Toxoplasmosis
LEVELS: Sometimes occurs; Severe; Housing alone keeps it out; Laboratory-dependent; Negligible; Negligible; Little; Available, or not needed
| Register id | toxoplasma_gondii |
| Type | parasite |
| Scientific name | Toxoplasma gondii |
| NCBI taxid | 5811 |
| Evidence | 2 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
Toxoplasma gondii is a protozoan parasite whose only definitive hosts are cats: they alone shed the environmentally resistant oocysts, and infected juvenile cats are considered the main source of infection for farm pigs, with infected rodents the other. In pigs the infection is almost always silent. Clinical disease is rarely reported, though sows infected during pregnancy can abort. The parasite's importance is to people. Tissue cysts form in most edible cuts of pork and remain viable for years, probably for the life of the pig, and pork is regarded as a source of human infection in many countries; congenital infection causes intellectual disability and loss of vision in children. The good news is that modern production has largely closed the route — congenital infection in confinement-reared pigs runs below 0.01%, and a recent survey of 20,209 market-age pigs across ten US states found 0.7% prevalence — but small establishments still have infected pigs. There is no vaccine and effectively no treatment; prevention is husbandry: keep cats out of pig buildings and feed stores, control rodents, remove dead pigs promptly, cover feed and prevent cannibalism. Cooking and freezing pork inactivate the cysts.
C1 Zoonotic potential
Likelihood of transmission from pigs to humans
Sometimes occurs: Known risk of transmission to humans (foodborne or occupational), but requires failure of one or more control points for transmission to humans
Basis. ch60 60.2: "Toxoplasmosis is zoonosis, and pork is considered a source of T. gondii infection for humans in many countries." "Tissue cysts are found in many edible tissues of infected pigs... Tissue cysts remain viable in pork for many years and probably the life of the pig." 60.2.5: "Following experimental infection, viable tissue cysts of T. gondii can be found in most commercial cuts of pork, and studies have shown that the tissue cysts will be viable for at least 2.5 years. Cooking and freezing pork will inactivate tissue cysts."
How this level was decided
Level 3, matching the worked example in criteria_levels.yml. Pork is named as a human source, viable cysts sit in most commercial cuts and persist for years, and the control points are stated and effective -- cooking and freezing both inactivate them. Transmission follows their failure.
C2 Zoonotic impact
Severity of human illness caused by the agent
Severe: Infection carries a substantial risk of life-threatening illness, death, or permanent disability, even if human infections are uncommon
Basis. ch60 60.2.5: "Control of T. gondii infection in pigs is important because of public health concerns regarding human infections. Toxoplasmosis causes intellectual disabilities and loss of vision in congenitally infected children."
How this level was decided
Level 4. Congenital toxoplasmosis produces permanent intellectual disability and blindness, which is the level 4 test -- substantial risk of permanent disability -- and it is the recognised presentation rather than a rare extreme. The chapter names it as the reason control in pigs matters at all.
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. ch60 60.2: "Cats (and other felids) are the only animals that can excrete environmentally resistant oocysts in their feces and are important in the transmission of T. gondii to pigs and other animals." 60.2.4: "The presence of T. gondii-infected cats and infected rodents was identified as the main source of T. gondii infection for pigs... infected juvenile cats are considered to be the main source of T. gondii for farm pigs."
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. oocysts from cat faeces and infected rodents. Confinement is what drove US pork seroprevalence down, and the chapter names infected cats and rodents as the main source for pigs.
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. Level 4, and it is the standing note SCORING/introduction_risk that puts it there rather than the ch61 default. The test is where the contamination came from, and here it can only have come from a cat -- felids are the ONLY animals that shed the oocyst. Not raising pigs on dirt does not exclude this agent, because the oocyst arrives with the cat. Contrast cystoisospora_suis in this same batch, at level 3, where the oocysts are pig-shed.
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
Basis. ch60 60.2.1: "Infections in swine are usually subclinical." 60.2.3: "Cytology, histopathological lesions, immunohistochemical staining (IHC), and detection of T. gondii DNA can aid in diagnosis... IHC staining can confirm the diagnosis within a few hours." "PCR can aid detection of T. gondii DNA." "Several serological tests are available for determining antibodies to T. gondii in pigs." "Greater than 10,000 organisms per gram of tissue are needed to find them in a 5 um histological section."
How this level was decided
Infection is usually subclinical so nothing prompts a test, ruling out level 1. But the diagnostic toolkit is broad and conventional -- Giemsa or Diff Quick cytology, routine histopathology, IHC returning an answer within hours, PCR, and several serological tests -- all standard veterinary diagnostic laboratory work. Level 2. The sensitivity limits the chapter records are real but are limits on an available test.
C5 Production cost
Financial impact on the infected farm's cost of production
Negligible: No measurable effect on cost of production
Basis. ch60 60.2.1: "Infections in swine are usually subclinical. Abortions due to T. gondii, although uncommon, may occur in sows infected during pregnancy." "There are few reports of clinical disease in pigs in most parts of the world", with epidemics reported in East Asia. 60.2.4: "The prevalence of congenital T. gondii infection in pigs raised in confinement is often less than 0.01%. In the most recent survey of 20,209 market-age pigs slaughtered in 10 US states, the prevalence was 0.7%."
How this level was decided
Usually subclinical, abortion uncommon, few reports of clinical disease outside East Asia, and congenital infection under 0.01% in confinement against 0.7% seroprevalence at market age. Under the standing note SCORING/production_cost_impact this is scored on modern US production, where it does not register as a cost. Level 1. The weight of this entry is entirely on the human side, at C1 and C2.
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, no evidence to suggest diagnosis would affect markets"
Basis. Section 60.2 records no trade measure, movement control or consumer response. 60.2: "Although the prevalence of T. gondii in feeder pigs in the United States is declining, there are still small pig establishments with T. gondii infection."
How this level was decided
INFERRED. Under the standing note SCORING/market_impact, T. gondii is long present in US pigs, routinely surveyed, and declining, with no market response on record. 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, no clinical or market imperative to support treatment development"
Basis. ch60 60.2.5: "Because porcine toxoplasmosis is usually subclinical, little is known about the treatment of the disease in swine. Ponazuril (closely related to toltrazuril) is effective against acute toxoplasmosis in mice and should also be effective in swine." Control is by husbandry -- excluding cats, rodent control, covering feed, preventing cannibalism.
How this level was decided
INFERRED. Under the standing note SCORING/treatment_potential, level 1 covers the case where there is no imperative to treat, and the chapter gives exactly that reason: porcine toxoplasmosis is usually subclinical, so little is known about treating it because nobody needs to. The second clause of level 1. NOTE: a treatment aimed at clearing tissue cysts to protect the consumer would be a food safety intervention rather than a pathogen-directed treatment improving the pig outcome -- the distinction Eric confirmed on campylobacter_spp and yersinia_enterocolitica.
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 clinical or market imperative to support vaccine development"
Basis. ch60 60.2.5 states plainly: "There is no vaccine." Prevention "can be achieved by practicing good husbandry" -- keeping cats out of pig buildings and feed stores, rodenticides, prompt removal of dead pigs, no uncooked garbage, covered feed.
How this level was decided
INFERRED. No vaccine exists, and under the reworded criterion the question is whether one is warranted. The chapter answers it by describing a husbandry programme that works and is the reason US feeder-pig prevalence is declining -- the same shape as trichinella_spiralis in the ch61 batch, where Eric ruled level 1 because "excluding it from farms is easy". Level 1 on the second clause. FLAGGED as arguable: the public health prize here is congenital disease in children, which is a stronger argument for a vaccine than anything in the pig.
Levels and evidence are generated from data/assignments/toxoplasma_gondii.yml; the overview is authored in data/overviews/toxoplasma_gondii.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.