Chlamydia spp.
LEVELS: Rarely occurs; Mild; Already in the herd; Laboratory-dependent; Minor; Negligible; Some; Available, or not needed
| Register id | chlamydia_spp |
| Type | bacteria |
| Scientific name | Chlamydia |
| NCBI taxid | 810 |
| Evidence | 4 document(s) |
| Assigned | 2026-09-04, against criteria version 093366e352a2 |
Overview
This entry covers Chlamydia suis and C. abortus, with C. pecorum and C. psittaci occasionally found alongside them. The pig is the natural host only of C. suis, and carriage is extremely common — over 90% of fattening pigs positive on faecal samples and about half on conjunctival swabs in one Swiss study, 62% of rectal samples in a Chinese one. What that carriage means for the pig is genuinely unsettled, and this register keeps two reviews that disagree side by side: one concludes chlamydial infections in sows, boars and piglets are more frequent than thought and economically important, the other that the rate of actual disease among carriers is difficult to pin down and that most enteric infection appears to be asymptomatic. Where disease is described it is conjunctivitis, enteritis, reproductive loss and, for C. abortus, occasional abortion. Infections are often missed because chlamydial testing is not routine in veterinary laboratories and the clinical signs are not distinctive; PCR is the practical diagnostic. C. suis has been detected in pig farmers and abattoir workers, but only rarely and without illness. One point of practical concern is the tetracycline resistance carried by C. suis, unusual in the genus, which pushes treatment towards fluoroquinolones.
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. CFSPH 2017: "C. suis mainly occurs in domesticated pigs and other members of the pig family, such as European wild boar", and it was recovered from pig farmers and abattoir employees. Hacker 2024 calls the detections "albeit rare". Schautteet 2011 reported none at all.
How this level was decided
RE-READ AND HELD AT L2 2026-09-04 AGAINST THE REWORDED LABELS. cfsph_2017 records C. suis recovered from pig farmers and abattoir employees. That is a pig-attributed occupational report, rarely made.
PREVIOUS ANSWER, kept because the evidence behind it has not changed, only the level boundary: Level 2 unchanged. Three documents now agree on the shape: transfer from pigs to people happens, is occupational, is rarely reported, and nothing in pork production is directed at preventing it. Both clauses of level 2.
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
Basis. CFSPH 2017: "C. suis was also found (by PCR and/or culture) in ocular, nasal and pharyngeal swabs and stool samples from pig farmers, and in the conjunctiva of two asymptomatic employees at a pig abattoir." Schautteet 2011: for C. abortus and C. psittaci "transmission of these pathogens from pigs to humans has not been reported", and for C. suis "no reports on zoonotic transmission were published". Hacker 2024 records the same detections as "albeit rare".
How this level was decided
Level 2 unchanged but now SUPPORTED rather than inferred, on a third independent document. The CFSPH sheet is the most explicit of the three on the point that matters: the people carrying C. suis were ASYMPTOMATIC. Three reviews spanning 2011 to 2017 record pig-to-human transfer of the organism and none records illness from it. Level 1 remains arguable and level 3 is not.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Already in the herd: Present in most herds, or carried by pigs themselves, so there is no introduction event to prevent
Basis. Hacker 2024: "The pig is considered the natural host only of C. suis". "There appears to be a very high prevalence in pigs: one study reported over 90% positivity of fattening pigs in fecal samples and around 50% in conjunctival swabs in Swiss farms (Hoffmann et al., 2015). A Chinese study reported positivity of 62.4% of rectal samples (Li et al., 2017)." On C. abortus in pigs: "Abortion in swine due to C. abortus-infection does not appear to be very frequent but has been described." On C. pecorum: "In pigs, C. pecorum is typically found together with C. suis."
How this level was decided
CHANGED, level 4 to level 1, and this is the cell the new papers move furthest. The old level 4 came from applying the C3 escalation rule to C. abortus and C. psittaci as ruminant and avian reservoirs outside the pig industry. Hacker 2024 reframes the entry: the pig has its own natural chlamydial species, C. suis, it is in over 90% of fattening pigs, and C. pecorum travels with it. C. abortus in pigs is infrequent. So the organism that defines this entry is already carried by the pigs and there is no introduction event to prevent, which is level 1. NOTE FOR ERIC: this removes one of the three cells that turned on the C3 escalation rule. The rule question still stands for klebsiella_pneumoniae and treponema_pedis, but it is no longer load-bearing here -- the entry resolves on its own evidence.
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, most labs able to test for this agent without having to refer"
Basis. Schautteet 2011: "Chlamydiaceae infections are often unnoticed because tests for Chlamydiaceae are not routinely performed in all veterinary diagnostic laboratories and Chlamydiaceae are often found in association with other pathogens, which are sometimes more easily to detect." Hacker 2024 describes PCR and sequencing as the working diagnostic tools throughout. ch58 58.5.4: "Presumptive diagnosis is difficult because clinical signs of chlamydial infection are not distinctive." "PCR is currently preferred for confirmation... Species-specific real-time PCR assays are also available and are sensitive enough to detect 10 DNA copies without cross-reaction."
How this level was decided
CHANGED, level 3 to level 2. The old level 3 read ch58's "most veterinary diagnostic laboratories do not routinely test" as amounting to referral. Schautteet states the same fact but explains it differently: tests are not routinely performed because infections go unnoticed and other agents are easier to find -- a demand problem, not a capability problem. Species-specific real-time PCR exists, is sensitive and is commercially described. Clinical signs are non-distinctive so level 1 is out, but a regional laboratory can confirm it once someone asks. Level 2. This also fits Eric's account of why the agent is under-tested: the field does not think it matters, so nobody looks.
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
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5l2, field evidence suggests negligble disease but that is anecdotal. peer review evidence supports c5l2 so we go with that"
Basis. The two reviews disagree, deliberately placed side by side. Schautteet 2011 abstract: "recent studies have demonstrated that Chlamydiaceae infections in breeding sows, boars and piglets occur more often than thought and are economically important." Hacker 2024 abstract: "all three species bring a substantial burden of disease to pigs" -- but its body says "The rate of disease among pigs carrying C. suis is difficult to pin down", "the majority of in particular enteric infections appears to be asymptomatic (Li et al., 2017)", and of C. pecorum "it is probably fair to say that not enough information is available to assess the importance of carriage and infection of swine with this chlamydial species."
How this level was decided
Level 2 unchanged, but now held against the strongest statement of the opposite case rather than in the absence of one. Both reviews assert economic importance in their abstracts, and neither quantifies it anywhere in the body. Against those assertions sit over 90% carriage with the majority of infections asymptomatic, a disease rate the 2024 review says cannot be pinned down, and an explicit admission that C. pecorum's importance is unassessable. An organism in nine pigs out of ten that has not produced a measurable herd cost in two review cycles is not moving cost of production. Level 2. INFERRED because it rests on the absence of a figure in papers that had every reason to supply one. Eric on the sheet: "chlamydia zealots have been trying to make a real disease out of this agent for decades in pigs, but the field experience aligns with the 2011 paper - it just doesn't stack up as a significant pathogen." NOTE: level 1 follows if the asymptomatic-carriage reading is taken all the way.
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. "I think will end up to be c6l1, but as above re-evaluate in light of the two new papers in inbox"
Basis. Neither review reports a trade measure, movement control, export restriction or consumer response attaching to porcine chlamydiosis. Hacker 2024 records over 90% prevalence in fattening pigs; Schautteet 2011 records seroprevalence rates from 6.9% to 63% across European surveys. Eric, on the sheet: "c6l1" confirmed on the previous pass.
How this level was decided
Level 1 unchanged, and Eric confirmed it on the previous sheet -- he wrote "I think will end up to be c6l1, but as above re-evaluate in light of the two new papers in inbox". The new papers do not disturb it: an organism carried by most pigs across two continents, with no reportable status and no control programme, cannot move pork supply or demand. Now supported rather than inferred, because two independent reviews surveying the field across thirteen years record no market event of any kind.
C7 Treatment potential
Potential for treatment to improve outcomes
Some: Treatment exists but is inconsistent, requires extralabel use, or works only in some circumstances
Basis. Hacker 2024: "True, acquired antibiotic resistance is rare in Chlamydia, and antibiotic therapy of chlamydial infection is therefore relatively straightforward. The notable exception is the tetracycline-resistance in C. suis... Tetracyclines are commonly used to treat infections, and this resistance is therefore relevant. However, the tetracycline-resistant strains retain sensitivity to other antibiotics such as rifamixine and fluoroquinolones (Borel et al., 2016). The quinolone enrofloxacine is considered an alternative for infections with tetracycline-resistant strains."
How this level was decided
Level 2 unchanged and now much better supported. Effective treatment plainly exists -- the 2024 review calls chlamydial antibiotic therapy relatively straightforward -- which rules out level 3. What holds it at level 2 is the one documented exception, tetracycline resistance in C. suis specifically, which is the pig-adapted species and the one that matters here, requiring a switch to enrofloxacine. Treatment that exists but works only in some circumstances.
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 economic imperative to develop a vaccine"
Basis. ch58 58.5.5: "No vaccines are commercially available", and the efficacy of ruminant C. abortus vaccines in swine is undetermined. Neither Schautteet 2011 nor Hacker 2024 reports a commercial porcine chlamydial vaccine. Eric, 2026-08-27: "it just doesn't stack up as a significant pathogen".
How this level was decided
MOVED L3 -> L1 by the rewording. INFERRED. Over 90% of fattening pigs carry C. suis and the majority of infections are asymptomatic, with Hacker conceding the disease rate "is difficult to pin down". An organism in nine pigs out of ten that has not produced a measurable herd cost in two review cycles does not justify a vaccine.
Levels and evidence are generated from data/assignments/chlamydia_spp.yml; the overview is authored in data/overviews/chlamydia_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.