Screwworm

Executive summary

“Screwworm” refers primarily to two obligate wound-myiasis flies whose larvae feed on living tissue rather than necrotic tissue: New World screwworm (Cochliomyia hominivorax) in the Americas and Old World screwworm (Chrysomya bezziana) in Africa, the Middle East, the Indian subcontinent, Southeast Asia, and Papua New Guinea. Both belong to the blowfly family Calliphoridae, subfamily Chrysomyinae, and both are WOAH-listed transboundary animal-health threats because they infest livestock, wildlife, pets, and occasionally humans. In contrast, so-called “secondary screwworms” and other facultative myiasis flies usually exploit already necrotic or preconditioned wounds rather than initiating aggressive invasion of healthy tissue. citeturn17view6turn8search5turn35view0

The core biology that makes screwworm so dangerous is simple and brutal. Females are attracted to wounds or moist body openings, deposit tightly packed egg masses, and larvae hatch within roughly 12–24 hours, penetrate deeply, and complete three larval instars on the host over about 5–7 days. Larvae then drop to soil to pupate; the pupal period is highly temperature-dependent, ranging from about 7 days in warm tropical conditions to weeks in cooler settings, and for New World screwworm the adult may emerge after 7–54 days depending on temperature and humidity. Because females generally mate only once, sterile insect technique (SIT) can collapse populations when implemented area-wide with surveillance, wound inspection, animal movement control, and rapid treatment. citeturn18view2turn12search6turn17view0turn18view3turn15view0

Historically, New World screwworm extended through the southern United States, Mexico, Central America, the Caribbean, and most of South America. It was eliminated from the continental United States by 1966 and then progressively pushed south through Mexico and much of Central America, with the Darién Gap in Panama serving as a biological barrier. That success has now been partially reversed. Since 2023, a northward resurgence has moved through Central America and Mexico; WOAH reported more than 20,000 outbreaks in WAHIS by August 2025, and USDA confirmed U.S. animal detections in Texas in June 2026, with APHIS maintaining a current dashboard and sterile-fly response. Old World screwworm remains endemic across broad parts of Africa and Asia, with recent official reporting highlighting serious outbreaks such as Senegal in 2023–2024. citeturn31view0turn31search2turn11search5turn9search0turn10search0turn36view0turn10search2turn10search5

Clinically, screwworm causes rapidly enlarging, painful, foul-smelling wounds with serosanguineous discharge, visible larvae, depression, anorexia, weight loss, and sometimes death. In livestock, common entry sites include branding, castration, dehorning, ear tagging, parturition injuries, navels of newborns, and tick bites. In humans, infestation can occur in skin wounds and also the nose, ears, mouth, eye orbit, genitalia, and anorectal region. Diagnosis still rests primarily on morphologic identification of larvae, especially mature third instars, supported in some settings by PCR. There are no standardized serologic tests for diagnosis and no vaccines currently available. citeturn35view0turn29view1turn28search0turn14view2turn18view5turn34search4

Treatment has two layers. The non-negotiable layer is mechanical removal of all eggs and larvae, wound cleansing, and repeat inspection after 24–48 hours. In animals, this is typically combined with topical larvicides and species-appropriate systemic antiparasitics. Official U.S. options now include a growing set of FDA conditionally approved or emergency-authorized drugs, including doramectin for cattle and nitenpyram for dogs and cats, though label, species, and residue constraints matter. In humans, CDC emphasizes that all larvae must be physically removed; it notes that ivermectin use is anecdotal and that no controlled human studies prove benefit for NWS myiasis. citeturn20view1turn20view2turn25view0turn21view2turn27view0turn40view0turn30search0

The strongest evidence in the entire screwworm literature concerns eradication, not pharmacology. SIT-based campaigns eliminated New World screwworm from island settings, the United States, Mexico, and most of Central America, and an emergency international campaign eradicated an introduced New World screwworm population from Libya, officially declared complete in June 1992, only 16 months after program inception. Economically, U.S. eradication has been estimated to yield annual producer benefits of $796 million and broader economic benefits of $2.8 billion in 1996 dollars; ecologically, failure to control outbreaks can hit wildlife hard, as seen in the 2016–2017 Florida Keys outbreak where 135 positive cases involved endangered Key deer. citeturn43search2turn43search1turn14view3turn38view0

The main strategic conclusion is that screwworm control is not a “treat-the-case” problem; it is an area-wide One Health systems problem. Surveillance, immediate reporting, movement controls, treatment access, wildlife monitoring, and above all sufficient sterile-fly capacity determine whether countries stay free or lose decades of eradication gains. Current weak points include sterile-fly production capacity, incomplete wildlife surveillance, limited controlled treatment data across species, sparse country-by-country official status data for Old World screwworm, and the risk that climate shifts, animal movement, trade, and parasite-control resistance will widen the suitable range. citeturn32search15turn32search0turn33search2turn40view0turn34search2turn34search1turn33search1turn42search15

Taxonomy, species distinctions, and morphology

The two principal screwworms are Cochliomyia hominivorax and Chrysomya bezziana. WOAH describes both as obligate parasites of mammals, including humans, during their larval stages, and places them within Calliphoridae. They cause wound or traumatic myiasis, can infest pre-existing wounds or mucous membranes, and must be distinguished from each other and from facultative wound flies because control, epidemiology, and regulatory consequences differ. citeturn17view6turn35view0

The term “screwworm” is sometimes used loosely, but analytically it is useful to separate true obligate screwworms from secondary or look-alike myiasis flies. In North America, Cochliomyia macellaria is the “secondary screwworm” because it usually follows primary tissue damage rather than initiating aggressive obligate invasion; in Eurasia and Africa, Wohlfahrtia magnifica is an important obligate traumatic myiasis agent but is not one of the two canonical screwworm species. Old World diagnostic keys also routinely discriminate C. bezziana from non-target blowflies such as Chrysomya albiceps, Chrysomya rufifacies, and Lucilia spp. in surveillance and diagnosis. citeturn8search5turn8search0turn17view3turn16view0

Adult identification is based more on diagnostic morphology than on size alone. WOAH notes that adults in the genus Cochliomyia can be recognized by a metallic blue-to-green body with three dark longitudinal thoracic stripes; this separates New World screwworm adults from many other wound-myiasis flies, although not by itself from all congeners. For C. bezziana, the Australian diagnostic manual describes adults as metallic, with a predominantly orange head and burgundy compound eyes, and reports body length up to 10.0 mm and head width up to 4.1 mm depending on larval nutrition. In the official sources prioritized here, precise adult body-length measurements for C. hominivorax were less consistently tabulated than larval measurements; most official diagnostics emphasize thoracic striping, coloration, and bristle patterns instead. citeturn18view5turn16view1

Larval morphology is the most important diagnostic stage because it is what clinicians, veterinarians, and laboratories actually recover from wounds. For New World screwworm, CDC’s bench aid describes third-instar larvae as 6.5–17 mm long, muscidiform, with encircling bands of short spines on each segment and—most importantly—darkly pigmented dorsal tracheal trunks extending anteriorly across at least two full body segments. For Old World screwworm, the defining combination is different: characteristic spinulation, an anterior spiracle with four to six papillae, and pigmented secondary tracheae. These distinctions are central because wound surfaces often contain other blowfly larvae, and shallowly collected specimens may represent the wrong species. citeturn14view2turn17view5turn16view0turn17view6

Egg and puparial measurements are diagnostically useful and, unlike some adult metrics, are explicitly tabulated in the Australian manual. C. bezziana eggs are reported at about 1.25 mm long and 0.26 mm wide; C. hominivorax eggs at about 1.04 mm long and 0.22 mm wide. The C. bezziana puparium is reddish-brown to nearly black and reaches up to 10.1 mm in length and 3.6 mm in diameter. For C. hominivorax, official fact sheets emphasize the barrel-shaped puparium but the sources reviewed here were less explicit about a standard puparial size range than they were for larvae. That asymmetry itself is a small but real data-gap when trying to build uniform comparison tables from official sources alone. citeturn15view1turn16view3

Species comparison

Life cycle and environmental determinants

The life cycle of screwworm is a tightly wound interaction between host wounds, temperature, and adult reproductive behavior. Females locate wounds or body openings by odor, deposit egg masses at the wound edge, and larvae immediately begin feeding on wound fluids and then healthy tissue. The larvae deepen and enlarge the wound as they mature, often attracting more gravid females and amplifying the lesion. That positive feedback is why a lesion may look disproportionately severe relative to the original trauma. citeturn18view2turn28search5turn29view1

flowchart LR
    A[Wound or moist body opening] --> B[Egg mass laid at wound edge]
    B -->|12-24 h| C[Larvae hatch]
    C --> D[Instar I and II]
    D --> E[Instar III in wound]
    E -->|about 5-7 d total on host| F[Larvae leave host]
    F --> G[Burrow into soil]
    G --> H[Pupation]
    H -->|NWS about 7-54 d depending on temperature and humidity| I[Adult emergence]
    H -->|OWS about 7 d at warm tropical temperatures but slower in cool seasons| I
    I --> J[Mating]
    J --> K[Female oviposition after maturation]
    K --> A

For New World screwworm, WOAH and CDC agree on the major timing steps: eggs hatch in 12–24 hours; larvae reach maturity in about 5–7 days; adults emerge from pupae after 7–54 days depending on temperature and humidity; and the whole cycle may be completed in less than 3 weeks in the tropics. CDC also notes that a female may lay 200–300 eggs at a time and up to about 3,000 eggs over a 10- to 30-day lifespan, while WOAH gives a higher average first batch for NWS of 343 eggs and emphasizes that females mate only once. The minor numerical differences reflect source type and averaging methods, not a biological contradiction. citeturn18view2turn18view3turn29view1turn18view1

For Old World screwworm, the Australian manual gives an especially useful stage-resolved picture. Eggs are laid in masses of up to 250; the first and second instars each occupy one day; the third instar lasts 3–5 days; larvae that leave the wound burrow 2–3 cm into soil and pupate; and under ideal conditions the life cycle can be completed in about 20 days. A figure in the manual shows roughly 12–20 hours to egg hatch, about 6–7 days on the host, and about 7–8 days in soil before adult emergence. Adults may survive around 15 days on average, though some survived 40 days in laboratory conditions at 28°C. citeturn15view0turn17view0

Temperature is the dominant environmental limiter. WOAH states plainly that off-host development is shorter at higher temperatures. The Old World manual adds stronger seasonal resolution: in Sudan, during winter months with mean daily temperatures of 12–19°C, only 21–44% of adults emerged and pupal development extended to 3–4 weeks; by contrast, in spring and autumn pupal development finished in 5–9 days and 54–77% of flies emerged. The same manual also notes that no stage is resistant to freezing, helping explain why climatic suitability constrains permanent establishment in temperate zones. citeturn18view2turn15view0

Stage-by-stage measurements and durations

Distribution, hosts, and clinical presentation

The broadest historical picture is easiest to describe for New World screwworm. IAEA states that NWS remains endemic in almost all South American countries except Chile and in five Caribbean settings it names as Cuba, Jamaica, Haiti, the Dominican Republic, and Trinidad and Tobago. FAO and CDC both say the parasite is currently endemic in South America and parts of the Caribbean, while also emphasizing its recent re-establishment across Central America and Mexico. citeturn11search8turn11search5turn11search4

The current northern front is unstable and must be described as such. Panama first notified the modern Central American resurgence in June 2023. By August 2025, WOAH reported spread into Belize, Costa Rica, El Salvador, Guatemala, Honduras, Nicaragua, and Mexico, with more than 20,000 outbreaks recorded in WAHIS. CDC’s June 2026 situation page states that the outbreak has moved northward through Central America and Mexico since 2023, and USDA confirmed the first U.S. animal case of the current outbreak in Texas on June 3, 2026; APHIS now maintains a live dashboard for U.S. detections and sterile-fly operations. citeturn9search0turn11search1turn10search0turn36view0

For Old World screwworm, the highest-confidence official framing is regional rather than country-by-country. Animal Health Australia and the Canadian Food Inspection Agency place C. bezziana across parts of Asia, tropical and sub-Saharan Africa, and some Middle Eastern countries, with broader descriptions that include Southeast Asia, the Indian subcontinent, Papua New Guinea, the Middle East, and tropical/subtropical Africa. Australia remains free of establishment but treats OWS as a major exotic threat and has documented at least one vessel-associated introduction risk event in 1988. Recent FAO reporting from Senegal underscores that OWS remains an active contemporary African animal-health problem rather than a merely historical one. citeturn10search5turn10search8turn14view1turn42search0

flowchart TB
    NWS[New World screwworm]
    OWS[Old World screwworm]

    NWS --> N1["Historic range: southern U.S. through Mexico, Central America, Caribbean, most of South America"]
    NWS --> N2["Long-term eradication success: U.S., Mexico, most of Central America"]
    NWS --> N3["Current endemicity: South America and parts of Caribbean"]
    NWS --> N4["Current resurgence: Central America and Mexico since 2023"]
    NWS --> N5["Recent U.S. detections: Texas and neighboring response zone in 2026"]

    OWS --> O1["Current broad range: Africa"]
    OWS --> O2[Middle East]
    OWS --> O3[Indian subcontinent]
    OWS --> O4[Southeast Asia and Papua New Guinea]
    OWS --> O5[Not established in Australia or Western Hemisphere]

Screwworm attacks almost any warm-blooded host. APHIS states that all mammals, and rarely birds, can be infested. USDA prevention materials stress risk to livestock, pets, wildlife, and occasionally birds and people. Common livestock hosts include cattle, sheep, goats, horses, and swine; wildlife hosts range from deer and tapirs to feral pigs and numerous other mammals; and humans are incidental but medically important hosts. citeturn35view0turn37search2turn37search1

Clinical presentation is remarkably consistent across hosts because larvae do the same thing in each: they tear and burrow into living tissue. APHIS case definition documents drainage, suppuration, blood-serum discharge, a distinctive odor, visible egg masses at wound edges, and larvae positioned head-down in the wound. Affected animals often show discomfort, lethargy, depression, herd separation, anorexia, and decreased milk production. CDC describes a human syndrome of rapidly worsening, foul-smelling, painful wounds, sometimes with swelling, bloody discharge, and a sensation of movement, and notes that infestation may also involve the eyes, ears, nose, mouth, genitals, and anorectal area. citeturn35view0turn29view1

Wildlife impact can be severe and sometimes conservation-relevant. During the 2016–2017 Florida outbreak, APHIS recorded 145 positive cases in total, of which 135 involved endangered Key deer; one raccoon and several pets were also affected. In Colombia, CDC’s Emerging Infectious Diseases documented infestation in wild mountain tapirs, illustrating that screwworm pressure can reach remote wildlife populations and not merely peri-domestic settings. citeturn38view0turn39view1turn37search1

Host groups and typical signs

Diagnosis and treatment

Diagnosis in the field begins with pattern recognition, but confirmatory diagnosis still depends on the larvae. APHIS instructs veterinarians to suspect screwworm when a wound is malodorous, suppurative, bloody or purulent, and to collect larvae from several sites and depths, especially the deepest part of the lesion because superficial larvae may belong to secondary myiasis species. CDC gives parallel advice for humans and recommends submitting at least 10 larvae if available. citeturn19view0turn29view1turn40view0

Laboratory diagnosis is primarily morphologic. For NWS, CDC’s DPDx bench aid emphasizes the pigmented tracheal trunks and the incomplete posterior spiracular peritreme of third-instar larvae. WOAH notes that the mature third instar is the most reliable stage for diagnosis, and APHIS notes that laboratory confirmation in official veterinary channels relies on morphologic identification of egg mass, larva, or adult fly. There are no standardized serologic tests currently indicated for routine diagnosis. citeturn14view2turn18view5turn35view0

Molecular methods have become more important for surveillance and exclusion testing, especially in trap catches. The Australian Old World screwworm manual discusses PCR as an ancillary screening or confirmatory tool for trap catches, and a 2024/2025 publication reported a novel real-time PCR for C. hominivorax with non-destructive extraction suitable for surveillance. Even so, the operational gold standard in regulatory response remains morphology plus chain-of-custody specimen submission. citeturn15view0turn34search2

Treatment principles are similar across species and jurisdictions: remove all larvae and eggs, clean and disinfect the wound, kill escaped larvae safely, re-examine the wound, and reduce reinfestation pressure. APHIS requires quarantine of confirmed animal cases until effective treatment is completed, plus cleaning and disinfestation of the premises and at least a month of post-quarantine monitoring with fly paper or traps. For human NWS, CDC states that treatment “requires physical removal of all larvae,” which may require surgical extraction for deeply embedded larvae; patients with secondary infections may need supportive care. citeturn20view1turn20view2turn29view1turn40view0

For veterinary pharmacology, the evidence base is uneven and highly species-specific. WOAH still describes the classical approach as organophosphorus insecticides applied to wounds for kill plus residual protection, with preventive use of organophosphates and, more recently, avermectins such as doramectin in at-risk animals. In the United States, FDA’s NWS page now lists a rapidly expanding menu of conditionally approved and emergency-authorized options, but the strongest dosing data in the official sources reviewed here are for doramectin in cattle and nitenpyram in dogs/cats. citeturn18view4turn41view1

For cattle, FDA’s conditional approval summary states that Dectomax-CA1 (doramectin injection) is indicated for prevention and treatment of New World screwworm in cattle and prevention of reinfestation for 21 days, at 1 mL per 110 lb body weight (200 mcg/kg) by subcutaneous or intramuscular injection in the neck. The September 2025 FDA announcement also notes a 35-day slaughter withdrawal and dairy/veal restrictions. citeturn25view0turn21view1

For dogs and cats, FDA’s June 2026 EUA authorizes generic nitenpyram tablets for treatment of NWS in animals at least 2 lb and at least 4 weeks old. FDA says most larvae are killed within hours; a second dose 6 hours later is advised, and the product does not prevent reinfestation. The weight-based tablet strengths from the FDA-approved nitenpyram label are 11.4 mg for dogs or cats 2–25 lb and 57 mg for dogs 25.1–125 lb; veterinary removal of residual larvae and wound care may still be needed. The label consulted did not provide a higher-weight cat-specific category because cats ordinarily fall within the 2–25 lb range. citeturn21view2turn27view0

For humans, the strongest official statement is conservative. CDC says all eggs and larvae must be removed and that no medication has been proven effective in controlled human studies. CDC notes anecdotal off-label ivermectin use but does not recommend it as standard human therapy for NWS. At the same time, peer-reviewed case reports and reviews do document ivermectin use as an adjunct in severe myiasis, including reports using about 200 µg/kg oral dosing in selected cases; these reports support plausibility, but not guideline-level certainty. citeturn40view0turn30search0turn28search4turn28search10

Diagnostic workflow

1. Suspect clinically when a wound enlarges rapidly, smells foul, drains blood/serum, or contains visible larvae or shingled egg masses. citeturn35view0turn29view1
2. Collect correctly from multiple sites and especially from the deepest portion of the wound. Preserve in 70% ethanol or at least 70% isopropanol; APHIS and CDC both warn against casual disposal of live larvae. citeturn19view0turn29view1turn40view0
3. Confirm morphologically using third-instar features, with PCR as adjunct where available. citeturn14view2turn16view0turn34search2
4. Report immediately to the appropriate animal-health or public-health authority because this is a regulatory emergency, not merely a wound infestation. citeturn9search1turn9search2turn35view0

Treatment comparison

Prevention, control, and eradication experience

Prevention has three concentric layers: prevent wounds, protect wounds, and suppress flies. USDA and CDC both stress rapid wound inspection and prompt treatment of even very small lesions. CDC recommends covering all wounds and using insect precautions in areas with circulation of NWS, including EPA-registered repellents, 0.5% permethrin on clothing and gear, and avoiding sleeping outdoors. In livestock systems, inspection intensity must increase during high-risk periods such as calving, lambing, branding, docking, and rainy seasons that increase wound occurrence and fly activity. citeturn29view1turn40view0turn9search4

Chemical control still matters, but usually as part of an integrated program rather than as a stand-alone eradication strategy. WOAH describes organophosphorus wound treatment and preventive spraying or dipping, with avermectins such as doramectin becoming important preventive tools. APHIS SOP also requires insecticidal treatment of bedding and holding areas in official response settings. FDA, however, warns that drug development for NWS remains incomplete, that no medicated feeds are approved for prevention or treatment of NWS in any species, and that antiparasitics should be used judiciously because of resistance concerns. citeturn18view4turn20view2turn21view0

There are no vaccines currently available for screwworm. WOAH explicitly states that no biological products such as vaccines are available, although vaccine research has been pursued experimentally. WHO-style eradication for screwworm therefore remains dependent not on host immunity but on population suppression and reproductive collapse of the fly. citeturn18view5turn34search4

That brings the discussion to sterile insect technique, the central strategic technology. SIT exploits the fact that female screwworm flies usually mate only once. By flooding an area with sterile males, programs push matings toward infertility and drive population decline. APHIS, IAEA, FAO, and historical reviews all describe SIT as the proven method that enabled continental-scale NWS eradication. APHIS’s 2016 ready-reference guide notes that, in every successful eradication program it summarizes, SIT was used. citeturn29view1turn42search12turn42search7turn44view0

Modern SIT is only as strong as its production and dispersal infrastructure. COPEG’s Panama production plant is designed to produce up to 100 million sterile pupae per week, and APHIS reports a dispersal facility at Moore Air Base, Texas, capable of dispersing up to 100 million sterile flies per week. USDA also broke ground in 2026 on a Texas production facility intended to reach an initial 100 million/week and later 300 million/week. These numbers matter because current outbreaks can outrun sterile-fly capacity if geography expands faster than production. citeturn32search15turn32search0turn32search4

timeline
    title Major screwworm control and eradication milestones
    1954 : Curaçao becomes first country to eradicate NWS with SIT
    1957 : Southeastern U.S. eradication program starts
    1959 : NWS eradicated from Florida / southeastern U.S.
    1962 : Southwestern U.S. eradication program begins
    1966 : NWS eradicated from continental United States
    1972 : Mexico-U.S. program advances barrier southward
    1986 : NWS pushed to southern Mexico border
    1988 : NWS introduced into Libya
    1991 : Last detected case in Libya in April; sterile releases stop in October
    1992 : Libya officially declares eradication in June
    1994 : Belize and Guatemala eradicated
    1995 : El Salvador eradicated
    1996 : Nicaragua eradicated
    1999 : Honduras eradicated
    2000 : Costa Rica declared free
    2006 : Darién Gap biological barrier maintained with continuous sterile releases
    2016 : Florida Keys reintroduction affects Key deer
    2017 : Re-eradication achieved in Florida
    2023 : Central American resurgence begins in Panama
    2025 : WOAH reports >20,000 outbreaks in WAHIS during resurgence
    2026 : USDA confirms current-outbreak U.S. animal detections in Texas

The New World eradication story is one of the great successes in applied entomology. APHIS and the National Agricultural Library document successive milestones: the southeastern U.S. campaign beginning in 1957, elimination from the United States by 1966, southern movement through Mexico, and later clearance of Belize and Guatemala (1994), El Salvador (1995), Nicaragua (1996), Honduras (1999), and Costa Rica (2000). The Darién Gap then served as the southern biological barrier protecting Central and North America from persistent South American endemicity. citeturn31search13turn31search2turn44view0

The Libya campaign remains the canonical example of emergency international eradication after transcontinental introduction. FAO and IAEA report that the last case in Libya was recorded on 7 April 1991 and official eradication was declared on 22 June 1992, only 16 months after the eradication program began. Contemporary analyses judged the programme decisive in protecting North Africa, the Mediterranean Basin, and the Middle East from establishment. citeturn43search0turn43search2turn43search3

For Old World screwworm, the evidence base is thinner on full eradication and stronger on surveillance, preparedness, and regional control. Iraq studies describe trap-based surveillance and seasonal fly monitoring; regional Middle East campaigns have been reported, but the official and peer-reviewed material consulted here did not provide the same clear, well-documented continent-scale eradication outcomes that exist for NWS in the Americas and Libya. In Africa, FAO’s Senegal report documents major recent OWS impact and response needs, but not eradication. That contrast is itself important: OWS remains a major endemic-control problem rather than a clearly solved eradication problem. citeturn42search15turn42search2turn42search0

Eradication and control case studies

Impacts, surveillance, emerging threats, and research gaps

The economic burden of screwworm is historically huge. APHIS estimates that before eradication, producers in the southeastern United States lost $10–20 million per year before the 1959 eradication, while southwestern producers lost $50–100 million per year before area-wide elimination. For Texas in 1976, APHIS cites $132.1 million/year in producer spending to manage outbreaks and a $283–375 million total impact on the Texas economy. Looking the other way, APHIS estimates annual benefits from U.S. eradication at $796 million to producers and $2.8 billion to the wider economy in 1996 dollars, not inflation adjusted. citeturn14view3

Broader international estimates are also large, though they are less frequently updated in official dashboards. A major SIT review chapter estimated annual producer benefits after eradication at $292 million for Mexico and $77.9 million for Central America, and suggested that eradication in South America could save about $3.592 billion per year. For Libya, the same synthesis reported a benefit-cost ratio of 5:1 in the infested zone and 10:1 nationwide. These are model- and era-specific estimates, but they make the central point clearly: screwworm is a macroeconomic livestock pest, not merely a veterinary nuisance. citeturn43search11

Ecological costs are harder to monetize but often more alarming. APHIS’s Florida report documents how an outbreak centered in wildlife rapidly affected vulnerable endemic fauna, especially Key deer. CDC’s 2025 mountain tapir report shows that screwworm can threaten species of conservation concern far from routine livestock monitoring. That means under-detection in wildlife is not just an ecological blind spot; it is also an epidemiologic blind spot that can compromise eradication. citeturn38view0turn37search1

Surveillance and reporting systems are now multilayered. Internationally, outbreaks are reported through WOAH WAHIS. In the United States, suspected animal cases are immediately reportable to State Animal Health Officials and the APHIS Area Veterinarian in Charge, with official identification at NVSL; suspected human cases should be reported immediately to local or state public health authorities, with species confirmation available through CDC DPDx. APHIS also maintains a current dashboard and status page for U.S., Mexico, and Central America, while Australia maintains active preparedness surveillance in high-risk northern areas despite remaining free of the pest. citeturn9search0turn9search1turn19view0turn9search2turn36view0turn9search3turn9search20

The main emerging threats fall into three coupled classes. The first is range expansion under environmental change. Modeling studies for C. bezziana project future spread into additional climatically suitable regions under climate-change scenarios, and recent NWS reviews tie re-emergence risk to climate-sensitive habitat suitability. The second is movement risk from trade, smuggling, cross-border livestock movement, and human travel; CDC’s 2026 HAN specifically linked the recent northward NWS spread to unregulated cattle movement, increased movement through the Darién Gap, and expansion into new farming areas. The third is treatment and operational resistance, not yet demonstrated as a single global failure mode but serious enough that FDA repeatedly warns about antiparasitic resistance and urges judicious use. citeturn33search1turn42search25turn40view0turn33search2

A final threat is capacity mismatch. Successful SIT requires enough sterile males, enough release geography, enough surveillance, and enough field staff. COPEG’s current output of around 100 million sterile pupae per week and APHIS’s Texas dispersal capacity are substantial, but policymakers have already warned that a wider outbreak could require much higher volumes. USDA’s current build-out of new production facilities is therefore not just an optimization project; it is core national preparedness. citeturn32search15turn32search0turn32search4turn32search6

Open questions and limitations

Several important gaps remain in the evidence base and in public official reporting. First, country-by-country current endemic status for Old World screwworm is less standardized in official pages than regional descriptions, so highly granular “current map” claims for Africa and Asia should be treated cautiously unless tied to national surveillance reports. Second, adult-stage metric data for C. hominivorax were less consistently provided in the official diagnostic sources prioritized here than larval and egg-stage metrics. Third, controlled therapeutic trials remain sparse across many host species, especially in wildlife and humans; for human NWS, CDC explicitly states that no medication has proven efficacy in controlled studies. Fourth, OWS has many surveillance and control reports, but the literature and official material consulted here did not yield a comparably clear, modern, official set of eradication outcomes analogous to the NWS campaigns in the Americas and Libya. Finally, FDA itself has highlighted priority data gaps on effectiveness, safety, and practical delivery systems for animal drugs across species, confirming that pharmacologic control remains a developing area rather than a settled one. citeturn21view0turn30search0turn34search2turn34search1turn42search15

Disclaimer: The information provided in this article is for educational purposes only and is not a substitute for professional veterinary advice, diagnosis, or treatment. Always consult with your veterinarian regarding any health concerns for your pet.