Percutaneous Interdigital Injection of Mycobacterium bovis as a Model for Tuberculous Lesion Development in Wild Brushtail Possums (Trichosurus vulpecula)

Abstract

Brushtail possums (Trichosurus vulpecula) are the major wildlife reservoir of Mycobacterium bovis, the causative agent of bovine tuberculosis (BTB), in New Zealand. Primary diagnosis of BTB in wild possums is by palpation to detect peripheral lymphadenomegaly followed by necropsy examination, which frequently identifies gross tuberculous lesions in the peripheral lymph nodes and lungs. Experimental infection studies were conducted with wild possums in an attempt to emulate field BTB, focussing on percutaneous administration of virulent M. bovis in the paws. In a preliminary study, viable M. bovis bacilli were recovered from lymph nodes draining fore- or hindlimbs 12 days after percutaneous injection. Subsequently, 21 wild possums were injected interdigitally with 500 colony forming units (cfu) of M. bovis, radio-collared and released; 17/18 possums recaptured 8 weeks later had an established M. bovis lymphatic infection, with 16 having culture-positive gross lesions in the superficial and/or deep axillary lymph nodes. A dual-site infection model was established, involving simultaneous interdigital injection of 100 cfu of M. bovis into front and rear paws of 19 wild possums; this identified that the average degree of lymphadenitis involved 30-fold enlargement of the draining lymph node by 7-8 weeks post injection (wpi). A time-course study demonstrated establishment of M. bovis infection in peripheral lymph nodes of 9/11 possums at 3-5 wpi of doses ranging from 60 to 190 cfu, but with no development of gross lesions; by 7 weeks, 8/8 animals injected similarly had both an established infection and gross lesions of peripheral lymph nodes. The incidence and progression of peripheral lesion development, together with indications of sequential infection of the lungs, liver and mesenteric lymph nodes(MLNs), indicates that a low-dose percutaneous M. bovis infection model is likely to emulate natural disease in possums.