Fact Sheet: Why do farms have to be quarantined for so long?
When a farm is identified as having animals that may have been exposed to bovine TB, the Canadian Food Inspection Agency (CFIA) orders quarantines and movement controls so that additional animals on other farms are not at risk of infection.
Quarantines remain in place until the herd is determined to be free of bovine TB. This determination is based on:
- the results of herd testing; and
- an epidemiological evaluation of the risk of having the disease.
1. Herd testing - How the CFIA tests for bovine TB
Because of the challenge of detecting bovine TB in live animals, the CFIA is using two tests on each test-eligible animal (over one year of age) under quarantine. Each test checks for different immune system responses from the animal and increases the confidence that any infected animals will be identified.
The two tests used by the CFIA are:
Caudal fold skin test
The caudal fold skin test is a tuberculin skin test that is used throughout the world to test for bovine TB. A small amount of tuberculin (a sterile protein extract from cultures of bovine TB bacteria) is injected under the skin of the animal. If the animal has been exposed to tuberculosis, there will be swelling and/or discolouration of the skin around the injection site.
The skin reaction can happen if the animal has been exposed to other types of tuberculosis (usually avian tuberculosis) and in some cases without any exposure to tuberculosis. These animals are called "reactors." In a disease-free livestock population, there will be reactors in 1% to 3% of the animals tested.
This test requires that the animal be examined again 72 hours after the tuberculin is injected. This means this test requires a total of three days to be completed.
Enzyme-linked immunosorbent assay (ELISA) blood test
An ELISA test detects and measures antibodies in a blood sample. CFIA inspectors take blood samples for the ELISA test at the same time as they administer the first step of the skin test.
The blood samples are sent to the CFIA's Ottawa Animal Health Laboratory for testing. Whenever possible, blood test results are sent to the inspectors conducting on-farm testing in time for the completion of the second part of the skin test. This will allow CFIA inspectors to segregate reactors to the blood test that did not react to the skin test and limit the number of times animals need to be handled.
It takes three to five days for the blood test results to be available.
Additional testing for reactor animals
Reactors to either the skin test or the blood test are humanely destroyed, and compensation is paid to the owner.
A detailed post-mortem examination is then conducted on the animal to look for visible signs of TB infection, such as lesions in the lungs and lymph nodes. However, it is also possible for animals to be infected without any visible lesions.
Tissue samples are taken from lesions and normal tissues for additional testing at CFIA's laboratory. Tissue samples are incubated and cultured to enable the growth of the bovine TB bacteria. The test is positive if growth of the bacteria is confirmed.
The culture test takes 8 to 12 weeks to complete. During this time, non-reactors are eligible to move to inspected slaughter, subject to permission from the CFIA.
An animal may be declared positive on the basis of the results of:
- the post-mortem exam;
- histopathology (an examination of tissue sections that detects the presence of tuberculosis bacteria);
- the polymerase chain reaction (PCR) test (which tests the DNA of the bacteria to identify the species and strain of mycobacteria); or
- the culture test.
An animal will be considered free of bovine TB when the culture test is negative.
2. Epidemiological evaluation
Given the nature of this disease and its long incubation period, this is a process that can take considerable time. For all implicated animals, there is typically a requirement to trace 5 years of animal movements.
The information that the CFIA gathers during the investigation and the tracing activities is key to determining the testing priority of animals and premises based on risk. This information is collected and analysed on a case-by-case basis.
For example, animals kept together for winter feeding with prolonged exposure to the herd mates of a positive animal would present a higher risk of exposure than animals that shared common summer pasture.
Release from quarantine
When all reactors from a herd have tested negative for bovine TB and where supported by the epidemiological evaluation, the herd can be released from quarantine under direction from the CFIA.
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