New World Screwworm (NWS) (Animal)

Primary reference(s)

OIE, 2020. Terrestrial Animal Health Code: New world screwworm (Cochliomyia hominivorax). Chapter 8.12. World Organisation for Animal Health (OIE). Accessed 19 October 2020.

Additional scientific description

The New World screwworm, Cochliomyia hominivorax, is a blow fly that is an obligate parasite of warm-blooded animals in tropical and subtropical regions of South America and the Caribbean. Females seek animals and lay their eggs on the skin, often near open wounds. Larvae feed on the animal’s 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’ (OIE, 2013).

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, from Guatemala, Belize, El Salvador, Honduras, Nicaragua, Costa Rica, and Panama. New World screwworm is presently found in the northern countries of South America southwards to Uruguay, northern Chile and northern Argentina. The parasite spread from the Americas to Libya when it was identified in 1988 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 screwworm myiasis disease in Libya in March 1989 (Lindquist and Abusowa, 1991).

Many cases of screwworm myiasis have been documented in humans. The disease can quickly become debilitating if it affects the eyes, mouth, nasal or frontal sinuses, or the ears (CFSPH, 2016).

Clinical presentation of screwworm is always associated with a variety of pre-established wounds and should be considered in the event of any myiasis. Wounds may manifest draining, suppuration and/or enlargement; signs of infestation with serosanguineous discharge often accompanied by distinctive odour. Animals with screwworm myiasis often display discomfort and appear unthrifty and depressed; separate from 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 toxicity and/or secondary bacterial infection (OIE, 2013).

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

Not identified.

Examples of drivers, outcomes and risk management

Drivers: presence/introduction of screwworm fly (Cochliomyia hominivorax), introduction of infected livestock or pet.

Outcomes: Death of infected animals, spread of infection, trade ban.

Risk management: control programme using sterile insect technique (SIT), movement control, improved biosecurity at farms.

New World screwworm is a disease listed by the World Organisation for Animal Health (OIE) in its Terrestrial Animal Health Code and must be reported to the OIE (OIE, 2019).

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 if necessary, 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 OIE Terrestrial Animal Health Code, is necessary (OIE, 2013).

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 screwworm 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).

No vaccine is currently available.

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 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).