Post-mortem studies show that TB lesions in badgers can affect nearly all organ systems. In badgers that have gross (visible) lesions, the most frequently affected site is the lungs. The lymph nodes of the thorax (chest) and body are also frequently affected. All post-mortem studies also report finding infected bite wounds on some animals, with the proportion of badgers affected varying with the population studied and the time of year. Bite wounds occur mainly at the tail base or sides of the neck and are more frequently reported in male than female badgers. The proportion of badger reported with kidney lesions due to TB varies greatly between studies.
These findings correspond to the isolation of M. bovis (the pathogen that causes bovine TB) from the sputum, faeces (probably as a result of swallowing sputum), urine, bite wounds and draining abscesses.
The majority of badgers infected with M. bovis do not show any obvious clinical signs of TB. Based on the lesions found in badgers, clinical signs may include abnormally enlarged lymph nodes (particularly on the neck), bite abscesses, respiratory signs and in the small minority of cases that go on to develop advanced disease, emaciation and death.
A long-term study of a badger population involving individual identification of the badgers and regular capture/examination and release found that male badgers were more likely to become infected with TB than females and had a higher probability of disease progression. Male badgers also experience a more rapid increase in mortality rates following infection. Reasons suggested for these differences were the greater risk of bite wounds in males due to territorial behaviour, the observation that males range further than females putting them at more risk of encountering infection, and the possibility that males may have weaker immune responses to TB infection. Evidence from this study also indicates that the behaviour of infected badgers may differ from that of uninfected animals, although whether this is a consequence or a cause of infection is not clear.