Protecting the herd

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The new world screwworm (NWS) can be a significant threat to livestock, creating challenges for cattle producers. This parasitic insect, scientifically known as Cochliomyia hominivorax, is notorious for its devastating impact on livestock health, business operations, and the broader agricultural economy. The new world screwworm is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals, including cattle. Female flies lay eggs in open wounds, and within hours, the larvae hatch and begin burrowing into the host’s flesh, feeding on live tissue. The infestation can quickly escalate, leading to severe health issues, secondary infections, and, in some cases, death if untreated.  

Understanding the risks and implementing effective measures is crucial for producers to protect their herds and livelihoods. Historically, the United States eradicated NWS in the 1960s through a sterile insect technique, meaning many cattle producers have gone their whole careers without encountering it. 

In spring 2025, however, cases were identified in Mexico, and although it hasn’t happened yet, there is concern NWS will make its way across the southern border of the United States. The United States Department of Agriculture (USDA) issued the immediate suspension of live cattle, horse, and bison imports through United States ports of entry along the southern border on May 11, 2025, in an attempt to curb this spread, a suspension that is ongoing.

Containment efforts 

Federal and state agencies play a critical role in controlling NWS outbreaks. Policies surrounding quarantines, livestock movement, and funding for eradication programs are vital in containing the threat.   

The USDA has announced a multi-national plan to prevent the screwworm from entering the United States. This includes the opening of a sterile fly dispersal facility in Edinburg, Texas, and investing twenty-one million dollars to upgrade a facility in Mexico. These efforts aim to produce millions of sterile flies weekly to halt the parasite's northward migration and complement existing efforts in Panama and Mexico, where sterile flies are already being produced.

The sterile insect technique, which involves releasing sterilized male flies to suppress breeding populations, has been a cornerstone of past eradication efforts. Continued investment in this method, along with research into new control technologies, is crucial. Colin Woodall, CEO of the National Cattlemen's Beef Association, emphasized the importance of building domestic sterile screwworm fly facilities, stating, “We must produce sterile flies ourselves here on our soil to effectively eradicate the new world screwworm.”  

The government plans to disperse the sterile flies across area about 50 miles into Texas, along the U.S. border with the state of Tamaulipas, Mexico. To ensure officials can tell the difference between sterile and wild NWS flies, USDA will dye the sterile pupae, and the dye will transfer to the sterile flies when they hatch. The fluorescent dye will glow under UV light and may also be visible to the naked eye. If a sterile fly is captured in a trap, this dye will allow animal health officials to quickly rule the fly out as a threat. 

Livestock health impact 

Infestations by NWS can cause excruciating pain, significant stress, and severe health complications for affected animals. If untreated, the larvae can burrow deeper into the tissue, leading to necrosis, secondary bacterial infections, and systemic illnesses. 

Woodall noted, “If the screwworm infestation is not caught early enough, you could have cattle that would die four to seven days after the eggs are laid. The lesions left untreated are just gruesome.” 

Infested animals often exhibit visible discomfort, such as excessive scratching, reduced grazing, and signs of distress. These symptoms can lead to weight loss and reduced productivity in terms of milk or meat output. In severe cases, untreated infestations can result in death, posing a serious threat to herd health and producer livelihoods.
 
In March 2026, the FDA issued an Emergency Use Authorization (EUA) for F10 Antiseptic Wound Spray with Insecticide (benzalkonium chloride, polyhexanide, and cypermethrin topical solution) for the prevention and treatment of New World screwworm. 

The agency has concluded the spray may be effective against NWS myiasis in cattle, horses and minor species of hoof stock (e.g., sheep, goats, deer), along with some addition al animals. The potential benefits of the product outweigh its known and potential risks.

Safety measures:

• Cattle, goats, and sheep must not be slaughtered for human consumption within 30 days of treatment with for F10 Antiseptic Wound Spray with Insecticide
• Milk taken from cows, goats, or sheep during treatment and for 10 days after treatment must not be used for human consumption.
• A withdrawal period has not been established for this product in pre-ruminating calves. Treated calves and calves born to treated cows must not be processed for veal.

This treatment is designed to be used once per week. When spraying the wound, wear disposable gloves and wash hands thoroughly after use. Use only in well-ventilated areas. Clean the wound prior to application and use maximum of five applications per animal with multiple wounds. It is imperative to only spray the wound site. F10 Antiseptic Wound Spray with Insecticide is water soluble, so be sure to reapply if animal is exposed to rain. Talk to your vet if wounds get worse.

Additional treatment options

Other medications on the market are either conditionally approved or approved under the EUA to treat cattle with New World screwworm. As an MWI customer, you can purchase many of these items from our online store.

Veterinarians are also empowered to prescribe extra-label treatments for New World screwworm. 

What do all these terms mean, you may be wondering? According to the FDA, under a conditional approval the manufacturer must prove “reasonable expectation of effectiveness.” A full approval is “substantial evidence" of effectiveness.

Under an EUA, the FDA can fast track approvals of certain drugs to benefit the public health when they determine there are no adequate alternatives on the market.

Extra-label drug use permits practitioners, including veterinarians, to prescribe medications for reasons not explicitly stated on the product label. These treatments are limited to situations where the animal’s health is in jeopardy and the animal may suffer or die without treatment. The prescribing veterinarian must have a VCPR in good standing and only medications under full FDA approval qualify for extra-label use. For production animals, additional steps must be taken to ensure substantial withdrawal periods before the animals re-enter the food chain.

You can also stop the spread of the disease through the use of insecticides. The Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) permits their use for NWS.

5 best practices to control NWS spread 

Preventing the spread of NWS requires a proactive approach, including diligent herd management practices and collaboration with veterinary professionals. Producers must stay informed about regulatory updates and participate in reporting suspected cases. Collaboration between the livestock industry and government agencies ensures a coordinated response to outbreaks. 

1. Wound management: Regularly inspect livestock for open wounds, including minor abrasions, as these are prime sites for NWS egg-laying. Promptly treat and close wounds to minimize the risk of infestation.
2. Fly control measures: Use insecticides and repellents to reduce fly populations around livestock. Producers should also implement fly traps and maintain clean environments to discourage fly breeding.
3. Quarantine and inspection: Isolate new or returning animals before introducing them to the herd. This includes thorough inspection for signs of infestation, as larvae can be transported unknowingly.
4. Collaboration with veterinarians: Work closely with veterinary professionals to identify and treat infestations early. Producers should also report any suspected cases to state or federal animal health authorities to aid in outbreak tracking and containment. Veterinarians play a critical role in identifying potential infestations, treating wounds, and reporting cases to state agencies like the Texas Animal Health Commission (TAHC).
5. Education and monitoring: Stay informed about local outbreaks and warnings from agricultural agencies. Awareness of high-risk seasons and regions can help producers take timely preventive measures. 

Erin Robinson, Director of Communications for the TAHC, highlighted the importance of biosecurity and good management practices in an email to MWI Animal Health. “The most important thing producers can do right now is assess their management practices and start adjusting strategies to mitigate the spread of NWS.” She recommended monitoring animals regularly for signs of maggots, improving facilities to prevent wounds, and scheduling procedures like castration or dehorning during cooler months when flies are less active. 
 
Michael Catangui, Ph.D., Manager, Entomology Integrated Technical Services for MWI Animal Health, emphasized, “Many younger producers may not know about screwworms anymore. It’s important to educate them on the risks and management practices necessary to protect their herds.

Business impact

An outbreak of NWS can disrupt business operations in significant ways. Quarantines or movement restrictions on livestock from affected areas can limit market access and reduce sales opportunities. Woodall stated, “We are seeing the economic impacts already today with the border closure. The Secretary of Agriculture closed our southern border to Mexico, which has economic consequences for producers.” Additionally, the costs of treatment, preventive measures, and increased veterinary interventions can strain producers' budgets.
 
For cattle producers, healthy livestock translates to profitability. NWS infestations lead to weight loss, reduced milk production, and even the loss of animals, directly impacting revenue. Woodall remarked, “For producers, it is valuable time spent monitoring and protecting against new world screwworms that adds to their operational costs.” 

The economic toll of a widespread NWS outbreak can be staggering. Direct costs include veterinary treatments, insecticide applications, and wound care, while indirect costs stem from lost productivity, restricted trade, and increased labor demands. 

Historical data from previous outbreaks shows that producers in affected regions experienced significant financial losses due to decreased livestock productivity and increased mortality rates. Dr. Catangui shared, “During a previous outbreak, producers spent tens of thousands of dollars on fly control measures alone, not including the cost of lost cattle.” 

The new world screwworm outbreak serves as a reminder of the importance of vigilance and proactive management in livestock operations. By staying informed and implementing best practices, cattle producers can protect their herds and sustain their businesses during this challenging time.