Transmission and Development of Bovine TB
The disease primarily affects the respiratory tract but can also spread to other parts of the body. The primary route of transmission is the exchange of respiratory secretions between infected and uninfected animals. This can be achieved through nose-to-nose contact or by the inhalation of aerosol droplets that have been exhaled by an infected animal. Animals may also become infected with M. bovis by ingesting the bacteria. This could occur by ingesting feeds that have been contaminated with M. bovis by other infected animals. Carnivores may become infected with bovine TB by eating infected carcasses.
Various factors affect the efficiency in which M. bovis is spread within a cattle herd. The number of infected animals shedding the organism, as well as the number of susceptible animals present within a herd can have an impact on the transmission of the bacteria. For example, the more animals within a herd that are shedding M. bovis, the greater the chance of an uninfected animal coming into contact with an infected animal. The animal density of a herd also influences the efficiency of M. bovis transmission. Transmission of M. bovis among animals housed in confinement facilities may be greater because of close contact. Cattle infected with bovine TB may shed bacteria in their feces, urine and milk, but these are felt to be a minor source of bacterial transmission.
Environmental contamination with M. bovis may play a role in the spread of bovine TB. Survival of M. bovis in the environment is primarily affected by exposure to sunlight. Reports on the length of survival of M. bovis vary from 18-332 days at temperatures ranging from 54-75 F. Under laboratory conditions, M. bovis has been isolated for up to 8 weeks from various feeds kept at 75 F and 14 weeks from various feeds kept at 32 F. However, under field conditions, it is difficult to isolate M. bovis from pastures grazed by animals known to be infected with bovine TB.
Tuberculosis is due to a small bacterium known as Mycobacterium. There are many different types of Mycobacterium but the two most important, M. tuberculosis and M. bovis are the only ones which can be spread between humans and animals. Examples of other types of Mycobacterium include M. avium, M. kansasii and M. fortuitum. Many of these can infect other animals and may also be recovered from soil and water. For example, M. avium organisms are widespread in nature and have been grown from water, soil, plants, and other environmental sources. Bacteria are very small and cannot be seen without using a microscope. Scientists look at bacteria to see their size, shape, internal structures and to see how they move. This information allows the laboratory scientist to make an early determination about what kind of bacteria are present. To see them with a microscope, bacteria must be stained so that they have more contrast and can be more easily seen.
The Mycobacterium are unique among the bacteria because they have a lot of waxy material in their cell walls. Because of the waxy material (known as mycolic acid), the usual stains for looking at bacteria with a microscope do not work. The mycolic acids give the Mycobacterium the ability to hold onto special bacterial stains, allowing them to be seen with a microscope. The special stain is called an acid fast stain and the Mycobacterium will be red while the other non-Mycobacterium will be blue.
Disease caused by mycobacteria often develop very slowly and may take months to years to develop. These bacteria grow very slowly and only replicate every 12-20 hours. While the pathogenic species (those which can cause disease) such as M. tuberculosis and M. bovis can infect a human or an animal, often the individual does not know that he has been infected. In humans, those infected but without active disease can be treated to prevent disease from occurring. To prevent other animals from being infected, a diseased animal must be culled from the herd.