Considering the spread of Koi carp, and their value as pet animals, an improvement in the veterinary diagnostic process is needed. Neoplastic diseases are described in these animals, and gonadal tumors should be considered in cases of coelomic swelling in koi carp. In the literature, gonadal tumors in koi carp are described but as exposed in a recent study by Ott Knüsel et al. conducted in 2016, these tumors are mostly represented by ovarian predominantly sex cord stroma tumors, whereas tumors originating from germ cells account for only 2.5% of coelomic neoplasms being relatively rare although reported in the literature (Ott Knüsel et al. 2016). The causes of the onset of these tumors are not yet known. However, the request for particular color varieties has increased the selection and inbreeding of the species; thus, a genetic predisposition has been suggested (Ott Knüsel et al. 2015). Moreover, since few studies exist on these tumors, environmental factors such as toxic compounds, or viral causes cannot be excluded (Sirri et al. 2021). In fish, organic pollutants are often absorbed through the gills and skin, and accumulate in lipid-rich tissues, such as liver, brain, gonads, and hypodermal lipid storages. (Baines et al. 2021) In particular, exposure to these substances: PAH (7,12-Diniethylbenz[a]anthracene), Ethlynitrosourea, N-methyl-N’-nitro-N-nitrosoguanidine (MNNG), PCB’s, pesticides (ß-endosulfan and α-endosulfan), hydrocarbons (oil), heavy metals, has been described as related to the occurrence of gonadal tumors in fish, particularly seminomas and dysgerminomas (Baines et al. 2021; Bunton & Wolfe 1996; Spitsbergen et al. 2000a; Spitsbergen et al. 2000b).
In the present case, the neoplasia described was ultrastructurally composed of cells that resemble normal germinal epithelium and have oval nuclei, straight cell borders, a distinct Golgi complex, scarcity of cytoplasmic organelles and the presence of intercellular bridges. These aspects, together with the presence of intercellular bridges, as seen in normal germinal cells, are present in seminomas. (Maxie 2015) These histological and cytological features allow the clear distinction of these tumors from the differential diagnoses of other testicular tumors such as interstitial or Sertoli cell tumors. Given the histological and cytological findings observed in this case, the present neoplasia was diagnosed as a spontaneous seminoma.
Seminomas in fish are reported in literature and are described as tumors composed of typical germ cells similar to those from humans and equivalent mammalian tumors. This enables the comparative oncologist to classify fish tumors on the same bases as mammal tumors (Masahito et al. 1988).
According to the WHO International Histological Classification of the Tumors of the Genital System of Domestic Animals (Kennedy et al. 1998), the present seminoma could be classified as diffuse, statin the evidence of a lobular arrangement of neoplastic cells divided by a stromal component infiltrated by lymphocytes suggested a similarity with the diffuse form. However, the high malignancy of our seminoma and the probable origin of the neoplastic cells from undifferentiated seminal cells suggest that the present seminoma is ascribable to the classical type, according to the WHO classification of testicular tumors in humans (Mostofi & Sesterhenn 1998).
As occurs for mammals, gonadal tumors are diagnosed histologically supported by cytological examination. However, in order to confirm the cytological and histological diagnosis and, above all, to classify seminomas according to classifications in human and veterinary medicine, an immunohistochemical panel, previously tested in a previous case of seminoma in a koi carp described by Sirri et al. in 2010 together with the PAS staining, is available (Sirri et al. 2010). This panel included several markers including in particular cytokeratin, vimentin, c-KIT, placental alkaline phosphatase (PLAP), and neuronspecific enolase (NSE), revealing an immunoreactivity of seminomatous germ cells to vimentin, PLAP, and c-KIT, but not to NSE and cytokeratin (Feitz et al. 1987; Foster & Ladds 2007; Grieco et al. 2007; Sirri et al. 2010). PLAP, which is produced ectopically by a variety of malignant tumors including human seminoma, was found to be a specific antibody for neoplastic cells of a classical histotype (Lange et al. 1982; Grieco et al. 2007). c-KIT, which is normally expressed by germ cells, has been validated as a marker to distinguish seminoma from Sertoli cell tumors, as it is also expressed by undifferentiated neoplastic seminal cells (Grieco et al. 2010; Yu et al. 2009; Sirri et al. 2010). However, the use of mammalian antibodies in fish tissues has certain limitations related to their specificity, and the immunohistochemical panel is useful for classifying the neoplasm, which diagnosis from first cytological then histopathological on the sample taken is in itself quite accurate given the particularity of the neoplasm and its very different appearance from its main differential diagnoses in both cytological and histological terms. Therefore, the cytological examination, which is quick, inexpensive and can be performed at the surgical site, is an excellent first-stage diagnostic tool. Little is known about the prognosis of these neoplasms as there is only one case report in the literature of a black sea bass in which surgery was performed to remove a seminoma diagnosed by histological examination. In that case, surgery was successful, as an improvement in the patient's vital parameters and the absence of a recurrence of the neoplasm during follow-up diagnostic investigations eight weeks after surgery have been described (Weisse et al. 2002). The cases described in the literature concerning surgical procedures for the removal of seminomas in koi carp and their post-operative prognosis are rare. The present report does not provide any further information in this respect as the koi died during the surgical procedure. There are currently studies in the literature in which new anaesthetic protocols are being tested with the aim of reducing the already high anaesthesiological risk in fish. This risk depends on several factors such as the sensitivity of these species to anaesthetics, drug dosage, anaesthesia monitoring and post-operative hospitalisation (Gladden et al. 2010).