New World Screwworm (NWS)
The New World screwworm (NWS), Cochliomyia hominivorax (Coquerel), is an obligate parasite of mammals, including humans, during their larval stages. Larvae feeding on the skin and underlying tissues of the host cause a condition known as wound or traumatic myiasis, which can be fatal (adapted from PAHO, no date and WOAH, no date).
Primary reference(s)
PAHO, no date. Miasis por larvas de Cochliomyia hominivorax Pan American Health Organization (PAHO). Accessed 27 May 2025.
WOAH, no date. New World Screwworm (Cochliomyia hominivorax). Accessed 27 May 2025.
Annotations
Additional scientific description
he New World screwworm, Cochliomyia hominivorax Cocquerel, is a blowfly that is an obligate parasite of warm-blooded animals in tropical and subtropical regions of South America and the Caribbean. Females seek animals or humans and lay their eggs on the skin, often near open wounds (PAHO, no date: WOAH, no date). Larvae feed on the animal's living tissues, enlarging the wound, which can cause death if not treated. The period between oviposition at a wound site and the expression of disease due to burrowing larvae can be as short as one to two days. After cessation of feeding, larvae leave the animal and pupate in the soil. The life cycle takes approximately three weeks. Infestation of tissue by fly larvae is known as 'myiasis'.
Human
NWS infestations in humans are rare but very painful. If you have an NWS infestation, you may see maggots (larvae) around or in an open wound. They could also be in your nose, eyes, or mouth. Tell your healthcare provider of any recent travel to Central or South America, or the Caribbean.
Symptoms can also include
- Unexplained skin lesions (wounds or sores) that do not heal.
- Skin wounds or sores that worsen over time.
- Painful skin wounds or sores.
- Bleeding from open sores.
- Feeling larvae movement within a skin wound or sore, nose, mouth, or eyes.
- Seeing maggots around or in open sores.
- A foul-smelling odour from the site of the infestation.
Secondary bacterial infections sometimes occur and may cause fever or chills (CDC, 2024).
Animal
In 1988, the parasite spread from the Americas to Libya; it was identified by Libyan veterinarians in the northwest part of the country. The mode of introduction and the country from which the pest came remain unknown. Positive taxonomic identification of the pest was made by British scientists, and the Food and Agriculture Organization of the United Nations (FAO) confirmed the presence of the new world screwworm myiasis disease in Libya in March 1989 (Lindquist & Abusowa, 1991). Many cases of screwworm myiasis have been documented in humans.
Economic losses due to screwworm infestation of livestock are significant. In 2005, it was estimated that in South America alone annual losses were approximately USD 3.6 billion (Scott et al., 2020). New World screwworm eradication programmes have eliminated this myiasis from Curacao, Puerto Rico, the Virgin Islands, the USA, Mexico and in Central America (Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica), down to Panama on the southern part of the country where a buffer zone was established.
Since 2021, the New World screwworm fly has been making a dramatic comeback (reinfestation) in regions where it had been eradicated for over 30 years. All Central American countries and, more recently, Mexico have reported its reappearance. This resurgence poses a significant threat to livestock, wildlife, and public health. As of 2024, New World screwworm is found in all countries in Central America (Belize, Costa Rica, El Salvador, Guatemala, Honduras, México, Nicaragua, Panamá), three in Caribbean (Cuba, Dominican Republic, Jamaica) out of its 13 sovereign states, and 10 out of 12 countries from South Americas (Argentina, Bolivia, Brazil, Colombia, Ecuador, Paraguay, Peru, Suriname, Uruguay, Venezuela; and suspected in Guyana).
The disease can quickly become debilitating or fatal if it affects the eyes, mouth, nasal or frontal sinuses, or ears (CFSPH, 2016). Clinical presentation of screwworms is always associated with a variety of pre-established wounds and should be considered in the event of any myiasis. Wounds may manifest draining or enlargement; signs of infestation with serosanguineous discharge are often accompanied by a distinctive odour. Animals with screwworm myiasis often display discomfort and appear unthrifty and depressed; separate from the group and may also manifest anorexia and reduced milk production. Morbidity, although variable in areas with high screwworm populations, may reach up to 100% in naval wounds of newborns. If untreated, animals may die within one to two weeks due to over-infestation, toxic shock and/or secondary bacterial infection.
Laboratory diagnosis is by identification of the parasites under the microscope. Field diagnosis of screwworm larvae, even with a microscope or magnifying glass, is difficult. Adult screwworms are rarely seen. They are also difficult to distinguish from other flies. Other techniques used mainly in research laboratories include cuticular hydrocarbon analysis, analysis of mitochondrial DNA, and random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) assays. Serology is not used (CFSPH, 2016).
Screwworm infestations can be treated in both endemic and non-endemic regions; euthanasia of the animal is not usually required. Although some wounds may be surgically excised, most are treated with a suitable larvicide and allowed to heal without closure. Treatment is usually repeated at intervals until the wound has healed. Removal of necrotic tissue may be necessary, and antibiotics may be given when secondary bacterial contamination is present. In non-endemic regions, the animal is quarantined until treatment is complete and the wound has healed. Treatment of the environment may also be necessary. Larvae that are removed from the wound must be placed in alcohol preservative or destroyed. If any larvae leave an infested wound and mature into adults, screwworms can become established in an area (CFSPH, 2016).
Metrics and numeric limits
As an example of metrics and numeric limits, FAO reports that in the US and relevant Caribbean countries, metrics are used to determine incidence of outbreaks (FAO, no date).
Key relevant UN convention / multilateral treaty
United Nations. 2023. UN Recommendations on the Transport of Dangerous Goods - UN Model Regulations Model Regulations. https://unece.org/transport/dangerous-goods/un-model-regulations-rev-23 Accessed 1 January 2025.
WTO Agreement on the Application of Sanitary and Phytosanitary Measures (SPS Agreement) (WTO, 1994). https://www.wto.org/english/res_e/booksp_e/agrmntseries4_sps_e.pdf Accessed 1 January 2025.
International Health Regulations (2005), 3rd ed. (WHO, 2016).
Drivers
Human
NWS is endemic in the American tropics and subtropics which includes the countries of South America, Cuba, Haiti, and the Dominican Republic. There have been recent cases in Central American countries for the first time in many years. People at higher risk include
- Those living in rural areas in regions or countries where NWS is endemic, and where livestock are raised.
- People who frequently work with livestock.
- Anyone with open sores or wounds, including from a recent surgery, as the flies will lay eggs on open sores.
- Vulnerable populations, including people who are immunocompromised, those at extremes of age, and people experiencing malnutrition.
Animal
NSW fly may spread together with NWS-infected livestock or pets of displaced people when they move (or move back). The presence/introduction of screwworm fly (Cochliomyia hominivorax), and the introduction of infected livestock or pets are other drivers.
Impacts
Human
NWS larvae cause extensive damage by tearing into the host's tissue with sharp mouth hooks. The wound can become larger and deeper as more larvae hatch and feed on living tissue. As a result, NWS can cause serious damage to the animal or person it infects. Bacterial superinfection can also occur because of the NWS infestation (CDC, 2024).
Animal
Death of infected animals, spread of infection, trade ban.
Multi-hazard context
Losses caused by screwworm infestation are not only those resulting from morbidity and mortality of livestock but also from the costs of continuous surveillance and treatment of animals. Increased costs of livestock management practices, dictated by NWS presence, are difficult to estimate (FAO, no date).
Risk Management
Human
Clinicians can diagnose NWS by finding fly larvae (maggots) in tissue. Identification of the genus or species involves comparing certain morphological structures on the larvae, including the anterior and posterior spiracles, mouthparts, cephalopharyngeal skeleton, and cuticular spines. Travel history can also be helpful for genus or species-level identification.
The best way to prevent New World Screwworm (NWS) is to avoid exposure. Healthcare providers should
- Be alert for NWS symptoms in people with a history of recent travel to regions where NWS is endemic.
- Encourage patients to avoid spending time around livestock or where livestock are housed while in rural areas in NWS endemic regions.
- Advise patients to clean and cover wounds, wear loose-fitting, long-sleeved shirts, pants and socks, and use Environmental Protection Agency (EPA)-registered insect repellents.
- Advise patients to avoid sleeping outdoors and protect sleeping quarters with screens or bed nets, especially where people have wounds or active nasal or ocular discharges. Screening of hospital windows and doors is essential (CDC, 2024).
Animals
Control programme using sterile insect technique (SIT), movement control, and improved biosecurity at farms. New World screwworm is a disease listed by the World Organisation for Animal Health (WOAH) in its Terrestrial Animal Health Code and must be reported to the WOAH (WOAH, 2024b).
For control of screwworms, the implementation of voluntary and regulatory actions can prevent the introduction of screwworms to pest-free areas. Screwworms can enter non-endemic areas in infested animals or as adult flies. Vehicles that may contain adults or immature screwworms should be sprayed with insecticides. Imported animals (including pets) must be inspected for infestations and treated, before they are allowed to enter. As a precaution, wounds that do not appear to be infested may be treated with an insecticide. Animals may also be sprayed or dipped. Any infestations that become apparent after an animal enters the country must be treated promptly (CFSPH, 2016). To prevent the spread of screwworms beyond their present geographical distribution, strict observation of the requirements for international trade, as set out in the WOAH Terrestrial Animal Health Code, is necessary (WOAH, 2024a; 2024b). In endemic areas, animals must be inspected for screwworms every few days. Livestock can also be protected by regular spraying or dipping with insecticides, or by subcutaneous injections of ivermectin and related compounds. Insect growth regulators have also shown good results. Organophosphate insecticides are effective against newly hatched larvae, immature forms and adult flies. Other insecticides, such as carbamates and pyrethroids have also been used. Acaricide resistance can be an issue. In areas where screwworms are seasonal, breeding can be scheduled to avoid births when these flies are numerous. Indirect prevention of screwworm infestation includes avoidance of wounding procedures at the times of year when screwworms are numerous; careful handling of livestock to minimise wounding; removal of sharp objects (e.g., wire strands) from livestock pens; and use of measures to reduce other wound-causing parasites, in particular ticks (e.g. by dipping and by insecticide impregnated ear-tags) (CFSPH, 2016).
Screwworms can be eradicated from a region by repeatedly releasing sterile male flies that mate with wild female screwworms to produce unfertilised eggs. Because it is usually impractical to separate irradiated male and female flies, both males and females are usually released. This technique, known as the sterile insect technique (SIT), leads to a reduction in screwworm numbers and eventually results in eradication. In addition, infested animals are treated, and their movements are controlled (CFSPH, 2016).
Monitoring
The section and the table below offer an overview of monitoring new world screwworm. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
| Which institution(s) produce(s) Disaster Risk Data/Information? | Ministry of Agriculture, Ministry of Livestock, Ministry of Health, FAO Reference Centres, WOAH Reference Centres |
How is the Hazard Observed/Monitored/Forecast? | Through its global early warning system, FAO has been supporting Members with risk monitoring, assessment and forecasting for animal health threats to enhance preparedness and response to animal health threats:
FAO empres-i+ https://empres-i.apps.fao.org/diseases WOAH WAHIS https://wahis.woah.org/#/event-management |
References
CDC, 2024. Clinical Overview of New World Screwworm Myiasis. Accessed 27 May 2025.
CFSPH, 2016. Screwworm Myiasis. Center for Food Security and Public Health. (CFSPH). Accessed 27 May 2025.
Lindquist, D.A., Abusowa, M., 1991. The New World Screwworm in North Africa. Accessed 27 May 2025.
PAHO, no date. Miasis por larvas de Cochliomyia hominivorax. Pan American Health Organization (PAHO). Accessed 27 May 2025.
Scott, M.J., Benoit, J.B., Davis, R.J., Bailey, S.T., Varga, V., Martinson, E.O., Hickner, P.V., Syed, Z., Cardoso, G.A., Torres, T.T., Weirauch, M.T., 2020. Genomic analyses of a livestock pest, the New World screwworm, find potential targets for genetic control programs. Communications Biology, 3:1-14.
WHO, 2016. International Health Regulations (2005), 3rd ed. World Health Organization (WHO). Accessed 27 May 2025.
WOAH, no date. New World Screwworm (Cochliomyia hominivorax) World Organisation for Animal Health (WOAH). Accessed 27 May 2025.
WOAH, 2024a. Manual of Diagnostic Tests and Vaccines for Terrestrial Animals, 13th edition. World Organisation for Animal Health (WOAH). Accessed 27 May 2025.
WOAH, 2024b. Terrestrial Animal Health Code. 32nd edition. World Organisation for Animal Health (WOAH). Accessed 27 May 2025.
WTO, 1994. The WTO Agreement on the Application of Sanitary and Phytosanitary Measures (SPS Agreement). World Trade Organization (WTO). Accessed 26 May 2025.
WTO, 2007. The WTO and the World Organization for Animal Health (OIE) G/SPS/GEN/775. World Trade Organization (WTO). Available from: WTO | Sanitary and phytosanitary measures - gateway. Accessed 26 May 2025.