Table 2.

Hypothetical stages of mycobacterial infection and host–pathogen interactions in domestic and wild animals. Infection stages may vary in different species depending on the host’s immune response and risk factors, as well as strain and infecting dose of Mycobacterium bovis. (Adapted and adjusted from [25,204]).

bTB Stage		Clinical Signs	Host–Pathogen Interactions
Uninfected		Absent	-No immune response, and no viable bacteria
Infected	Cleared infection	Absent	-Innate and/or adaptive immune system clears the bacteria -There may be no memory response, or the immune memory response may last for some time after the pathogen has been eliminated -No viable bacteria present
	Latent infection	Absent	-Detectable immune responses present -Viable and slow or non-replicating bacteria possibly present
	Subclinical infection	Absent	-A transition state from latency or initial/early infection to active disease -Viable and slow or non-replicating bacteria likely present along with a small proportion of actively replicating populations -Microbiological evidence of culturable bacteria -Radiographic, gross, or histopathological evidence of infection
	Active TB disease	Present	-Viable and replicating bacteria present -Clinical signs (e.g., coughing, weight loss, and lymphadenopathy) -The individual may be infectious (intermittent shedding) -It is relatively easy to find culturable bacteria depending on the location of infection and the type of samples used for analysis