Given the relationship between IFN-γ expression and chlamydial infection, the low levels of IFN-γ expression seen in our vaccinated koalas could possibly be explained by the low to no detectable levels of C. pecorum. Therefore, from this data, it appears that IFN-γ is not a direct marker for infection or disease and does not seem to be responsible for the resolution of infection, in these ocular diseased koalas. Here, IFNG is linked to chlamydia trachomatis infectious disease.