Bovine Tuberculosis Control Programs in South Africa
Bovine tuberculosis in South Africa is currently controlled according to the guidelines contained in the Interim Bovine Tuberculosis Manual, dated October 6, 2016. This manual will remain in use until such time that the Tuberculosis Advisory Group constituted to deal with this matter formulates a new control strategy for BTB in South Africa.
The objectives of tuberculin testing are multifold and include the detection, control, and eventual eradication of BTB from a country. In South Africa, the single cervical tuberculin test (SIT) is used to screen herds for the presence of BTB. With this test, if any animal shows a reaction of more than 6 mm increase in the skin thickness at the test site, the entire herd is retested after 3 months using the CCT for increased specificity.
The following programs are currently employed to control BTB in South Africa:
• Maintenance program
• Diagnostic program
• Infected herd program
Participation in the maintenance and diagnostic programs is voluntary, while the infected herd program is compulsory. That means, if a BTB-infected herd is detected in any of the test programs, the herd is automatically incorporated in the “infected herd” program. Further tests and other actions required for eradication then become compulsory, and these are enforced in conformity with Act 7 of 2002 (the Animal Health Act).
In South Africa, the SIT is currently used for screening or diagnostic purposes in BTB-free herds. In all herds with a confirmed BTB-positive or suspect BTB status, the CCT is used. Briefly, the CCT involves the preparation of two injection sites about 12-15 cm apart, on either side of the mid-neck by shaving the skin with a clipper, and measuring of the skinfold thickness with a calibrated caliper, followed by the intradermal injection of 0.1 ml of avian tuberculin PPD containing 2500 IU/ ml at the cranial site and 0.1 ml of bovine tuberculin PPD containing 3000 IU/ml at the distant site using preset automatic McLintock syringes.
The skin reactions are evaluated after 72 h, and the skinfold thickness is remeasured by the same operator. For interpretation of the skin test result, the standard OIE method (World Organisation for Animal Health 2008) is used according to which an animal is considered positive if the increase in skinfold thickness at the bovine injection site is at least 4 mm and is at least 4 mm greater than at the avian site, inconclusive when the reaction difference is from 1 to 4 mm, and negative when the bovine reaction is less than or equal to the reaction to the avian PPD. In addition to measuring the extent of the reaction, changes in the skin at the site of the reaction such as the presence of necrosis, edema, and pain are assessed since the physical characteristics of this reaction also influence the interpretation of the test. At the discretion of the responsible state veterinarian, the commercial IFN-γ assay (Bovigam®) may be used as an ancillary test for diagnostic purposes.The South African BTB control schemes are described in the following sections, with an emphasis on the requirements for being admitted into the various test programs, the required activities in each program, the expected measures of compliance, and how to deal with BTB-positive herds.
19.3.1 Maintenance Program
The maintenance program includes all the BTB-free herds including those herds that were infected and cleaned. The program does not require individual animal identification. All animals of all sexes above the age of 12 months are retested every 2 years. Cattle that develop suspect reactions to the tuberculin skin test are tagged with official ear tags to identify them permanently for further tests. If any of the cattle react positive to the skin test, the herd is transferred to the infected program and all the animals in the herd are then identified with official ear tags.
New herds that enter the program must undergo two consecutive negative tests with an interval of not less than 3 months before being awarded a BTB-negative status.
The initial tests can be done by state officials and are free of charge.The declaration of BTB negative only certifies that the animals were tested negative for BTB on the date of the second test, but it does not certify freedom from the disease. This declaration is valid for 2 years, and animals must be retested every 2 years. It is the owner’s responsibility to keep the herd free from BTB.
19.3.2 Diagnostic Program
The diagnostic program is designed for farmers who want to determine the tuberculosis status of their herds or that of individual animals. State officials, to determine the prevalence of BTB in a herd, district, or municipality, also use this program. For this purpose, a state veterinary officer subjects all animals in a herd, older than 18 months, to one CCT at the state’s expense. Where selected individual animals are tested, a private veterinarian, at the owner’s expense, must preferably do the tests. Animal identification is preferred but not required for these tests, but official ear tags are attached to BTB-suspect or -positive animals following the specific test. When suspect reactors are detected, the entire herd is retested after 3 months, and if positive reactors are then found, the herd is incorporated in the infected herd program. These herds may be transferred to the maintenance program once the whole herd has had two consecutive negative CCTs, at least 3 months apart.
Imports Cattle that are imported and are kept at one of the registered quarantine stations must undergo a compulsory CCT done by the officer-in-charge of the quarantine station.
Exports Most importing countries require a negative BTB test before issuing import permits, or as a requirement to allow cattle into their country. Cattle that are destined for export are subjected to a CCT at the owner’s expense.
19.3.3 Infected Herd Program
A herd is regarded as infected if any of the following procedures indicate infection with M. bovis: meat inspection, milk examination, postmortem examination, clinical examination, a positive tuberculin skin test, or the introduction of animals from a confirmed BTB-infected herd.
The infected herd is placed under official supervision and quarantined. Necessary steps are then taken to confirm and eradicate the infection according to Act 7 of 2002 (the Animal Health Act), the Regulations enacted under the Act, the Bovine Tuberculosis Scheme regulations (R1953 of 1988 as amended), and the BTB Scheme Manual (South African Government 1984; Department of Agriculture, Forestry and Fisheries 1988; Department of Agriculture, Forestry and Fisheries 2013). Trial slaughter of tuberculin test-positive reactor animals to collect tissue samples for mycobacterial culture is recommended, but not mandatory, to confirm M. bovis infection in a herd.Procedures for Handling BTB-Positive Herds The state veterinarian in charge of the process has the responsibility of implementing quarantine measures, supervising all testing and diagnoses, communication with the farmer, record-keeping, and implementing all applicable control measures in the infected herd.
According to existing legislation, every effort should be made to deal with positive reactor animals in such a way that there is no further spread of the disease within and beyond the herd. Immediate slaughter of the reactors is mandatory. Should a large number of reactor animals be involved that will seriously affect the owner financially should they all be slaughtered immediately, disrupt the supply of milk to the community, or threaten the family’s livelihood, slaughtering may be postponed, but all reactors must be slaughtered within 12 months. Extension may also be considered in situations in which valuable animals such as high milk producers are involved, if more than 20% of the animals are infected, if the reactors are valuable pregnant cows, or those with small calves at foot. During the interim, milk produced by positive reactors must be boiled, pasteurized, or sterilized before it can be used for human or animal consumption. Cows that are known to produce infected milk are slaughtered immediately. Should the animal be condemned at the abattoir because of generalized tuberculosis, farmers may apply for compensation that, at present, would be the slaughter value of the animal that he/she would have been paid at the abattoir.
Following the removal of all known reactors, quarantine can be lifted after two additional consecutive negative CCTs, 3 months apart.
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