Large Roundworm
LEVELS: Rarely occurs; Mild; Housing alone keeps it out; Routine; Minor; Negligible; Little; Needed but not available
| Register id | ascaris_suum |
| Type | parasite |
| Scientific name | Ascaris suum |
| NCBI taxid | 6253 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Ascaris suum, the large roundworm, is among the most prevalent parasites of pigs anywhere in the world. Adult worms live in the small intestine and the females lay eggs in prodigious numbers; those eggs are extraordinarily tough, survive temperature extremes, remain infective for years and are unaffected by essentially every disinfectant available, so the environment is heavily contaminated wherever pigs are kept. Most piglets are thought to be infected by swallowing eggs in manure stuck to the sow's udder. The cost is mostly invisible: adult worms compete with the pig for nutrients and interfere with absorption, so even light infections measurably reduce daily gain and feed efficiency without any pig looking ill. The visible cost comes at slaughter, where larval migration through the liver leaves the white spots that lead to trimming or condemnation. Diagnosis is easy — the eggs are abundant and float readily. Treatment is straightforward and, unusually among the ascarids, A. suum in pigs remains susceptible to every anthelmintic class in use, whereas the equine and poultry ascarids have developed resistance. It is also the pig parasite with the clearest zoonotic link, cross-infecting with the human roundworm A. lumbricoides, though most infections in both species still appear host-adapted.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Rarely occurs: Human infection from pigs is plausible but has only rarely been reported, and specific control points are not commonly implemented to prevent transmission to humans
Basis. Mugo 2025: "zoonotic transmission is most clearly established for A. suum and Toxocara spp.", "cross-infections between A. suum and A. lumbricoides have been reported in both experimental and natural settings", and "molecular data indicate occasional hybridization and gene flow between human and pig populations". But: "The relative epidemiological importance of such events remains uncertain, as most infections in humans and pigs still appear host-adapted."
How this level was decided
RE-READ AND HELD AT L2 2026-09-04 AGAINST THE REWORDED LABELS. mugo_2025: "cross-infections between A. suum and A. lumbricoides have been reported in both experimental and NATURAL settings", and "zoonotic transmission is most clearly established for A. suum". Pig-attributed human infection is established, which is what level 2 now requires.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2 unchanged, better supported. Pig-to-human transmission is established rather than theoretical -- cross-infection is documented in natural settings and the genomes show gene flow. What keeps it at level 2 rather than level 3 is the review's own qualifier: most infections on both sides remain host-adapted, so the event is real but uncommon.
C2 Zoonotic impact
Severity of human illness caused by the agent
Mild: Human illness is usually mild and self-limiting, and serious disease occurs only rarely
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "assess the relevant paper i have put in the inbox. reassess criteria but it seems most human infections with A.suum are mild or subclinical and therefore it should be at c2l2." The Mugo review was placed and assessed; it is a control-and-resistance paper and says nothing about human clinical severity, so this level rests on his judgment rather than on that paper. It did supply direct evidence for C7.
Basis. The chapter establishes that pig Ascaris can infect humans but does not describe the resulting human illness.
How this level was decided
INFERRED. Human ascariasis is typically a mild or subclinical intestinal infection, occasionally causing larval migration symptoms. The folder does not say so -- this is general parasitological knowledge, not evidence from the source.
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 61.2.3.2: "The eggs are resilient and long-lived, so the environment is likely to be heavily contaminated wherever swine exist." "Eggs are very resistant to temperature extremes and may remain infective for years. Most chemicals have no effect on eggs." "It is thought that most piglets are exposed when they ingest eggs in manure stuck to the sow's udder." 61.7.1: "There are currently no disinfectants available that are capable of killing A. suum eggs."
How this level was decided
MOVED L3 -> L2 on 2026-09-01. The reasoning that put it at L3 is unchanged and still right as far as it went: every egg on the premises was shed by a pig, so the source is pig-origin and not a reservoir outside the industry, and no disinfectant kills the eggs. What changed is that the old level 3 asked only whether the route could be blocked, and the honest answer for Ascaris is that it can -- by a housing decision rather than by a disinfectant. Eggs on the ground do not reach pigs that are never on the ground. 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. NOTE: this cell was scored L4 before 2026-08-28 and L3 after, and is now L2. The entry was signed off on 2026-08-27, so this raises REVIEW AGAIN a second time. 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
Routine: The disease would be suspected from clinical signs, history and epidemiology in normal practice, and confirmed by tests that local or regional laboratories run routinely
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "l1, routine diagnosis at local laboratories"
Basis. "The diagnosis of ascariasis is straightforward when dealing with patent infections since the prodigious oviposition rate of the female makes eggs easy to find by standard flotation methods." (ch61 §61.2.3.2)
How this level was decided
Fecal flotation is a routine in-practice test. The chapter notes prepatent infections are missed, but the criterion asks about recognising and confirming infection in the usual case, which is straightforward.
C5 Production cost
Financial impact on the infected farm's cost of production
Minor: Small and generally short-lived losses with little effect on overall cost of production
Basis. "Adult ascarids compete with the host for nutrients and probably interfere with nutrient absorption... thereby causing insidious production losses." "At slaughter, there are losses due to condemnation or trimming of livers." "experimental infections demonstrate some compromise of average daily gain and feed efficiency, even at low infection levels." (ch61 §61.2.3.2)
How this level was decided
Real, measurable losses through feed efficiency and liver condemnation, but described as insidious and incremental rather than threatening. Small and manageable -- level 2.
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
Basis. Losses described are liver condemnation and trimming at slaughter. Nothing on supply, demand or trade. (ch61 §61.2.3.2)
How this level was decided
Liver condemnation affects an individual carcass, not the pork or pig market. No market disruption arises from ascariasis occurring on a farm.
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. Mugo 2025, Table p.7, anthelmintic susceptibility by host and species. Ascaris suum in swine is AS -- anthelmintic susceptible -- across every drug class listed: ivermectin, moxidectin, fenbendazole, levamisole and pyrantel. The same table records Parascaris spp. in horses as AR, anthelmintic resistant, to four of the five, and Ascaridia galli and Heterakis gallinarum in poultry as resistant to fenbendazole. Legend: "AR, anthelmintic resistance; AS, anthelmintic susceptibility".
How this level was decided
Level 1 unchanged, but on direct evidence instead of general reasoning. Effective pathogen-directed treatment exists and still works: A. suum is the ascarid that has NOT developed resistance, and the paper sets it beside close relatives that have. This is one of the few criteria this review touches -- it is a control-and-resistance paper, not a clinical one.
C8 Vaccine availability
Availability of effective vaccines or bacterins
Needed but not available: No effective vaccine is available in the US or has been developed, and the disease would justify vaccinating if one existed
Basis. The folder documents anthelmintic control and resistance concerns but no commercial vaccine against A. suum. Liver white spot causes carcass condemnation and the parasite is among the most prevalent in swine worldwide.
How this level was decided
STAYS L3. A highly prevalent parasite with real carcass and performance losses, controlled by anthelmintics whose efficacy is under pressure. A vaccine would be used if it existed, so the absence is a genuine gap. Level 3 under the reworded test.
Levels and evidence are generated from data/assignments/ascaris_suum.yml; the overview is authored in data/overviews/ascaris_suum.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.