final report - ARCHIVE: Defra

Diagnosis
of bTB. If test results from particular sample types can be correlated with the local prevalence and
incidence of bTB in both cattle and badgers, this approach could provide a valuable tool in the control
effort.
2.7.2 The results reported to date indicate a surprisingly high prevalence of sample positivity and level of
contamination (Sweeney et al., 2007; Young et al., 2005). At 10 4 –10 6 genome target copies per gram of
sample this represents the highest DNA signal consistently recorded for a member of the Mtb complex
in diagnostic samples. It has proved particularly challenging to achieve good sensitivity in the diagnosis
of human TB by specific DNA detection. Available assays do not consistently outperform microscopy,
which, at best, detects 10 3 target bacteria per gram, in most published studies. Problems with obtaining
good DNA extraction and inhibition of amplification reactions are regularly encountered. These
problems do not appear to have been encountered in the Mb study. However, the surprisingly high
numbers of bacilli detected, combined with PCR’s potential for giving misleading results, means that
the data will need to be particularly solid if they are to be used for policy decisions.
2.7.3 Immunofluorescence detection and culture results following immunomagnetic capture enrichment
appear to corroborate the DNA detection results (Sweeney et al., 2006, 2007).
2.7.4 If robust methods based on the work reported can be established and replicated in other laboratories
then this approach could be exceptionally valuable. These issues appear to be addressed in a recently
established validation study (Defra project SE3231).
2.7.5 Whether the bacterial cells responsible for positive DNA signals were capable of transmitting infection
at the time of sampling, and how long these signals persist in the environment, remain open questions
and reflect generic problems in molecular diagnostics as applied to infection (Barer & Harwood, 1999).
Considerable work will be necessary to develop reliable interpretive criteria for the present assay. Such
work should be undertaken following the outcome of the established validation study.
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