Massive Unilateral Pyonephrosis with Extreme Leukocytosis in a Cat: A Case Report

Pyonephrosis is a rare, life-threatening suppurative disease of the kidney characterised by the accumulation of purulent exudate within the renal pelvis and the destruction of renal parenchyma. It is typically associated with azotaemia and urinary tract obstruction. Extreme leucocytosis (>100 × 103/µL), known as a leukemoid reaction, has been rarely reported in feline pyonephrosis. A 13-year-old neutered domestic shorthair female cat was brought to a veterinary clinic with symptoms of lethargy and apathy, as well as a significantly enlarged abdominal cavity. Physical examination revealed dehydration and a large, non-painful mass occupying the left abdominal cavity. Haematology revealed severe leucocytosis of 123.22 × 103/µL and anaemia with haematocrit (HCT) 18.6% and serum biochemistry showed elevated SDMA 17 µg/dL, AST 97 U/L, with hypokalaemia, hypochloraemia and hypoproteinaemia. Creatinine (1.5 mg/dL) and blood urea nitrogen (BUN) (23 mg/dL) remained within reference values. Abdominal X-ray and ultrasound revealed a large mass of anechoic structure encompassing the entire left side of the abdominal cavity, consistent with pyonephrosis. Microbiological studies of purulent material obtained by ultrasound-guided fine-needle aspiration and culturing revealed the presence of Escherichia coli. The day after admission, the patient underwent surgical laparotomy with unilateral nephrectomy. Histopathology confirmed severe inflammatory infiltration of the mucosa, connective tissue proliferation and fibrosis, with partial glomerular hyalinisation and without cellular atypia. Six days after surgical intervention, the leukocyte count decreased to 44.81 × 103/µL. The patient was alive and clinically healthy 3 months after surgery. This case illustrates that massive unilateral pyonephrosis can occur in geriatric cats without azotaemia due to the compensatory function of the contralateral kidney. Marked leucocytosis of 123.22 × 103/µL probably represents a leukemoid reaction secondary to severe suppurative inflammation. Moreover, pyonephrosis should be considered in the differential diagnosis of large abdominal masses in cats, even in the presence of a normal creatinine level.