Mink are the most common definitive host for Dioctophyma renale, the largest known nematode, which has a worldwide distribution. Many other species, including dogs and people, can become infected. The definitive host contracts the parasite by ingesting encysted larvae in raw fish (eg, pike, bullhead) or frogs or by ingesting an infected annelid worm. The larvae penetrate the bowel wall and migrate first to the liver and later to the kidneys. In dogs, the parasite often fails to reach the kidneys and may be found free in the abdominal cavity. Kidney worms grow larger in dogs than in mink, reaching up to 103 cm.
Female worms are larger than male worms, and both are blood red. Both male and female worms must be present in the same kidney to complete the life cycle. Barrel-shaped, yellow-brown eggs with a thick, pitted shell measuring 71–84 × 45–52 μm are shed into the urine.
In the kidneys, the worm(s) cause obstruction, hydronephrosis, and destruction of the renal parenchyma. The right kidney is most commonly affected. Kidney failure can result if both kidneys are parasitized. Chronic peritonitis, adhesions, and liver disease are also possible. Clinical signs are hematuria, pollakiuria, weight loss, and renal or abdominal pain. Urinalysis may reveal proteinuria, hematuria, and pyuria. Ultrasonography or IV pyelography shows the enlarged hydronephrotic kidney(s). The diagnosis is made by finding the eggs in the urine sediment if both sexes of the nematode are present in the kidney and the ureter is patent. Alternatively, exploratory laparotomy may reveal the diagnosis. Worms may be found in the peritoneal cavity, between the lobes of the liver, or within the affected kidney(s) via nephrotomy.
Unilateral nephrectomy is the treatment of choice if the opposite kidney is unaffected. Preventing ingestion of raw fish or other aquatic organisms is recommended in areas where the parasite is known to infect wild animals.