completion at 2 months. Once the induction phase was completed, the regimen was changed to isoniazid and rifampin for 4 months. Currently, the patient in still on treatment.

Since Mycobacterium bovis is intrinsically resistant to pyrazinamide (1), the strain was sent to a reference laboratory in order to repeat susceptibility testing using the agar proportion method and for species identification using gyrB and hsp65 PCR-restriction fragment length polymorphism (RFLP)-based analysis and multiplex PCR to determine the presence/absence of the RD9 and RD1 regions (2–4). The isolate was finally identified as non-BCG M. bovis.

M. bovis is a member of the M. tuberculosis complex and is the primary cause of tuberculosis in cattle. It may also cause human disease in the setting of ingestion of infected dairy products or via airborne-particle inhalation (5). Human-to-human transmission is rare. When the infection is acquired via ingestion of contaminated products, extrapulmonary localization is more likely than pulmonary disease. Extrapulmonary sites include lymph nodes (6), pleural space (7), joints (8), and the central nervous system (9).

Regarding the acquisition of M. bovis infection in this case, the patient did not report contact with infected animals or humans. The extrapulmonary localization of the infection suggests that consumption of unpasteurized products could have been the route of the transmission acquisition. Although the possibility of consumption of imported contaminated dairy products could not be excluded, locally produced dairy products were unlikely to be contaminated with M. bovis. The characteristics of the patient (elderly and born in Spain) suggest reactivation of latent infection rather than a primary infection.

Finally, we postulate that the patient’s impaired cellular immunity due to both the immunosuppressive medication and the unexplained persistent lymphopenia might have contributed to the reactivation of this latent infection.

See https://doi.org/10.1128/JCM.00648-18 in this issue for photo quiz case presentation.