History
A 4-month-old sexually intact female Roborovski hamster (Phodopus roborovskii) was evaluated because of multiple firm subcutaneous nodules in all 4 feet. The nodules were first identified 1 month earlier and had rapidly increased in size. The hamster was reared alone in a plastic cage with no toys or ornaments. The hamster was fed a commercial diet, and its bedding consisted of pine wood shavings.
Clinical and Gross Findings
On physical examination, all 4 feet were swollen and erythematous with multifocal to coalescing masses that were firm, nonpruritic, and sessile (Figure 1). These changes extended to the carpal and tarsal joints. Cytologic examination of fine-needle aspirates revealed mixed populations of degenerate neutrophils and macrophages; occasional macrophages contained intracytoplasmic coccoid bacteria. Bacteriologic culture of the masses revealed coagulase-positive, methicillin-susceptible Staphylococcus intermedius. Treatment with enrofloxacin (5 mg/kg, IM, q 12 h) was initiated, but during the following month, the lesions grew more severe and the owner elected euthanasia. At necropsy, all 4 feet contained multiple coalescing subcutaneous masses up to the level of the carpal and tarsal joints; the masses often extended into the underlying muscle. No other gross abnormalities were observed. Tissue samples were collected, fixed in neutral-buffered 10% formalin, and submitted for histologic examination.
Photographs of the forefeet of a Roborovski hamster (Phodopus roborovskii) that was evaluated because of multiple firm subcutaneous nodules in all 4 feet. In both forefeet, multiple erythematous coalescing nodules extend to the carpal joints; similar nodules were present in the hind feet. Inset—A view of the right forefoot at higher magnification.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Photographs of the forefeet of a Roborovski hamster (Phodopus roborovskii) that was evaluated because of multiple firm subcutaneous nodules in all 4 feet. In both forefeet, multiple erythematous coalescing nodules extend to the carpal joints; similar nodules were present in the hind feet. Inset—A view of the right forefoot at higher magnification.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Photographs of the forefeet of a Roborovski hamster (Phodopus roborovskii) that was evaluated because of multiple firm subcutaneous nodules in all 4 feet. In both forefeet, multiple erythematous coalescing nodules extend to the carpal joints; similar nodules were present in the hind feet. Inset—A view of the right forefoot at higher magnification.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →
Histopathologic Findings
Microscopically, the nodules consisted of multi-focal to coalescing pyogranulomas in the dermis and panniculus, which occasionally extended into the underlying skeletal muscle. The pyogranulomas were characterized by a deeply basophilic granular center containing densely packed, gram-positive coccoid bacteria surrounded by a rim of dense, brightly eosinophilic, radiating and clubbed material (a result of the Splendore-Hoeppli phenomenon; Figure 2). These microscopic findings were consistent with cutaneous staphylococcal pseudomycetoma.
Photomicrographs of a section of a subcutaneous nodule from the right forefoot of the hamster in Figure 1. A—Microscopically, there was evidence of multifocal to coalescing areas of pyogranulomatous inflammation with multiple densely basophilic cellular cores. B—At higher magnification, centrally located bacterial colonies (asterisk) surrounded by a rim of dense, brightly eosinophilic, radiating to clubbed material (Splendore-Hoeppli phenomenon [arrow]) are visible. Large numbers of neutrophils and macrophages are present at the periphery. H&E stain; in panels A and B, bar = 200 and 50 μm, respectively.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Photomicrographs of a section of a subcutaneous nodule from the right forefoot of the hamster in Figure 1. A—Microscopically, there was evidence of multifocal to coalescing areas of pyogranulomatous inflammation with multiple densely basophilic cellular cores. B—At higher magnification, centrally located bacterial colonies (asterisk) surrounded by a rim of dense, brightly eosinophilic, radiating to clubbed material (Splendore-Hoeppli phenomenon [arrow]) are visible. Large numbers of neutrophils and macrophages are present at the periphery. H&E stain; in panels A and B, bar = 200 and 50 μm, respectively.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Photomicrographs of a section of a subcutaneous nodule from the right forefoot of the hamster in Figure 1. A—Microscopically, there was evidence of multifocal to coalescing areas of pyogranulomatous inflammation with multiple densely basophilic cellular cores. B—At higher magnification, centrally located bacterial colonies (asterisk) surrounded by a rim of dense, brightly eosinophilic, radiating to clubbed material (Splendore-Hoeppli phenomenon [arrow]) are visible. Large numbers of neutrophils and macrophages are present at the periphery. H&E stain; in panels A and B, bar = 200 and 50 μm, respectively.
Citation: Journal of the American Veterinary Medical Association 239, 5; 10.2460/javma.239.5.583
Morphologic Diagnosis
Severe chronic multifocal to coalescing pyogranulomatous dermatitis and panniculitis with intralesional bacterial cocci (S intermedius) and evidence of the Splendore-Hoeppli phenomenon.
Comments
The genus Phodopus includes 3 species of hamsters: Russian dwarf hamster (Phodopus sungorus), Campbell dwarf hamster (Phodopus campbelli), and Roborovski hamster (P roborovskii). Roborovski hamsters (desert dwarf hamsters) are the smallest and fastest dwarf hamsters. They are sandy-gold in color over the back without the dorsal stripe that is evident in all other dwarf hamsters, have distinctive white eyebrows, and grow to approximately 2 inches long. They are rarely used as laboratory animals but are commonly kept as pets.
Bacterial pseudomycetoma (botryomycosis [from the Greek botryo, meaning grape cluster]) is a rare, chronic, pyogranulomatous disease caused by nonbranching, nonfilamentous bacteria. Staphylococcus spp, Pseudomonas spp, Proteus spp, or Streptococcus spp are commonly isolated from bacterial pseudomycetomas.1 Pseudomycetoma is distinct from mycetoma, which is a chronic granulomatous dermatitis that is caused either by fungi (eumycetoma) or Actinomyces spp (actinomycetoma). Mycetomas have clinical and microscopic characteristics similar to those of pseudomycetomas.2 In addition, bacterial pseudomycetoma should be differentiated from dermatophytic pseudomycetoma. In general, dermatophytic pseudomycetoma develops in Persian cats infected with Microsporum canis,1 although a case involving a domestic longhair cat has been recently reported.3
Bacterial pseudomycetoma may develop in sites other than the skin. Visceral (intra-abdominal) bacterial pseudomycetoma has been reported rarely,4 as has botryomycosis of the lungs,5,6 pericardium,6 and intestines.7 There are reports of cutaneous bacterial pseudomycetoma in several species, including cows,8 dogs,9,10 cats,10 mice,11 horses,12 pigs,13 and hamsters.14,15 Previous cases of this disease in hamsters involved 2 Siberian dwarf hamsters.14,15 In both cases, infection with Staphylococcus spp was suggested as a possible cause on the basis of microscopic examination findings; however, bacteriologic culture of samples from either animal was not performed and the causative agents were not definitively identified. Coagulase-positive S intermedius was cultured from the skin nodules in the hamster of this report. To the authors' knowledge, this is the first definitive identification of a causative agent for pseudomycetoma in hamsters. Staphylococcus intermedius is a commensal bacterium commonly found on the skin and in the oral and nasal cavities that intermittently causes clinical disease. Staphylococcus intermedius has been cultured from cutaneous pseudomycetomas in 2 dogs9 and from an intra-abdominal pseudomycetoma in another dog.4
Microscopically, bacterial pseudomycetomas are characterized by tissue grains (granules). In H&E-stained sections, tissue grains consist of a deeply basophilic granular center (bacterial colonies) surrounded by brightly eosinophilic, radiating or clubbed material.1,9 The Splendore-Hoeppli phenomenon that results in the production of this material is a unique inflammatory process associated with infectious diseases caused by fungi, parasites, and certain bacteria, all of which can cause pseudomycetomas or mycetomas (the latter caused by Actinomyces spp). The reaction can also occur around silk sutures and is sometimes associated with noninfectious diseases such as hypereosinophilic syndrome and allergic conjunctival granulomas in humans.16 The composition of Splendore-Hoeppli material is variable, and its exact nature is not fully understood. Depending on the disease process, the material components can include various combinations of pathogens, antibodies (IgG, IgM, or IgA), complement elements, eosinophilic major basic protein, neutral glycoproteins combined with lipid and calcium, and inflammatory cell debris.16
Bacterial pseudomycetomas are presumed to result from local trauma. Although the hamster of this report had no history of trauma and was raised alone in a plastic cage without toys or ornaments, local trauma is believed to have been the inciting cause. Because all 4 feet were equally affected, the most likely source of trauma was the bedding composed of wood shavings. Wood shavings are a relatively popular choice of bedding material for hamsters, but it has been reported that shavings may cause footpad dermatitis with granulomatous inflammation.17,18 In a recent study,19 bedding material composed of wood shavings caused pawpad wounds in almost all exposed hamsters, although shavings caused fewer injuries than did wood chips.
The treatment of choice for bacterial pseudomycetomas is excision. Systemic antimicrobial treatment alone is generally ineffective because granulomatous inflammation and Splendore-Hoeppli material hinder penetration of antimicrobials to the location of the bacteria in the tissues.9 In the 2 previously reported cases of pseudomycetomas in hamsters14,15 as well as in the hamster of this report, long-term systemic antimicrobial treatment (duration, 10 days to 4 weeks) failed to ameliorate clinical signs and all 3 affected animals were euthanized.
References
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