Tuberculosis (TB) is an infectious, granulomatous disease caused by acid-fast bacilli of the genus Mycobacterium . Although commonly defined as a chronic, debilitating disease, TB occasionally assumes an acute, rapidly progressive course. The disease affects practically all species of vertebrates, and before control measures were adopted, was a major disease of humans and domestic animals. Bovine TB is still a significant zoonosis in many parts of the world. Signs and lesions are generally similar in the various species.
Tuberculosis in cattle is usually due to MYCOBACTERIUM BOVIS but the avian form is ocasionally found. This is a very important disease worldwide. Signs vary with the location of the lesions. To safeguard public health affected cattle are slaughtered.
The clinical signs reflect the extent and location of lesions plus the underlying toxemia. Generalized signs include progressive emaciation, lethargy, weakness, anorexia, and a low-grade, fluctuating fever. The bronchopneumonia of the respiratory form of the disease causes a chronic, intermittent, moist cough with later signs of dyspnea and tachypnea. The destructive lesions of the granulomatous bronchopneumonia may be detected on auscultation and percussion. Superficial lymph node enlargement may be a useful diagnostic sign when present. Affected deeper lymph nodes cannot always be palpated, but they may cause obstruction of the airways, pharynx, and gut, leading to dyspnea and ruminal tympany.
In pigs, lesions caused by M avium avium are most often seen in lymph nodes associated with the GI tract, although generalized disease does not occur.
The single most important diagnostic test for TB is the intradermal tuberculin test. Diagnosis on clinical signs alone is very difficult, even in advanced cases. Radiography is useful in nonhuman primates and small animals. Microscopic examination of sputum and other discharges is sometimes used. Necropsy findings of the classic tuberculous granulomas are often very suggestive of the disease. Confirmation of diagnosis is by isolation and identification of the organism, with culture usually taking 4-8 wk, or by PCR, which requires only a few days.
The delayed-type hypersensitivity response of the host, responsible for much of the pathology of TB, is fundamental to the tuberculin skin test that is widely used for diagnosis in large animals. The single intradermal (SID) test involves inoculation of mycobacterial antigen prepared from a filtrate of cultures of either M bovis or M tuberculosis . Purified protein derivative (PPD) preparations of the mycobacteria improve specificity.
In a reactor, the antigen stimulates a local infiltrate of inflammatory cells and causes skin swelling that can be detected by palpation and measured by calipers. The reaction is read at 48-72 hr for maximum sensitivity and at 96 hr for maximum specificity. Test sites used vary in sensitivity and between countries and include the neck region, anal or caudal fold at the tail base, and vulval lip. One disadvantage of the M bovis SID test is its poor specificity, with cross-reactions occurring in animals infected with M avium , M tuberculosis , M paratuberculosis , and even Nocardia spp .
In areas with a high incidence of either avian TB, atypical mycobacteriosis, or paratuberculosis, the comparative test can be used, with biologically balanced M bovis and M avium PPD tuberculins inoculated simultaneously but at separate sites in the neck. The agent causing sensitization provokes the greater skin reaction. Other diagnostic tests used for TB include the thermal test, which may detect a pyrexic peak (104F [>40C]) at 6-8 hr after SC inoculation with tuberculin. The Stormont test uses an intradermal inoculation of PPD followed by a second inoculum at the same site 7 days later. The test is read for swelling 24 hr later.
False-negative results may occur in animals with poor immunity such as those in the early stages of infection, anergic cases in advanced disease, or old animals. Cattle that have recently calved may also have false negative results. Current research is focused on the identification of antigens such as secretory proteins of M bovis for use in improved diagnostic tests. Serologic tests such as ELISA appear to be of limited diagnostic use, consistent with the lesser role of antibody compared with the cellular immune response in TB. In vitro cellular assays have been developed (ie, interferon-? assay) using blood lymphocytes stimulated with M bovis antigen and show promise as an alternative to the widely used SID test; however, they have not come into widespread use.