Stephanurus dentatus are stout-bodied worms (2–4.5 cm long) found encysted in pairs along the ureters in the perirenal fat and in the kidney. The kidney worm is found worldwide, particularly in tropical and subtropical areas. It is primarily a parasite of swine raised outdoors in the southeastern and south central USA. The eggs hatch shortly after being passed in the urine and reach the infective stage in 3–5 days. The larvae are susceptible to temperature extremes, desiccation, and sunlight. Infection is by skin penetration or ingestion of the infective larvae (earthworms may serve as paratenic hosts). The larvae enter the liver, where they migrate extensively for 3–9 mo. Larvae then penetrate the capsule and migrate through the peritoneal cavity to the perirenal area, where they find their way into the ureters, kidney, and perirenal fat. Occasionally, some larvae errantly migrate to other tissues and organs or to developing fetuses. Infections usually become patent in 9–16 mo but may be found as early as 6 mo.
When present in large numbers, kidney worms may adversely affect growth. The principal economic loss results from condemnation of organs and tissues affected by migrating larvae. The most severe lesions are usually in the liver, which shows cirrhosis, scar formation, extensive thrombosis of the portal vessels, and a variable amount of necrosis. Kidney and lung damage are also possible.
When worms are in the kidney or in cysts that open into the ureter, eggs may be recovered in the urine. Prepatent infections are difficult to diagnose, and a definitive diagnosis depends on demonstration of the worms or lesions at necropsy.
Good control practices are indicated in areas where the worm is found. Because of the long prepatent period, control may be achieved with a “gilts only” breeding program, which prevents patent infection from developing. Older boars are replaced with young boars from clean herds, and only gilts are bred and then sold after weaning. Eradication is possible within 2 yr. More commonly, anthelmintics and sanitation (rearing on concrete or in confinement) are used to control kidney worm.
Ivermectin (in-feed for 7 days at 1.8 g/ton), fenbendazole (in-feed for 3–12 days at 9 mg/kg/day), or levamisole (in-feed at 0.36 g/ton) are effective against Stephanurus sp. Doramectin (single injection at 300 mcg/kg) is also approved for use against this worm.