Breed: Bichon Frise
Gender: MALE, INTACT
Source/History: 2 cm intradermal mass at right lateral thigh. History of 1 cm lipomas at the left sternum and left lateral thorax just caudal to axilla.Click here for the IDEXX Pathology Report on this case.
Slides are moderately cellular and consist of inflammatory cells in a pale blue tissue fluid background with scant to moderate blood (image 1). Inflammatory cells consist of variable proportions of histiocytes, small lymphocytes and non-degenerate neutrophils. Histiocytes are often noted phagocytizing globular magenta material (image 3) which is also noted free in the background (images 3,4). Binucleated and multinucleated giant form histiocytes (image 4) are also noted. No infectious organisms or overtly neoplastic cells are seen.
Mixed inflammation with foreign material
The magenta material is most consistent with vaccine adjuvant (added to vaccines to enhance immunogenicity) and this is most likely a local injection site/vaccine reaction. These typically can take up to 3 months to resolve; however, the lesion should be getting smaller as time passes. If the lesion fails to regress, biopsy with histopathology is recommended.
Injection site reactions are uncommon in both dogs and cats. These reactions may occur weeks to months after vaccination. The underlying etiology is suspected to be a combination of foreign body (granulomatous inflammation with giant cells) and Type IV or delayed hypersensitivity (lymphocytic +/- eosinophilic inflammation) reactions. Vaccines are the most common cause, but other injections (antibiotics, potassium bromide, etc.) can have similar reactions. Grossly these lesions are solitary subcutaneous nodules that vary greatly in size, are freely moveable or fixed, and can be soft or firm. Occasional pain and alopecia may be noted. Differential diagnoses include neoplasia, cyst, and traumatic panniculitis.
Gross TL, Ihrke PJ, Walder EJ, et al: Skin Diseases of the Dog and Cat, Clinical and Histopathologic Diagnosis. 2nd ed. Oxford UK, Blackwell Science, 2005; pp.541-542.
Moore GE, HogenEsch H. Adverse vaccinal events in dogs and cats. Vet Clin Small Anim. 2010; 40(3); 393-407.