The swine kidney worm (Stephanurus dentatus) is a historically important parasite that is adapted to survive in wild swine populations in warm climates. It is of no importance to pig populations kept in confinement. However, it remains a potential agent of disease in any backyard or extensively reared pigs, and it can occur at high prevalence in wild and feral populations. Infections are mostly subclinical, although they can affect growth and productivity. Diagnosis is via detection of kidney worm eggs in the pig's urine or detection of adult worms at postmortem examination.
Etiology of Swine Kidney Worm Infection
Courtesy of Dr. Ausraful Islam.
Courtesy of Dr. Ausraful Islam.
Stephanurus dentatus are stout-bodied worms (2–4.5 cm long) that encyst in pairs along the ureters in the perirenal fat and in the kidney of pigs. The swine 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 US.
The eggs hatch shortly after being passed in the pig's urine and reach the infective stage in 3–5 days. The larvae are susceptible to temperature extremes, desiccation, and sunlight, and do not survive at temperatures < 5°C. 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 months. 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 can become patent as early as 6 months but the usual time is 9–16 months.
Clinical Findings and Diagnosis of Swine Kidney Worm Infection
Poor growth
Definitive diagnosis via postmortem examination
When present in large numbers, kidney worms may adversely affect the growth of pigs. 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 postmortem examination.
Control of Swine Kidney Worm Infection
Antiparasitic treatments, sanitation, and management changes
Good control practices are indicated in areas where the swine kidney 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 years. More commonly, anthelmintics and sanitation (rearing on concrete or in confinement) are used to control kidney worm.
Administering medications by feed to food animals is heavily regulated in the US. Any antimicrobial administered in feed must be with an approved medicated feed and must occur under a Veterinary Feed Directive (VFD). In the US, ivermectin for swine is available as a medicated Type C VFD feed. If administered to swine via feed, it must occur under a VFD. Ivermectin is only available as a medicated feed combined with either bacitracin1 or lincomycin.2 Both of the combination medicated feeds contain ivermectin at 1.8 g/ton of feed to provide ivermectin at 0.1 mg/kg, daily, for 7 days. Ivermectin injection is not approved for swine kidney worm in the US but extralabel use has shown efficacy as a dosage of 0.5 mg/kg, SC, once. Additionally, doramectin or fenbendazole are effective against this worm. Doramectin (300 mcg/kg, IM) is an FDA-approved over-the-counter drug labeled for use in swine for kidney worm.3 The VFD type C medicated feeds deliver fenbendazole at 10-300 g/ton in feed to provide fenbendazole at 9 mg/kg/day, in feed, for 3–12 days.4 Levamisole (0.36 g/ton, in feed) is effective against adult worms only; however, this represents extralabel use in the US, and all levamisole products labeled for swine have been voluntarily withdrawn from the US market. Treatment should be integrated with management strategies to control or eliminate the parasite in the population. Repeated antihelminthic treatments at 4–6 months should prevent contamination of the environment.
References
Key Points
Swine kidney worm is a parasite limited to wild, feral, or outdoor pig populations, particularly in warm climates.
Swine kidney worms are susceptible to most anthelminthic compounds.
The long prepatent period of swine kidney worm infection facilitates control via management.
For More Information
Iowa State University College of Veterinary Medicine: Kidney Worm Infection
