Heterobilharzia americana infection as a cause of hepatic parasitic granulomas in a horse.

The research article discusses a case regarding a 22-year-old American Paint Horse gelding, which despite being treated for a perirectal squamous cell carcinoma, was severely infected by the parasite Heterobilharzia americana leading to numerous disseminated granulomas in its liver, presenting a starry-sky appearance, and resulting in its euthanization.

Case Description & Clinical Findings

• The horse, from the Gulf Coast region of Texas, was evaluated for regrowth of a perirectal squamous cell carcinoma, which had been surgically removed 11 months before.
• A necrotic and ulcerated mass was discovered below the horse’s anus, which upon histologic examination revealed as squamous cell carcinoma. Complications included paraphimosis and difficulty in urination.
• Due to the non-feasibility of adequately resecting the mass and its closeness to the urethra, the horse was put to rest.

Necropsy Outcome & Testing

• During necropsy, along with the carcinoma, hundreds of nodules ranging from white to pale yellow were discovered throughout the horse’s liver, creating a unique starry-sky appearance. Similar granulomas were observed in the right caudal lung lobe and small intestinal serosa.
• Most intriguing was the presence of a pair of adult schistosomes within a singular, larger-sized granuloma in the horse’s liver.
• Hepatic granuloma specimens were subjected to PCR amplification and sequencing, using primers specific to a portion of the Heterobilharzia americana small subunit rRNA gene, resulting in the amplification of a 487-base pair product that had a 100% sequence identity with H. americana.

Clinical Relevance

• The research paper shines a spotlight on the severe cases of liver granulomas in horses, which can result in a liver with an abnormal starry-sky pattern.
• This is notably the first documented report associating the appearance of granulomas with H. americana infection and adult schistosomes in a horse’s liver.