Melanoma in pigs

Melanin is a substance that melanocytes produce in the skin surface, which results in skin pigment and protects the skin from UV radiation. Melanoma is a type of cancer that develops in the melanocytes.

Melanoma in pigs, while generally rare, can occur in pigs with black genes such as Duroc and Duroc crossbreeds. Melanomas are malignant tumours, but studies have found that where melanomas have not progressed beyond the immediate draining lymph node, depending on the extent, the lesion or the affected carcass part can be trimmed and condemned. The remainder of the carcase and parts can be passed for human consumption. (Refer Photo 1). Carcases with extensive tumours (See Photo 2) are to be condemned.

A study involving 130,000 pigs found that:

• 1.53% had visible melanoma lesions on the skin.
• 0.044% had related lesions in the immediate lymph nodes.
• 2.86% of the pigs with skin melanoma had related lesions in the immediate lymph nodes.
• No pigs had lesions beyond the draining lymph nodes.

Disposition

In 2020, Schedule 3 AS4696: 2007 was amended for the disposition judgement criteria for pigs with melanoma. (see highlighted in table below)

Subsequently, to be able to make the correct disposition for melanoma, meat inspectors will need to have a good knowledge of the lymph system of pigs especially the lymph drainages regions of the skin and the location of their respective regional and terminal lymph nodes.

A paper written by Ran Ito, M.D., Ph.D. & Hiroo Suami, M.D., Ph.D. Houston, Texas on Lymphatic Territories (Lymphosomes) in Swine: An Animal Model for Future Lymphatic Research identified seven Lymphosomes (lymph drainage regions) of a pig and their respective regional lymph nodes. (See diagram 1). This information would be a useful tool for meat inspectors when making a disposition for pig melanoma.

It should be noted that no lymph nodes existed in the superficial axillary region, which commonly exists in other mammals. The lymphatic vessels in both of the forelimbs and the ventral cranial torso in pigs run through the axillary region and connected to the Prepectoral (Ventral cervical) lymph node.

 No lymphatic vessels are found to cross the midlines in either the ventral or dorsal aspects and therefore the midlines are lymphatic watersheds.

Diagram 1: 1 Parotid 2 Submaxillary (Mandibular) 3 Prescapular (Dorsal Superficial Cervical) 4 Prepectoral (Ventral cervical) 5 Precrural (Sub Iliac) 6 Superficial Inguinal 7 Popliteal

If melanotic material is found in any lymph node beyond the immediate draining lymph node or in tissue other than the primary lesion, it is defined as systemic/malignant/metastatic and the carcase and its parts are to be condemned.

The following are examples of dispositions that could apply when melanoma is detected in one of the specific areas (A to G) indicated in Diagram 1 above.

 Area A

If melanoma has not spread to the parotid LN or has spread to the parotid LN but not spread to the atlanta (lateral retropharyngeal) LN, remove melanoma lesion and judge carcase on its merits, If it has spread to the atlantal LN (next draining lymph node) condemn carcase and parts.

Area B

If melanoma has not spread to the submaxillary (mandibular) LN or has spread to the Submaxillary LN but not spread to the prepectoral LN, remove melanoma lesion and judge carcase on its merits, If it has spread to the prepectoral LN (next draining lymph node & terminal lymph node) condemn carcase and parts

Area C

If melanoma has not spread to the prescapular (dorsal superficial cervical) LN or has spread to the prescapular LN but not spread to the prepectoral (ventral superficial cervical) LN remove melanoma lesion and judge carcase on its merits, If it has spread to the prepectoral LN (next draining lymph node) condemn carcase and parts.

Area D

If melanoma has not spread to the prepectoral (ventral superficial cervical) LN or has spread to the prepectoral LN but not spread to the cranial, middle, or deep cervical (commonly known as the variable chain of three) LNs remove melanoma lesion and judge carcase on its merits, If it has spread to the cranial, middle, or deep cervical LNs (next draining lymph nodes) condemn carcase and parts.

Area E

If melanoma has not spread to the precrural (sub iliac) LN or has spread to the Precrural LN but not spread to the external iiiac (lateral iiiac) LN, remove melanoma lesion and judge carcase on its merits, If it has spread to the external Iiiac LN (next draining lymph node & terminal lymph node) condemn carcase and parts

Area F (1)

If melanoma has not spread to the superficial inguinal /supra mammary LN or has spread to the superficial inguinal/mammary LN but not spread to the external iliac (lateral iliac) LN, remove melanoma lesion and judge carcase on its merits, if it has spread to the external iliac LN (next draining lymph nodes) condemn carcase and parts.

Area F (2)

If melanoma has not spread to the superficial inguinal /supra mammary or the deep popliteal LNs or has spread to the superficial inguinal /supra mammary or the deep popliteal LNs but not spread to the external iliac next draining LN, remove melanoma lesion and judge carcase on its merits, if it has spread to the external iliac LN (next draining lymph nodes) condemn carcase and parts.

Area G

If melanoma has not spread to the deep popliteal or superficial popliteal LN or has spread to the deep popliteal or superficial popliteal LN but not spread to the Ischiatic (gluteal) LNs remove melanoma lesion and judge carcase on its merits, If it has spread to the Ischiatic LN (next draining lymph nodes) condemn carcase and parts.

If the melanoma has spread to the superficial popliteal but not to the deep popliteal remove the melanoma and judge carcase on merits if it has progressed to the deep popliteal (next draining lymph node) condemn the carcase and parts

The table below outlines the international names and the corresponding Australian names (including those used in the Australian Standard) of lymph nodes in pigs.

The following diagram and photos can be used to determine “next draining lymph node(s)” and location of lymph nodes in a dressed carcase.

Diagram 2. Lymph drainage system (Spurgeon’s Colour Atlas of Large Animal Anatomy – The Essentials)

Lymph Nodes

Photo 3.  Lymph node locations

RTOs should include information on pig lymphosomes, the location of their respective regional lymph nodes and practical training in incision of these lymph nodes when training students in AMPMSY403 Perform ante and post-mortem inspection – Porcine.

Meat inspectors performing post-mortem inspection of pigs must be able to identify lymphosomes, the location of their respective regional lymph nodes and be able to inspect them by incision when determining if a melanoma has spread beyond the immediate draining lymph node.