Exploring the cause of initially reactive bovine brains on rapid tests for BSE

ABSTRACT

Bovine spongiform encephalopathy (BSE) is an invariably fatal prion disease of cattle. The identification of the zoonotic potential of BSE prompted safety officials to initiate surveillance testing for this disease. In Canada, BSE surveillance is primarily focused on high risk cattle including animals which are dead, down and unable to rise, diseased or distressed. This targeted surveillance results in the submission of brain samples with a wide range of tissue autolysis and associated contaminants. These contaminants have the potential to interfere with important steps of surveillance tests resulting in initially positive test results requiring additional testing to confirm the disease status of the animal.

The current tests used for BSE screening in Canada utilize the relative protease resistance of the prion protein gained when it misfolds from PrP ^C to PrP ^Sc as part of the disease process. Proteinase K completely digests PrP ^C in normal brains, but leaves most of the PrP ^Sc in BSE positive brains intact which is detected using anti-prion antibodies. These tests are highly reliable but occasionally give rise to initially reactive/false positive results. Test results for these reactive samples were close to the positive/negative cut-off on a sub set of test platforms. This is in contrast to all of the previous Canadian positive samples whose numeric values on these same test platforms were 10 to 100 fold greater than the test positive/negative cut-off. Here we explore the potential reason why a sample is repeatedly positive on a sub-set of rapid surveillance tests, but negative on other test platforms.

In order to better understand and identify what might cause these initial reactions, we have conducted a variety of rapid and confirmatory assays as well as bacterial isolation and identification on BSE positive, negative and initially reactive samples. We observed high levels of viable bacterial contamination in initially reactive samples suggesting that the reactivity may be related to bacterial factors. Several bacteria isolated from the initially reactive samples have characteristics of biofilm forming bacteria and this extracellular matrix might play a role in preventing complete digestion of PrP ^C in these samples.