# Screwworm


## Key Takeaways

- Screwworm myiasis is caused by obligate wound-myiasis flies, primarily *Cochliomyia hominivorax* (New World) and *Chrysomya bezziana* (Old World), which infest living tissue. Larval development from egg hatch to pupation on the host typically spans 5-7 days, with environmental temperature significantly influencing pupal development and adult emergence.
- Diagnosis relies heavily on morphologic identification of larvae, particularly third instars, characterized by specific features like pigmented tracheal trunks (New World) or anterior spiracular papillae (Old World); standardized serologic tests and vaccines are currently unavailable.
- Clinical presentation involves rapidly enlarging, painful, foul-smelling wounds with serosanguineous discharge and visible larvae, affecting livestock, wildlife, and occasionally humans, with common entry points including natural orifices and iatrogenic wounds.
- Treatment necessitates mechanical removal of all eggs and larvae, wound cleansing, and often topical larvicides or systemic antiparasitics; in the U.S., FDA has authorized options like doramectin for cattle and nitenpyram for dogs and cats, with specific dosage and withdrawal considerations.
- Eradication efforts have historically centered on Sterile Insect Technique (SIT), which exploits the female's single mating event to collapse populations; successful campaigns eliminated New World screwworm from the continental U.S. and Libya, underscoring its efficacy as an area-wide control strategy.
- Current challenges include the northward resurgence of New World screwworm through Central America and Mexico, with recent U.S. detections, and the need for robust surveillance, sterile-fly production capacity, and integrated One Health approaches to prevent further spread and re-establishment.

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## 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

| Feature | New World screwworm | Old World screwworm | Other important comparators |
|---|---|---|---|
| Accepted species | *Cochliomyia hominivorax* citeturn17view6turn35view0 | *Chrysomya bezziana* citeturn17view6turn35view0 | *Cochliomyia macellaria* usually secondary/facultative in wounds; *Wohlfahrtia magnifica* is an obligate traumatic myiasis fly but not one of the two classical screwworms. citeturn8search5turn17view3 |
| Parasitic strategy | Obligate myiasis in living tissue. citeturn17view6 | Obligate myiasis in living tissue. citeturn17view6 | Many “secondary screwworm” or wound flies are facultative and often exploit necrotic tissue or preconditioned wounds. citeturn8search5 |
| Adult hallmarks | Metallic blue-green body with 3 dark thoracic stripes; official sources prioritize morphology over a single standard body-length figure. citeturn18view5 | Adult body length up to 10 mm; orange head, burgundy eyes, metallic body. citeturn16view1 | Look-alikes vary by region and require key-based identification. citeturn17view3turn16view0 |
| Egg size | ~1.04 × 0.22 mm. citeturn15view1 | ~1.25 × 0.26 mm. citeturn15view1 | Varies; egg morphology can aid exclusion of non-target Calliphoridae. citeturn15view1 |
| Mature larva | Usually 6.5–17 mm; dark pigmented tracheal trunks are key. citeturn14view2turn13search0 | Final instar typically up to ~17–18 mm in comparison sources; diagnostic emphasis is on spinulation, 4–6 anterior spiracular papillae, and pigmented secondary tracheae. citeturn13search12turn16view0 | *W. magnifica* has robust mouth hooks and thorn-like black spines; *C. macellaria* lacks the heavily pigmented tracheal trunks of NWS. citeturn17view3turn16view0 |
| Geographic frame | Western Hemisphere. Historically southern U.S. to South America; currently endemic in South America and parts of the Caribbean, with resurgence through Central America, Mexico, and recent U.S. detections. citeturn11search5turn11search4turn10search0turn36view0 | Africa, Indian subcontinent, Southeast Asia, Middle East, Papua New Guinea; not established in Australia or the Western Hemisphere. citeturn10search5turn10search8 |

## 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

```mermaid
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](/knowledge/diagnostics/molecular/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

| Stage | New World screwworm | Old World screwworm | Main environmental dependence |
|---|---|---|---|
| Egg | ~1.04 × 0.22 mm; average first batch ~343 eggs in WOAH summary. citeturn15view1turn18view1 | ~1.25 × 0.26 mm; egg masses often 95–245 eggs, up to ~250. citeturn15view1turn15view0 | Wound availability and humidity at oviposition site. citeturn18view2turn15view0 |
| Egg-to-hatch interval | 12–24 h. citeturn18view2turn12search10 | 12–20 h in the manual figure; text also aligns with ~12–24 h. citeturn17view0turn15view0 | Warmer conditions accelerate development. citeturn18view2turn15view0 |
| Larvae in wound | Third instars usually 6.5–17 mm; mature larvae average ~15–16 mm. Total on-host larval period about 5–7 days. citeturn14view2turn13search1turn18view2 | First and second instars 1 day each; third instar 3–5 days; total on-host period roughly 5–7 days. Mature larvae commonly reach ~17–18 mm in comparison sources. citeturn15view1turn13search12 | Wound temperature and host tissue conditions. citeturn18view2turn15view0 |
| Soil phase / puparium | Pupal stage 7–54 days depending on temperature and humidity. citeturn12search6turn29view1 | Puparium up to 10.1 mm × 3.6 mm; adult emergence after ~7 days at 28°C but markedly delayed in cool weather, including 3–4 weeks at 12–19°C in field observations. citeturn16view2turn15view0 | Soil temperature, soil moisture, and season. citeturn15view0 |
| Adult maturation | Males mature quickly; females mature over ~6–7 days and oviposit a few days later. Adult lifespan often 10–30 days in summaries. citeturn18view2turn29view1 | Manual figure indicates ~6–7 days to pre-oviposition; average lifespan ~15 days, some 40 days at 28°C. citeturn17view0turn15view0 | Nectar/protein access, temperature, and humidity. citeturn18view1turn15view0 |

## 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

```mermaid
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

| Host group | Common entry points | Typical signs | Notes |
|---|---|---|---|
| Cattle, sheep, goats | Branding, castration, dehorning, ear tagging, parturition injuries, navels of newborns, tick bites, fencing cuts. citeturn35view0turn18view2 | Enlarging wound, foul odor, serosanguineous discharge, visible larvae, pain, anorexia, depression; decreased milk in dairy animals. citeturn35view0turn7search21 | Neonatal navels and perineal/vulvar areas are classic high-risk sites. citeturn18view2turn12search10 |
| Horses | Limb wounds, tack injuries, surgical wounds, body openings. citeturn40view0turn35view0 | Similar rapidly progressive wound myiasis; significant pain and secondary infection risk. citeturn28search0turn35view0 | Horses are repeatedly highlighted in North American preparedness materials. citeturn40view0 |
| Pets | Cuts, scratches, bite wounds, surgical sites, ear disease, body openings. citeturn21view2turn35view0 | Painful worsening lesion, visible larvae or egg masses, lethargy, appetite loss. citeturn21view2turn37news42 | Dogs and cats now have specific FDA-authorized U.S. options, but veterinary wound care remains essential. citeturn21view2turn41view1 |
| Wildlife | Fighting wounds, rut injuries, predation wounds, tick bites, neonatal navels. citeturn15view0turn37search0 | Often detected late; severe tissue destruction and mortality can occur. citeturn38view0turn37search1 | Wildlife complicates eradication because detection and treatment are difficult. citeturn37search3turn34search25 |
| Humans | Small skin wounds, surgical sites, cancers, chronic ulcers, nose, ears, eyes, mouth, genitals, anorectal area. citeturn29view1turn40view0 | Pain, foul odor, bloody discharge, swelling, visible larvae, sensation of movement; can be fatal if untreated. citeturn29view1turn40view0 | Higher risk in people with open wounds, outdoor exposure, incapacity, unstable housing, or proximity to livestock. citeturn40view0 |

## 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

| Host / context | Core treatment | Drug options with dose where available | Important cautions |
|---|---|---|---|
| Livestock with suspected screwworm | Remove all visible eggs/larvae; clean and disinfect wound; recheck after ~24 h; quarantine if under official response. citeturn20view1 | Classical/WOAH approach: topical organophosphorus insecticides into wounds; avermectins such as doramectin used preventively/therapeutically in some programs. citeturn18view4 | Drug choice depends on species, food-animal residue rules, and jurisdiction. citeturn21view0 |
| Cattle in U.S. | As above, plus official program measures. citeturn20view1turn20view2 | **Dectomax-CA1 (doramectin)**: **1 mL/110 lb** (**200 mcg/kg**) SC or IM in neck; indicated for prevention/treatment and 21-day reinfestation prevention in cattle. citeturn25view0 | 35-day slaughter withdrawal; dairy and veal restrictions apply. citeturn21view1 |
| Dogs and cats in U.S. | Veterinary wound care plus removal of residual larvae. citeturn21view2 | **Nitenpyram** EUA: eligible animals **≥2 lb**, **≥4 weeks**; repeat second dose at **6 h**. FDA-approved tablet strengths on nitenpyram label: **11.4 mg** for dogs/cats **2–25 lb**; **57 mg** for dogs **25.1–125 lb**. citeturn21view2turn27view0 | Short-acting; does **not** prevent reinfestation. citeturn21view2 |
| Other U.S. animal options | Species-specific, many under EUA/conditional approval. citeturn41view1 | FDA’s June 2026 page lists Dectomax/Dectomax-CA1 EUA for dairy cattle, horses, swine, sheep, deer; topical sprays/ointment/powder; lotilaner products for dogs/cats; Exzolt Cattle-CA1; Ivomec cattle EUA. citeturn41view1turn41view2 | Exact dosing varies by product and species; label/EUA controls. citeturn41view1turn21view0 |
| Humans | **Physical removal of all larvae and eggs**; may require surgery; wound care and supportive care for secondary infection. citeturn29view1turn40view0 | CDC: no medication proven effective in controlled human studies. Case reports describe off-label **ivermectin ~200 µg/kg** as adjunct in severe cases. citeturn40view0turn28search4 | CDC does **not** recommend ivermectin as standard treatment for human NWS at this time. citeturn30search0 |

## 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

```mermaid
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

| Program / setting | Main species | Approach | Outcome | What it shows |
|---|---|---|---|---|
| Curaçao, first SIT island campaign | NWS | Area-wide SIT. citeturn44view0 | First successful NWS SIT eradication in **1954**. citeturn44view0 | Small isolated geographies can validate technology before mainland scale-up. |
| Southeastern United States | NWS | SIT plus animal-health operations. citeturn31search13 | Program initiated **1957**; eradication in the Southeast by **1959**. citeturn14view3turn31search13 | Proof that continental eradication was feasible. |
| Continental United States | NWS | SIT expansion westward and border control. citeturn14view3turn31search13 | U.S. eradication completed by **1966**. citeturn14view3turn31search13 | One of the landmark insect-eradication achievements in agriculture. |
| Mexico and Central America | NWS | Progressive southward elimination with multinational SIT. citeturn31search7turn44view0 | Southern Mexico by **1986**; Belize/Guatemala **1994**; El Salvador **1995**; Nicaragua **1996**; Honduras **1999**; Costa Rica **2000**. citeturn14view3turn31search2 | International coordination and synchronized field operations are essential. |
| Libya / North Africa | NWS introduced from the Americas | FAO-led emergency program with SIT, surveillance, quarantine, and wound treatment. citeturn43search2turn43search3 | Last case **7 Apr 1991**; eradication declared **22 Jun 1992**. citeturn43search0turn43search2 | Rapid, internationally financed emergency eradication can prevent regional establishment. |
| Florida Keys 2016–2017 | NWS | SIT plus intensive wildlife and domestic-animal surveillance/treatment. citeturn38view0 | Reintroduced NWS was eradicated again by early **2017** after a wildlife-centered outbreak. citeturn14view3turn38view0 | Even “eradicated” countries remain vulnerable to reintroduction. |
| Middle East / Iraq regional control | OWS | Surveillance traps, seasonal monitoring, regional campaign activity. citeturn42search15turn42search2 | Publicly available outcome data are much less standardized than for NWS eradication campaigns. citeturn42search15turn42search2 | OWS control evidence is comparatively fragmented. |

## 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

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*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.*

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