An 8-month-old hamster was presented with 2 nodules on a forefoot of approximately 1 month's duration. Otherwise the hamster appeared healthy. The nodules were freely movable in the skin. Fine-needle aspiration revealed chronic inflammation (neutrophils and macrophages). A bacterial infection was suspected, and the animal was treated with trimethoprim-sulfamethoxazole and topical antibiotics. After lack of response, the nodules were excised and histologic examination revealed coalescing pyogranulomatous inflammation consistent with a bacterial pseudomycetoma. Gram stain revealed clusters of large gram-positive cocci.

COMMENTARY: One of the most common causes of nodules in hamsters is neoplasia; however, it is important to consider other causes in young animals. This hamster had a bacterial infection of the skin that formed a cutaneous bacterial granuloma or bacterial pseudomycetoma. The pathogenesis is not well-understood, but the disorder is believed to be caused by host-agent interaction in which the host is able to identify and isolate the invader but not eradicate it. Typical of pseudomycetoma is something referred to as Splendore-Hoeppli reaction. Clusters of agents are found in the center surrounded by a brightly eosinophilic material. Pseudomycetomas can also be caused by fungal agents, with the most commonly encountered being Microsporum canis pseudomycetoma in long-haired cats. Treatment usually involves excision of the nodule, culture, and appropriate antimicrobial treatment.

Bacterial pseudomycetoma in dwarf hamster Phodopus sungorus. Martorell J, Gallifa N, Fondevila D, Rabanal RM. VET DERMATOL 17:449-452, 2006.