Liver Fluke
LEVELS: Highly unlikely; Moderate; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Some; Available, or not needed
| Register id | fasciola_hepatica |
| Type | parasite |
| Scientific name | Fasciola hepatica |
| NCBI taxid | 6192 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Fasciola hepatica, the liver fluke, is a trematode with a very broad host range that includes pigs, ruminants and people. Its life cycle requires water and a lymnaeid snail: eggs develop in water, hatch, infect the snail, and the parasite eventually encysts on vegetation as a metacercaria that is eaten by the grazing animal. That requirement is why it barely touches modern pig production — no snail habitat, no wet grazing, no infection — and metacercariae die quickly under dry conditions. In the pigs that do become infected, immature flukes tunnelling through the liver leave necrotic haemorrhagic tracts, and the adults in the bile ducts cause the thickening and fibrosis known as pipestem liver. Heavy infections cause anaemia and low blood protein from liver damage and the flukes' blood feeding, but most infections are silent apart from some loss of gain; the consequence is condemnation of the liver at slaughter. The prepatent period is ten to twelve weeks. No flukicide appears in any labelled swine indication, and there is no vaccine — control is grazing and snail management.
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
Basis. ch61 61.4.1: "Eggs passed in the feces develop in water. A ciliated miracidium hatches and penetrates a lymnaeid snail... Each cercaria encysts on vegetation to form an infective metacercaria. After being ingested by the final host, the metacercaria penetrates the intestinal wall and the liver capsule." "Ruminants and humans can also be infected."
How this level was decided
Humans are infected the same way pigs are -- by swallowing metacercariae encysted on wet vegetation, characteristically watercress. The infective stage is on the plant, not in the pig, and eating pork liver cannot transmit it because the fluke in the liver is an adult, not the encysted metacercaria. The pig is a parallel dead-end host rather than a source, which is the pattern Eric ruled on for chikungunya. Level 1.
C2 Zoonotic impact
Severity of human illness caused by the agent
Moderate: Clinically important illness is common and may require medical treatment or hospitalization, but death or permanent disability is uncommon
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. REVIEWED ON C1_C2_RECLASSIFICATION.md, 2026-09-04, after the C1/C2 rewording that separated exposure from consequence. Eric marked up every moved cell and reversed his own earlier setting on twenty of the twenty-one: "In almost every case, I agreed with new opinion and reversed my old setting." HIS MARKUP HERE WAS "l3". Reverses the 2026-09-01 move to level 1, restoring the pre-rescoping level. Human fascioliasis is biliary disease requiring treatment. C1 records separately that people acquire it from metacercariae on watercress, in a snail cycle cattle and sheep drive.
Basis. ch61 61.4.1 describes the pathology it causes in its host: "Migrating adolescent flukes cause necrohemorrhagic tracts in the liver parenchyma... Adults cause bile duct hyperplasia and fibrosis, leading to the characteristic pipestem liver lesions." "Clinical signs associated with fasciolosis are anemia and hypoproteinemia due to liver damage and hematophagia by adult flukes." It does not describe the human clinical course.
How this level was decided
RE-SCORED 2026-09-04. C2 now grades the severity of the illness the agent causes in a person, independent of how the person was exposed -- the 2026-09-01 pig-attribution reading is reversed, and C1 alone carries the source question. Returns to the level it held before 2026-09-01. Human fascioliasis is biliary disease requiring treatment, rarely fatal.
PREVIOUS ANSWER, written under the pig-attribution reading: MOVED L3 -> L1 on 2026-09-01. Human fascioliasis is acquired by eating metacercariae encysted on watercress and other aquatic plants, from a snail cycle that cattle and sheep drive. The pig is another accidental definitive host rather than a source, which is why C1 is level 1. Moved by the 2026-09-01 C2 rescoping, which Eric accepted: "i accept the c2_review changes". C2 now grades the severity of illness acquired FROM PIGS, so an entry whose pig is a dead-end host has no pig-attributable illness to grade. THE LEVEL MUST NOT BE READ AS A FINDING THAT THE AGENT IS MILD -- it records that pigs are not a 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 61.4.1: "Fasciola hepatica is a cosmopolitan trematode that has a broad host range that includes swine." "Eggs passed in the feces develop in water. A ciliated miracidium hatches and penetrates a lymnaeid snail... Metacercariae may remain infective for a long period of time but are killed quickly under dry conditions."
How this level was decided
MOVED L5 -> L2 on 2026-09-03, when C3 was reduced to four levels and the whole level 5 block was re-read against the exclusion test rather than the reservoir test. Eric: "C3 is about exclusion, not reservoirs per se, though the two are related", and "who cares about reservoirs? Reservoirs really don't matter as long as you keep domestic pigs from becoming infected through biosecurity implementation." THE PREVIOUS LEVEL WAS NEVER JUDGED: this entry carried a RENUMBERED L4 -> L5 stamp from 2026-09-01, meaning it was moved past the newly created housing level rather than assessed against it. metacercariae encysted on wet pasture in a snail cycle. No pasture, no exposure -- the cleanest housing case in the register.
EARLIER REASONING, kept because it is the evidence read: RENUMBERED L4 -> L5 on 2026-09-01, when C3 went from four levels to five. The argument below is unchanged and so is the judgment. What changed is the scale it sits on. C3 now runs on the EFFORT REQUIRED TO EXCLUDE: L1 already in the herd, L2 housing alone keeps it out, L3 routine biosecurity keeps it out, L4 extraordinary biosecurity required, L5 exclusion may not be achievable. The reservoir is a snail population in wet ground, sustained by cattle and sheep as much as by pigs, and it is entirely outside the pig industry. Level 4. As with the rest of the pasture parasites in this batch, dry lot or slatted housing removes the exposure while leaving the reservoir untouched.
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 61.4.1: "Infected individuals may have weight loss or poor gains, but infections are often silent. The liver is condemned at slaughter." "The eggs are large (130-150 by 65-90 um), oval, yellowish brown, and operculated." "The prepatent period is 10-12 weeks."
How this level was decided
INFERRED. Infections are often silent, so nothing prompts a diagnosis, and the pipestem liver is found at slaughter. The egg is large and distinctive once seen, but fluke eggs are heavy and do not recover well on the standard flotation used for nematodes -- a sedimentation technique is needed, which the chapter does not spell out for swine. That is a method change rather than a referral, so level 2 rather than level 3. Level 1 is out because there is no clinical presentation to recognise.
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 is a well controlled disease in productions settings where exposure may occur, production costs are negligible at the industry level"
Basis. ch61 61.4.1: "Infected individuals may have weight loss or poor gains, but infections are often silent. The liver is condemned at slaughter." The parasite requires a lymnaeid snail and wet grazing, both absent from confined production. 61.1: modern facilities "decreased parasitism by denying pigs access to soil and/or parasite intermediate hosts".
How this level was decided
INFERRED. Fasciola in pigs requires wet pasture shared with a snail intermediate host, which confined US production does not provide, and the chapter says infections are often silent even where they occur. Scored on modern US production under the standing note SCORING/production_cost_impact. Level 1, with the cost when it does occur being condemnation of the individual liver.
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 is a traditional disease, infrequent in pigs. Diagnosis would not have any effect on markets"
Basis. Section 61.4.1 records no trade measure, movement control or consumer response. The consequence named is condemnation of the affected liver.
How this level was decided
INFERRED. Liver condemnation affects an individual carcass, not the pork or pig market, following ascaris_suum. Level 1.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c7l2, treatable but I believe only with extralabel use of existing medications"
Basis. No flukicide appears anywhere in ch61 61.7.2 -- every compound listed is a nematode drug, and none is described as active against trematodes. Fasciola is not named in any labelled swine indication.
How this level was decided
INFERRED, and this entry is scored a level above the nematodes for a specific reason: the treatment gap here is real rather than notional. Triclabendazole and albendazole are the standard flukicides in ruminants and in human medicine, so effective pathogen-directed treatment for this organism plainly exists -- it simply has no swine label and would be extralabel use in a food animal. "Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances" is level 2 word for word. NOTE: the same reasoning would apply to paragonimus if that entry is ever registered.
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 liver fluke vaccine is described in ch61. The chapter addresses trematodes through the intermediate host and grazing management.
How this level was decided
INFERRED. No vaccine exists, and none is warranted for a parasite that confined production already excludes by denying pigs access to snail-bearing wet pasture. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/fasciola_hepatica.yml; the overview is authored in data/overviews/fasciola_hepatica.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.