New World Screwworm in the United States: Current Status, Risks to Cattle and Research Advances

New World screwworm in the United States: biology, cattle risks, surveillance, response and research.

Last reviewed: 30 July 2026

Sources and update policy: Current case counts, affected zones, movement restrictions and veterinary authorisations are checked against USDA APHIS, CDC and FDA. Stable biological and chemical sections are revised when the underlying peer-reviewed evidence changes.

The New World screwworm, Cochliomyia hominivorax, is a parasitic fly whose larvae feed on the living tissue of warm-blooded animals. After the parasite had been eliminated from the continental United States in the 1960s, a northward-moving outbreak reached the country again in 2026. The resulting animal-health response is particularly relevant to cattle producers in Texas and other southern states.

This article explains the current U.S. situation, the biological and economic risks to cattle, the principal control measures and the role of scientific research. It also describes a documented ChiroBlock contribution to related parasitology research while clearly distinguishing that work from direct research on New World screwworm.

Quick answer: New World screwworm has been detected in animals in the United States, but the available evidence does not indicate establishment across the country. As of 24 July 2026, CDC reported 37 domestically acquired animal cases and no locally acquired human cases. The principal risks concern animal health, cattle welfare, production losses and livestock movement. Surveillance, rapid reporting, veterinary treatment, movement controls and releases of sterile flies are central to the response.

Key Takeaways

• New World screwworm larvae consume living tissue and can turn a small wound into a severe or fatal infestation.
• The first confirmed U.S. animal case in the current outbreak was reported in Texas on 3 June 2026.
• CDC reported 37 domestically acquired animal cases as of 24 July 2026, while the overall U.S. risk remained very low.
• Cattle are particularly vulnerable at umbilical wounds, injuries, tick bites and sites created by husbandry procedures.
• Rapid detection and official reporting are essential because infested animals can transport larvae to new areas.
• The Nature Communications study discussed below concerns parasitic nematodes, not the New World screwworm fly.

What Is the Current New World Screwworm Situation in the United States?

New World screwworm has been confirmed in U.S. animals, with response and surveillance measures concentrated in affected areas of Texas and New Mexico.

The first confirmed U.S. case in the current outbreak involved a three-week-old calf in Zavala County, Texas. USDA announced the detection on 3 June 2026 after larvae were found in the calf’s umbilical area. Additional animal cases were subsequently identified.

Announced change: On 24 July 2026, USDA announced a conditional, phased reopening of southern cattle ports, beginning with Douglas, Arizona, on 24 August 2026. Because the timetable depends on implementation of the Joint Action Plan, the current port status should be checked with USDA APHIS before animal movements.

These figures are a dated snapshot. Case numbers, infested zones and movement requirements can change rapidly. The USDA APHIS New World screwworm status page and the CDC outbreak situation summary should be consulted for current information.

A confirmed animal case does not automatically prove that a self-sustaining fly population is established throughout a county or state. Authorities evaluate the animal’s movement history, nearby cases, wildlife observations, fly-trapping results and other epidemiological evidence. An individual animal may also recover while the surrounding surveillance or infested zone remains active.

USDA states that the current situation is not a food-safety issue. The primary concerns are animal health, animal welfare, production continuity and prevention of further spread.

What Is New World Screwworm and How Does It Cause Disease?

Cochliomyia hominivorax causes traumatic myiasis when its larvae invade a wound or body opening and feed on viable tissue.

Female flies are attracted to wounds, mucous membranes and vulnerable sites such as the umbilicus of a newborn animal. Eggs deposited at the wound edge can hatch within approximately one day. The larvae then enter the wound, feed collectively and enlarge the damaged area. Mature larvae eventually leave the host, pupate in the soil and develop into adult flies.

This biology distinguishes New World screwworm from many other fly larvae that primarily consume dead or decomposing material. The progressive destruction of living tissue can cause:

• deepening and enlargement of the wound;
• pain, inflammation, bleeding and discharge;
• a characteristic foul odour;
• secondary bacterial infection;
• reduced feeding, weakness and impaired production; and
• death if a severe infestation remains untreated.

Any warm-blooded animal can potentially be affected, including cattle, sheep, goats, horses, wildlife, pets and, less commonly, people. New World screwworm is not transmitted directly from one animal or person to another. However, moving an infested animal can transport larvae into a previously unaffected area.

How Can New World Screwworm Affect Cattle Production?

The main consequences for cattle operations arise from tissue damage, animal losses, treatment requirements, additional labour and restrictions on animal movement.

Newborn calves are particularly vulnerable because an incompletely healed umbilicus provides an attractive site for egg deposition. Other potential entry sites include injuries, tick bites, surgical wounds and areas affected by castration, dehorning, branding or identification procedures.

Farm-level consequences may include:

• veterinary examination, wound treatment and medication costs;
• additional inspections and labour requirements;
• reduced weight gain or milk production in affected animals;
• loss of breeding animals or valuable young stock;
• delayed transport, sale or movement of livestock; and
• biosecurity and documentation costs within regulated zones.

The wider economic effect can exceed the direct loss caused by individual infestations. Surveillance programmes, movement controls, closed livestock ports and changes to breeding or management procedures can affect producers, transporters, veterinary services and regional supply chains.

Reliable estimates of future losses depend on the geographical extent and duration of an outbreak. Historical loss figures should therefore not be presented as direct forecasts for the current situation.

How Can Livestock Owners Detect and Respond to New World Screwworm?

Regular inspection, prompt wound care and immediate reporting of suspicious larvae are the most important practical measures available to livestock owners.

Risk-reduction measures

• Inspect animals regularly, paying particular attention to newborns and existing wounds.
• Clean and manage wounds promptly in consultation with a veterinarian.
• Reduce avoidable injuries and review the timing of procedures that create wounds.
• Control ticks and other external parasites that can damage the skin.
• Inspect animals before transport and comply with official movement requirements.
• Monitor pets and susceptible wildlife as well as livestock.

Warning signs

Possible signs include visible larvae, a wound that rapidly becomes larger or deeper, bleeding, discharge, a foul smell, repeated licking or biting of the affected area, restlessness, head shaking, reduced feeding and unusual weakness.

Not every larva found in a wound is C. hominivorax. Official laboratory identification may be required to distinguish New World screwworm from other fly species.

Response to a suspected case

1. Contact a veterinarian immediately.
2. Avoid unnecessary movement of the affected animal.
3. Follow instructions for collecting or preserving larvae; do not discard potentially diagnostic material.
4. Notify the relevant state animal-health authority or USDA as directed.
5. Inspect other susceptible animals on the premises.
6. Use only treatments authorised for the species and situation.

USDA provides current instructions for reporting suspected New World screwworm cases. Current regulatory information on veterinary products is available from the FDA animal-drug information page.

Human infestations require prompt professional medical care. People should not attempt to remove larvae or egg masses without medical guidance.

How Is New World Screwworm Controlled?

Effective control combines treatment of affected animals with surveillance, movement controls, wildlife monitoring and the Sterile Insect Technique.

The Sterile Insect Technique, or SIT, is an area-wide control method. Large numbers of New World screwworm flies are produced under controlled conditions, sterilised and released over affected or at-risk areas. Matings between sterile males and wild females do not produce viable offspring. Repeated releases reduce reproduction in the wild population and can ultimately interrupt transmission.

SIT was central to the historic elimination of established New World screwworm populations from the United States. It is most effective when combined with rapid case detection, treatment of infested animals, fly trapping, animal-movement controls and continuous evaluation of the release area.

USDA explains the method and its role in the current response on its page about using the Sterile Insect Technique to control screwworm.

How Does Scientific Research Support Parasite Control?

Research supports more efficient sterile-fly production, faster diagnostics, improved surveillance, new attractants and safer veterinary treatments.

Direct New World screwworm research includes genomics, population tracing, molecular diagnostics, trap and lure development, insect behaviour, sterile-insect technologies, epidemiological modelling and veterinary product development. Defined chemical compounds are often needed to investigate biological pathways, validate analytical methods or test new control concepts.

Related research on natural anthelmintics

A peer-reviewed study published in Nature Communications examined avocado-derived fatty alcohols and related acetates as potential anthelmintic compounds:

Fahs, H. Z. et al. “A new class of natural anthelmintics targeting lipid metabolism.” Nature Communications 16, 305 (2025). DOI: 10.1038/s41467-024-54965-w.

The study investigated effects on free-living and parasitic nematodes and examined lipid metabolism as a potential biological target. According to the paper’s chemical-sources section, ChiroBlock GmbH synthesised avocadane acetate and ¹³C-labelled avocadene acetate for the experimental work.

Why the isotope-labelled compound mattered: The study reported that NMR analyses using ¹³C-labelled avocadene acetate showed no degradation in C. elegans. The authors interpreted this result as consistent with the possibility that the avocado-derived fatty alcohols and acetates were not metabolised in the model organism. This provides a concrete, study-specific example of how isotope-labelled compounds can clarify compound fate in biological research.

Scientific boundary: The publication concerns nematodes. It did not investigate the New World screwworm fly and does not demonstrate activity against Cochliomyia hominivorax.

The documented contribution illustrates how custom-synthesised research reagents and isotope-labelled compounds can enable biological experiments when suitable materials are not commercially available. The biological study design, experiments and scientific interpretation were the responsibility of the research authors.

Further documented examples are available in ChiroBlock’s overview of scientific expertise in custom synthesis.

Scientific Summary

New World screwworm is the parasitic blowfly Cochliomyia hominivorax. Its larvae cause traumatic myiasis by feeding on living tissue in wounds or body openings of warm-blooded hosts. Following renewed spread through Central America and Mexico, the first U.S. animal case of the current outbreak was confirmed in Texas in June 2026. CDC reported 37 domestically acquired U.S. animal cases and no locally acquired human cases as of 24 July 2026. Cattle risks include tissue injury, secondary infection, animal losses, treatment costs, additional labour and movement restrictions. Control depends on rapid reporting, veterinary treatment, surveillance, movement controls and sterile-fly releases. Related parasite research may require non-commercial compounds and isotope-labelled research materials. In the cited Nature Communications study on nematodes, ChiroBlock synthesised avocadane acetate and ¹³C-labelled avocadene acetate. That publication was not direct research on New World screwworm.

Frequently Asked Questions

Conclusion

The return of New World screwworm detections to the United States requires vigilance but also careful interpretation. Confirmed animal cases justify intensive surveillance, rapid reporting and coordinated control; they do not demonstrate nationwide or statewide establishment.

For cattle producers, the immediate priorities are wound prevention, frequent inspection, veterinary support and compliance with current movement requirements. For researchers, advances in diagnostics, chemical ecology, genomics, sterile-insect technology and veterinary treatment remain important.

ChiroBlock supports scientific projects by providing chemical custom synthesis for complex and non-commercially available compounds. Researchers who require a defined research compound or isotope-labelled material can request a confidential project assessment.

Primary Sources and Further Information

• USDA APHIS: Current Status of New World Screwworm
• CDC: New World Screwworm Outbreak Situation Summary
• USDA APHIS: New World Screwworm Prevention for Animals
• USDA: Using the Sterile Insect Technique to Fight Screwworm
• FDA: Animal Drugs for New World Screwworm
• Texas Animal Health Commission: New World Screwworm
• Nature Communications: A New Class of Natural Anthelmintics Targeting Lipid Metabolism

Author and Scientific Review Information

Article author: Alexander Gutsche (Chemist and Projekt Manager, Business Development at ChiroBlock) https://www.linkedin.com/in/alexander-gutsche-8aaa2013a/

Scientific review: Dr. Oliver Seidelmann (Chemist, Managing Partner ChiroBlock) https://www.researchgate.net/profile/Oliver-Seidelmann

Editorial review: ChiroBlock GmbH

Relevant expertise: Synthetic chemistry, chemical custom synthesis, research compounds, chemical biology and scientific collaboration

Last reviewed: July 30, 2026

Author and Review Information