Other Diagnostics
HISTOLOGY ACID–FAST STAINING
Histological examinations of the granulomas are performed to identify
the accumulation of macrophages and bacteria. Acid–fast staining
allows visualization of the bacteria. The tissue is ground into small
pieces, treated to destroy non–mycobacteria, concentrated by
centrifugation, and applied to a microscopic slide to be examined. The
bacteria appear as short red or pink rods. As other bacteria can also
absorb acid–fast stain, further diagnostics are necessary.
POLYMERASE CHAIN REACTION
Polymerase chain reaction (PCR) test can be performed on those samples containing acid–fast bacteria. PCR is used to detect the presence of specific M. bovis genetic material. PCR is the amplification of portions of the M. bovis DNA. The benefit of PCR is the ability to detect low levels of bacteria. PCR results are available in approximately 48 hours.
CULTURE
Culturing M. bovis is the definitive diagnosis of TB. Samples are cultured under specific conditions, which favor M. bovis growth. M. bovis is a slow growing organism and takes from 8–16 weeks to grow. Once Mycobaterium is isolated, the sample undergoes biochemical testing to ensure the species is M. bovis.
DNA FINGERPRINTING
In recent years, DNA fingerprinting techniques by restriction fragment length polymorphism (RFLP) has been developed. RFLP is a technique which produces a pattern of bands similar to a bar code. This pattern represents the fingerprint of that particular M. bovis isolate. The similarity of fingerprints indicates a relationship between two isolates. Fingerprinting is of great importance to determine the source of TB infections in cattle herds.
Recent research has led to the development of a blood test (gamma interferon), a test that measures cell–mediated immunity in cattle. This test has recently been approved as a confirmatory test for cattle that may be used instead of the CCT. The test is expensive ($8–10/head) and the blood is difficult to handle. There is no reliable antibody–antigen test that measures humoral immunity for the diagnosis of TB because cattle have few circulating antibodies.
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Contact us for more information:
California Department of Food and Agriculture
Animal Health and Food Safety Services, Animal Health Branch
1220 N Street, Room A-107
Sacramento, California 95814
Telephone: (916) 654-1447
Fax: (916) 653-2215
or send an email to: ahbfeedback@cdfa.ca.gov