Transmission of Tb is mainly by close contact between animals, with infected animals breathing or coughing out fine droplets laden with Tb bacteria. This bacterium is then inhaled by another animal.

Milk from heavily infected cows can be infectious, and historically unpasteurised milk was an important cause of infection for humans. 

Occasional cases in humans also occur following handling infectious animal material (such as hunters cleaning a carcass or skinning a possum), and bacteria entering the body via the mouth or an open wound.

Animals which die of Tb provide a source of infectious carrion for scavengers like pigs or ferrets.

New Zealand differs from most other countries in the world as the major source of Tb infection for domestic cattle and deer herds is from wild animals, primarily possums. This usually occurs through respiratory infection.

To find out more about the role of possums, click here.