Northern Mexico Infestation Prompts U.S. Health Alert for Screwworm Threat

The Centers for Disease Control and Prevention (CDC) has issued a Health Alert Network advisory due to a recent outbreak of New World screwworm (NWS) in the Mexican state of Tamaulipas, which borders Texas. While no related infestations have been identified in the United States to date, the CDC is urging heightened awareness among healthcare professionals, public health officials, and the public about the potential for geographic spread and the necessary precautions.

New World screwworm is a parasitic pest where flies lay eggs in the wounds of warm-blooded animals and people. These eggs hatch into larvae, commonly known as maggots, which feed on living flesh. While primarily affecting livestock like cattle and horses, these flies can also infest humans, and untreated cases in people can be fatal. The current outbreak, spanning Central America and Mexico, has resulted in over 1,190 human cases and seven deaths as of January 20, 2026. Mexico has reported 601 active animal cases and 24 hospitalizations in people. Tamaulipas, the border state to Texas, has identified eight active animal cases of NWS.

Historically, NWS posed a significant economic threat to livestock in the United States. The U.S. Department of Agriculture (USDA) successfully eradicated the pest through a strategy involving the release of sterile male flies, which rendered female flies unable to produce viable offspring. This method, previously used to eliminate NWS from the U.S. and Mexico, was also employed in 2017 to address a reintroduction in the Florida Keys. A biological barrier maintained at the Darien Gap between Panama and Colombia since 2006 aimed to prevent its reintroduction into Central America. However, a surge in NWS cases in Panama in 2023, coupled with increased cattle movement and unregulated passage through the Darien Gap, contributed to its rapid northward spread into Mexico. NWS also remains prevalent in South America and parts of the Caribbean.

The USDA’s ongoing strategy to control and re-eradicate NWS involves releasing sterile male flies, enforcing animal movement controls, and enhancing surveillance and public outreach in affected regions. A critical component of this strategy is the swift identification and appropriate clinical management of human infestations to prevent further spread. The CDC is collaborating with various federal, state, local, and international entities to support these control efforts through public health actions.

Individuals may be at a higher risk of developing NWS if they are in areas where the NWS fly is present and have open wounds, including minor scratches, cuts, or insect bites, or surgical wounds. Those with medical conditions that impede wound healing, such as skin cancer, psoriasis, or eczema, or who are very young or elderly, may also be more vulnerable. Malnourished individuals and those who sleep outdoors, particularly during daytime hours when the flies are most active, are also at increased risk. Furthermore, people who live, work, or spend extended periods in close proximity to livestock or other warm-blooded animals in these affected areas face a greater chance of infestation.

Clinicians are advised to consider NWS in patients presenting with visible larvae or egg masses in wounds or body orifices, tissue destruction, a sensation of movement, foul odor, bloody discharge, swelling, and pain, especially if the patient has recently traveled to regions where NWS is prevalent. In areas where NWS is a concern, individuals should be advised to meticulously clean and cover all wounds, regardless of size or location. Protective measures include wearing loose-fitting, long-sleeved clothing and pants, socks, and hats to minimize exposed skin, and utilizing EPA-registered insect repellents. It is recommended to avoid areas where livestock are housed whenever possible and to refrain from sleeping outdoors, particularly during the day. If windows are open and not screened, using bed nets can provide protection for sleeping areas, and screening windows and doors in healthcare facilities is essential.

Healthcare providers are instructed to immediately report any suspected human cases of NWS to their respective state, tribal, local, or territorial health departments. Collaboration with health departments is crucial for seeking confirmatory diagnosis through the CDC’s Diagnostic Parasitology Lab (DPDx). For clinical inquiries and patient management questions, clinicians can contact the CDC’s Parasitic Diseases Hotline during business hours or the CDC’s Emergency Operations Center outside of regular hours. Non-clinical questions can be directed to a dedicated email address.

For patients with larval infestations, clinicians should consult with their health department and the CDC to collect and submit larvae for diagnosis and confirmation, as multiple fly species can cause myiasis. At least 10 larvae should be submitted, or all if fewer than 10 are available. If different larval stages are present, a representative sample from each should be included. Specimen submission instructions can be obtained by emailing dpdx@cdc.gov. Telediagnosis may be available for faster diagnosis. Prompt removal and killing of all larvae and eggs are critical; failure to do so could lead to the reintroduction and spread of NWS. Larvae and eggs should be placed in a leak-proof container, submerged in 70% ethanol (preferred) or another suitable preservative, and then sealed in a plastic bag for disposal in the trash. Surgical extraction may be necessary for deeply embedded larvae. Follow-up examinations within 24–48 hours are recommended to ensure no live larvae remain.

Health departments are tasked with emphasizing the importance of prompt larval and egg removal and proper disposal to medical professionals and the public, advising against discarding them in trash or outdoors. They should also encourage the collection of at least 10 larvae for submission to the CDC for confirmation. Human infestations may suggest local fly circulation, prompting health departments to alert state animal health officials to monitor for NWS in animals and coordinate containment efforts. Outreach to high-risk communities, such as those with unstable housing or agricultural workers, is also advised. Case reporting forms for human NWS cases should be submitted to the CDC via the One CDC Data Platform. Suspected animal cases must be reported to state animal health officials and the APHIS office.

Laboratories are reminded to ensure all larvae are properly preserved and to report suspect human and animal cases immediately to the appropriate health and animal health authorities. Diagnostic assistance is available through the CDC.

The public is advised that prevention is key in areas where the NWS fly is present. This includes keeping all wounds clean and covered, wearing protective clothing, using EPA-registered insect repellents, and treating clothing and gear with permethrin. Sleeping indoors with screened windows or using bed nets is recommended. Anyone who sees or feels maggots in a wound should seek immediate medical care and should not attempt to remove them personally. Improper disposal of larvae or eggs can contribute to further spread. Individuals with questions should contact their local health department.

Veterinarians must immediately report suspected animal NWS cases to their state animal health official and the APHIS office. Larvae from animal cases can be submitted to the USDA’s National Veterinary Services Laboratory for identification. The FDA has issued an Emergency Use Authorization for a product to treat NWS infestations in dogs and cats. Similar to human cases, prompt removal and proper disposal of larvae and eggs from animals are crucial to prevent environmental spread.

Article by Mel Anara, based upon information from the Centers for Disease Control and Prevention (CDC)

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