Congenital tumours in horses

The case report on the occurrence of a melanoma in the oesophagus of a young foal (Caston and Fales-Williams 2010) brings to the attention of the reader the possibility of congenital tumours in our patients. The differential diagnosis of neoplasia is commonly excluded when examining neonatal or young animals. However, after reading the accompanying case report, and reviewing the equine literature there are a surprising number of case reports of congenital tumours occurring in both horses and other large animals. Most commonly reported are epithelial tumours such as melanomas or papillomas but also tumours of internal organ or musculoskeletal tissues. The presence of neoplasia in neonates can occur due to placental transmission of neoplastic cells or as a primary disease. In man congenital neoplasia also occurs by similar mechanisms. Melanoma is the most common neoplasia to have placental transmission in man but this only occurs when the mother has metastatic melanoma herself (Altman et al. 2003). Placental transmission is rare but fetal transmission is even rarer. It is speculated that at a certain point the fetus becomes immunologically competent and can recognise the tumour cells as foreign and prevent transmission. In the case described in this report the mare did not undergo a full examination for the presence of internal melanoma but had no obvious external tumours or clinical signs. The placenta was not examined microscopically as there was no indication to do so. However, with future births from mares with external tumours or systemic disease, this report serves to indicate that submission of the placenta may be a worthwhile procedure to facilitate early diagnosis and treatment of the foal. The foal in this report may not have been able to have the melanoma safely excised due to its extensive nature and location in the oesophagus. There have been reports in foals of congenital melanomas that have been successfully excised without short-term recurrence (Cox et al. 1989; McMullen et al. 2008). The detection of neoplastic cells in the placenta would allow foals to be examined early and potentially treated with surgical excision prior to growth of the melanoma. Most reports in horses are those of congenital tumours without a history of maternal involvement. Congenital tumours were reported as a cause for late-term abortion along with other miscellaneous conditions in 26/3527 horses but there are many reports of live foals with congenital tumours (Giles et al. 1993). Congenital tumours can be malignant, benign, or in the form of hamartomas, which are benign growths that occur in the tissue of origin. They do not grow rapidly or spread to other areas but are areas of disorganised growth that often affect the surrounding tissue by a local mass effect. Hamartomas have been reported consistently in both veterinary and human medical literature. They are often of epithelial origin but can occur in other organs such as liver. Fetal hamartomas occur in horses as well as other large animals, with epithelial hamartomas as well as those of internal organs such as liver or ovaries having been reported (Machida et al. 2001; Brown et al. 2007). Hamartomas are not malignant and do not metastasise but can have a local mass effect and require surgical intervention. Primary congenital or fetal benign and malignant tumours have also been reported in foals such as rhabdomyomas, teratomas or haemangiomas/haemangiosarcomas (Misdorp 2003; Meyerholz et al. 2004). Many of these tumours have also responded to surgical intervention with success despite histopathological evidence of malignancy (Cox et al. 1989; McMullen et al. 2008). A few isolated reports have also reported metastasis particularly with congenital melanoma (Foley et al. 1991). It is interesting that the mare was grey in colour and the foal was also appearing to be light coloured as well. The connection of melanoma and grey colour in horses is well established but most commonly in the form of melanocytomas or slow growing external masses in older horses. Melanocytic naevi have been reported in younger horses and both grey and nongrey horses as benign occurrences (Valentine 1995; Schoniger and Summers 2009). There is some disagreement concerning the exact classification of melanomas in grey horses but all agree that they are more commonly slow growing and in dermal or epidermal locations. Melanoma has been shown to become metastatic in grey horses after a prolonged period with a large external mass with spread to internal Corresponding author email: yvonne.elce@gmail.com EQUINE VETERINARY EDUCATION 391 Equine vet. Educ. (2010) 22 (8) 391-392 doi: 10.1111/j.2042-3292.2010.00105.x