Giardia duodenalis
LEVELS: Highly unlikely; Moderate; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | giardia_duodenalis |
| Type | parasite |
| Scientific name | Giardia duodenalis |
| NCBI taxid | 5741 |
| Evidence | 3 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Giardia duodenalis is the commonest intestinal parasite of humans and livestock worldwide, and pigs carry it at rates between about 0.1% and 20%. It divides into genetic assemblages, and this matters for interpreting the pig's role: assemblages A and B infect people, assemblage E infects pigs and other livestock, and surveys worldwide find that most infected pigs carry E with few carrying the zoonotic types — so the pig's contribution to human giardiasis is unclear. In the pig itself, the evidence points to no disease at all. Three independent studies looked for an association between infection and clinical illness in pigs and none found one; an Ohio survey noted that a well-known list of six common causes of newborn pig diarrhoea did not include Giardia, and a Czechoslovak study found infected piglets asymptomatic. That Ohio work also showed how effectively housing removes it: a modern farm with wire and mesh floors, where pigs had minimal contact with their faeces, yielded zero Giardia in twenty litters and nineteen sows. Cysts are shed intermittently, so diagnosis needs several faecal samples over a week. No drug is approved for treating giardiasis in swine and no vaccine exists.
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. 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 "l1". Reverses his 2026-09-02 level 2. Most pigs carry assemblage E, which does not infect people, and ch60 60.5.1 calls the role of swine as a source for human giardiasis unclear.
Basis. ch60 60.5.1: "Only assemblages A and B infect humans, are zoonotic, and occur in a wide range of mammals, including pigs... Assemblage E infects swine and other livestock." "Surveys worldwide indicate that most pigs infected with Giardia have assemblage E with few having other assemblages including zoonotic assemblages, A and B." "The role of swine and other domestic animals as sources for G. duodenalis in humans is unclear."
How this level was decided
MOVED L2 -> L1, re-read 2026-09-04 AGAINST THE REWORDED LABELS. FLAG. ch60 60.5.1: "most pigs infected with Giardia have assemblage E", which does not infect people, and "THE ROLE OF SWINE AND OTHER DOMESTIC ANIMALS AS SOURCES FOR G. DUODENALIS IN HUMANS IS UNCLEAR". The zoonotic assemblages A and B do occur in pigs, so this is unclear rather than excluded.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: The zoonotic assemblages do occur in pigs, so a pig-sourced human infection is plausible -- but most pigs carry assemblage E, which does not infect people, the chapter calls the role of swine unclear, and nothing in pork production is directed at preventing it. Plausible, rarely reported, no control point. Level 2. Chapter 60 places Giardia under "Other Protozoa of Minor Importance or Potentially Transmissible to Humans", and potentially is the operative word.
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. "c2l3, agree with assessment and my reason above" EARLIER RULING, 2026-09-01, kept because a re-confirmation does not erase the reasoning it rests on: CONFIRMED. "c2l3, can be quite severe and usually not self resolving in people, generaally requires treatment"
Basis. ch60 60.5.1: G. duodenalis is "the most common intestinal parasite of humans and livestock worldwide". Neither document describes the human clinical course.
How this level was decided
INFERRED -- the folder establishes that Giardia is a major human parasite but says nothing about severity. Human giardiasis is a prolonged diarrhoeal illness with malabsorption and weight loss that commonly requires specific treatment, and death or permanent disability is rare. Clinically important illness commonly needing medical attention is level 3. FLAGGED: this rests on general parasitological knowledge rather than on evidence in the folder, and level 2 is defensible if the self-limiting cases are weighted more heavily.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c3l2, agree with assessment and my reasons above" EARLIER RULING, 2026-09-02, kept because a re-confirmation does not erase the reasoning it rests on: "c3l2, i have uploaded the relevant paper to inbox that was cited in DOS. the detail is important. Once in a farm, perhaps the organism will persist even on perforated floors but the incursion itself almost had to come originally from soil or outdoor exposure - the paper is unclear about this herd history. the paper is very old and from two small herds in Ohio, well before 'modern production'. I think we can reliably say the organism is unlikely to be on a typical us indoor farm. We won't let this single paper artifically inflate the risk."
Basis. ch60 cites the Ohio survey for Giardia prevalence in US pigs. That paper is now in the folder. xiao_1994_crypto_giardia_ohio_farms.md, the whole result: two central Ohio farms sampled by direct immunofluorescence in January and February 1993. FARM 1 was "a well-managed farm with modern facilities and good hygiene. Sows on the farm were kept in elevated deck farrowing crates with woven wire floors, while weanlings were raised in groups of 15-20 head in pens with mesh floor. Thus animals on Farm 1 had minimal contact with their excrement." It yielded ZERO Giardia in 20 litters of nursing piglets, zero in 19 sows and 1 of 30 weanlings. FARM 2 "had older facilities with porous concrete floors. One to two sows were housed together with nursing piglets... Weanlings and some sows were kept in 6 m x 8 m pens WITH ACCESS TO AN OUTDOOR LOT of similar size... the rough cement surface of the farrowing and weanling pens precluded complete removal of waste." It yielded Giardia in 4 of 24 litters, 3 of 24 sows and 8 of 32 weanlings. The difference in weanlings was significant at P<0.05, and the authors conclude that "Giardia infection was also delayed on the farm with better hygiene".
How this level was decided
MOVED L4 -> L2 on Eric's ruling of 2026-09-02, and the paper he asked for is the reason. His words: "the detail is important. Once in a farm, perhaps the organism will persist even on perforated floors but the incursion itself almost had to come originally from soil or outdoor exposure - the paper is unclear about this herd history. the paper is very old and from two small herds in Ohio, well before modern production. I think we can reliably say the organism is unlikely to be on a typical us indoor farm. We won't let this single paper artificially inflate the risk." THE DETAIL THE CHAPTER DROPS IS THE FARM CONTRAST. Diseases of Swine cites this survey as a prevalence figure -- 8.3% of preweaned piglets -- which is the Farm 2 number carried without its condition. The farm that looked like modern US confinement had none in the piglets, none in the sows and one weanling in thirty; the farm with porous concrete and an outdoor lot had it in every group. That is the C3 level 2 label exactly: housing alone keeps it out, no ongoing biosecurity programme is needed, and only the outdoor herd stays exposed. NOTE WHAT THIS DOES NOT SAY -- it does not claim the organism cannot persist once introduced, and Eric explicitly allows that it might. Level 2 is about the effort needed to exclude it, and for this organism that effort is the barn.
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. "c4l2, agree with assessment"
Basis. ch60 60.5.1: "Detection of motile trophozoites (10-20 um in length) can be seen in saline suspensions of loose or diarrheic feces. Ovoid to ellipsoid cysts (9-15 um in length) are best detected after concentration in feces using zinc sulfate flotation." "Because cysts might be excreted intermittently, multiple fecal samples should be collected and examined at intervals within a week." "Assemblages are determined based on genetic markers."
How this level was decided
Nothing points here clinically, since the chapter reports no association between Giardia and clinical illness in pigs. Detection is a saline smear or zinc sulfate flotation, which any laboratory runs. Level 2. Assemblage typing needs genetic markers and is reference work, but that answers the zoonotic question rather than the diagnostic one.
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, agree with assessment"
Basis. Two independent studies looking for an association between Giardia infection and clinical illness in pigs, and both finding none. xiao_1994_crypto_giardia_ohio_farms.md adds a third and an American one: "Most animals sampled had no diarrhea", and "In our study, no association between the occurrence of diarrhea and Cryptosporidium or Giardia infection was found." The authors note that "Johnson et al. (1992) implicated six enteropathogens as common causes for diarrhea in newborn pigs; Cryptosporidium and Giardia were NOT included", and that Koudela's Czechoslovak study likewise "reported that Giardia-infected piglets were asymptomatic".
How this level was decided
Level 1, and the Ohio paper strengthens it rather than changing it. Three studies on two continents have looked for an association with clinical illness in pigs and none has found one, including this one where infected piglets carried heavy cyst burdens -- one had a 4+ infection -- without diarrhoea. That is an examined negative rather than a silence, which is the distinction that separates a supported level 1 from an unassessed cell. No measurable effect on cost of production.
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 at level 1 on the 2026-09-02 re-read. He wrote "c5l1" against this cell, which names C5; the cell is C6 and is on level 1, and C5 immediately above it was answered separately, so it is read as c6l1. His words: "agree with assessment." EARLIER RULING, 2026-09-01, kept because a re-confirmation does not erase the reasoning it rests on: CONFIRMED. "c6l1, old agent well recognised, no evidence to suggest diagnosis would have a market impact"
Basis. Section 60.5.1 records no trade measure, movement control or consumer response. Prevalence in pigs runs between 0.1 and 20% worldwide.
How this level was decided
INFERRED. An endemic protozoan with no association to clinical illness in pigs 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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c7l1, agree with assessment"
Basis. ch60 60.5.1: "No drugs are approved for treating giardiasis in swine. However, fenbendazole and albendazole have been used to treat Giardia in dogs." "no association was found between Giardia infections and clinical illness in pigs."
How this level was decided
Level 1 on the second clause, under the standing note SCORING/treatment_potential. Extralabel options exist -- the benzimidazoles used in dogs -- which would ordinarily make this level 2 on Eric's rule. What holds it at level 1 is the other half of his rule: there is no imperative to treat, because two studies found no association with clinical illness. Nothing to improve.
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, agree with assessment and my reason above" EARLIER RULING, 2026-09-01, kept because a re-confirmation does not erase the reasoning it rests on: CONFIRMED. "c8l1, no clinical or market imperative to develop a pig vaccine"
Basis. No vaccine for swine giardiasis is described in ch60 60.5.1. Control is by sanitation, desiccation, quaternary ammonium compounds, bleach and boiling water.
How this level was decided
INFERRED. No vaccine exists and none is warranted for an infection with no demonstrated association to clinical illness in pigs. Level 1 on the second clause.
Levels and evidence are generated from data/assignments/giardia_duodenalis.yml; the overview is authored in data/overviews/giardia_duodenalis.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.