A new research paper, conducted by CVS’ Finn Pathologists and RVC student Rebecca Hederer, has reported on the underlying histopathological diagnoses of over 600 biopsy samples from guinea pig lumps. It is expected to further support clinicians’ understanding of guinea pig health.
Small mammals are increasingly popular as companion animals. Guinea pigs were reported the fifth most common household pet in the UK in 2021. Various skin diseases, including palpable masses, are a major reason for presentation for veterinary care. However, few studies have been published detailing spontaneously arising disease or reporting the prevalence of different conditions when kept as pets – to provide information for clinicians about conditions in this species.
The Lumpy bumpy guinea pigs: a retrospective study of 619 biopsy samples of externally palpable masses submitted from pet guinea pigs for histopathology study aimed to retrospectively analyse a large number of biopsy samples submitted for histopathology from pet guinea pigs that had clinically presented with externally palpable masses. It aimed to determine the most common tissue origins of these masses and the most frequent pathological diagnoses, together with signalment data for the most commonly diagnosed lesions.
Of the 619 samples submitted from 493 animals, 54 (8.7%) had arisen from the mammary glands and 15 (2.4%) from the thyroid glands, with the remaining 550 (88.9%) involving the skin and subcutis, muscle (n = 1), salivary glands (n = 4), lips (n = 2), ears (n = 4) and peripheral lymph nodes (n = 23).
Forty-seven (7.6%) of the masses were diagnosed as inflammatory in nature, including dermatitis, panniculitis, lymphadenitis, cheilitis, myositis, sialoadenitis, abscess and chronic inflammation with osseous metaplasia. Non-neoplastic, non-inflammatory lesions accounted for a further 31 (5.0%), namely cysts, hamartomas, hyperplastic lesions, polyps, ectopic bone formation and salivary gland steatosis.
The remainder of the samples were neoplastic in nature (541; 87.4%), with 99 masses classified as epithelial in origin, 347 as mesenchymal, 23 as round cell, five as melanocytic and eight as unclassified malignant neoplasms (Table 1). Of these, mesenchymal neoplasms were further subdivided into benign (288) and malignant (59). Lipomas were the most common neoplasm diagnosed, accounting for 286 of all samples submitted.
Of all 619 samples included in the study, the eight most commonly diagnosed lesions were lipoma (46.2%), trichofolliculoma (12.3%), sarcoma (of various subtypes: 9.5%), inflammation (all sites 7.6%), lymphoma / leukaemia (3.4%), mammary carcinoma (4%), mammary adenoma (3.2%) and thyroid carcinoma (1.5%). Overall these eight diagnoses accounted for 87.7% of all submissions from the externally palpable masses from the pet guinea pigs submitted for histopathology.
Melanie Dobromylskyj, Diagnostic Histopathologist at Finn Pathologists who authored the study, said: “Guinea pigs are an increasingly popular household pet. Skin diseases, including palpable masses, are a common reason for veterinary presentation. But few studies have been published to date that provide information for clinicians about these conditions.
“This study illustrates the importance of neoplasms of the skin and subcutis in pet guinea pigs, with over 87% of the cases analysed having one of eight diagnoses. While most of these neoplasms are benign, a substantial minority are malignant and identifying those lesions is especially important in treatment planning. Clinicians dealing with pet guinea pigs should also be aware that some externally palpable masses may arise from the thyroid or mammary glands rather than the skin or subcutis, and that male guinea pigs may also develop mammary lesions.”
To conduct the study, biopsy data on externally palpable masses in pet guinea pigs were retrieved from the archives of Finn Pathology’s laboratory between November 2013 and July 2021. Samples from guinea pigs comprised 0.24% of all submissions to the laboratory over the study period. The total number of histopathology submissions from guinea pigs with externally palpable masses received by the laboratory was 619 from 493 individual animals. For the 415 animals for which the age was specified on the submission form, the median age was 3 years (range 5–152 months). Of the 493 individual animals in the study, 249 (50.5%) were recorded as female (of which 19 were recorded as neutered) and 195 (39.6%) as male (29 neutered); the sex was not indicated on the submission form for the remaining 49 (9.9%) individuals.
The Lumpy bumpy guinea pigs: a retrospective study of 619 biopsy samples of externally palpable masses submitted from pet guinea pigs for histopathology paper has been selected as ‘Editor’s Choice’ by the Journal of Comparative Pathology and will be free to access until September on Lumpy, bumpy guinea pigs: a retrospective study of 619 biopsy samples of externally palpable masses submitted from pet guinea pigs for histopathology – ScienceDirect.
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 As part of her final year studies
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 A. Minarikova, K. Hauptman, E. Jeklova, Z. Knotek, V. Jekl. Diseases in pet Guinea pigs: a retrospective study in 1000 animal. Vet Rec, 177 (2015), p. 200.
 Masses arising from mammary glands accounted for 54 lesions in the study, with some guinea pigs having more than one mammary lesion biopsied. Of those 54 masses, 28 came from females (5 neutered), 23 from males (1 neutered) and three were from cases for which the sex had not been specified.