Small mammals, including guinea pigs (Cavia porcellus), are increasingly popular as companion animals and are commonly presented for veterinary care. UK Pet Food released pet population data in April 2022 and guinea pigs were reported to be the fifth most common household pet, with an estimated 900,000 living in homes in the UK in 2021 [1].

There are relatively few publications detailing spontaneously arising disease or reporting the prevalence of different conditions in guinea pigs when kept as either pets or as laboratory animals. In terms of spontaneously arising disease in pet guinea pigs, recent large-scale investigations have started to address this, with one retrospective study in Poland [2] looking for the presence of clinical disease in 1,000 guinea pigs kept as pets. This study reported a high prevalence of dental disease, skin disease and diseases of the genital system and eyes, in particular. Another study from Poland [3] investigated cutaneous and subcutaneous tumours of small pet mammals generally, including guinea pigs, rats, rabbits, ferrets, hamsters and other species, and found that lipomas were the most commonly arising tumours in guinea pigs.

The aim of the current study was to analyse retrospectively a large number of biopsy samples submitted for histopathology from pet guinea pigs that had clinically presented with externally palpable masses. The study 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.