Surveillance and diagnostics are critical for the early detection, containment and eradication of exotic pests. For the screwworm fly, this is usually via targeted surveillance and exclusion testing of trap-caught flies, as well as the identification of larvae associated with myiasis wounds. We present a specific and sensitive real-time polymerase chain reaction (PCR) assay for the detection of the New World screwworm fly, Cochliomyia hominivorax Coquerel (Diptera: Calliphoridae). The assay targets the cytochrome oxidase subunit 1 (cox1) gene from adult flies or larvae and retains high analytical sensitivity when multiplexed with an existing assay for the Old World screwworm fly, Chrysomya bezziana Villeneuve (Diptera: Calliphoridae), achieving a limit of detection of less than 1 copy per microlitre of reaction. To assess its utility for surveillance and diagnostics, a novel non-destructive DNA extraction method was performed on spiked trap catches of field-caught flies, and on boiled and unboiled specimens of larval instars. The multiplexed assay detected 95% of spiked flies, and all screwworm flies from positive samples were retrieved and morphologically identified. Results from larval instars confirmed that the assay can be used for larvae, with higher sensitivity observed for unboiled larval instars. This molecular assay enables the simultaneous detection of Co. hominivorax and Ch. bezziana, offering a reliable alternative to existing single-target and destructive methods of bulk fly testing. It also holds potential for broader application in the identification of larval stages.
New World screwworm, Cochliomyia hominivorax (Coquerel), is an obligate parasite of warm-blooded animals and a serious threat to livestock, wild animals, and humans. The sterile insect technique (SIT) was used to eradicate screwworm from North and Central America during the mid-1900s to the early-2000s. Traps are an important tool for screwworm control programs, where they are used for surveillance, evaluation of production strains, and to estimate the ratio of wild to sterile flies following aerial and ground releases. Here, we tested two trap designs for capturing New World screwworm. The vertical sticky trap, the trap currently used, is a triangle-shaped trap with 1,710 cm2 surface area coated with insect adhesive. The cylinder sticky trap is a cylinder-shaped trap with 1,700 cm2 surface area coated with the same insect adhesive. Both traps were baited with swormlure-4 as an attractant. We used a release-recapture study with a randomized block design to test the efficacy of the two trap designs. The cylinder sticky trap captured significantly more C. hominivorax than the vertical sticky trap, likely due to an increase in the ability of the flies to land on the surface of the cylinder-shaped trap.
The New World screwworm, Cochliomyia hominivorax (Coquerel, 1858; Diptera: Calliphoridae), was eradicated from North and Central America through the first application of the sterile insect technique. The sterile screwworm adult fly releases were mixed sex, but releasing only males could be much more effective. Here, we describe screwworm transgenic sexing strains (TSSs) with expected embryo lethality. Strains were developed with a Tetracycline-off (Tet-off) system utilizing either the Lucilia cuprina (Wiedemann, 1830) nullo (DR6) or Cochliomyia macellaria (Fabricius, 1755) CG14427 (DR7) gene promoter and a tTA-activated effector to promote female lethality. The promoter activity was highest in 2 to 3 h embryos and low in larvae and adults. However, these strains also had unexpected high expression in pupae. Evaluation on two doxycycline (Dox) suppression regimens found that inclusion of Dox in the last larval feeding rescued females of the subsequent generation, likely by maternal transfer of Dox. All TSSs produced only males on a reduced Dox feeding regimen, but the female lethal period for the DR6 TSS was too late in development to reduce diet costs. Production parameters were met by all strains in colony, but strains had lower male fly survival than current production strains after removing Dox. In noncompetitive mating success trials, DR6 strains performed poorly, but DR7 performed equally to production strain males. However, males from all TSSs fared poorly in mating competition tests against production males. Our study highlights the importance of tightly regulated gene promoters and stage-specific antibiotic feeding schemes for the development and evaluation of TSSs based on the Tet-off system.
The New World screwworm (Cochliomyia hominivorax) is an obligate parasitic fly whose larvae invade and consume living tissue, causing myiasis with severe consequences for human health, animal welfare, and agricultural productivity. Although eradicated from the United States in the 1960s through the sterile insect technique (SIT) and contained thereafter by a Panama-based biological barrier, the parasite remains endemic in parts of South America and the Caribbean. Endemicity in these regions sustains the risk of reintroduction into screwworm-free areas. A recent travel-associated case of human screwworm in the United States highlights this ongoing threat, though no evidence of local transmission has been detected. While the public health risk in the United States remains low, the agricultural stakes are substantial: screwworm outbreaks can cause significant morbidity and mortality in livestock, necessitating costly interventions and resulting in considerable economic losses if detection and control are delayed. This review synthesizes the biology and pathogenesis of C. hominivorax, the historical trajectory of eradication efforts, details of the most recent U.S. case, the ongoing risks to public health and agriculture, and current and emerging prevention strategies. We emphasize that future resilience will depend on embedding surveillance and control in a One Health framework, maintaining redundant SIT capacity, integrating climate-informed risk models, and fostering international collaboration. The recent U.S. case should be interpreted as a sentinel event that reaffirms the fragility of eradication gains and the need for sustained vigilance.
Myiasis is the infestation of humans and other warm-blooded animals with dipterous larvae. In the current study, a total of 40 maggots were collected from the cutaneous lesions of the Chinese Milu (Elaphurus davidianus) in China. Based on morphological identification, the third-instar larvae were classified as Chrysomya bezziana. To accurately identify this species, genomic DNA from the larvae was analyzed using the mitochondrial cytochrome c oxidase subunit I (COI) gene. The findings indicated that the examined larvae were unequivocally consistent with the Old World screwworm fly, C. bezziana. This study firstly documented the cases of cutaneous myiasis caused by C. bezziana in Chinese Milu, and provided effective strategies for the prevention and control of myiasis.
The New World screwworm, Cochliomyia hominivorax, is an obligate parasite of warm-blooded animals and a major pest of livestock and wildlife in the Americas. The first genome assembly for C. hominivorax enabled substantial progress in key areas including gene expression related to fly behavior and physiology and gene editing technologies. However, the first genome was sequenced prior to several technological advances that result in fewer errors and better genome annotations. Here, we used the trio-binning approach to produce haplotype-resolved genome assemblies of C. hominivorax. A single male progeny from the cross of a Panama line male with a production strain female was sequenced using PacBio HiFi and scaffolded using Hi-C chromatin conformation, while Illumina NextSeq 2000 was used for short-read sequencing of both parents to facilitate trio-binning. We produced a linear haploid reference assembly by transferring a copy of the X chromosome and mitochondrial genome to the paternal haplotype. This assembly is comprised of 5 autosomes, 2 sex chromosomes, the mitogenome, and 75 unplaced scaffolds spanning 455.6 Mb, which is closer to the predicted size based on flow cytometry (443.8 Mb) than the previous assembly of 534.4 Mb. NCBI's external Eukaryotic Genome Annotation Pipeline (EGAPx) was used to annotate the protein-coding and non-coding genes in the linear haploid reference and the maternal haplotype assemblies. Due to the better resolution of the sex chromosomes and updated genome annotations, these improved assemblies will advance future experiments aimed at understanding sex determination, gene expression, and the evolution of parasitism in the Calliphoridae.
Infestations of Cochliomyia hominivorax, the New World Screwworm (NWS), has become a reemerging veterinary concern in the United States due to the recent northward expansion of fly detections as far as northern Mexico. Rapid, accurate and validated detection pipelines need to be developed in the case of an incursion into the United States. Confirmatory cases are evaluated by morphological identification with no paired test to verify identifications. With the frequency of submissions of non-ideal samples, particularly from fly traps, a molecular tool would be necessary for species identification. In this manuscript, we develop and assess a pipeline including three real-time PCR assays targeting the ribosomal RNA and five sets of Sanger primers targeting the mitochondrial genome that would be used as a paired tool with morphological identification. Two of the assessed real-time PCR assays are highly specific, sensitive and repeatable requiring < 1 copy per reaction for detection. Four of the five Sanger primer sets were assessed, optimized and results evaluated for potential use in preliminary geographic analysis of specimens. This workflow will expedite screening of samples, provide a method to verify results using different tools and help understand genetic variations within the mitochondria for NWS outbreaks.
The New World Screwworm (Cochliomyia hominivorax) has reemerged in Central America, posing risks to livestock and public health. Recent outbreaks highlight its northward spread, raising concerns about its potential reintroduction into Mexico. This study uses linear regression models to examine the role of animal movement in the parasite's spread, with a particular focus on human-mediated factors. Our analysis determined a spread rate of 1.2-1.9 km/day for screwworm outbreaks. Furthermore, consecutive outbreak events occurring more than 50 km apart suggest a strong association with animal movements rather than natural dispersal mechanisms. Based on these findings, we identified potential routes for its reintroduction into Mexico. To mitigate further outbreaks, enhanced control measures, active surveillance, and regional cooperation are urgently required.
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New World Screwworm (NWS), or Cochliomyia hominivorax, is a potentially fatal disease due to severe larvae infestation and tissue destruction. A resurgence has been reported. This narrative review provides a focused overview of NWS for the emergency clinician, including the epidemiology, presentation, diagnosis, and management of the disease. NWS is endemic to tropical and subtropical America. While previously eradicated in the U.S., several human cases have been recently reported. This insect primarily infests livestock but may also affect humans. Female flies are attracted to mucosal surfaces and wounds and lay hundreds of eggs. Once hatching, larvae burrow into these surfaces and cause extensive damage by ingesting live tissue. Humans at risk include those with significant immunocompromise, open wounds, and those around animals. Infested patients present with severe swelling, bleeding, pain, and open, malodorous wounds. Larvae are usually visible in the wound. Bacterial superinfections may also occur. Suspected cases should be reported to the local or state public health department and the Centers for Disease Control and Prevention. Diagnosis includes morphologic identification of larvae removed from a wound. Larvae should be placed in a leak-proof container with 70% ethanol. Serum laboratory testing may also be needed in patients with evidence of bacterial superinfection and sepsis. Treatment includes removal of all larvae and often extensive debridement. Reevaluation of the wound within 48 h is recommended. Antibiotics are necessary for those with bacterial superinfection. Emergency clinicians should be aware of the presentation, diagnosis, evaluation, and management of NWS to ensure optimal outcomes.
We describe New World screwworm (Cochliomyia hominivorax) infestation in 2 injured mountain tapirs (Tapirus pinchaque) from a protected area in the Central Andes, Colombia. Screwworms were not a known threat to mountain tapirs. Community outreach is needed to raise awareness on effects of this parasite on humans, domestic animals, and wildlife.
Oral myiasis is a rare medical condition in which soft tissues of the oral cavity are invaded by the larvae (maggots) of several flies. We report a fatal case in a 75-year-old man from a subtropical rural region of Ecuador who presented to a public health center with respiratory distress, peripheral cyanosis, dyspnea, and a Glasgow Coma Scale score of 9 of 15. The oral cavity emitted a fetid odor and contained bloody secretions with extensive larval invasion affecting the gingiva, lower lip, tongue, and soft palate. More than 300 larvae were manually extracted and morphologically identified as third-instar Cochliomyia hominivorax. The patient had a known history of epilepsy managed with carbamazepine (400 mg/day). Despite supportive management, including oxygen therapy and fluid resuscitation, the patient died approximately 8 hours after admission. The rapid deterioration was probably owing to airway compromise caused by larval migration toward the oropharynx. This case underscores the importance of prompt diagnosis and aggressive management of oral myiasis in vulnerable elderly individuals.
Tetracycline repressible (Tet-Off) male-only sexing systems have great potential for improving the efficacy of sterile insect control programs in addition to reducing rearing cost. The relationship between insects and their microbial symbionts, as well as potential physiological stress caused by tetracycline class antibiotics, pose concerns that Tet-Off strains could have reduced fitness in nature. Here, we evaluated the biological performance, midgut microbiome, and midgut transcriptomes of wild-type screwworm, Cochliomyia hominivorax, reared in diet without Tc (control), with Tc (on-Tc), or a pseudo "male-only" condition where offspring of the Tc fed line were reared without Tc (post-Tc), like Tet-Off strains previously developed for this species. Biological performance was not significantly changed by the inclusion of Tc in the diet, and in most cases the flies reared with Tc were generally more fit than lines reared without Tc. The gut microbiome and transcriptome revealed interesting and similar patterns. In both surveys, the greatest changes were between both control and post-Tc treatments and the on-Tc treatment. Very few differences were observed between control and post-Tc treatments, suggesting that there are few negative persistent effects of Tc exposure to fly colonies, and flies revert to their natural state rapidly after the removal of Tc. These results suggest there are limited negative impacts of Tet-Off regulatory systems for use by the C. hominivorax eradication program, and differences observed in Tet-Off strain performance are likely not related to Tc exposure.
Myiasis is the invasion of mammalian tissues by dipterous larvae. Cutaneous myiasis is the commonest type, and less commonly, internal tissues and organs are affected. Here, we report the first case of oro-nasopharyngeal myiasis caused by the third-instar larvae of Chrysomya bezziana (Old-World screwworm) in Sri Lanka. A retired 71-year-old female tea-plucker presented to the hospital with a 4-day history of nasal bleeding and neck discomfort. Except for well-controlled hypertension, she had no significant medical history. On examination, there was an ulcerated area in the posterior oro-nasopharynx filled with maggots. Hematological analysis showed a high C-reactive protein level (24 mg/dL) and white blood cell count (17.5 × 109/L) with 80% neutrophils. Computed tomography showed severe inflammation of the pharynx with no features of a neoplasm or parapharyngeal abscess. An examination was performed under anesthesia followed by manual removal of larvae, surgical debridement with biopsy, and treatment with antiparasitic medicines and antibiotics. Histopathology excluded neoplasm and only showed inflammatory changes. All extracted larvae were morphologically similar and identified as third-instar larvae of C. bezziana using a trinocular zoom stereomicroscope. The patient recovered and was discharged from the hospital on the 15th day and was well at 2 months follow-up.
The reemergence of the New World screwworm (Cochliomyia hominivorax) poses a significant threat to animal and public health with minimal regulatory oversight. This study analyzes the potential distribution and reemergence of this pest, which is endemic to South America but was previously eradicated in North America. We first developed bioclimatic suitability models, and then incorporated these findings along with reemergence records and inspection point data to simulate possible dispersal routes into Mexico and the United States. Our results document the historical distribution of C. hominivorax across the Americas and recent reemergence events in Panama (2023) and Mexico (2024-2025). Findings indicate high invasion potential from Central America, with significant risk along Mexico's Pacific and Atlantic coasts and the Yucatan Peninsula. In the United States, Texas and Florida face the highest risk. Regions with high livestock density in both countries demonstrate considerable climatic suitability for the pest. Our simulations identify Chiapas, Campeche, Tabasco, and Veracruz as critical northern dispersal points. The invadable areas contain substantial populations of domestic hosts, primarily cattle and horses. These findings will enable governmental authorities to develop comprehensive prevention and control strategies to address this emerging threat.
Chrysomya bezziana, the Old-World Screwworm, is a parasitic blowfly that causes myiasis, an infestation where larvae feed on live tissues of vertebrate hosts. Myiasis caused by C. bezziana poses a threat for public health and livestock economy in Asia with a potential risk of global spread by human travelers and animals. Genome resources for this parasite are urgently needed as these can facilitate the development of novel control methods to mitigate the impact of C. bezziana on public health and livestock industries. Here, we present the first genome assembly and annotation for C. bezziana. The genome exhibits high-quality with a size of 472 Mb and an N50 of 1.8 Mb and 98.4% of BUSCO single and 0.5% of duplicated genes, showing high completeness and haploid resolution. This initiative, when combined with other blowfly genomes available, will contribute to the ongoing effort to understand the evolutionary origin and molecular underpinnings of parasitic behavior in blowflies. Furthermore, it will also contribute to the development of pest management strategies to control the spread and the impact of this harmful parasite.
Screwworm flies are mass-reared and released along the Panama-Colombia border to prevent reinfestation of Central and North America. The cost of the production facility, labor, and diet materials makes mass-rearing the most expensive component of the program. The mass-rearing diet has a large impact on the quality and quantity of insects produced, both of which are necessary for the successful implementation of the sterile insect technique. The diet currently used to rear screwworm flies in Panama contains dried bovine red blood cells, dried bovine plasma, egg powder, milk replacement powder, cellulose (thickening agent), formaldehyde (antimicrobial), and water. Here, we tested an alternative diet containing 2 chicken by-products, which cost less and are locally available, to replace the egg powder and milk replacement powder currently used in the diet. We used 2 screwworm colony strains in our test, the current production strain (Jamaica) and an early female-lethal strain. The chicken diet performed similarly to the production diet with the Jamaica strain, while further optimization will likely be needed for transgenic strain. Finally, nutritional analysis conducted on 7 diet ingredients will assist with diet optimization and the identification of alternative diet ingredients.
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The New World screwworm is an obligate parasitic fly and a significant economic pest of livestock in the Americas. Although eradicated from the USA using the Sterile Insect Technique (SIT), enhancing SIT efficiency remains a priority. A promising approach involves conditional female-lethal genetic strains that produce only males in the absence of tetracycline, ideally eliminating females early in development to reduce larval diet costs. However, while some strains match wild-type production levels, lower male fitness reduces the net benefit of replacing the current wild-type strain with one of these genetic-sexing strains. This study aimed to improve strain performance through female-specific expression of both the driver and effector components of the lethality system. We tested four transgenic strains using early embryo-specific promoters from the Chhalo and g6451 genes. Strains with the Chhalo promoter driving tTA expression exhibited early-stage female lethality under a modified doxycycline regimen but suffered from reduced male fitness. In contrast, one strain with the g6451 promoter produced males with excellent fitness but female lethality occurred at the late pupal stage. Despite imperfect female lethality timing, the overall fitness characteristics of this strain makes it a good candidate for future sterile or fertile male release genetic control programs.