New World Screwworm
LEVELS: Highly unlikely; Moderate; Extraordinary biosecurity required; Laboratory-dependent; Moderate; Temporary disruption; Little; Available, or not needed
| Register id | new_world_screwworm |
| Type | parasite |
| Scientific name | Cochliomyia hominivorax |
| NCBI taxid | 115425 |
| Evidence | 1 document(s) |
| Assigned | 2026-09-06, against criteria version 093366e352a2 |
Overview
New World screwworm is a fly, not a germ, and what makes it dangerous is that its maggots eat living flesh rather than dead. A female lays 200 to 300 eggs around the edge of any break in the skin — and the breaks that matter on a pig farm are the ones we make ourselves: castration, tail docking, ear notching or tagging, needle teeth clipping, any injection site, the navel of a newborn piglet, a tusk trim that bleeds, a shoulder sore, a bite from a louse or a tick. The eggs hatch within half a day, the larvae burrow into the wound and feed for five to seven days, and the damage they cause draws more flies to the same animal. An untreated pig can die in seven to fourteen days. The United States eradicated screwworm in the 1960s, but confirmed cases have been moving north through Central America and Mexico since late 2024, which is why SHIC published guidance for pig producers in January 2026 and why USDA projects that an outbreak in Texas alone could cost livestock producers more than $1.8 billion. Housing helps but does not settle it: the flies prefer to be outdoors, yet a warm, humid barn is good habitat for the larvae to pupate in — in floor cracks, under feeders, in dried manure — and the ordinary manure-based fly controls do not work, because this fly has no interest in manure. Practical control is minimising wounds, sealing or treating the ones that cannot be avoided, and inspecting every pig every day. Treatment works and approved drugs exist; the disease is reportable, suspect animals must be quarantined and never moved, and a healed, treated, withdrawn pig is safe for the food chain. People can be infested the same way animals are, by a fly finding their own wound, and the resulting myiasis is painful and needs medical removal — but that is the fly reaching a person directly rather than a disease caught from pigs.
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. "c1 agree". The pass scored L1 and flagged it, because human screwworm myiasis is common and well reported and the only question is whether the PIG is the source. It is not -- a person is infested by a gravid female landing on their own wound, exactly as a pig is. The argument put to him for L2, and declined, was that an infested herd amplifies the local fly population and that livestock amplification is why the eradication programme targeted livestock.
Basis. shic_2026: NWS "is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals including livestock, pets, wildlife, and people." "Adult NWS flies can travel 6 to 12 miles to find a suitable host... In typical circumstances, daily flight distance depends on host density and environmental conditions." "Transportation of infested animals by humans can allow NSW flies to move long distances in a single day."
How this level was decided
LEVEL 1, AND FLAGGED FOR ERIC -- this is the one cell on this entry I would not defend without a ruling. Human NWS myiasis is real and well reported, so the question is only whether the PIG is the source, which is what C1 asks. It is not: a person is infested by a gravid female fly landing on their own wound, exactly as a pig is, and the factsheet describes people as one more warm-blooded host alongside livestock, pets and wildlife rather than as downstream of livestock. That is the L1 clause verbatim -- reported, but not attributed to pig or pork exposure. THE ARGUMENT FOR L2, which Eric may prefer: an infested herd is an amplifier, larvae drop, pupate and emerge as more flies on that premises, so a pig genuinely does raise the local human risk, and C1 counts vector routes. Livestock amplification is why the eradication programme targeted livestock. I have scored L1 because no human case is attributed to pig exposure and L2 requires that attribution to have been made at least rarely, but the amplification argument is not weak and this is his call.
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
Basis. shic_2026: larval infestations "can occur in any broken or damaged skin and cause rapidly progressing, painful wounds that can lead to serious injury or death." "As the larvae grow in size, the animal will experience severe pain and tissue damage." "Death occurring within 7 to 14 days in untreated cases." FDA-approved treatments exist and are listed.
How this level was decided
Level 3. C2 scores the severity of illness the agent causes in a person regardless of where the exposure came from, and judges a typical-to-severe bout rather than the tail. Human NWS myiasis is painful, destructive of living tissue and progressive, and it requires medical intervention -- larvae have to be removed and the wound debrided -- which is "may require medical treatment or hospitalization" squarely. It is not level 4: with treatment the outcome is recovery, and death or permanent disability is the untreated or neglected case rather than the typical one. The severe tail is real, particularly orbital and nasopharyngeal infestation, but the tail does not set the score.
C3 Herd introduction risk
Effort required to keep the agent out of the herd
Extraordinary biosecurity required: Exclusion needs measures beyond the routine, such as air filtration, thermally treated feed and bedding, or repeated costly testing to find carriers, and may still fail
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Corrected by Eric from L3 Routine biosecurity keeps it out. Reason: MOVED L3 -> L4. Eric, 2026-09-06: "c3 hard but I think push to l4 (even indoor farms have plenty of flies so I assume it would take filters to exclude them)." The pass had reasoned that the measures the SHIC factsheet actually asks for are the routine list -- quarantine of suspect animals, transport inspection, a maintained buffer, monitoring -- and scored L3. His ruling is that those measures address the CONSEQUENCE of a fly getting in rather than the entry itself, and that excluding a free-flying insect from a barn is a screening problem, which is named under filtration in the L4 list. The factsheet supports him on the premise: "Indoor housing does not eliminate the threat of NWS infestation... the stable warm and humid environment of pig barns is a good climate for fly development." Consistent with the standing rule that housing does nothing when the route is something that flies.
Basis. shic_2026: "Indoor housing does not eliminate the threat of NWS infestation. Although, confinement-raised pigs are at lower risk since blow flies prefer to live outside. But the stable warm and humid environment of pig barns is a good climate for fly development if given the opportunity." "Good farm biosecurity is a cornerstone of NWS prevention. Maintaining a clear line of separation between the farm and the surrounding areas can reduce contact with wildlife that may be infested with the fly... a well-maintained buffer has the potential to lower the risk of infestation." "Transportation of infested animals by humans can allow NSW flies to move long distances in a single day."
How this level was decided
Level 3. The standing rule decides the L2 boundary outright: housing closes the route when the route is the ground the pig stands on, and DOES NOTHING when the route is something that flies. The factsheet says so in its own words -- indoor housing does not eliminate the threat, and a barn is in fact good habitat for the larval stage. So this cannot be level 2 however much confinement helps. Against level 4: the measures the factsheet actually asks for are the routine list -- quarantine of suspect animals, inspection of animals in transit, a maintained buffer, environmental cleanliness and monitoring -- and not the extraordinary list of filtration, treated feed or repeat carrier testing. The long-distance route is a truck carrying an infested pig, which is transport control. NOTE FOR ERIC: the case for L4 is that excluding a free-flying insect from a building is an insect-screening problem and screening is named under filtration in the L4 list. I have not taken it because the factsheet treats confinement as protective rather than as requiring screening, but the reading is available.
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. shic_2026: signs include "Visible larvae in wounds", "Wounds with the smell of rotting tissue", and "Wounds with abnormal appearance that progressively worsen". But: "Confirmation of this fly is conducted by trained personnel and includes the identification of several other features which require a microscope", NWS is "easily mistaken for the secondary screwworm (Cochliomyia macellaria)", and "Rules for collecting larvae for diagnostic confirmation may vary by state. Producers may be allowed to collect samples themselves, or this may be restricted to veterinarians or authorized regulatory officials."
How this level was decided
Level 2. Suspicion is easy and clinical -- larvae are visible in the wound and the smell is distinctive -- so this is not level 3 or 4. But suspicion is not confirmation: the factsheet is explicit that the fly is easily mistaken for the secondary screwworm, that adult colour "is a poor indicator", and that species identification needs a microscope and trained personnel. That is the level 2 shape, a validated method that has to be run somewhere other than the barn once the clinician suspects it. The method itself is morphological and long established, so there is no question of an unvalidated assay.
C5 Production cost
Financial impact on the infected farm's cost of production
Moderate: Losses measurably increase cost of production but remain manageable within normal farm operations, whether acute, chronic, or associated with endemic disease
CONFIRMED by Eric. The assignment reached this level from the evidence, and he has reviewed and ratified it.
Confirmed by Eric. CONFIRMED. "c5 agree". Scored on modern US confinement per the standing note. Confinement lowers the risk, but every routine procedure -- castration, tail docking, ear notching, injection, the piglet umbilicus -- creates an egg-laying site, so the control is "inspecting every pig every day" for as long as the fly is in the region. Past level 2 short-lived losses; short of level 4 because labelled treatment works, and because the $1.8 billion Texas projection is an all-livestock figure dominated by range cattle.
Basis. shic_2026: "Death occurring within 7 to 14 days in untreated cases." "Managing the risk of NWS requires inspecting every pig every day for signs of wounds and infestations." Risk factors are routine husbandry -- "Piglet processing, including ear notching or tagging, castration, tail docking, tattooing, or needle teeth clipping", "Any needle injection", "Umbilical sites in newborn piglets". "In Texas alone, projections estimate that an NWS outbreak could result in more than $1.8 billion in livestock losses (USDA APHIS, 2025)."
How this level was decided
LEVEL 3, AND THE CELL I AM LEAST SURE OF AFTER C1. The standing note requires scoring modern US confinement rather than the clinical picture in general, and on that reference the factsheet itself says confinement pigs are at lower risk. What confinement does not remove is the labour: every routine procedure the industry performs -- castration, tail docking, ear notching, injection, the piglet umbilicus -- creates an egg-laying site, so the control is inspecting every pig every day for as long as the fly is in the region, plus topical treatment of every wound. Those are real and sustained costs with deaths behind them, which is past level 2 short-lived losses. I have not gone to level 4 because treatment works and is labelled, so the losses are manageable rather than potentially unsustainable, and because the $1.8 billion Texas projection is an all-livestock figure dominated by range cattle, not a pig figure. If Eric reads the daily whole-herd inspection burden as unsustainable, this is a 4.
C6 Market impact
Duration of material disruption to pork and pig markets
Temporary disruption: Material negative effect on supply or demand lasting less than a month when disease occurs on one or more farms
INFERRED — reasoned, not stated. The evidence implies this level rather than stating it. The reasoning is below.
Corrected by Eric from L3 Significant disruption. Reason: MOVED L3 -> L2. "c6l2, any incursion is not likely to be widespread and I suspect market effect would be managed within a month." The pass scored 3 on the movement-control machinery -- quarantine, certificated movement, a national continuity-of-business framework -- and reasoned from the historical US eradication campaign, which ran for years. His ruling is about SCALE rather than about the machinery: an incursion would be contained rather than national, so the market effect resolves inside a month, which is the level 2 label word for word.
Basis. shic_2026: "suspected cases of NWS must be reported to state animal health officials and the USDA immediately." "suspect infested animals should be quarantined and never be transported off the premises as this could greatly increase the spread of the fly." Movement during a response is governed by the "New World Screwworm National Continuity of Business Standardized Animal Health Certificate Guidance". Against that: "NWS is not a food safety concern... Because NWS larvae only feed on living tissue, there is no risk of contamination of carcasses or pork products at slaughterhouses."
How this level was decided
Level 3. There IS a precedent to reason from, which is what the standing note requires before scoring this cell at all rather than nulling it: NWS was endemic in the US until the 1960s and is currently moving north through Mexico, and a national continuity-of-business movement framework already exists in draft. So the disruption is knowable and is a MOVEMENT disruption -- quarantine of affected premises, certificated movement, regional controls running through an eradication campaign, which historically is measured in months to years. It is NOT a pork-product disruption: the factsheet says outright that there is no food safety concern and no carcass contamination risk, which is what keeps this off level 4. Scored as inferred rather than supported because the factsheet documents the response machinery without reporting what the current Mexican outbreak has done to pig markets.
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. shic_2026 links "a list of FDA approved treatment options" and cites "US Food and Drug Administration. Animal Drugs for New World Screwworm." Wound management is "apply topical treatments that promote healing and protect the wound site from fly activity, including commercially available pesticide sprays containing permethrin". "Consult with your herd veterinarian if animals treated do not improve."
How this level was decided
Level 1. The C7 label test decides this cleanly and in the favourable direction for once: the FDA maintains a page titled "Animal Drugs for New World Screwworm", so a labelled product plainly names the agent, which the standing note says is level 1 rather than the extralabel level 2. Treatment is also the expected response rather than an aspiration -- the factsheet assumes infested animals are treated and healed, and that treated, healed, withdrawn animals re-enter the food chain normally. There is no gap here for research to close.
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
Basis. shic_2026: the entire control frame is exclusion, wound management, treatment and eradication -- "Rapid detection and reporting to veterinarians and state animal health officials will be key to promptly eradicate NWS populations", "Prevention depends heavily on minimizing wounds", and FDA-approved drugs are listed. No vaccine or immunological control is mentioned anywhere in seven pages. "Although NWS was eradicated from the United States in the 1960s..."
How this level was decided
Level 1, on two of the four routes to "no imperative" in the standing note, arriving at the same place independently. FOURTH ROUTE, the response to an incursion would be eradication rather than vaccination: the US eradicated NWS once already and the entire national apparatus described here -- immediate reporting, quarantine, a response playbook, certificated movement -- is an eradication response, which is the swine vesicular disease shape Eric ruled on. SECOND ROUTE, eradicable and excludable by other means: labelled treatment cures the individual animal and wound management prevents the infestation, which is the Sarcoptes and Haematopinus reasoning. This is NOT the unsafe use of level 1 the note warns about -- the claim is affirmatively supported by a documented alternative control that has already worked at national scale, not asserted from an absence of information.
Levels and evidence are generated from data/assignments/new_world_screwworm.yml; the overview is authored in data/overviews/new_world_screwworm.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.