Kidney Worm
LEVELS: Highly unlikely; No human illness; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | stephanurus_dentatus |
| Type | parasite |
| Scientific name | Stephanurus dentatus |
| NCBI taxid | 321369 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Stephanurus dentatus, the kidney worm, is a large nematode whose adults live in cysts around the kidney and ureters, shedding eggs into the urine rather than the faeces. Infective larvae survive on pasture for several months and either are eaten or bore through the skin. What makes the parasite destructive is the journey the larvae take to reach the kidney: they wander widely through the abdomen and liver, leaving necrotic and haemorrhagic tracts behind them with considerable abscess formation and scarring, and the liver spots they cause are much more prominent than the roundworm's. Livers and kidneys are condemned as a result. The prepatent period is very long, nine to twelve months, so a young pig can carry the migrating stages for most of its life without shedding an egg. Confinement rearing has dramatically reduced the incidence, and where it persists a gilts-only breeding system — selling every female before she is old enough to shed — has eradicated it in under two years. Avermectins, fenbendazole and levamisole are all effective. No human infection is described.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Highly unlikely: Human infection with the agent has never been reported, or has been reported but is not attributed to pig or pork exposure
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c1l1, I cannot find any evidence that suggests S.dentatus can infect humans"
Basis. Section 61.5.1 covers the kidney worm in full and makes no mention of human infection.
How this level was decided
INFERRED from the chapter structured silence. Level 1.
C2 Zoonotic impact
Severity of human illness caused by the agent
No human illness: Agent does not cause illness in people
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c2l1, I cannot find any evidence that suggests S.dentatus can infect humans"
Basis. No human disease is described in 61.5.1.
How this level was decided
INFERRED. C2 scores level 1 whenever the agent does not cause disease in people.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Housing alone keeps it out: Raising pigs indoors and off soil breaks the cycle, so no ongoing biosecurity program is needed and only outdoor herds stay exposed
Basis. ch61 61.5.1: eggs "are found in urine". "Infective larvae may survive on pasture for up to several months and can either be ingested or penetrate the skin." "confinement rearing has dramatically reduced the incidence of kidney worm." 61.7.1: a gilts-only breeding system "was used to achieve the eradication of kidney worms in less than 2 years".
How this level was decided
MOVED L3 -> L2 on 2026-09-01, and this is the clearest of the eight because the chapter proves it directly rather than by inference. Larvae come from the urine of infected pigs and persist on pasture for months, but the kidney worm has been effectively eliminated from US commercial production by confinement -- the change in housing did what no anthelmintic programme did. An agent that the industry has already excluded by a housing decision is the definition of level 2. Scored under the 2026-09-01 C3 rework: level 2 is where the farm's own housing decision closes the route permanently, and an outdoor herd's exposure is a consequence of that production choice rather than a property of the agent. Eric: "accept being an outdoor herd itself is the biosecurity failure/risk, not the agent per se, and covers things like most gut parasites." Level 3 is reserved for routes no housing system blocks. CONFIRMED AT L2 by the five-level rework later the same day. The four-level L2 had held two different amounts of work -- this case, where housing alone does it, and agents like progressive atrophic rhinitis that need a running biosecurity program and are equally excludable from an outdoor herd. Eric spotted it: the qualifier "but outdoor herds may still be exposed" is true here and false there. Those agents moved up to the new L3 and this one stayed, on a level that now says only what is true of it.
C4 Detection difficulty
Difficulty of recognizing and confirming infection
Laboratory-dependent: The presentation does not point to this disease specifically, but local or regional laboratories can confirm it with a validated assay once it is suspected
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "l2, straightforward to diagnose"
Basis. ch61 61.5.1: "Their eggs are typical for strongyles but can be differentiated from those of hookworms, nodular worms, and other strongyles because they are found in urine." "The prepatent period is long (9-12 months)." "Migrating larvae produce necrohemorrhagic tracts wherever they go... including much more prominent hepatic milk spots than those caused by ascarids." 61.2.3.2 adds that ascarid milk spots "must be differentiated from fibrosis associated with kidney worm migration".
How this level was decided
The egg is distinctive once found, because it appears in urine rather than faeces -- but that means routine faecal flotation misses it entirely and nobody screens pig urine. A prepatent period of nine to twelve months means a growing pig can be slaughtered before any egg appears. Diagnosis in practice is the liver and kidney lesions at slaughter, which must be told apart from ascarid milk spots. Not recognised clinically, confirmable locally once suspected. Level 2.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5l1, this disease is not a feature of modern US production systems and is unlikely to rank as a costly disease even in the rare instances where it may be present in some outdoor herds."
Basis. ch61 61.5.1: "confinement rearing has dramatically reduced the incidence of kidney worm." "Migrating larvae produce necrohemorrhagic tracts wherever they go, producing considerable abscessation, eosinophilia, and fibrosis." "In systems where the problem persists, a gilts only breeding system can be implemented in order to control the parasite."
How this level was decided
INFERRED. Where it occurs the damage is real -- the chapter calls the migratory lesions considerable and worse than ascarid milk spots -- but it is scored on modern US production under the standing note SCORING/production_cost_impact, and the chapter says confinement has dramatically reduced incidence. Level 1. FLAGGED as the cell in this batch where the note costs the most information: this is a genuinely damaging parasite that housing has removed rather than one that was never important, and 61.7.1 records that a gilts-only system eradicated it in under two years where it did persist.
C6 Market impact
Duration of material disruption to pork and pig markets
Negligible: Little or no material disruption to supply or demand when disease occurs on one or more farms
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c6l1, this disease is not a feature of modern US production systems and is unlikely have any impact on markets if/when it occurs."
Basis. Section 61.5.1 records no trade measure, movement control or consumer response. Liver and kidney condemnation is a carcass-level cost.
How this level was decided
INFERRED. Organ condemnation affects the individual carcass, not the pork or pig market, following ascaris_suum. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Little: Effective pathogen-directed treatment is available and works, or animals recover acceptably without it
Basis. ch61 61.7.2.1: doramectin is labelled for "Stephanurus (adults)". 61.7.2.2: fenbendazole is labelled for Stephanurus in the United States. 61.7.2.3: levamisole "is effective for the treatment of Ascaris, Oesophagostomum, Metastrongylus, Strongyloides, and Stephanurus."
How this level was decided
Three effective options, two of them US-labelled for this parasite by name. Effective treatment is available and works. Level 1. NOTE: the chapter prefers a management solution -- the gilts-only breeding system that exploits the long prepatent period -- but that does not change the availability of a working anthelmintic.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Available, or not needed: Effective vaccines are widely available in the US or secured for national outbreak response, or the disease does not clinically or economically justify vaccinating
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c8l1, there is no market imperative to develop a vaccine for this disease"
Basis. No kidney worm vaccine is described in ch61. Control is by confinement, the gilts-only breeding system, and anthelmintics.
How this level was decided
INFERRED. No vaccine exists and none is warranted: confinement has already removed the parasite from most US production, a gilts-only system eradicates it where it persists, and two labelled anthelmintics treat it. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/stephanurus_dentatus.yml; the overview is authored in data/overviews/stephanurus_dentatus.md. Do not hand-edit this page — corrections go in data/assignments/CORRECTIONS.yml.
Quoted material marked Basis is from Zimmerman JJ, Karriker LA, Ramirez A, Schwartz KJ, Stevenson GW, Zhang J, eds. Diseases of Swine, 12th edition. Hoboken: Wiley Blackwell, 2025 — except where another source is named in the quotation itself.