Actinomycosis of the jaw (“lumpy jaw”) may be the most common infectious disease in 
kangaroos and wallabies. While also seen in wild kangaroos, it is much more common in zoo animals, presumably because commonly provided grass feed can lead to gum disease thus providing a nidus for infection. The organism, always present in the oral cavity as part of the normal mouth flora, penetrates into deeper layers and causes destruction of local tissues and bone.
Importantly, the infection interferes with the ‘molar progression’ whereby teeth migrate from the back towards the front of the mouth, which is the normal process of tooth eruption and dental renewal in kangaroos [1].
There is still some confusion whether the fairly aggressive disease – it can present as local destructive disease but also with bacteremia which is often fatal – is actually caused by actionmyces alone or in concert with other mouth pathogen. Diagnosis based on stains of the lesion cannot definitively differentiate actinomyces from nocardia or fusobacteria, all pathogens often found in the oral cavity together.[2] Hence, a synergistic infection with several bacteria may be responsible.
References:
[1] http://www.2ndchance.info/raisepossum-Johnson-Delaney2006.pdf
[2] P Ladds. Pathology of Australian Native Wildlife. Csiro Publishing 2009
http://books.google.com/books?id=CR-MEFTR0PMC&pg=PT169&lpg=PT169&dq=kangaroo+actinomycosis&source=bl&ots=frGwrZ603L&sig=tAnfLydQAygwNRy-OtzL4HTZHL4&hl=en&sa=X&ei=2jgPVJi4KOa1sQSNkIGoAw&ved=0CDEQ6AEwAzgK#v=onepage&q=kangaroo%20actinomycosis&f=false
