Tuberculosis in cattle
Jai Bhagwan1 , Mahavir Chaudhari2, Sandeep Sangwan3
1Assistant Professor, Department of veterinary medicine, 2 Extension Specialist, Pashu Vigyan Kendra, Sirsa, Lala Lajpat Rai University of Veterinary and Animal Sciences, Hisar, 3 Assistant professor, Khalsa Veterinary College, Amritsar
Tuberculosis (TB) is an infectious granulomatous disease of animals and humans that is due to acid-fast bacilli of the genus Mycobacterium, particularly Mycobacterium tuberculosis complex (human and mammalian TB) and Mycobacterium avium complex (avian TB). The disease affects many species of vertebrates. The widespread occurrence of multidrug-resistant (MDR) strains and extensively drug-resistant (XDR) strains of M tuberculosis is of concern to clinicians and public health and regulatory officials involved in the control of disease. Bovine TB is still an important zoonotic disease in countries without control programs in cattle and without extensive milk pasteurization
Etiology: The main recognized types of the Mycobacterium tuberculosis complex (mammalian tubercle bacilli) are M tuberculosis, M canettii, M bovis, M caprae, M pinnipedii, M microti, M mungi, M suricattae, M africanum, and the yet-unnamed dassie bacillus and chimpanzee bacillus.
- The Mycobacterium aviumcomplex includes M avium avium (avian tubercle bacilli), M avium hominissuis (isolated from humans, swine, and other mammals), and M intracellulare, among other species. All Mycobacterium types may produce infection in host species other than their own.
- M tuberculosisis the most host-specific; it produces progressive disease only rarely in animals other than humans and nonhuman primates (mainly dogs, pigs, cattle, and elephants).
- M boviscan cause progressive disease in many mammalian species, including humans.
- M caprae, an organism closely related to M bovis, has been isolated from humans, goats, cattle, and several wildlife species in Europe.
- M avium aviumis the species of most importance in birds; however, it has a wide host range and is also pathogenic for pigs, cattle, sheep, deer, mink, dogs, cats, certain exotic hoofed animals, and some cold-blooded animals.
- M avium hominis suisis the cause of TB in immunocompromised humans, swine, and other animals.
- M intracellularecauses disease in cold-blooded animals and has been isolated from many other species.
Pathogenesis
Inhalation of infected droplets expelled from the lungs is the usual route of TB infection; however, the bacteria can also be ingested, particularly via contaminated milk, water, or food. Transmission via infective bites is common in badgers. Intrauterine and coital methods of infection are less common.
Inhaled bacilli are phagocytosed by alveolar macrophages that may either clear the infection or allow the mycobacteria to proliferate. In the latter instance, a granuloma may form, consisting of dead and degenerate macrophages surrounded by epithelioid cells, granulocytes, lymphocytes, and eventually, multinucleated giant cells. The purulent to caseous necrotic center may calcify, and the lesion may become surrounded by granulation tissue and a fibrous capsule (forming a “tubercle”). The initial granulomatous lesions in the organ and regional lymph node are known as the “primary complex.” Infection by the oral route can lead to a primary complex in the pharynx or mesenteric lymph nodes or, less commonly, in the tonsils or intestines. The cellular composition and presence of acid-fast bacilli in tuberculous lesions differ between and within host species. Dissemination through vascular and lymphatic channels may be generalized and rapidly fatal, as in acute miliary TB. Nodular lesions may form in many sites, including the pleura, peritoneum, liver, kidney, spleen, skeleton, mammary glands, reproductive tract, and CNS.
Clinical findings: Generalized clinical signs include progressive emaciation, lethargy, weakness, anorexia, diarrhea, 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 and draining abscesses may be present. Affected deeper lymph nodes cannot always be palpated; however, they may cause obstruction of the airways, pharynx, and gut, leading to dyspnea and ruminal tympany.
Diagnosis
- Cellular immune response
- Identification of the pathogen via culture or molecular tests
The single most important diagnostic test for TB is the intradermal tuberculin test. Purified protein derivatives (PPDs) prepared from the culture filtrate of M bovis or M avium. The delayed-type hypersensitivity response of the host, responsible for much of the pathology of TB, is fundamental to the tuberculin skin test widely used for diagnosis in large animals. For the single intradermal tuberculin test (SITT), the animal is inoculated with bovine PPD. In a reactor (ie, an animal who reacts to the test), 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 hours for maximum sensitivity and at 96 hours for maximum specificity. Test sites vary in sensitivity and by anatomic location; they include the neck region, caudal fold at the tail base, and vulval lip. One disadvantage of the bovine SITT is that cross-reactions may occur in animals infected with M kansasii, M avium, M tuberculosis, M avium paratuberculosis, or other mycobacteria that share some antigenic determinants with M bovis. In areas with a high incidence of avian TB or other mycobacterial infections such as paratuberculosis, the single comparative intradermal tuberculin test (SCITT) can be used, with biologically balanced bovine and avian PPD tuberculins inoculated simultaneously at separate sites in the neck.
Control: Test-and-cull and abattoir surveillance
Treatment: Because of zoonotic risk it is rarely attempted in pets and endangered species
Conclusion: The main reservoir of M bovis infection is cattle. However, bovine TB often persists in a multihost community that includes wildlife and other domestic animals. Control is particularly challenging when M bovis or M caprae is present in a multihost community. The main approaches to the control of TB in production animals are test-and-cull and abattoir surveillance. Test-and-cull relies on routinely scheduled testing, after which reactors to the intradermal tuberculin test or interferon-gamma assay are slaughtered. Alternatively, test-and-cull may be carried out prior to any animal or herd transport. In an affected herd, testing every 2 months is recommended to rid the herd of individuals that can disseminate infection.
Routine hygienic measures aimed at cleaning and disinfecting contaminated premises, as well as strict biosecurity, are useful. Test-and-slaughter is mandatory in many countries, in some cases leading to eradication. In some countries, where test-and-cull is impractical, test-and-segregate strategies have been used for control. No reliable TB vaccine is available for any species. Live attenuated, heat-inactivated, protein subunit, and DNA vaccines have all been developed; they are not administered, however—in part because they interfere with the diagnostic assays used in eradication programs.
