Definition
This is a chronic, granulomatous disease of swine, poultry, wild birds, many domestic and wild animals and people.
Occurrence
Tuberculosis (TB) occurs frequently in man, domestic and wild animals, poultry and wild birds. It occurs in almost all vertebrates and in some cold-blooded animals. In the US, the disease in domestic species and people occurs less often than in developing countries. The disease is present worldwide throughout the year.
In 1995, the incidence of TB condemnations in 100 million swine slaughtered in federally inspected abattoirs of the US was 0.003%.
Historical information
In the United States, TB once was a common disease of farm poultry flocks, cattle, swine and people. Swine often were exposed to other infected species or materials contaminated by them.
A major effort to eradicate tuberculosis in cattle and in people markedly reduced incidence of the types of tuberculosis usually seen in cattle and people. Changes in poultry production largely eliminated the raising of farm poultry flocks, which was the usual source of exposure of swine. A traceback system identified the source of infected swine for slaughter and reduced infection at points of origin. As a consequence of these measures, TB in swine was reduced substantially in incidence. Reduction was achieved without an eradication program directed specifically at swine or poultry.
Increasing prevalence of TB in immunocompromised people, coupled with resistance of tuberculosis to many therapeutic agents, is now of great public health concern. Avian type TB once was believed to be rare in people but more recently some of the serovars of avian type TB found in confinement-raised swine have been found in people. At slaughter, those serovars have caused marked condemnation losses to pork producers. These developments may translate into increased concern for TB in both swine and poultry.
Etiology
Classification of the tuberculosis bacteria (mycobacteria) is incomplete; a numerical system of classification is emerging but classical terminology is used here.
As a genus, mycobacteria are Gram-positive, acid fast bacilli. They are highly resistant to most environmental factors, many disinfectants and a large and increasing number of antibiotics. Avian type mycobacteria (Mycobacterium avium) have survived for more than four years in poultry lot soil, in cages, and in sawdust used as litter.
The three Mycobacterium species that cause tuberculosis in swine are Mycobacterium avium, Mycobacterium bovis and Mycobacterium tuberculosis. These three species infect poultry, cattle and people, respectively. However, each of these three occasionally infects other animal species.
Mycobacterium avium usually is the organism that infects swine. There are 19 known serovars. In the US serovars l, 2, 4 and 8 are the most common isolates from swine. Serovars 4 and 8 have been found in poultry, large confined swine herds and people.
Epidemiology
The marked resistance of mycobacteria to environmental factors makes contaminated premises a long-term threat to swine. The TB organisms usually are spread to swine by tuberculosis-infected poultry, wild birds, cattle or people, or by soil or bedding materials contaminated by them. The route of exposure usually is by ingestion although spread by inhalation occurs occasionally. Infectious feeds that have spread TB bacilli to swine include offal from infected poultry and cattle, uncooked garbage, unpasteurized milk and dairy products. Exposure of swine to lots contaminated by the feces of tuberculosis-infected poultry often has led to their infection by M. avium.
Other less common but proven routes of exposure include exposure to serovars of avian type contained in sawdust or wood shavings used as bedding, congenital exposure of fetuses of infected, pregnant sows, and animal-to-animal exposure among swine. Some infected swine have TB lesions in their tonsils or intestine and shed the organisms in their feces.
Pathogenesis
The complex lipid nature of the organisms may explain the difficulties that body defenses have in destroying mycobacteria though phagocytosis and oxidative killing. In mononuclear phagocytes, toxic lipids in the mycobacteria, or factors released by them, appear to interfere with phagolysosome formation or to inactivate lysosomal enzymes. If macrophages fail to destroy ingested mycobacteria, they can carry them to other sites. There, the bacteria are liberated upon death of the macrophages and released organisms set up new foci of infection.
In swine, TB bacilli appear to infect the tonsils and intestinal mucosa initially and then spread to the regional lymph nodes, especially those of the cervical area, less often to mesenteric nodes. Lesions in the nodes tend to develop slowly and, in most cases, the bacilli are successfully walled off. Only occasionally does the infection generalize, usually in older breeding stock infected with M. bovis.
Clinical signs
In swine with localized TB infections, there usually are no signs. Since most swine are slaughtered at an early age, there is little time for generalization of infection and the appearance of signs. Condemnation of carcasses at slaughter is the usual presentation. In older breeding stock, TB generalization does occur occasionally. Affected animals tend to waste away despite adequate feed. They eventually become emaciated and may die from tuberculosis.
Lesions
In localized TB, grossly visible lesions usually are apparent only in cervical, submaxillary or mesenteric lymph nodes. Lesions vary considerably in appearance, depending on the type of infection. Localized avian and human type infections tend to produce enlarged, firm nodes with no discrete purulent foci. Calcification is seldom apparent and lesions are not easily enucleated. Nodes affected by M. bovis tend to be well encapsulated, relatively easy to enucleate, and there often is marked calcification in the lesions.
Mycobacterium bovis tends to generalize more readily than do other forms of tuberculosis. Lesions in the organs vary considerably. Initially, the lesions are miliary and widely distributed. Those in the lungs tend to develop along pleural and septal lymphatics and resemble dewdrops. Those in the liver tend to have caseated or liquefied centers. Splenic lesions occur commonly and, when well developed, they often protrude. Serous membranes seldom are affected.
It is not possible to accurately identify tuberculosis, or the particular variety of tuberculosis on the basis of gross or microscopic lesions. Lesions, including discrete granulomas containing acid-fast organisms, can be caused by other agents and are easily mistaken as tuberculosis.
Diagnosis
A presumptive diagnosis often can be made on the basis of history, study of gross and microscopic lesions and the presence of acid-fast bacteria within discrete granulomas or granulomatous tissue. However, errors will be made. Firm, unequivocal diagnosis can be made only after isolation, identification and typing of the bacteria. Polymerase chain reaction (PCR) and immunohistochemistry (IHC) are increasingly useful for confirmation.
Control
Prevention is by avoiding exposure of swine to TB bacilli (see under Epidemiology). Eradication of avian TB would reduce the likelihood of exposure to TB bacilli but is not possible. Raising either swine or poultry in confinement can reduce the most common exposure of swine to infection. Conversely, close confinement of swine with careless exposure to wild birds or poultry can provide an opportunity for greater exposure. All swine, as well as bedding, should be protected from birds.
No effort should be made to treat TB in swine.