Flesh-Eating Parasite Prompts US Halt on Mexican Livestock Imports

The United States has temporarily suspended the import of livestock from Mexico due to the detection of the New World Screwworm (NWS), a parasitic fly capable of causing severe tissue damage in warm-blooded animals. The U.S. Department of Agriculture (USDA) announced the import suspension on Sunday, May 11, under the direction of Secretary Brooke Rollins. This action aims to protect the American livestock industry from a pest that was eradicated from the U.S. decades ago.

The New World Screwworm, a fly about the size of a common housefly, poses a significant threat to livestock and other warm-blooded creatures. Distinguishable by its orange eyes, metallic blue or green body, and three dark stripes along its back, the NWS earns its name from the destructive behavior of its larvae, or maggots. These maggots burrow into open wounds, essentially "screwing" their way into the flesh and feeding on the living tissue. This feeding process inflicts substantial damage, tearing at the host’s tissue with sharp mouth hooks, as described by the Animal and Plant Health Inspection Service (APHIS), an agency within the USDA.

The current suspension was prompted by the discovery of NWS infestations in remote farms located approximately 700 miles south of the Mexico-U.S. border. The USDA will reevaluate the suspension on a month-by-month basis, considering the ongoing situation and the effectiveness of control measures implemented in Mexico. The aim is to resume normal trade relations as soon as it is deemed safe for the U.S. livestock population.

Understanding the New World Screwworm

The New World Screwworm is a parasitic fly that can infest a wide range of warm-blooded animals, including livestock, pets, wildlife, birds, and, in rare cases, humans. While these flies typically inhabit forests and wooded areas, they are drawn to animals in pastures and fields, particularly those with open wounds. Even minor injuries, such as those caused by tick bites, can attract the flies and initiate an infestation.

The life cycle of the NWS begins when a female fly lays her eggs near an open wound on a warm-blooded animal. A single female can lay up to 3,000 eggs during her lifespan. Within hours, the eggs hatch into larvae, or maggots, which immediately begin to burrow into the wound and feed on the surrounding living tissue. This feeding continues for approximately seven days, after which the larvae drop to the ground, burrow into the soil, and pupate.

The pupation stage lasts between seven and 54 days, depending on environmental conditions. During this time, the larvae transform into adult flies within their pupal casings. Once fully developed, the adult flies emerge from the soil and the cycle begins anew.

Signs of Screwworm Infestation in Animals

Livestock owners and animal handlers need to be vigilant in observing their animals for signs of screwworm infestation. Infected animals often exhibit specific symptoms that can alert caregivers to the presence of the parasite. These signs include:

• Restlessness and Irritation: Animals may exhibit unusual restlessness, constantly scratching or rubbing the affected area.
• Foul-Smelling Discharge: The wound will produce a foul-smelling discharge due to the maggots feeding on the tissue and the subsequent bacterial infection.
• Visible Maggots: The maggots themselves can be seen within the wound, appearing as small, white or cream-colored larvae.
• Enlarged Wound: The wound will become progressively larger and deeper as the maggots continue to feed on the tissue.
• Loss of Appetite: Severely infested animals may lose their appetite and become weak.

Screwworm Infestation in Humans

While rare, New World Screwworms can infest humans. This is more likely to occur in individuals traveling to regions where the flies are prevalent, such as South America and the Caribbean. Symptoms of a screwworm infestation in humans include:

• Painful Lesions: The initial symptom is the development of painful lesions on the skin, typically around open wounds or sores.
• Itching and Irritation: The affected area will be intensely itchy and irritated.
• Visible Maggots: Maggots may be visible within the lesions.

Secondary bacterial infections can occur as a result of the screwworm infestation, leading to fever or chills. Prompt medical attention is crucial for individuals suspected of having a screwworm infestation.

History of Screwworm Eradication in the U.S.

The New World Screwworm first emerged as a significant problem in the United States in 1933, following the shipment of infested animals from the Southwest. By the 1950s, the federal government recognized the severity of the threat and implemented an innovative eradication strategy known as the Sterile Insect Technique (SIT).

The Sterile Insect Technique involves mass-producing male flies in a laboratory and sterilizing them through radiation. These sterile males are then released into the target areas to mate with wild female flies. When a sterile male mates with a fertile female, the resulting eggs are unfertilized and do not hatch. Over time, this process reduces the population of the pest until it is eventually eradicated.

The Sterile Insect Technique proved to be highly successful in eliminating the New World Screwworm from the United States. By the early 1960s, Southeastern producers had experienced significant economic losses due to screwworm infestations, estimated between $50 and $100 million. The successful eradication program, which cost the federal government $32 million, ultimately eliminated the New World Screwworm from the U.S. by 1966, preventing further economic hardship and protecting the nation’s livestock industry.

The Current Situation and Future Outlook

The recent detection of New World Screwworms in Mexico and the subsequent import suspension underscores the importance of continued vigilance and international cooperation in preventing the re-establishment of this destructive parasite in the United States. The USDA is working closely with Mexican authorities to implement control measures and monitor the situation.

The month-by-month reevaluation of the import suspension will allow the USDA to assess the effectiveness of the control measures and determine when it is safe to resume normal trade relations. The goal is to protect the U.S. livestock industry while minimizing disruption to international trade.

The re-emergence of the NWS serves as a reminder of the potential threats posed by invasive species and the importance of maintaining strong biosecurity measures. The success of the Sterile Insect Technique in the past provides a valuable tool for future eradication efforts, should the need arise.