USDA APHIS Veterinary Services

Chronic Wasting Disease Program – Post Mortem Sample Collection Guidance

Procedure for Removal of Obex & Medial Retropharyngeal Lymph Nodes (MRPLN)

Chronic Wasting Disease (CWD) has not been shown to infect humans, but other cervid diseases, such as

rabies, can be zoonotic diseases. Therefore, personal protective equipment (PPE) should be worn when

handling animal tissues. Dedicated or disposable instruments, work area covers, and PPE are

recommended for sampling.

Recommended PPE

Gloves, eye protection, coveralls

Equipment

Extraction spoon Formalin Jars

Scalpel Whirl-paks

Forceps Icepacks

Scissors Coolers

Knife Bleach

Permanent Marker

Obex & MRPLN are required tissues for APHIS CWD 

Herd Certification Program (HCP) CWD testing

Samples MUST be submitted within 7 days of collection

Sampling Procedure for Obex:

1. Incise the head of the animal at the atlanto-occipital joint

(between skull and first vertebra). Cut behind the back of the

ears and extend the cut around and through the front of the

larynx. Sever the brainstem as far caudally as possible during

the removal process.

2. Position the head upside down (ventral side up). Locate the

occipital condyles and foramen magnum (FM). Locate the

brainstem inside the FM. Trim the dura mater around the brainstem and cut the attached cranial nerve trunks.

3. Gently lift the brainstem with forceps and insert the spoon into

the dorsal aspect of the FM between the brainstem and dorsal

calvarium.

4. Advance the spoon 2-3 inches rostrally until it contacts bone to

sever the cerebellum.

5. Re-position the spoon in the ventral aspect of FM between the

brainstem and the ventral calvarium. Advance the spoon until it

contacts bone and transversely sever the brainstem.

6. Remove the brainstem using the spoon and forceps. Examine to

ensure the proper obex sample (Bifurcation or “V”) is preserved.

7. Further trim the brainstem section by making a transverse cut

3/4 inch in front of the “V” shape bifurcation and an equal

distance behind the bifurcation for good fixation.

SUBMIT

FIXED: [formalin] (IHC) place the trimmed obex and a brainstem piece

in 10% buffered formalin jar (10:1 ratio of formalin to tissue sample).

FRESH: [chilled or frozen] Place the second piece of brain stem and place in a labeled whirl pack.

For ELISA testing – place the second piece of brainstem sample and trimmed pieces in a conical tube.

Please see Sample Submission Procedure section in the HCP Program Standards for further details.

Ventral Side Up

Occipital Condyles

Obex

Foramen Magnum Brainstem

Brainstem

Bifurcation “V” Photos courtesy: Indiana BOAH
SUBMIT

FIXED: [formalin] (IHC) place half of the left, and half of the right

MRPLN in the same10% buffered formalin jar as the obex

(10:1 ratio of formalin to tissue sample).

FRESH: [chilled or frozen] place the other halves of the MRPLNs in

a labeled whirl pack. For ELISA testing – place MRPLNs in a labeled

whirl pack.

Obex & MRPLN are required tissues for APHIS CWD 

Herd Certification Program (HCP) CWD testing

Samples MUST be submitted within 7 days of collection

Sampling Procedure for Medial Retropharyngeal Lymph Nodes (MRPLN)

1. With the head positioned ventral side up, locate the esophagus

and trachea above the foramen magnum (FM).

2. Lift the trachea and dissect muscles forward of the foramen

magnum (rostrally). Locate the left and right Medial

Retropharyngeal Lymph Nodes (MRPLN) half-way between each

corner of the jaw bone and the FM, caudal to the naso-pharynx,

and deep to the salivary gland. Lymph node (LN) consistency is

much firmer and rounder than the surrounding tissue.

3. Remove each left and right MRPLN and longitudinally incise each

LN. Place half of the left and half of the right node into formalin,

and the other halves into a whirl-pak.

Occipital Condyles

FM

MRPLNs

Naso-pharynx

Sampling Procedure for ID and Skin 

FRESH: [chilled or frozen] place the official ID with attached skin (ear or hide) attached into a labeled

whirl-pak and submit with other fresh tissue. DO NOT PLACE IN FORMALIN

1. Remove the official ID with a 1” X 1” piece of ear attached for

each animal sampled and place in a labeled whirl-pak. If the animal

is a trophy animal, submit the official ID and attach a new official

ID to another piece of skin. If no official ID is present, the collector

must affix one and record the information. Microchips must be in tissue.

Sample Submission Procedures: 

1. Label sample containers / bags with sample number, producer name, official ID number, species, date, 

etc. Refer to appendix V in the current HCP Program Standards for a specifics on required paperwork, and 

sample labeling.) 

2. Complete VS 10-4 Form. List any notes or clinical signs in the Additional Data section.

3. Sample Shipping: visit www.fedex.com/us/packaging/guides/Clinical_fxcom.pdf for packaging instructions.

Packages containing biological specimens must be labeled as a Category B (UN3373) for shipping.

4. Ship to a VS approved CWD laboratory 

Fixed: [formalin] Obex and MRPLN Fresh: Obex, MRPLN, and tag w/skin

Ship samples in formalin jars Keep samples chilled on ice packs

and use overnight delivery

SUBMIT: