Dioctophyme renale, or, the Giant Kidney Worm, in the kidney of a dog
Dioctophyme renale is the largest parasite to infect humans, though its natural host is wild carnivores. Humans can contract the parasite by eating raw or undercooked fish or dog meat, and infection is most prevalent around the Caspian Sea and Iran.
Significant pain around the lower back, loin, and groin is often endured by those afflicted, though the fact that generally only one kidney is infected means that their kidney function is not completely lost.
As the female worms lay eggs, those eggs are excreted in the urine, which ends up in local waterways, and eventually in the environments of fish once again, re-starting the cycle of infection. The eggs in the urine are also how doctors diagnose infection with Dioctophyme renale. Once diagnosed, surgical removal is the only cure - if one leaves the worm in there indefinitely, their maximum lifespan is 5 years, and they will decompose and destroy local tissue once they die.
Top: The Principles of Pathology. J. George Adami, 1912.
Bottom: Dioctophyme renale removed from medium-sized dog, Vetnext.com.