Case of the month #2
by Francisco Conrado (DipACVP)
A 4-year-old spayed female German Shepherd dog was presented for evaluation of chronic, non-healing ulcerations and mass-like lesions affecting the entire right hindlimb. The lesions included necrotic areas and draining tracts. Despite approximately four months of treatment with various courses of antibiotics and anti-inflammatory medications, significant improvement was not achieved. Due to a lack of response and concerns about her quality of life, the owners opted for limb amputation.
Approximately two months post-amputation, the dog was presented again for evaluation of the surgical incision site (see gross image). Examination revealed an oblong, firm, warm, and erythematous soft tissue swelling. The swelling featured multifocal punctate draining tracts with eschar overlying the previously healed incision.
To further investigate the nature of the swelling, a fine needle aspirate (FNA) was performed and stained with aqueous Wright-Giemsa. The FNA results are shown at various magnifications (see images attached). What’s your diagnosis?
Fig 1 A-D: Macroscopic and cytological appearance of a non-healing ulcerations and mass-like lesions affecting the entire right hindlimb, Wright Giemsa, 50x
Cutaneous pythiosis.
This is cutaneous pythiosis, as indicated by cytologic findings of marked mixed inflammation (predominantly pyogranulomatous, with lesser eosinophilic, lymphoplasmacytic, and mastocytic components) and intralesional pseudofungal/oomycetal organisms. Microscopically, Pythium spp exhibits fungal-like hyphal structures but is an infective aquatic oomycete, phylogenetically closer to diatomeae and algae than true fungi. Similar oomycete organisms include Lagenidium spp and Paralagenidium spp. Despite cytomorphological challenges in specific speciation, a positive serological test for Pythium spp with a titer of 62% at a 1:1000 dilution supported the diagnosis in this case.
Cytologically, the organisms appear as negatively stained, large branching hyphal-like structures, occasionally slightly basophilic. They may exhibit slight ballooning with bulbous extremities or appear more linear and nonbranching, with a diameter ranging from 5 to 12 μm. This dog’s previously amputated leg had numerous cutaneous and subcutaneous lesions with marked pyogranulomatous inflammation and intralesional hyphal-like structures, as depicted in attached histopathologic photomicrographs. Surgical removal and histopathologic evaluation of the right inguinal and medial iliac lymph nodes indicated disease dissemination.
Fig 2A-B Special stain and histopathological picture of a non-healing ulcerations and mass-like lesions affecting the entire right hindlimb.
Pythiosis, sometimes called “swamp cancer,” is a rare but serious disease caused by an oomycetal organism that naturally inhabits wetlands, ponds, and swamps. It usually occurs in the southeastern United States, particularly in swampy areas around the Gulf of Mexico, though cases have been diagnosed as far west as California and even in dogs with no significant travel history. It has global geographic distribution, with cases reported in several countries of Southeast Asia, South, Central, and North Americas, and the Caribbean islands. It is a zoonotic disease, with cases reported in humans and several species, but most cases occur in otherwise immunocompetent horses and dogs. The infection develops in the form of a cutaneous or gastrointestinal disease, although cases of localized lung and bone infection, as well as systemic dissemination through lymph nodes are also described. Gastrointestinal pythiosis causes weight loss, vomiting, diarrhea, and abdominal pain, and is invariably fatal. Cutaneous pythiosis forms non-healing, ulcerating wounds on the legs, tail, head, and other areas. Early diagnosis and treatment are crucial. Prognosis in dogs is guarded due to misdiagnoses and variable efficacy of available treatments.
Currently, there is no consensus on the “gold standard” treatment for pythiosis, with aggressive surgical resection being recommended whenever possible, followed by adjunctive long-term medical therapy to control microscopic disease and prevent recurrence. Adjunctive medical therapy historically consists of systemic antifungals variably combined with prednisone. However, the reported efficacy of antifungals is poor. Because of the lack of definitive, predictable, and effective treatment options, alternative therapies have been sought in the treatment of pythiosis. Mefenoxam, an agricultural antifungal and anti-oomycotic compound, has gained popularity as a potential breakthrough therapy in the treatment of this disease. The dog in this case, despite treatment with a variety of medications, including prednisone and mefenoxam, had recurrence of the disease and was humanely euthanized.
Here are some references that might be useful.
On the overall features of this disease:
https://pubmed.ncbi.nlm.nih.gov/20800978/
https://pubmed.ncbi.nlm.nih.gov/12910739/
https://pubmed.ncbi.nlm.nih.gov/35817539/
On the geographic distribution of Pythium:
https://pubmed.ncbi.nlm.nih.gov/35205934/
https://pubmed.ncbi.nlm.nih.gov/34968184/
On current treatment strategies: