Threadworm
LEVELS: Highly unlikely; No human illness; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | strongyloides_ransomi |
| Type | parasite |
| Scientific name | Strongyloides ransomi |
| NCBI taxid | 553534 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Strongyloides ransomi, the threadworm, is a minute nematode with a worldwide distribution that is now rarely seen in modern indoor production, and it matters most in tropical and subtropical climates. It is particularly a parasite of the suckling pig, and the reason is an unusual route of infection: larvae lie dormant in the sow's mammary fat until she farrows, then move into the udder and are shed in colostrum and milk. Piglets taken from the sow at birth are worm-free while their nursing littermates are not. Heavy infections in young pigs cause poor growth, haemorrhagic diarrhoea and death. Diagnosis rests on finding larvated eggs on faecal flotation, distinguished from other parasites by culturing them and looking at the hatched larvae; the adult worms are only three to five millimetres long and easily confused with the larval stages of other nematodes. Because transmission is through the sow, the practical control is to treat her ten to fourteen days before farrowing, which also prevents transmission of other nematodes and of mange.
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, no evidence this is a zoonosis"
Basis. Section 61.2.3.1 covers the threadworm in full and makes no mention of human infection. Chapter 61 names the human angle wherever one exists.
How this level was decided
INFERRED from the chapter structured silence. S. ransomi is the pig-adapted species; the human threadworm is S. stercoralis and is not this entry. 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, no evidwence this is a zoonosis"
Basis. No human disease is described in 61.2.3.1.
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
Corrected by Eric from L1 Already in the herd. Reason: MOVED L1 -> L2, and he chose it after being shown the conflict. On the C3 review sheet he wrote "L1, i do not care about carrier pigs, that is overemphasised in the literature. A free farm will remain free, unless it comes into contact with the organism via soil exposure" -- a level 1 with a level 2 reason, and the SAME reason he used in the same sheet to move clostridium_tetani UP to level 2. THE CELL WAS HELD AND PUT TO HIM rather than written, because the existing level 1 rested entirely on the carrier route he was dismissing: ch61 61.2.3.1 has larvae dormant in the sow mammary fat, mobilised at parturition and shed in colostrum, with "piglets separated from their dam at birth are worm-free as opposed to nursing littermates" -- a separation experiment rather than a literature claim. Shown that, he chose level 2. SO THE RULE IS THE ROUTE INTO A CLEAN HERD, NOT THE ROUTE WITHIN AN INFECTED ONE. Transcolostral transmission is how the parasite moves from sow to piglet once a herd has it; it is not how a free herd acquires it, and a free herd acquires it from soil. That distinction now governs any agent whose within-herd route and between-herd route differ. Strongyloides joins the other soil helminths at level 2 rather than standing alone at level 1.
Basis. ch61 61.2.3.1: "The most important route of infection in neonatal pigs is transcolostral. In sows, larvae accumulate in a hypobiotic state in the mammary fat until the time of parturition, whereupon the larvae become mobilized, enter mammary alveoli, and are shed in colostrum and milk." "Piglets separated from their dam at birth are worm-free as opposed to nursing littermates."
How this level was decided
ALONE IN THIS BATCH AT LEVEL 1, and the chapter is unusually clean about why. The dominant route is not the environment at all -- larvae sit dormant in the sow mammary fat and pass to the piglet in colostrum, which the separated-littermate observation proves directly. The parasite is carried by the pigs themselves, from dam to offspring, so there is no introduction event to prevent. Every other helminth in the ch61 batch is scored level 4 on an environmental or intermediate-host reservoir; this one is genuinely different.
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 61.2.3.1: "Diagnosis of patent infections is by observation of embryonated (larvated) eggs in fecal flotation. However, these larvated eggs must be differentiated from other parasites such as spiruroids. This can be accomplished by culturing eggs to allow hatching and observation of rhabditiform L1 larvae." "the adult nematodes are so small that they may be difficult to identify amidst ingesta." "Adult Strongyloides may be found in mucosal scrapings, but the adults are very small (3-5 mm) and may be confused with larval stages of other nematodes."
How this level was decided
Flotation is local and routine, but the larvated egg has to be told from a spiruroid egg by culturing it out, and the adult is small enough to be mistaken for another worm larva. The clinical picture -- scour in suckling pigs -- points at half a dozen commoner agents first. Not recognised clinically, confirmable locally with an extra step. 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, disease is exceedingly rare in modern production systems, negligible effect"
Basis. ch61 61.2.3.1: "Strongyloides ransomi, the minute threadworm, is a rhabditoid nematode that has a cosmopolitan distribution but is rarely present in modern indoor swine production facilities. It is particularly pathogenic in suckling pigs and has a greater importance in tropical and subtropical climates." "Heavy infections in juvenile animals can lead to a poor rate of gain, hemorrhagic diarrhea, and death."
How this level was decided
INFERRED, and the two halves of the chapter sentence pull opposite ways. Heavy infections kill suckling pigs, which read alone is level 3. But the same sentence says the parasite is rarely present in modern indoor facilities and matters more in tropical climates, and the standing note SCORING/production_cost_impact directs scoring on modern US production. Level 1. FLAGGED alongside stephanurus_dentatus as a cell where the note is doing heavy lifting -- this is a damaging parasite of suckling pigs that US housing has largely removed, not one that was never important.
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, old disease and diagnosis would have no effect on markets"
Basis. Section 61.2.3.1 records no trade measure, movement control or consumer response. Losses fall on pre-weaning pigs that never enter the market.
How this level was decided
INFERRED. A non-zoonotic parasite killing suckling pigs on farm has no route to pork supply or demand -- affected animals die before weaning and nothing enters the food chain. 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: avermectins control "S. ransomi (adults)" and doramectin is labelled for it. 61.7.2.3: levamisole "is effective for the treatment of Ascaris, Oesophagostomum, Metastrongylus, Strongyloides, and Stephanurus." 61.7.2.6: dichlorvos has "slightly lower efficacy against Strongyloides". 61.7.2: "Treatment of sows 10-14 days before farrowing has been shown to be effective in preventing transmission of nematodes and Sarcoptes scabiei."
How this level was decided
Effective treatment exists in three drug classes, and more to the point the chapter names a treatment that breaks the specific route this parasite uses -- treating the sow before farrowing stops the transcolostral transfer that infects the litter. That is a pathogen-directed intervention aimed exactly where the damage happens. Level 1.
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, no market imperative for vaccine development"
Basis. No threadworm vaccine is described in ch61. Control is by housing, sow treatment before farrowing, and anthelmintics.
How this level was decided
INFERRED. No vaccine exists and none is warranted for a parasite that modern indoor housing has largely removed and that pre-farrowing sow treatment prevents. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/strongyloides_ransomi.yml; the overview is authored in data/overviews/strongyloides_ransomi.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.