Streptococcus equi subspecies equi pyelonephritis in a pony

Streptococcus equi subspecies equi (SEE) causes a highly contagious disease colloquially known as "strangles" in equids. This disease is typically characterized by pyrexia, mucopurulent nasal discharge, and abscessation of the submandibular and retropharyngeal lymph nodes. Although typically localized to the upper respiratory tract, the infection can disseminate via lymphatic or hematogenous routes, leading to disseminated strangles involving peripheral lymph nodes and thoracic and abdominal organs.

A 21-year-old Welsh Pony cross gelding presented for urine dribbling and presumptive cystolithiasis due to urine retention and bladder distention. Urinary catheterization and urethrocystoscopy revealed a large volume of hematuria, no urinary calculi, and an abnormal ureteral opening. Imaging revealed nonspecific renal abnormalities consistent with chronic kidney disease, with secondary acute kidney injury, and a mass near the aortic bifurcation. Aerobic urine culture yielded SEE identified via matrix-assisted laser desorption/ionization time-of-flight mass spectrometry with a score >2.0 and confirmed by PCR. The pony was euthanized due to poor prognosis, and autopsy results revealed pituitary pars intermedia hyperplasia and microadenoma and neutrophilic, hemorrhagic, and necrotizing pyelonephritis. The pyelonephritis is likely due to hematogenous or lymphatic dissemination of the organism to the kidney. We speculate that immunosuppression related to pars pituitary intermedia dysfunction may have predisposed this animal to this atypical manifestation.

There are limited reports of metastatic strangles presenting as pyelonephritis in equids. This case highlights the potential for a common equine pathogen to manifest as an unexpected presentation and underscores the need to consider strangles in the differential diagnosis of equine renal disease.