Screwworm Invasion: The Silent Killer Threatening U.S. Livestock!

Ladies and Gentlemen, BUYERS BEWARE! The U.S. is under attack from an old foe, the New World Screwworm (NWS), and it's a threat that could cost us BILLIONS! On May 11, 2025, the U.S.-Mexico border was closed to cattle, bison, and horses from Mexico to protect our country from this deadly pest. NWS was eradicated from the U.S. in 1966 and eliminated as far south as Panama by 2000, but it's back with a vengeance, moving north through Central America and Mexico. This is not just a health threat; it's an economic time bomb waiting to explode!

WHAT IS THE NEW WORLD SCREWWORM?

NWS is a fly endemic to Cuba, Haiti, the Dominican Republic, and countries in South America. It's spread northward to Costa Rica, Guatemala, Nicaragua, Belize, El Salvador, and, more recently, Mexico. The fly lays eggs on a warm-blooded animal, typically on or near a wound. When the eggs hatch, the larvae feed on the surrounding flesh as they burrow into the wound, which is how the pest got its name. NWS can infest any warm-blooded animal, including livestock, poultry, wildlife, domestic animals, and, though rarely, humans. This pest does not discriminate among warm-blooded hosts, and wildlife such as deer, feral hogs, and even birds can quickly spread NWS. If left untreated, NWS infestations can be fatal. Animals can die of trauma, toxicity, or other infections within two weeks.

THE ECONOMIC IMPACT

The potential economic impacts on the U.S. livestock industry if the New World Screwworm (NWS) were to establish a foothold in the country could be devastating. The U.S. livestock industry benefits by more than $900 million a year as a result of the eradication of the screwworm. Mexican producers and consumers saved about $2 billion from the beginning of eradication to 1991. A 1995 Texas A&M University study evaluated the direct benefit to the Central America livestock producers at $73 million a year and overall economic benefits to the region at $257 million annually. The benefits to Mexico were estimated at $275 million annually from 1991 to 1994. In addition there are the benefits of enhanced human and animal health and increased standards of living due to the removal of this deadly insect. Endangered wild species have a better chance of survival. Additional grazing areas previously subject to high rates of screwworm infestation are now available for use by livestock and wildlife. The quality of life for smaller farmers and ranchers, especially in Central America, was improved by relief from lost man-hours needed to treat livestock and risk of loss from screwworm infestation. These benefits were made possible by the close cooperation of federal and state governments, individual producers, and producers' alliances to financially support eradication, as well as with activities that tracked infestations and allowed them to be treated. USDA scientists, primarily from the Agricultural Research Service (ARS) and Animal and Plant Health Inspection Service (APHIS), achieved the scientific and technical advances that made eradication possible. The life cycle of the insect lasts about 21 days in warm weather and slightly longer in cooler climates. The adult female mates only once and lays her white, elongated eggs along the edges of wounds on warm-blooded animals. The Larval Stage Infestations are possible in smaller animals such as cats, dogs, and birds but the greatest economic impact is on larger animals such as pigs, goats, sheep, deer, and cattle. Even humans are susceptible to infestation, often in the nose or sinus cavities. Left untreated in humans, livestock, or wild animals, the egg masses hatch into swarms of larvae”the "worm" stage”which embed themselves in the host's flesh and consume the living tissue and fluids. The appearance of the larvae and the way they burrow into the host's flesh give the screwworm its common name. Feeding screwworms enlarge the wound and attract additional female flies, which deposit more and more eggs in the wound. If the infestation remains untreated, the host animal has little chance of surviving the secondary infections that often follow. The Pupal Stage The larvae gorge for several days to provide nourishment for the next stage in their development. Once mature, they drop to the ground and burrow shallowly into the soil. There they develop a dry outer shell and enter the pupal stage. The pupae undergo metamorphosis into adult flies. Adult Flies Adult flies emerge from the pupae and soon begin mating, to repeat the cycle. Screwworm flies are blue-green, have three dark stripes on their backs, and have orange eyes; they are about twice the size of a housefly. Theory and Research Leads to Eradication Techniques The work of many people and institutions over decades contributed to the eradication effort, which now has the screwworm in retreat as far south as Panama. Throughout, research in theory and practical application by ARS scientists and laboratories made the eradication process possible. In particular, three scientists”Dr. Emory Cushing, Dr. Edward F. Knipling, and Dr. Raymond C. Bushland”performed the basic analyses and created the techniques underpinning the eradication process. The eradication of NWS in the U.S. was a significant achievement, and the re-emergence of the pest poses a serious threat to the livestock industry. The economic impact could be severe, with potential losses in the billions of dollars. To mitigate these impacts, strategic investments in prevention and treatment measures are crucial. The Sterile Insect Technique (SIT) has been proven effective in eradicating NWS. The release of mass-produced sterile male flies in strategic areas can reduce the wild population of NWS. The only fly production facility in the world is in Panama, and it has the capacity to produce about 100 million flies per week. Additionally, treatment options such as Nitenpyram and Ivermectin have been successful in killing and expelling NWS larvae quickly. Dressing infested wounds with anti-parasitic agents like lindane or ronnel can also be effective. These measures, combined with increased surveillance and rapid response to any detected infestations, can help prevent the establishment of NWS in the U.S. and protect the livestock industry from significant economic losses.

THE THREAT IS REAL

NWS is typically endemic to warmer climates. In fact, the larvae cannot survive temperatures below 46 degrees Fahrenheit. However, a cold climate doesn’t eliminate the threat. Livestock and wildlife movement can transfer NWS to the northern regions of the country in the summer months where they can find hosts and spread until temperatures cool off during fall and winter. NWS flies are able to travel as far as 12 miles to find suitable hosts. This pest does not discriminate among warm-blooded hosts, and wildlife such as deer, feral hogs, and even birds can quickly spread NWS. If left untreated, NWS infestations can be fatal. Animals can die of trauma, toxicity, or other infections within two weeks.

THE TREATMENT

As addressed earlier, NWS was eradicated as far south as Panama by 2000 when a biological barrier was established using the Sterile Insect Technique (SIT). SIT is the release of mass-produced sterile male flies in strategic areas. The male flies mate with the wild females to produce unfertilized eggs. Since the female NWS flies only mate once, the wild population shrinks until the pest is eradicated. The only fly production facility in the world is in Panama.

While SIT is the only tool we have to eradicate NWS, there are options for treatment once NWS is detected. First, screwworm infestations must be reported to state and federal authorities. Treatment for NWS involves killing and removing the larvae from the infested wound. Nitenpyram is highly effective for killing and expelling NWS larvae quickly. Ivermectin has been successful in treating and even preventing NWS infestations. Screwworms can also be killed by dressing an infested wound with an anti-parasitic such as lindane or ronnel. All wounds must be dressed to promote healing and avoid reinfestation.

NWS is extremely difficult to eliminate and there have been some isolated incidents of re-emergence in the U.S. For example, NWS re-emerged in the Florida Keys in 2016. Officials reacted quickly by releasing sterile flies and by 2017 the pest was eradicated again. This is evidence of just how difficult NWS is to control.

BREACHING THRESHOLDS

When NWS was discovered in the Mexican state of Chiapas in November 2023, primary and secondary thresholds were established as breaking points for fly control. The Panama production facility had enough capacity (about 100 million flies per week) to release sterile flies inside the buffer zone between the primary and secondary threshold and push them back to the biological barrier in Panama.

Since November 2023, there have been over 1,400 detections of NWS in Mexico. On May 11, 2025, U.S. Agriculture Secretary Brooke Rollins announced the suspension of live animal imports from Mexico because NWS had been detected 127 miles outside of the secondary threshold in Oaxaca and Vera Cruz, Mexico. With the break in the secondary threshold, there is no longer enough sterile fly production capacity to keep NWS from moving further north.

WHAT CAN WE DO?

The Sterile Insect Technique (SIT) has been highly effective in controlling New World Screwworm (NWS) outbreaks in the past. For instance, the technique was successfully used to eradicate NWS from the U.S. by 1966 and from Panama by 2000. The SIT involves releasing mass-produced sterile male flies in strategic areas, which mate with wild females to produce unfertilized eggs, thereby reducing the wild population over time. This method was also effective in eradicating NWS from the Florida Keys in 2017 after an outbreak in 2016. The USDA released approximately 3 million sterile flies twice a week in the affected area, leading to the successful eradication of the pest.

However, the current re-emergence of NWS in Central America and Mexico presents significant challenges to the effectiveness of SIT. The Panama production facility, which is the only fly production facility in the world, had enough capacity to release sterile flies inside the buffer zone between the primary and secondary threshold and push them back to the biological barrier in Panama. However, since November 2023, there have been over 1,400 detections of NWS in Mexico, and on May 11, 2025, the U.S. Agriculture Secretary Brooke Rollins announced the suspension of live animal imports from Mexico because NWS had been detected 127 miles outside of the secondary threshold in Oaxaca and Vera Cruz, Mexico. With the break in the secondary threshold, there is no longer enough sterile fly production capacity to keep NWS from moving further north. This limitation highlights the need for increased production capacity and additional strategies to control the spread of NWS.

CONCLUSION

The threat of the New World Screwworm is real, and it's time to take action. The U.S. livestock industry is at risk, and we need to protect it at all costs. Invest in prevention and treatment measures, increase surveillance, and be prepared to respond quickly to any detected infestations. The future of our livestock industry depends on it!