Sterile nodular panniculitis (SNP) is a sterile inflammatory process involving the subcutaneous fatty tissue. This sterile condition is rare in dogs when compared to the panniculitis resulting from infectious organisms embedded in subcutaneous fat, introduced through puncture wounds or other traumas.
The subcutaneous fat undergoes hydrolysis resulting in the formation of glycerol and free fatty acids, which are pro-inflammatory. It is often difficult to identify the trigger in an individual; however, the condition has been associated with pancreatitis, pancreatic neoplasia and chronic skin disease (German et al., 2003; Gear et al., 2006; Kim et al., 2011). Other possible triggers are hepatic disease, systemic lupus erythematosus, lymphoplasmacytic colitis and adverse drug reactions (Miller et al., 2013).
Some dogs with SNP present with fever, anorexia and lethargy. Other signs such as vomiting, abdominal pain or discomfort, or even joint pain, have been reported in dogs with concurrent pancreatic disease.
Although there are no age or sex predilections, some breeds of dogs such as Dachshunds, Miniature Poodles and Collies are at increased risk of developing SNP.
Cutaneous lesions are characterised by single or multiple nodules that may be well circumscribed (Figure 1), or ill-defined. The distribution is mainly truncal but can occur at other sites (Figure 2). The nodules in the early stage of the disease are firm, and as the fatty tissue undergoes liquification they become soft and fluctuant. Some nodules may regress, others rupture and ulcerate and some develop draining sinuses that produce an oily or seropurulent discharge, sometimes with blood (Figure 3). Ulcerated lesions can subsequently become secondarily infected.
The history and clinical signs together with in-house and external laboratory work-up are all required to reach a reliable diagnosis. It is important to rule out infectious agents in all cases before arriving at a diagnosis of SNP.
Cytological examination of exudate and/or fine needle aspirates reveal neutrophils and foamy macrophages without any microorganisms (Figure 4). However, these findings by themselves are not sufficient to rule out infectious agents, so excision biopsy samples should be submitted for histology and bacterial and fungal cultures.
Histological findings reveal lobular inflammation of the panniculus in the early stages becoming more diffuse as the disease progresses. The predominant inflammatory cells are neutrophils and foamy macrophages. Obvious bacterial and fungal organisms are absent, but this should be confirmed using special stains such as Ziehl–Neelsen acid-fast stain, Gram stain and periodic acid–Schiff stains.
Additional tests are recommended and include haematology, biochemistry and urinalysis. In cases where pancreatic disease is suspected, serum amylase and lipase should be performed. Diagnostic imaging may be indicated in some cases especially when pancreatic neoplasia is suspected.
The first-line treatment for rapid results is prednisolone, 1 to 2mg/kg q24h until the lesions regress and then reduced to every other day. The aim is to reduce the total dosage and the frequency to the minimum effective dose and then to cease treatment once all the lesions have resolved. The prognosis is good and most lesions regress. In cases where longer-term treatment is required, ciclosporin 5mg/kg q24h is an option.
Other treatments such as vitamin E 400IU, azathioprine and tetracycline/nicotinamide combinations have been suggested; however, in the current climate of concern about antimicrobial resistance, the use of both tetracycline and doxycycline as immunomodulating agents is not recommended.
Favourable response to topical and intralesional dexamethasone has been seen, when used as alternative treatments (Kim et al., 2011).
Surgical excision of single nodules is successful in most cases.