Interferon gamma test (for Feline mycobacterial disease)
Synonym(s): Interferon gamma release assay IGRA, IFNG
- The test is an aid to the diagnosis of feline mycobacterial disease. In particular it provides guidance as to whether a suspected infection with acid fast bacteria is likely to be environmental, avian TB Mycobacterium avium , Mycobacterium microti,or Mycobacterium bovis Mycobacterium bovis or Mycobacterium tuberculosis Mycobacterium tuberculosis Feline tuberculosis , the latter two having the highest zoonotic potential.
- Not applicable, see below.
- Diagnosis of feline mycobacterial disease is complex and as for most infectious diseases relies on a combination of clinical signs, history and appropriate use of diagnostic tests. The test is of most use where mycobacterial disease is strongly suspected as it can provide a rapid indication of the infecting organism and therefore the zoonotic potential.
- There is no perfect test for feline mycobacterial disease. Culture although definitive if positive is slow and often not successful in identifying the causative agent. PCR PCR (Polymerase chain reaction) based technologies do not always distinguish between the different tuberculous group Mycobacteria Mycobacterium spp ( M. microtiand M. bovisin particular), and primers are not available for many of the different types of non-tuberculous Mycobacteria Skin: panniculitis due to rapidly growing mycobacteria that cats can become infected with.
- Treatment for feline mycobacterial disease is lengthy and expensive and requires substantial commitment from the owner. If the test indicates it may be a highly pathogenic strain of tuberculous Mycobacteria (with significant zoonotic risks) a decision to treat requires careful thought and appropriate risk assessment.
Confirmed bovine TB (culture or identification at PostMortem) is a Notifiable disease in any mammal in the UK.
Source of test material
- Whole heparin blood.
Quantity of test material
- Minimum 2 ml.
Sample collection technique
- Standard precautions for aseptic collection of blood apply. Sample must not be chilled.
Timing of test
- The test is currently set up twice a month and the laboratory should be contacted before sample collection to check the next test run. The test takes a minimum of 96 hours to complete, results can be expected 4-5 days after test set up.
- Not possible, test must be set up within a short time (40 hours) of sample collection.
- Normal postal regulations apply for diagnostic samples. Samples must be protected from extremes of temperature and must not be chilled. Insulated boxes are available from the laboratory. Overnight guaranteed delivery is recommended.
- The test method prepares feline peripheral blood mononuclear cells (PBMCs) from the whole blood submitted. The PBMCs are cultivated for 72 hours in 5 separate reactions: with control materials (positive and negative controls) and TB specific antigens (PPDA, PPDB and the ESAT6/CF10 peptide mix). The assay is run in duplicate. Following cultivation supernatant is collected from each well and tested for interferon gamma in an ELISA Enzyme linked immunosorbent assay (ELISA) .
- Test is currently available from Biobest Laboratories Ltd .
- 70-100% depending on precise criteria applied by the laboratory to each case.
- 95-100% depending on precise criteria applied by the laboratory to each case.
- Predictive value depends on the prevalence of disease in the population under test. This is directly affected by the skill of the clinician in selecting cases appropriate for testing. The test is intended to assist classitification of the type of mycobacterial infection and is most appropriate where acid fast bacteria are present or where mycobacterial diseases is strongly suspected due to history and clinical presentation.
Technique intrinsic limitations
- The test supports a diagnosis of feline mycobacterial disease and provides an indication of the class of mycobacteria involved. The test should be used to support diagnosis in the context of other supportive clinical and laboratory findings.
Normal (reference) values
- A normal cat will have no interferon gamma in the unstimulated control wells, clear interferon gamma production in the non-specific stimulation wells and no interferon gamma production in response to any of the Mycobacterial antigens.
- The test is interpreted by comparing the response of the PBMCs in producing interferon gamma when exposed to mycobacterial antigens in comparison to the unstimulated cells and cells non-specifically stimulated.
- PPDA-, PPDB-, ESAT6/CF10-:
- Pathogenic TB complex infection unlikely. Avian complex Mycobacterial infection unlikely. Environmental Mycobacterial infection possible if acid fast bacteria identified in lesions.
- PPDA+, PPDB-, ESAT6/CF10-:
- Likely exposure to environmental / avian complex Mycobacteria only.
- PPDA+, PPDB+, ESAT6/CF10-:
- Less-pathogenic TB complex, most likely Mycobacterium microtiin UK cats.
- PPDA+, PPDB+, ESAT6/CF10+ :
- Very pathogenic TB complex, likely to be Mycobacterium bovisin UK cats or Mycobacterium tuberculosis.
- The PPDB response is often higher than PPDA in pathogenic Mycobacteria infections.
Errors and artifacts
- In the absence of viable PBMCs the interferon gamma positive control will fail and test results will not be valid.
- Some cats have high background production of interferon gamma in the absence of stimulation. The non-stimulated control is positive in such cases. This may be due to high levels of natural killer cell activity.
- Recent references from VetMedResource and PubMed.
- Gunn-Moore D A, McFarland S E, Schock A, Brewer J I, Crawshaw T R, Clifton-Hadley R S, Shaw D J (2011) Mycobacterial disease in a population of 339 cats in Great Britain: II. Histopathology of 225 cases, and treatment and outcome of 184 cases. J Feline Med Surg 13, 945-952 PubMed.
- Rhodes S G, Gunn-Mooore D, Boschiroli M L, Schiller I, Esfandiari J, Greenwald R, Lyashchenko K P (2011) Comparative study of IFNG and antibody tests for feline tuberculosis. Vet Immunol Immunopathol 144, 129-134 PubMed.
Other sources of information
- Further information on mycobacterial disease in the dog and cat can be found in Greene C E (2011) Infectious Disease of the Dog and Cat. 4th edn. Saunders.
- Test is available from Biobest Laboratories Ltd who provide full support on how to use the test and interpretation of results.