El cattle screwworm It has become one of the unwelcome protagonists of current health and livestock issues in the Americas. What for decades was a controlled problem, even declared eradicated in some countries, has returned to the scene with outbreaks in Central America and Mexico, a first confirmed human case in the United States, and growing concern about its capacity to cause serious harm to humans, animals, and the rural economy. Far from being a mere biological curiosity, we are dealing with a parasite that literally devours living tissue.
Beyond the alarming headlines, understanding What is the screwworm, how does it live, how is it transmitted, what risks does it pose, and what is being done to stop it? This information is crucial for healthcare professionals, livestock farmers, and anyone living in or traveling to endemic areas. Below you will find a detailed and organized explanation of this parasite, the disease it causes (myiasis), its impact on public health and agricultural production, and the main strategies for prevention, control, and reporting to the authorities.
What exactly is the screwworm?
The call cattle screwworm, known in many places as maggot infestation, is actually the larval stage of a fly, scientifically called Cochliomyia hominivoraxThis species is characterized by being a obligate parasite of warm-blooded animals (domestic mammals, wild mammals, birds and even humans), which means that their larvae need to feed on living tissue to complete their life cycle.
Unlike other flies that lay their eggs in dead tissue or decaying matter, the screwworm fly It lays its eggs directly on open wounds, mucous membranes, or natural orifices. of the body. When the eggs hatch, the larvae begin to burrow into living tissue, creating veritable tunnels that resemble a "boring" in the flesh, hence the parasite's common name.
These The larvae have a creamy white color. These larvae, which may turn slightly reddish as they mature, have rows of spines and mouth hooks that they use to attach themselves and burrow into the tissue. They are arranged vertically and deeply embedded in the wound, making manual removal difficult and allowing them to continue feeding and growing for several days.
Myiasis: the disease caused by the screwworm
Screwworm infestation is called myiasis, a parasitic disease in which fly larvae invade the living tissue of a host. In the case of Cochliomyia hominivoraxThis is a particularly aggressive form of myiasis, because the larvae are specifically adapted to consume live flesh rather than being limited to necrotic areas.
When a female fly lays eggs 200 to 400 eggs in a wound (even as small as a tick bite or a minor skin lesion), the larvae hatch after about 24 hours. These larvae quickly burrow into the wound, cluster together in compact masses, and begin feeding on the tissues, generating deep, very painful wounds with a strong smell of rotting meat.
As they progress, the wound fills with bloody secretions and serous fluids, becomes inflamed, and turns into an ideal breeding ground for secondary bacterial infectionsThe result is a clinical picture that can range from moderate discomfort to a severe systemic infection, with fever, intense pain, weight loss and, in vulnerable animals or people, death within days if no timely intervention occurs.
Life cycle of the screwworm
El life cycle of the screwworm It lasts approximately 21 days, although this can vary depending on temperature and environmental conditions. Broadly speaking, it consists of five clearly differentiated stages, all of which are crucial for understanding how the parasite spreads and at what points control measures can be applied.
First, the adult female fly locates a wound or exposed tissue in a susceptible animal (cattle, horses, sheep, goats, pets, wildlife, or even humans). It deposits between 200 and 400 eggs in clusters on the wound. This preference for fresh wounds makes any livestock handling (dehorning, shoeing, ear tagging, castration) or birthing injuries (umbilical cord of newborn calves, udders of lactating females) ideal entry points.
In second place, After about 24 hours, the eggs hatch The tiny larvae emerge and begin to feed voraciously. For approximately seven days, these larvae develop within the tissue, penetrating deeper and deeper into the wound, which increases pain, inflammation, and drainage. It is during this period that the characteristic "plug" of white maggots is observed protruding from the lesion.
Third, when the larvae have reached maturity, they leave the wound and They fall to the ground, where they bury themselves to transform into pupaeIn this phase, they form a seemingly inert protective shell, but inside they are restructuring into what will become the adult fly. This pupal stage usually lasts about 10 days, although it can be longer if environmental conditions are less favorable.
Fourth, from the pupa emerges the fully formed adult flyIt will emerge from the ground, spread its wings, and search for food and a mate. One of the peculiarities of this species is that the female mates only once in her life, but that single copulation is enough for her to produce several clutches of eggs during her lifetime.
Finally, the adult fly, now fertilized, look for new wounds in warm-blooded animals in which to lay their eggs, thus restarting the cycle. In warm climates or with the increase in temperatures associated with climate change, this cycle can accelerate, allowing more generations per year and a faster spread of the parasite.
Clinical signs and symptoms in animals
The animals affected by screwworm They exhibit a number of fairly characteristic signs, although the lesions often go unnoticed in the first few days. Infested wounds have a bloody discharge, a pungent odor, and, if observed carefully, groups of larvae can be seen moving inside.
The larvae can colonize all types of injuriesThese range from simple tick bites to cuts caused by wire, dehorning wounds, castrations, ear marks, or blows. They can also enter through orifices such as the nostrils, mouth, eye sockets, ears, or genitals, further complicating the clinical picture and treatment.
Regarding behavior, infested animals tend to They separate from the group, showing signs of discomfort, pain, apathy, and loss of appetite.And in the case of dairy cows, a significant decrease in milk production is observed. If left untreated, they can lose weight rapidly, become weak, and, in severe infections, die within 7 to 14 days from toxicity, shock, or secondary bacterial infections.
Although practically All warm-blooded animals are susceptible.The greatest economic losses are observed in cattle, sheep, and goats due to their importance in meat and milk production. However, horses, pigs, dogs, cats, and wildlife can also be affected, resulting in a major ecological and animal welfare problem.
Manifestations in humans: the case of Maryland and other risks
Although screwworm myiasis is more frequent in cattleIt can also affect humans, especially in endemic areas or in people with open wounds living in vulnerable conditions. One case that attracted considerable attention was the first confirmed human case in the United States, recently diagnosed in a Maryland resident who had returned from a trip to El Salvador.
This patient presented with myiasis caused by Cochliomyia hominivorax after having been in a region where the parasite is present. US health authorities indicated that the case was treated appropriately, the patient recovered, and No additional infections or presence of the insect in local animals have been detected.According to the Department of Health and Human Services (HHS) and the Centers for Disease Control and Prevention (CDC), the risk to the general population in the United States remains very low.
Symptoms in humans usually begin with redness, swelling, and intense pain In the affected area, small holes appear through which white larvae may emerge. As the days pass, bloody discharge, fever, and general malaise develop, and if the infestation progresses without treatment, serious systemic complications can arise.
In most of the cases described, the Elimination of the larvae requires minor surgical proceduresThese procedures involve manually removing the lesions and thoroughly cleaning the wound. Antibiotics are also administered to prevent or treat secondary bacterial infections. The sooner this intervention is performed, the better the prognosis and the less permanent tissue damage.
Medical literature insists on the need to strengthen prevention measures for travelers For those from endemic areas: cover wounds properly, use insect repellents, avoid prolonged exposure in rural environments with high fly density, and see a doctor for any suspicious injury where internal movement or disproportionate pain is noticed.
Impact in Latin America and the return of the “villain” in Mexico
In Latin America, and very markedly in Mexico and Central AmericaThe screwworm has been a historical pest. Countries like Mexico, after enormous coordinated efforts with the United States, managed to eradicate the parasite in livestock in the early 90s using innovative strategies such as the sterile insect technique. For decades, the name of this parasite was practically relegated to the history books of animal health.
However, recent outbreaks in Central America They have reactivated the alarm. Warnings about the risk of reinfestation had been circulating since 2017, but the crisis intensified especially after March 2024, when a health alert was declared for screwworm in Nicaragua. Months later, Guatemala reported its first cases and, subsequently, Mexico confirmed the reappearance of the parasite in its territory, more than 30 years after it was officially eradicated.
This situation not only impacts the livestock sector. Authorities and specialists emphasize that it is a a public health problem that affects both humans and domestic and wild animalsHuman deaths from severe infestations have even been reported in the region, and new cases in people have been documented, such as the one that occurred in the state of Yucatán in early March 2025.
On the economic front, the blow could be considerable. Mexican livestock farming, for example, represents a value of hundreds of billions of pesos and directly employs more than 800.000 people. The presence of the screwworm increases production costs (treatments, animal losses, decreased weight and yield, trade restrictions) and can ultimately be reflected in the final price of meat and other animal products.
The magnitude of the problem has been such that, upon detection of the parasite, The United States went so far as to temporarily close its border to Mexican live cattle.to reduce the risk of reintroducing the screwworm into their territory. Although exports have been gradually resuming, the danger of new closures remains until the situation is fully under control.
Control strategies: the sterile insect technique and other measures
Among the More effective tools to combat the screwworm The well-known sterile insect technique stands out, having been successfully applied in historical campaigns in Mexico, the United States, and other countries. This method consists of mass-rearing male flies in specialized facilities and subjecting the pupae to non-lethal sterilizing radiation.
Once they emerge sterile adult malesThey are released in large numbers over the affected areas. These males compete with fertile wild males and mate with the females, but the eggs resulting from these copulations are non-viable, that is, they do not give rise to new larvaeWith sustained releases over time, the screwworm population decreases to very low levels and can even be locally eradicated.
In addition to the sterile insect technique, the affected countries are implementing other complementary measures: animal health fences in border areas to check livestock in transit, insecticide baths for animals, injection of antiparasitic drugs and training campaigns for producers so that they know how to identify, treat and report cases of myiasis in a timely manner.
In Mexico, for example, they have been installed checkpoints in southern states such as Chiapas, Campeche and VeracruzThere, specialized veterinary and canine personnel inspect the animals, identify wounds suspected of myiasis, provide immediate treatment, and record each case. These actions, combined with the release of sterile flies in vulnerable areas, aim to break the chain of transmission of the parasite before it becomes permanently re-established.
Role of health authorities and international organizations
In the event of a screwworm infestation, it is essential that there be a rapid and coordinated response between public health and animal health authoritiesIn each country there are usually institutions in charge of epidemiological surveillance, case reporting and the implementation of control plans in both humans and animals.
In the SICA region (Central American Integration System), the Ministries of Health National veterinary clinics in each country are the first point of contact for human cases, while official veterinary agencies handle cases in livestock and other animal species. Among the main national agencies that serve as reference points in this area are:
- Ministry of Health of El Salvador
- Ministry of Health of Belize
- Ministry of Health of Guatemala
- Ministry of Health of Honduras
- Ministry of Health of Costa Rica
- Ministry of Health of Panama
- Ministry of Health of the Dominican Republic
Furthermore, international organizations such as the World Health Organization (WHO), the Pan American Health Organization (PAHO) and World Organization for Animal Health (WHO) They play a key role by providing technical support, coordinating efforts between countries, facilitating diagnostic tools, and promoting eradication campaigns and continuous surveillance.
Many countries have also developed emergency phone lines, online forms, and mobile apps to quickly report any suspected cases of myiasis. The swift use of these channels allows for the activation of field investigations, the implementation of containment measures, the sampling of exposed animals and people, and the prevention of uncontrolled spread of the parasite.
The experience of the International Regional Organization for Agricultural Health (OIRSA) and agencies such as the United States Department of Agriculture demonstrates that without constant monitoring and regional cooperation It is very easy for the screwworm to reappear in territories that were already considered free, especially in a context of intense animal movement and climate change.
Prevention in humans and animals
La Prevention is the most effective tool To reduce the risk of screwworm myiasis in both people and domestic and farm animals. For the general population and travelers to endemic areas, the CDC recommends keeping all wounds clean. clean, disinfected and well coveredpreventing them from being exposed in rural environments where flies are abundant.
It is equally important minimize prolonged contact with livestock or wildlife In areas where the parasite is present, wear clothing that covers the skin, apply insect repellent, and go immediately to a health center if you notice that a wound becomes very painful, gives off a bad odor, or feels like it is "moving" inside.
In the veterinary field, the key lies in check the animals frequentlyEspecially after procedures such as dehorning, castration, ear tagging, or birthing. Any wound should be cleaned, disinfected, protected from flies, and monitored for several days to detect any signs of myiasis early.
Livestock producers and pet owners must be trained to identify the initial clinical signs (bloody discharge, strong odor, presence of larvae) and seek veterinary care immediately. Early treatment with larval removal, thorough wound cleaning, and administration of systemic antiparasitics can make the difference between a favorable outcome and the animal's death.
The screwworm has reappeared in several Central American countries and Mexico and has even made headlines with the first confirmed human case in the United States; therefore, This parasite is not an isolated problem in the countryside.but rather a threat that intertwines public health, animal welfare, and economic stability. With constant vigilance, collaboration among communities, livestock farmers, veterinarians, and health authorities, the intelligent use of technologies such as the sterile insect technique, and a strong culture of prevention and early reporting, it is possible to contain outbreaks, protect livestock and the most vulnerable people, and keep this old enemy at bay—an enemy that, like any good recurring villain, always seeks an opportunity to reappear.
