The nature, cause, origin, and progress of Tuberculosis.
Tuberculosis is caused by three specific types of bacteria that are part of the Mycobacterium group:
- Mycobacterium bovis - M. bovis, can be transmitted from livestock to humans and other animals. No other TB organism has as great a host range as bovine TB, which can infect all warm blooded vertebrates.
- Mycobacterium avium - M. avium can affect all species of birds, as well as hogs and cattle.
- Mycobacterium tuberculosis - M. tuberculosis primarily affects humans but can also be transmitted to hogs, cattle, and dogs.
In general, disease-causing mycobacteria live only a few weeks outside a host's body because they cannot tolerate prolonged exposure to heat, direct sunlight, or dry conditions. Under cold, dark, and moist conditions, the organisms can survive longer. Mycobacteria do not grow outside of a host except in cultured media, where they multiply approximately once every 20 hours. Because of this relatively slow rate of growth, the disease usually takes many months to develop. In some instances, the organisms lie dormant within the host's body for its lifetime, both in animals and in humans, without causing progressive disease.
TB is a chronic disease, seldom becoming apparent until it has reached an advanced stage in cattle, captive cervids, and swine. Some infected livestock seem to be in prime condition, showing no evidence of infection until they are slaughtered, yet they may be found so seriously infected during slaughter inspection that their carcasses must be condemned.
TB lesions may be found in any organ or body cavity of diseased animals. In early stages of the disease, these lesions are difficult to find, even during post mortem examination. But in later stages, the nodules or lumps caused by bovine TB become every evident in the lungs and associated lymph nodes and in the lymph nodes of the head and intestinal tract. Lesions may also appear in the abdominal organs, reproductive organs, nervous system, superficial body lymph nodes, and bones.