EXCEPTIONAL OCCURRENCE AND EXTENT OF MALI GNANT-MELANOMA IN PIG

The hereditary malignant melanoma (melanoblastoma) was recognised in a herd of miniature laboratory pigs kept at the Institute of Animal Phy siology and Genetics, Academy of Sciences of the Czech Republic. These pigs genetically came from the original Hormel strain imported from t he USA in 1967. Due to blood group studies, they were crossed with sev eral other breeds and strains (Gottingen miniature pig strain, Canadia n Landrace, Cornwall, Vietnamese pigs) during the past time. Selective breeding of animals bearing exophytic form of melanoma suggested a ge netic basis to this disease. Mating of halfsiblings, which have been h ealed of tumours by surgical ischaemization (devitalization), proved a simple genetic predisposition. The segregation of tuberonodular form of melanoma in these families showed a simple mendelian character (1 : 1). The fast growth phase after birth, strong agressivity and severit y of the tumour in some animals could be explained by matings of relat ed parents and generally by inbreeding in our miniature pig line (F = 0.30). In one case, we observed an exceptionally large tumour and broa d dissemination of the primary skin tumours. Metastatic lesions were g reater in size and extent in comparison with cases published previousl y. The case report of this tumour is documented morphologically and hi stologically. The piglet with this tumour was strongly wasted and died at the 81st day after birth due to massive embolization of the arteri a pulmonalis (sudden respiratory distress and dyspnoea). The two large st primary tuberonodular exophytic tumours (9 x 6 cm and 2.5 x 2.5 cm) were prominently elevated on the skin. Metastases were broadly dissem inated in various organs and tissues: lungs, liver, gall-bladder, sple en, kidneys, adrenals, urinary-bladder, uterus, ovaries, pancreas, thy mus, lymph nodes, thyroid gland and all segments of digestive tract in cluding rectum. Great size and unusual locations of metastases were ob served in organs where their occurrence in people or animal models is very rare or still has not been described (heart, diaphragm, aorta). L ight microscopy confirmed the direct macroscopic examination and sugge sted that the histopathological features of the tumour are similar to human melanoma.