Mycobacterium Fortuitum Infection Interference with Mycobacterium Bovis Diagnostics: Natural Infection Cases and a Pilot Experimental Infection

Abstract

Mycobacterium fortuitum and at least 1 unidentified species of soil mycobacteria were isolated from lymph nodes from 4 of 5 African buffalo (Syncerus caffer) that had been culled because of positive test results using the Bovigam assay. The buffalo were part of a group of 16 free-ranging buffalo captured in the far north of the Kruger National Park (South Africa) assumed to be free of bovine tuberculosis. No Mycobacterium bovis was isolated. To investigate the possible cause of the apparent false-positive diagnosis, the Mycobacterium isolates were inoculated into 4 experimental cattle and their immune responses monitored over a 13-week period, using the gamma interferon assay. The immune reactivity was predominantly directed toward avian tuberculin purified protein derivative (PPD) and lasted for approximately 8 weeks. During that period 3 of 4 cattle yielded positive test results on 1 or 2 occasions. The immune responsiveness was boosted when the inoculations were repeated after 15 weeks, which led to 2 subsequent positive reactions in the experimental animal that did not react previously. Including an additional stimulatory antigen, sensitin prepared from M. fortuitum in the gamma interferon assay, showed that it was able to elicit a detectable gamma interferon response in all 4 experimentally inoculated cattle when applied in parallel with bovine and avian tuberculin PPD for the stimulation of blood samples. The implications of occasional cross-reactive responses in natural cases of infection with environmental mycobacteria in the diagnosis of bovine tuberculosis in African buffalo and cattle in South Africa are discussed.

Approaches towards optimising the gamma interferon assay for diagnosing Mycobacterium bovis infection in African buffalo (Syncerus caffer)

“…We have recently reported the isolation of environmental mycobacteria from infected and uninfected buffalo as well as from surface water (Michel et al, 2007;Michel, 2008a) and some of the false positive reactivity in the buffaloes examined in the present study may have been caused by antigenic cross-reactivity with mycobacteria other than tuberculosis. Further optimisation of the IFN␥ test validity was therefore pursued in this study by examining 'a priori' exclusions which allow for some bovine reactors to be classified as test negative, based on the level of reactivity to avian or fortuitum PPD.…”

“…The decision what error rate in either direction is acceptable, entirely depends on the specific epidemiological setting and management objective. In wildlife farming operations where buffaloes are individually identified and accessible for testing, it is a further possibility to diagnose and separate multiple reactors from nonresponsive buffaloes and re-test them after eight weeks, in accordance with the kinetics of the cross-reactivity caused by exposure of buffaloes to non-tuberculosis mycobacteria (Michel, 2008a). According to our observations, uninfected buffaloes, previously non-specifically sensitized with the latter then no longer have detectable levels of IFN␥ above the cut-off value, irrespective of the antigen used for stimulation.…”

“…We have previously observed false positive test results in freeranging buffaloes when using the standard protocol for the gamma interferon assay. We have further established that false positive test results may be caused by sensitisation of the animals with environmental mycobacteria (Michel, 2008a). Subsequently the commercial assay was modified into a triple comparative test set-up.…”

Carcinoma de células escamosas en linfonódulo mandibular diagnosticado a la inspección en matadero como tuberculosis bovina

“…Lo anterior deja en evidencia que el diagnóstico diferencial de tuberculosis bovina versus otras patologías, incluyendo el diagnóstico diferencial con otros miembros el género Mycobacterium, son de capital relevancia al momento de la inspección en matadero (Michel 2008). Por tal motivo, es importante a futuro esclarecer el diagnóstico fidedigno y certero de los principales hallazgos mal diagnosticados como lesiones tuberculosas en matadero, por cuanto se genera un conocimiento valioso en cuanto a la estadística relacionada a las principales patologías que se confunden con lesiones tuberculosas, así como también para aquellos que realizan inspección en matadero.…”

“…Con los antecedentes anteriormente expuestos, se puede inferir que el diagnóstico erróneo de la enfermedad ocurre, aun cuando en la actualidad existen otros métodos de diagnóstico mucho más sensibles y específicos, que van desde el uso de la tuberculina, pasando por la histopatología y llegando en la actualidad a la técnica de reacción en cadena de la polimerasa (PCR) o el inmunoensayo de Interferón Gamma (Michel 2008, Michel y col 2010.…”