Ivermectin Gains Emergency Screwworm Authorization, Highlighting Unmet Need and a Critical Evidence Gap

FDA Authorizes Ivermectin for Screwworm Prevention in Horses

The U.S. Food and Drug Administration (FDA) issued an Emergency Use Authorization (EUA) on July 15, 2026, for Ivermectin Liquid for Horses (ivermectin oral solution). This authorization is for the short-term prevention of New World screwworm (NWS) infestations in horses, specifically when administered within 24 hours of birth or at the time of initial wound care. The EUA, part of the Trump administration's efforts to safeguard American agriculture, aims to protect horses and livestock from the parasitic fly whose larvae feed on living tissue, which can be severe and life-threatening if untreated. This marks the FDA’s 11th EUA against NWS.

• The Emergency Use Authorization for Ivermectin Liquid for Horses is strictly for short-term prevention of New World screwworm myiasis, not for treatment of existing infestations. It is authorized for administration within 24 hours of a horse's birth or during initial wound care, and its preventive window lasts no more than 24 hours, necessitating its use alongside other fly-control methods.
• This EUA is a component of the Trump administration's comprehensive strategy to combat and prevent the spread of New World screwworm, involving collaborative efforts between the U.S. Department of Health and Human Services (HHS), the U.S. Department of Agriculture (USDA), and the FDA. It represents the FDA's 11th such emergency authorization deployed against NWS, underscoring a proactive approach to agricultural protection.
• Ivermectin Liquid for Horses is a clear, ready-to-use oral solution that can be administered as a drench or via a nasogastric tube. It is available exclusively by prescription from a licensed veterinarian, emphasizing the need for professional guidance in its application and ensuring it is used only in horses, not other species.

Addressing Unmet Needs in New World Screwworm Prevention

Current strategies for managing New World screwworm (Cochliomyia hominivorax) infestations rely heavily on chemical interventions, particularly macrocyclic lactones and organophosphorus compounds. While these agents form the backbone of control programs, significant challenges related to efficacy, resistance, and the availability of validated alternatives create unmet needs for more durable and effective solutions.

• Suboptimal Efficacy of Ivermectin: Although it is the most frequently used preventive insecticide, ivermectin has demonstrated incomplete efficacy. In studies, subcutaneous administration at the recommended dose of 200 µg/kg failed to prevent C. hominivorax infestations in over 29% of treated incisions in cattle, whereas doramectin at the same dose provided 100% efficacy in preventing infestation for up to 21 days.

• Emerging and Existing Insecticide Resistance: The long-term viability of current chemical controls is threatened by insecticide resistance. Concerns have been documented for organophosphorus (OP) compounds, a primary class of larvicides. Furthermore, while still at a low frequency, the identification of the K242R SNP in the ChGluClα gene—a mutation near a known macrocyclic lactone resistance site—in surviving larvae raises concerns about the potential development of ivermectin resistance.

• Limited Validated Therapeutic Options and High Costs: There is a scarcity of novel, validated treatment modalities specifically for C. hominivorax. While newer agents like spinosad are used, and lotilaner has shown high efficacy against Old-World screwworm (Chrysomya bezziana), robust data for their use against NWS is limited. This reliance on a few chemical classes contributes to high control costs, estimated at US$2 million annually in Venezuela alone. Severe human cases often require aggressive manual extraction of larvae, sometimes under general anesthesia, underscoring the need for more effective pharmacological interventions.

Identifying Vulnerable Horses to New World Screwworm

Myiasis caused by the New World screwworm (Cochliomyia hominivorax) affects diverse patient populations, with its true incidence often underreported, especially in tropical and subtropical regions. Clinical data from studies in Peru and Jamaica help identify key demographic and clinical risk factors that increase a patient's vulnerability to infestation.

• Age as a Primary Risk Factor: Clinical data indicates a bimodal age distribution for myiasis susceptibility, with pediatric and elderly populations being the most affected. In a Peruvian study, eight of nine cases occurred in pediatric or elderly patients, while a larger Jamaican study found individuals under 10 years (52.8% of cases) and those 60 years and older (18.8% of cases) constituted the largest affected groups.

• Influence of Comorbidities: Specific pre-existing conditions are significant risk factors that predispose patients to myiasis at different anatomical sites. A study in Jamaica established a strong association between Tinea capitis and infestations of the scalp and head (p < 0.001) and identified diabetes mellitus as a major risk factor for myiasis of the lower limbs (p < 0.001; OR = 14.48).

• Age-Dependent Anatomical Location: The site of infestation varies significantly with patient age. In the Jamaican cohort, myiasis in patients under 10 years predominantly affected the scalp and head (96% of cases in this group). Conversely, in patients aged 60 and over, the lower limbs were the most common site of infestation (66.6% of cases in this group).

• Socioeconomic and Geographic Considerations: Broader factors also contribute to infestation risk. Studies on ectoparasitic infestations, which include myiasis, suggest that vulnerable groups such as the homeless have an increased risk of acquiring infestations. Furthermore, the global spread of myiasis-causing agents into nonendemic areas, facilitated by climate change and global transport, is a growing concern.

Ivermectin's Role in the NWS Prevention Landscape

The standard of care for New World screwworm (Cochliomyia hominivorax) myiasis primarily involves chemical control in animals and physical removal in humans, supplemented by treatment for secondary infections. In animal populations, particularly in regions like Brazil and Venezuela, control has historically relied on the application of insecticides. These include organophosphorus (OP) compounds, macrocyclic lactones, and more recently, formulations like a spinosad-based foamy spray. The economic burden of this approach is significant, with larvicide costs in Venezuela estimated at US$2 million annually. However, the continuous and sometimes improper use of these chemicals has raised concerns about the selection of insecticide-resistant strains. Evidence for this includes the identification of mutations in the E3 gene of C. hominivorax at positions (Gly137 and Trp251) known to confer OP resistance in other fly species, highlighting the need for resistance monitoring.

In contrast to the chemical-dependent strategies in livestock, human treatment focuses on the physical debridement of larvae. A 2024 case report from Costa Rica described the successful treatment of a human patient following the manual extraction of approximately 160 first- and second-instar larvae from ulcers. Healing was further facilitated by treatment with ciprofloxacin to manage a concurrent Staphylococcus aureus and Pseudomonas aeruginosa infection. This case underscores the importance of addressing secondary bacterial complications in clinical management. On a population level, the sterile insect technique (SIT) has been a cornerstone of NWS management, leading to its eradication from North and Central America. A biological barrier is currently maintained at the Panama-Colombian border to prevent re-infestation from South America.

Ongoing surveillance and modern diagnostics are critical components of NWS control and prevention. Surveillance programs, such as those using sentinel animals in Venezuela, help track the prevalence and seasonality of myiasis cases. Furthermore, a diagnostic single nucleotide polymorphism (SNP) panel has been developed to rapidly determine the geographic origin of flies from incursions by distinguishing between South American, Inner Caribbean, and Outer Caribbean clusters. Despite these efforts, challenges remain, including the presence of feral swine as a reservoir, though infestations in these animals have been noted as more superficial. The reintroduction of C. hominivorax into Costa Rica in 2023, after its official elimination in 2000, serves as a stark reminder of the persistent risk of northward expansion and the need for continued vigilance among clinicians and public health officials.