Although rectal biopsy (RB) is now being recognized as an effective option for screening scrapie, producers should know that testing a sample of brain tissue remains the gold standard. Similar to third eyelid sampling, RB involves collecting a sample of lymphoid tissue; however, it targets lymphoid tissue associated with the rectal-anal mucosa as opposed to lymphoid tissue associated with the third eyelid. Both sampling of the third eyelid and RB can be conducted on live animals.
RB was approved by the United States Department of Agriculture’s Animal and Plant Health inspection Service in January 2008 following a large-scale study conducted in 2007. The study evaluated RB as a means to collect lymphoid tissue for scrapie testing. Using live, high-risk sheep and goats, the study compared the test results from RB’s and third eyelid biopsies to test results obtained postmortem from the same animals on brain-stem, lymph node, tonsil and rectal biopsy. The study found that RB testing is an effective option for detecting scrapie. However, producers will only want to consider using RB when testing the entire flock/ herd. Similar to the third eyelid biopsy, false negative results can be reported from individual sheep in which the presence of the scrapie prion is restricted to brain tissue only, and is not present in the lymphoid tissue.
The US is currently trying to integrate some form of live testing into their Scrapie Flock Certification Program, however, no details on how this will be done have been confirmed.
The CFIA has implemented RB as an alternative live animal test to screen potential source flocks on confirmed scrapie infected premises. Canada’s Scrapie Flock Certification Program (SFCP) also incorporated RB in July 2008, making it part of Pathway 2. Prior to this date, producers enrolled on Pathway 2 were only permitted to complete third eyelid testing on all sheep/ goats aged 12 months or older but changes were made to allow RB as an alternative to the third eyelid test. This offers producers another option to the somewhat challenging and often time consuming third-eyelid test.
Even though RB is pricey, it’s slightly more cost-effective than the third eyelid test due to the fact that it takes less labour and time. Producer costs include bringing out a veterinarian who is trained to collect the sample, as well as laboratory testing fees. The lab will test the sample using immunohistochemistry, a method which demonstrates specific antigens in tissues by the use of markers that are either fluorescent dyes or enzymes. The test costs about $60 per sample.
The OIE code currently lists conditions for an establishment to be recognized as free from scrapie. The current conditions are identical to those of Pathway 1 of the SFCP and they must be maintained for a total of seven years. Producers who wish to engage in international trade of live sheep are encouraged to participate in Pathway 1 of the program. Due to the possibility of false negatives when using lymphoid sampling, it is highly unlikely that any country (including Canada) will accept the test results from a lymphoid sample as proof of disease freedom for the purposes of trade.
For more information about RB and/ or scrapie, please contact Scrapie Canada at 1-866-534-1302 or firstname.lastname@example.org