The U.S. Food and Drug Administration (FDA) has granted conditional approval for the injectable product Dectomax-CA1 (doramectin injection) to enable the prevention and treatment of New World screwworm (Cochliomyia hominivorax, NWS) larval infections in cattle. This marks the first U.S. approval of a drug specifically targeting this flesh-eating parasitic larva, which affects open wounds and mucous membranes, destroying living tissue. The conditional approval allows for immediate availability while the manufacturer, Zoetis, gathers further efficacy data. Given the ongoing spread of NWS in Central America and Mexico, which threatens U.S. livestock, this represents a crucial step in protecting agriculture.
The Threat of the New World Screwworm: A Historical and Current Danger
The New World screwworm is one of the most destructive parasites for warm-blooded animals and was last eradicated in the U.S. in 1966 through a successful sterilization campaign using genetically modified flies. The larvae, which hatch from the eggs of Cochliomyia hominivorax, burrow into wounds and feed on living tissue, leading to severe infections, secondary infections, and high mortality. Affected are not only livestock such as cattle but also wildlife, pets, and in rare cases, humans. In the past, the screwworm caused hundreds of millions of dollars in losses annually, as it enlarges wounds and delays healing until the larvae hatch and repeat the cycle.
Since 2023, the screwworm has spread explosively in Central America: from Panama through Costa Rica, Nicaragua, Honduras, Guatemala, Belize, and El Salvador to Mexico. In Panama, cases increased from an average of 25 per year to over 6,500 in 2023. In August 2025, the first human case in the U.S. was confirmed in a traveler from El Salvador, raising the alert level. Particularly alarming is the northward migration: on September 21, 2025, Mexico's SENASICA reported a case in Sabinas Hidalgo, Nuevo León – less than 70 miles from the U.S. border. This is the northernmost detection in this outbreak and poses a direct threat to U.S. livestock, especially in border states like Texas, which holds 12.5 million cattle (14 percent of the U.S. herd).
The economic risks are enormous: A USDA study estimates that an outbreak in Texas alone would cost $1.9 billion annually, including animal losses, treatments, and trade restrictions. For the entire US economy, losses could reach up to $11 billion, including impacts on the hunting and wildlife industry ($9 billion in Texas alone). As early as May 2025, the USDA closed the border to cattle, horse, and bison imports from Mexico to prevent introduction. Experts from Texas A&M AgriLife and the University of Florida warn that warm climates like Florida and Texas are particularly at risk, as the screwworm thrives in tropical and subtropical regions. Its spread correlates with climate change and reduced sterilization programs in the region.
Conditional Approval of Dectomax-CA1: Rapid relief with proven active ingredients
The FDA approval of September 30, 2025, is based on the expanded conditional approval pathway, which has been in place since 2018 for severe animal diseases with unmet needs. Dectomax-CA1 contains the active ingredient doramectin at a dose of 200 micrograms per kilogram of body weight, identical to the already fully approved Dectomac Injectable (NADA 141-434), which has been used for years against nematodes and arthropods in cattle and swine. The conditional approval covers the prevention and treatment of New World Screwworm larval infections, as well as protection against reinfestation for 21 days.
Since the safety data, manufacturing standards, and residue analyses from the original approval could be adopted, no additional testing was required. The FDA assessed the product as safe for cattle and with a high expectation of efficacy, as studies for full approval would be complex – for example, through field trials in infected areas. The drug is administered as a subcutaneous injection and is available in 250 ml and 500 ml bottles. The withdrawal period before slaughter is 35 days, identical to Dectomax. It is not approved for dairy cows over 20 months of age, and there is no established withdrawal period for calves in veal production. The labels will be combined to facilitate the transition.
Zoetis, the sponsor from Kalamazoo, Michigan, plans to launch the product in the first half of 2026. The FDA emphasizes that this is only the first step: further products for other animal species are under review to expand protection. The conditional approval requires Zoetis to collect additional efficacy data within one year to obtain full NADA approval.
Efficacy of Doramectin: Proven prophylaxis against screwworm
Doramectin, a macrocyclic antiparasitic from the avermectin group, acts systemically against larval stages of parasites by paralyzing their nervous system and inhibiting oxygen consumption. Studies from the 1990s in Brazil showed 100 percent efficacy in induced infections: an injection completely prevented larval development, and protection lasted up to 28 days. In controlled trials with calves, wounds were artificially infected; in doramectin-treated animals, third-instar larvae did not hatch, and wounds healed within 96 to 120 hours. Compared to placebos, where larvae developed fully and wounds worsened, doramectin reduced infections by 100 percent.
Comparable results exist for ivermectin, a related active ingredient routinely used in Mexico. A dose of 200 micrograms per kilogram killed 100 percent of larvae up to the second day and 70 to 21 percent of older stages. These data support the FDA's expectations for Dectomax-CA1, as doramectin has been successfully used against NWS in endemic areas like South America. Nevertheless, the FDA warns of resistance risks: excessive use could diminish efficacy against other parasites, which is why an integrated parasite management strategy is recommended, including inspections and alternative methods like sterile insects.
U.S. Government Actions: Border Security and Prevention
The USDA and APHIS (Animal and Plant Health Inspection Service) have taken comprehensive countermeasures since the outbreak in spring 2025. In May, the import of cattle, horses, and bison from Mexico was suspended to prevent transmission. On August 15, 2025, the USDA announced an $8.5 million plan, including the construction of a sterile insect production facility at Moore Air Force Base in Edinburg, Texas, which is expected to release up to 100 million sterile flies per week by the end of 2025. This builds on the successful cooperation with Panama (COPEG), which has maintained the barrier for decades.
Since February 2025, sterile insects have been deployed directly in Mexico to combat the northernmost outbreak. The USDA urges border regions to inspect animals for wounds, inflammation, and larvae – especially in the nose, ears, genitals, and navel area. Veterinary information from the FDA and CDC emphasizes early detection: in case of suspicion, seek immediate medical help for animals and humans. The American Farm Bureau Federation and the National Cattlemen’s Beef Association praise the measures as essential for the volatile cattle market, which could be further strained by the outbreak.
Global and Economic Implications: A Wake-Up Call for the Livestock Industry
The NWS outbreak highlights the vulnerability of global supply chains: Thousands of animals were treated in Mexico, and the case in Nuevo León triggered immediate inspections. In the US, states like Texas and Florida could be most affected, where the screwworm was already an exception in 2016/2017, decimating endangered species like the Key Deer. The FDA and USDA are working with international partners such as the FAO to strengthen surveillance and minimize trade barriers.
The conditional approval of Dectomax-CA1 finally offers cattle producers a specific tool that alleviates the pressure of import bans and potential outbreaks. In the long term, the strategy aims for a permanent barrier, but climate change and travel increase the risk. Experts are calling for investment in research and early detection to avert future threats.
Sources: FDA press release, USDA APHIS, American Farm Bureau Federation, CIDRAP, NPR, Texas A&M AgriLife, University of Florida, PubMed studies on Doramectin.
