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Tuberculosis is caused by Mycobacterium. Additional lessons about bacteria may be used as a supplement in this area. Mycobacterium is unique due to the high content of mycolic acid (waxy material) contained within its cell wall. This waxy substance inhibits the uptake of nutrients and causes the cells to clump. These factors contribute to the slow growth rate. Mycobacterium do not grow outside of a host except in cultured media. Mycobacterium multiplies approximately once every 20 hours. Do not get growth and survive confused. Mycobacterium can survive outside a host, but can not multiply.

Mycobacterium requires oxygen, is heat sensitive, and can be killed by a weak solution of common household bleach. It can remain viable for extended periods of time in cold weather. This is part of the reason that the baiting of deer has been eliminated. Infected deer feeding on a bait pile can cough out the bacterium which can then survive for extended periods of time and be ingested by other animals.

There are three common strains of Tuberculosis:

Mycobacterium bovis is the strain that we are finding in the deer and cattle in the upper northeast corner of Michigan's Lower Peninsula. Bovine TB can be transmitted from livestock to humans, deer, and other animals. It is not host specific. Bovine TB has the greatest range of hosts of any organism, and can infect all warm-blooded vertebrates.

Mycobacterium avium can affect all species of birds, and can also affect hogs and cattle.

Mycobacterium tuberculosis primarily affects humans, but can also be transmitted to hogs, cattle, and dogs.

Bovine TB can be transmitted from humans to animals, or vice versa. Young animals and humans can contract the disease by drinking raw milk from infected dams. Pasteurizing will kill the organism. The bacterium can be discharged through saliva and other discharges of infected animals. The most common means of transmission is respiration, i.e. coughed out.

Because the disease is spread by aerosol, tiny droplets that are coughed out and then inhaled by other animals or humans, animals kept in close contact with infected animals in enclosed areas like barns are at the greatest risk for exposure to bovine TB.

Once the bacteria has entered the host, the immune system recognizes it, and inflammatory cells called macrophages are sent to dispose of the bacteria. Mycobacterium is resistant to destruction. Once the bacteria have been ingested by the macrophage it may replicate and kill the macrophage. The animal immune system continues to send macrophages to help kill the bacteria. The bacteria continue to kill the macrophages. This results in the accumulation of living and dead macrophages at the site of the bacteria. This accumulation is called a tubercle. The formation of tubercles is where tuberculosis got its name.

Eventually a thick capsule may form around the tubercle; this is called a granuloma. The picture illustrates a pericardial granuloma.

The next step is the diagnosing of bovine TB. The Bovine Tuberculosis Testing Procedure ppt will outline the on-farm testing protocol. This power point will explain how TB is diagnosed in the lab, after a suspect animal has been identified by on farm testing.

Cattle that are suspected of being infected after the Caudal Fold Tuberculin (CFT) and Comparative Cervical Tuberculin (CCT) test are submitted to an animal diagnostic laboratory for necropsy. Necropsy is an animal autopsy. This necropsy consists of a gross examination (visual) as well as a histological (microscopic) examination.

Once an animal is submitted, it is closely examined for gross lesions suggestive of bovine TB. Gross lesions are visible to the naked eye. Lymph nodes are closely examined, as they are often sites of lesions. Multiple tissue samples are taken for histopathological examination.

The characteristic gross lesion of an animal infected with bovine TB would be the presence of tubercles within the body. These tubercles are a result of the immune system reaction explained above. A tubercle is generally white in appearance, 1mm-2cm in diameter and will appear within a lymph node or an organ. Due to the fact that TB is often spread through respiration, tubercles are commonly found in the thoracic cavity, but may also be fo9und in the liver or other major organs.

Due to the presence of mycolic acid within the cell wall, a special stain can be used to identify the bacteria. This stain, called acid-fast stain, allows for bacteria to be visualized. It may be a good supplemental exercise to have student's practice staining cells and then looking at them under a microscope. First tissue is ground into small pieces and then treated to destroy any non-mycobacteria. The sample is concentrated by centrifugation and applied to a microscope slide. The slide is then stained so that the mycobacteria can be seen.

If following the staining, the animal is suspect of being infected with bovine TB, attempts are made to grow Mycobacterium bovis from tissues samples that were collected during necropsy. Culturing must be done under specific conditions that favor the growth of the bacteria. Culturing usually takes 8 to 16 weeks due to the slow growth rate of the bacteria. A culturing lab could be performed so students can observe how cells replicate and grow in colonies.

A Polymerase Chain Reaction (PCR) is performed next. A PCR is used to detect the presence of DNA that is specific to the organism of interest. In other words, it makes sure that the DNA matches the DNA of Mycobacterium bovis. PCR amplifies the specific portion of DNA, which is then easily visualized using standard laboratory procedures. PCR is very sensitive and can detect a specific organism even if it is present at very low levels. The PCR is only used on tissues that have microscopic evidence of bovine TB. Results from a PCR are available within seven days. Positive results suggest that the animal is infected with TB, but culturing confirms if the strain is bovine TB or not.

The human disease is very similar to bovine TB is some ways, and very different in others. It is possible for a human to be infected with tuberculosis and not be aware of it. Due to the slow growth rate of the bacteria, the disease usually takes months to develop. It is possible for organism's to lay dormant within a host's body for its lifetime without every causing progressive disease. Infected humans who have not yet developed the active disease can be treated to prevent the disease from occurring.

The immune response in humans is very similar. The causative bacteria (this could include bovine TB) enter the body and settle in a clump on the host tissue. The bacteria multiply for several weeks, and then the immune system recognizes their invasion and an immune response begins.

The immune response is called "cell mediated". Cells of the lung begin to multiply to form a wall around the invading bacteria. This takes on the shape of a small tuber, tubercle. At this stage of infection the tubercle is commonly sealed off and transformed into scar tissue. The bacteria may become dormant and no longer infect the body.

The human host can continue to live normally with one or several dormant tubercles. As long as the immune system remains strong, the infection may be controlled and usually will not spread. If the immune system weakens, the tubercles will break open; pus will flow out and infect other regions of the lung and body with bacilli.

When infection occurs again, it begins to make holes in the lung. This forms many more tubercles, which develop into cavities or pits in the lungs. The pus from the infected lungs is often coughed up; this is referred to as "sputum". Anyone producing sputum from a tuberculous lung is considered infectious and may spread the disease to other people or animals.

Eventually the bacteria consume the lung and causes large lesions, which prevents the lungs from working properly. This can eventually lead to death in an untreated individual.

The Human TB in Michigan slide shows that of the 287 cases in 2000, 286 were caused by M. tuberculosis(human), and only one by M. bovis (a freign strain). In other words, the strain of TB that we are finding in the deer and cattle herds is not responsible for the cases of TB that we are finding in humans. Thus bovine TB remains an economic issue, and not one of human health. All cases of human TB in Michigan have been treated.