The screwworm is a parasitic disease caused by fly larvae that feed exclusively on the living tissue of warm-blooded animals, including humans. This obligatory myiasis is caused by two main species: Cochliomyia hominivorax, known as the New World screwworm, historically present in the Americas; and Chrysomya bezziana, the Old World screwworm, found in Asia, Africa, and parts of the Middle East.

Both species display a high invasive capacity and share an almost identical ecological niche, which represents a constant threat to animal health and public health in regions that have already achieved eradication. They also share sensitivity to cold climates, which limits their spread in areas with consistently low temperatures.

Unlike other forms of myiasis, in which larvae colonize necrotic or decaying tissue, the screwworm begins its cycle directly on healthy tissue, causing deep and rapidly progressing damage. This ability to penetrate intact tissue and multiply in minimal wounds — such as tick bites, mucosal skin lesions, or surgical procedures — makes this disease a silent but highly destructive risk.

The screwworm’s life cycle begins when the female fly lays its eggs (between 200 and 400 at a time) at the edges of an open wound or on moist mucous membranes. Larvae emerge within 12 to 24 hours and begin to penetrate the host’s living tissue, feeding actively while burrowing into deeper layers.

During this process, the wound may enlarge rapidly, show serosanguineous discharge, and emit a foul odor. In three days, the larvae may reach concentrations of up to 200–300 in a single wound, arranged vertically. In extreme cases, up to 3,000 larvae have been documented in a single wound. Mature larvae then drop to the ground, where they pupate, and after 7–14 days (depending on temperature), they emerge as adult flies. This cycle may be completed in less than three weeks in tropical climates or extended up to three months in colder regions.

Adult flies, especially females, can fly from 10–20 km in regions densely populated with livestock, to more than 300 km in arid environments, facilitating the parasite’s spread even into free zones.

Transmission occurs mainly when the female fly detects a fresh wound, moist mucosa, or even a cutaneous lesion with discharge, where it lays its eggs. Upon hatching, the larvae penetrate living tissue and begin feeding aggressively.

Both species, despite belonging to different genera, share this same behavior. Infested wounds attract more flies, favoring multiple and recurrent infestations.

The screwworm thrives in warm, humid climates. Cochliomyia hominivorax has been eradicated in the U.S., Mexico, Central America, and several Caribbean islands thanks to control programs, but it has reappeared in some countries through the importation of infested animals.

Chrysomya bezziana persists across much of Southeast Asia, India, sub-Saharan Africa, and some Middle Eastern countries. It has never become established in Europe or the Americas but remains a potential threat.

Very low or prolonged near-freezing temperatures eliminate pupae in the soil, limiting the parasite’s spread in colder regions.

In animals, infestation may begin with almost imperceptible lesions. Over time, deep wounds appear with suppuration, foul odor, fever, lethargy, separation from the herd, decreased appetite, and reduced milk production. Common sites include the navel in newborns, genitals, ears, anus, and surgical wounds.

In humans, larvae may invade nasal cavities, ears, eyes, skin, or genitals, causing intense pain, fever, inflammation, and, in severe cases, systemic infections. These infections, though rare, require immediate medical attention.

Diagnosis is based on visual inspection of wounds. Advanced-stage larvae appear cylindrical, with spines and dark bands. They resemble a screw and burrow deeply, making superficial detection difficult. Young larvae are cream-colored; mature larvae may turn reddish.

In laboratories, molecular tests such as PCR, mitochondrial DNA analysis, and RAPD-PCR are used. Serology is not employed.

Samples must be handled carefully, placed in 80% ethanol (never formalin), and collected from the deepest areas of the wound. Incorrect identification of superficial larvae could lead to errors.

Free countries must carry out strict border control, veterinary inspection of imported animals (including pets), vehicle disinfection, and preventive application of insecticides to apparently healthy wounds.

Immediate notification of suspected cases is mandatory, and quarantine protocols must be activated.

Screwworms can enter non-endemic zones in infested animals or as adult flies. Vehicles that may contain adult or immature screwworms must be sprayed with insecticides. Imported animals (including pets) must be inspected and treated if necessary before entry is allowed. As a precaution, wounds that do not appear infected are treated with insecticide. Animals may also be sprayed or dipped with insecticides. Any infestation that becomes evident after an animal has entered the country must be treated immediately.

In endemic zones, animals must be regularly inspected. Livestock can also be protected by regular spraying or dipping with insecticides, or by subcutaneous injections of ivermectin and related compounds. Organophosphate insecticides are effective against newly hatched larvae, immature forms, and adult flies. Carbamates and pyrethroids can also be used. In areas where screwworms are seasonal, management practices can be scheduled to avoid births during peak fly populations. Whenever possible, procedures that may cause wounds should not be performed during the screwworm season, and sharp objects should be removed from corrals. No vaccine is available.

One of the most innovative and effective strategies is the Sterile Insect Technique (SIT), which consists of the repeated release of sterile male flies that mate with wild females and produce infertile eggs. (Since it is usually not practical to separate irradiated male and female flies, both males and females are typically released.) This technique reduces the number of screwworms and eventually eradicates them. In addition, infested animals are treated, and their movement is controlled. This approach was successfully used to eradicate the screwworm from the southern U.S., Mexico, and most of Central America in past decades.

Treatment of screwworm infestation must begin immediately upon detection, as larval progression can be rapid and cause significant damage. Clinical management begins with the complete manual removal of larvae (especially from deep areas), ensuring that they are extracted whole to prevent complications from retained fragments. This procedure must be performed with extreme care, as breaking a larva within the tissue increases the risk of inflammation and secondary infection.

Once the larvae are removed, the wound must be thoroughly cleaned with antiseptic solutions and, if necessary, surgically debrided to eliminate necrotic tissue. The application of topical larvicides such as fenthion or coumaphos is recommended to ensure no residual larvae remain. In severe or difficult-to-access cases, local or general anesthesia may be required.

In countries where the screwworm has been eradicated, detection of a suspected or confirmed case constitutes a health emergency. Therefore, it must be reported immediately to the competent authorities, such as official veterinary services or the Ministry of Health. The affected animal must be kept in isolation to prevent larval spread, and the contaminated environment must be treated with appropriate products to prevent pupal maturation in the soil.

Compliance with quarantine protocols and activation of surveillance networks are key to avoiding reinfestation of free zones, especially in border areas or regions with high animal mobility.

Scientific research on the screwworm has focused on improving control strategies through the development of new technologies and products. Among the most relevant advances are long-acting, environmentally friendly insecticide formulations that allow more effective field control. In addition, controlled-release systems for chemical products, such as collars or topical devices, have been designed to provide continuous protection for high-risk animals.

Optimization of the Sterile Insect Technique (SIT) has also been a focus of improvement, aiming to reduce operational costs and expand its applicability to other regions of the world. In this context, trials have been conducted to adapt SIT to different climatic and ecological conditions, ensuring its effectiveness as an eradication tool.

Early diagnosis is vital for effective control of infestation. In this regard, rapid molecular tests are being developed to identify the presence of screwworm larvae at early stages, even before visible symptoms appear. These tools are being integrated into portable diagnostic kits, ideal for rural or hard-to-reach areas.

Technologies based on artificial intelligence and computer vision are also being explored to analyze wound images and detect patterns compatible with myiasis. These solutions could be integrated into mobile applications used by veterinary technicians and community health workers.

The screwworm represents one of the most costly threats to livestock in tropical and subtropical regions. Infestations cause direct economic losses due to animal deaths, reduced milk or meat production, and deterioration of product value. Added to this are indirect costs from treatments, medication use, veterinary care, and reduced herd reproductive efficiency.

In exporting countries, detection of cases may result in shipment suspensions, loss of international markets, and sanitary sanctions. These impacts directly affect local economies, especially small and medium producers.

Eradication of the screwworm not only eliminates control-related costs but also improves the country’s health status before international organizations. This translates into the opening of new markets, increased competitiveness in the agricultural sector, and strengthened national biosecurity.

Cost-benefit analyses have shown that investing in eradication programs, especially when applying sustained strategies such as SIT and border control, is more economical than facing recurrent outbreaks. Countries that have maintained free status have experienced long-term benefits in their rural economies and public health.

For those wishing to deepen their knowledge of the screwworm, there are multiple reliable sources offering technical, educational, and updated information on this parasitic disease and its control. International institutions such as the World Organisation for Animal Health (WOAH), the Center for Food Security and Public Health at Iowa State University (CFSPH), the Pan American Health Organization (PAHO), and the International Regional Organization for Plant and Animal Health (OIRSA) provide resources aimed at health professionals, veterinarians, livestock producers, and the general public.

These resources include technical manuals, fact sheets, surveillance protocols, diagnostic tools, and control strategies, especially useful for those involved in the prevention and eradication of the screwworm. They also provide data on the implementation of the Sterile Insect Technique, regional health policies, and alerts on outbreak occurrences.

Scientific studies and academic publications can also be accessed through platforms such as Google Scholar or PubMed, where topics such as the biology of Cochliomyia hominivorax, the economic impact of infestation, and the results of eradication programs in different countries are analyzed. This information is essential for understanding the magnitude of the problem, formulating effective public policies, and fostering regional and international collaboration.

For direct access, some of these resources are available at the following links:

• Center for Food Security and Public Health: https://www.cfsph.iastate.edu/

• World Organisation for Animal Health (WOAH): https://www.woah.org

• Pan American Health Organization (PAHO): https://www.paho.org

• International Regional Organization for Plant and Animal Health (OIRSA): https://www.oirsa.org

In case of suspected screwworm infestation, it is essential to immediately contact the national public health and animal health authorities. Each country has an entity responsible for surveillance, reporting, and response to zoonotic and parasitic diseases. These institutions usually have specific action protocols and trained personnel to manage suspected cases, coordinate health interventions, and prevent parasite spread.

In the SICA region (Central American Integration System), the Ministries of Health of each member country represent the first point of contact for reporting human cases, while veterinary authorities handle animal cases. It is recommended to have the following official contacts available:

MINISTRY OF HEALTH EL SALVADOR
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MINISTRY OF HEALTH BELICE
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MINISTRY OF HEALTH GUATEMALA
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SECRETARY OF HEALTH HONDURAS
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MINISTRY OF HEALTH COSTA RICA
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MINISTRY OF HEALTH PANAMÁ
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MINISTRY OF HEALTH DOMINICAN REPUBLIC
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Likewise, international organizations such as WHO, PAHO, and WOAH can provide technical guidance and logistical support during health emergencies. These institutions work in collaboration with national governments to strengthen surveillance systems, provide assistance during outbreaks, and facilitate access to diagnostic and treatment tools.

Many countries have also implemented emergency hotlines, online forms, or mobile platforms for reporting suspected cases in both humans and animals. Efficient use of these channels enables a rapid and coordinated response, which is key to controlling the spread of the screwworm and mitigating its impacts on public health and agricultural production. Active collaboration among communities, health professionals, veterinarians, and health authorities is essential to contain any outbreak and move toward sustainable eradication of the parasite.

Spickler, A. R. (2007, October). Screwworm myiasis [Technical factsheet]. Center for Food Security and Public Health, Iowa State University. Retrieved from https://www.cfsph.iastate.edu/Factsheets/es/miasis_por_el_gusano_barrenador.pdf