Lungworms
LEVELS: Highly unlikely; No human illness; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | metastrongylus_spp |
| Type | parasite |
| Scientific name | Metastrongylus |
| NCBI taxid | 55272 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Lungworms of the genus Metastrongylus live in the airways of pigs, and their life cycle explains why they have largely disappeared from modern production. Eggs are coughed up, swallowed and passed in the faeces, where they must be eaten by an earthworm; the larvae develop in the earthworm's tissues, and the pig is infected only by eating the earthworm. Slatted floors or concrete therefore prevent the infection outright, and the general decline of parasitism in modern facilities comes precisely from denying pigs access to soil and intermediate hosts. Where infection does occur — outdoor and small-scale herds — the signs are unimpressive on their own, but heavy burdens and infections complicated by bacteria cause coughing and laboured breathing, and concurrent bacterial pneumonia is not uncommon. Diagnosis by faecal flotation is awkward because the eggs do not float well; at post-mortem the worms are found by trimming strips from the edge of the diaphragmatic lobe and squeezing the adults out of the bronchi. 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 evidence suggesting the parasite is zoonotic"
Basis. Section 61.3.1 covers lungworms 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. 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 evidence suggesting the parasite is zoonotic"
Basis. No human disease is described in 61.3.1.
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
Basis. ch61 61.3.1: "Eggs are coughed up, swallowed, and passed in the feces. Certain earthworms, notably Eisenia and Allolobophora spp., ingest the eggs. Larvae hatch and invade the earthworm's tissues... When swine eat the earthworm, the lungworm larvae migrate to the lungs."
How this level was decided
MOVED L3 -> L2 on 2026-09-01. Same shape as the thorny-headed worm and for the same reason: the earthworm acquires the lungworm by eating eggs a pig shed, and a pig acquires it by eating the earthworm. Neither half of that happens indoors on perforated flooring. The chapter's own epidemiology says so -- this is a parasite of outdoor pigs. 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 61.3.1: "Clinical signs are not pronounced, but heavy infections and infections complicated with bacterial infections cause coughing and thumping." "Diagnosis of metastrongylosis is accomplished by finding the characteristic eggs on flotation, but the eggs do not float well. Postmortem diagnosis is accomplished by trimming 1 cm strips from the edge of the diaphragmatic lung lobe and squeezing to express adults from the bronchi." "Concurrent bacterial pneumonia is not uncommon."
How this level was decided
Coughing in a growing pig points at half the respiratory chapter before it points here, and the chapter says concurrent bacterial pneumonia is common, which masks it further. Flotation is a local test but the chapter warns the eggs float poorly, so a negative means little. Not recognised clinically but locally confirmable, with a caveat on sensitivity. 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, negligible presence and cost in the US industry"
Basis. ch61 61.3.1: "Clinical signs are not pronounced, but heavy infections and infections complicated with bacterial infections cause coughing and thumping." 61.7.1 states that slats or concrete prevent metastrongyloid infection outright. 61.1: "modern swine production facilities have decreased parasitism by denying pigs access to soil and/or parasite intermediate hosts".
How this level was decided
INFERRED. Scored on modern US production under the standing note SCORING/production_cost_impact. Lungworm requires a pig to eat an earthworm, which a slatted or concrete unit makes impossible, so the parasite is effectively absent from confined production -- the chapter says prevention is complete, not merely reduced. Level 1. NOTE: in outdoor systems this is a real cause of coughing and secondary pneumonia, and ch61 61.1 warns that "the recent resurgence of extensive or outdoor production practices offers risk for the re-emergence of parasite concerns".
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, diagnosis would not trigger any market response"
Basis. Section 61.3.1 records no trade measure, movement control or consumer response.
How this level was decided
INFERRED. A non-zoonotic lungworm absent from confined production has no route to pork supply or demand. 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 "Metastrongylus (adults)", and doramectin is labelled for it. 61.7.2.2: "Fenbendazole products are labeled for the treatment of A. suum, Metastrongylus, Oesophagostomum, Hyostrongylus, Stephanurus, and Trichuris 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 drug classes, one of them US-labelled for this genus by name. Effective pathogen-directed treatment is available and works. 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 to develop a vaccine"
Basis. No lungworm vaccine is described in ch61. Control is by housing and anthelmintics.
How this level was decided
INFERRED. No vaccine exists and none is warranted for a parasite that slatted flooring prevents outright and three labelled drug classes remove. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/metastrongylus_spp.yml; the overview is authored in data/overviews/metastrongylus_spp.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.