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The aim of this study was to investigate the incidence of spontaneous tumours in guinea pigs in biopsy and necropsy samples in the monitored period (2016--2022) on a representative number of cases and to determine their occurrence in relation to age and sex of the animals. A total of 204 samples from various pathological lesions were examined, while 153 cases (75.00%) were true tumours and 51 cases (25.00%) were pseudotumorous non-neoplastic lesions. The distribution of the incidence of true neoplasms in relation to sex was uneven. In the examined group, more neoplasms were diagnosed in males (n = 88, 57.52%) compared to females (n = 65, 42.48%). The mean age of guinea pigs with diagnosed tumour in our group was 4.2 years. Thyroid adenocarcinoma was the most frequent tumour. The majority of thyroid tumours (67.86%) were diagnosed in animals aged 4--5 years, as a significant result in terms of incidence in this age range, as well as a predisposition to tumour formation in males (78.57%). Mammary gland tumours occurred in the age range of 2--7 years, with a mean age of incidence of 4.7 years. In all cases, these were malignant variants and showing a significant higher incidence in males, in the age range of 4--5 years. The processing of a representative number of samples and epidemiological data on the incidence of spontaneous tumours in guinea pigs can contribute to the overall expansion of knowledge about this topic in veterinary medicine. [ABSTRACT FROM AUTHOR]

Equine melanocytic tumours are of great concern for horse owners, especially those who own grey horses. These tumours can be initially benign but data from literature suggest that all equine melanocytic tumours have a potential to undergo a malignant transformation and even develop distant metastases. This study aimed to diagnose and classify equine melanocytic tumours, assess their prevalence in relation to the horse's coat colour, sex, and age, and evaluate their occurrence sites. Over the period of four years (2020--2023), biopsies and necropsies of 203 horses were performed; melanocytic tumours were diagnosed in 34 cases (16.75%). Based on the morphological features of the neoplastic cells and presence of distant metastases, 3 horses (8.82%) were diagnosed with early-stage melanocytic lesions, 10 (29.41%) with melanocytomas, 4 (11.76%) with low-grade melanoma, 8 (23.53%) with intermediate-grade melanoma, 6 (17.65%) with metastatic melanoma and 3 (8.82%) with high-grade anaplastic melanoma. There was a significantly higher prevalence of the most malignant types of melanocytic tumours (metastatic melanoma, high-grade anaplastic melanoma) in horses over the age of 15 years (70%, P < 0.05). [ABSTRACT FROM AUTHOR]