Chapter 13 – Chronic Wasting Disease Voluntary Herd Certification Program
13.6 Appendix 1A: Brain Sampling Procedures
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Entire heads may be submitted fresh or frozen to a Canadian Food Inspection Agency (CFIA)-approved laboratory. (See Module 13.6, Appendix 1C.)
Samples removed by anyone other than an accredited veterinarian, provincial staff, or an approved laboratory (or a CFIA veterinarian/inspectorFootnote 3) will not be counted as submitted under the Chronic Wasting Disease Voluntary Herd Certification Program (CWD VHCP).
Both the obex and the retropharyngeal nodes (RPLNs) must be submitted for all farmed cervids tested for CWD under the CWD VHCP.
Visualize the approved identification device in situ prior to removing tissues for testing, record all individual animal identification information on the CWD form, and ensure the tag(s) are submitted with the sample to the laboratory.
Note: Visualization of this technique is available on the training CD entitled Transmissible Spongiform Encephalopathies: Surveillance and Specimen Collection published by the CFIA and available from district veterinarians and TSE veterinary program specialists in the various areas.
Obex Harvesting Technique
- knife for disarticulation of the head (if required);
- rat-toothed forceps;
- obex removal knife, spoon (plastic BioRad or metal) (Figure 1);Footnote 4
- scalpel (optional);
- scissors (optional).
Figure 1 - Examples of obex spoon
When removing the head from the body, all the flesh, including the spinal cord, should be cut cleanly 15 cm (6 inches) from the head.
1. Place the cervid head upside down (dorsal side down) with the nose pointing away from you and the foramen magnum facing you on a clean, disinfected surface (Figure 2).
Figure 2 - Cervid head, dorsal side down for correct orientation
2. Using the forceps, grasp the dura mater (which is the thick lining around the spinal cord). With the scissors, make a single cut down the centre line to form two flaps.
3. Remove the congealed blood from around the spinal cord.
4. Use the forceps to hold the dura. Sever the cranial nerves from the spinal cord. This can be done with a scalpel, an obex knife, or carefully with scissors (as shown in Figure 3) by moving the instrument gently around the cord.
Figure 3 - Severing the attachments to free up the spinal cord
5. This is the most important step in freeing up the spinal cord. The cord must be completely free from attachments by cranial nerves in all directions. If some cranial nerves are left attached, a portion of the obex will remain with that cranial nerve and be pulled off your sample.
6. Once the brain stem and spinal cord are free from cranial nerve attachments, turn the head so that it is dorsal side up on the table. Insert the spoon or spatula into the cerebral canal. Advance it cranially as far as you can until the tip lodges against parts of the skull and comes to rest between the cerebellum and the brain stem. (See Figure 4.)
Figure 4 - Head turned - dorsal side up, advancing the collection spoon into the canal for obex removal
7. Use your index finger to place downward pressure on the handle of the obex tool, and rotate the tool back and forth to sever the brain stem.
8. Keep the tip of the obex tool down and drag the tool backwards, gently bringing the severed portion of the brain stem with it. The obex is recognizable as a V-shaped depression. (See Figure 5.)
Figure 5 - Obex
Contact the laboratory to request information on specimen submission (fresh or frozen), and ensure that the individual animal's tag is submitted with the sample to the approved laboratory.
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