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Tuesday, September 9, 2025
Kim Vaughn

New World Screwworm: An emerging threat to mammals

The New World Screwworm (Cochliomyia hominivorax) may have an unusual name but it’s a very real parasitic fly that poses a serious threat to both animals and people. Although the U.S. successfully eradicated it decades ago, the fly has reestablished itself in Latin America and Mexico in the last two years threatening to continue northward into the US.  NWS has massive production and economic impacts for livestock producers and serious animal welfare implications for animals.  During the 20th century, the presence of NWS cost the US livestock industry more than $100 million annually.

Here’s what you need to know about this dangerous insect, how to protect yourself and your animals, and why Washingtonians should be aware—but not alarmed.

Washington state: Risk remains low

  • No known cases of New World Screwworm have ever occurred in Washington state.
  • The climate is less suitable for fly development.
  • The Washington State Department of Agriculture (WSDA) continues to monitor and respond to any animal health threats, including screwworm.

Still, with international travel and animal imports increasing, staying alert is important.

What is the New World Screwworm? 

The New World Screwworm is a species of blowfly whose larvae feed on living flesh. Unlike common blowfly that eat dead tissue, screwworm larvae invade and eat living tissue in  open wounds, umbilical cords, dehorning sites, insect bites or body orifices of warm-blooded animals—including livestock, pets, wildlife, and even people.  NWS is not contagious and cannot spread between animals and humans. Left untreated, infestations can become severe, causing deep, necrotic wounds and even death. 

Lifecycle: How screwworms spread

  1. Eggs: Female screwworm flies lay hundreds of eggs on the edges of wounds or mucous membranes such as the eyes, mouth, nostrils, and genitals. These wounds can include anything from natural injuries to surgical sites, such as castration wounds, dehorning sites, or new ear tags in livestock. One female can lay up to 3,000 eggs in her lifespan. 
  2. Larvae: Within 12–24 hours, the eggs hatch. The larvae burrow into healthy, living tissue, causing pain and extensive tissue damage.
  3. Pupation: After 3–7 days of feeding, mature larvae drop to the ground to pupate in the soil.
  4. Adult flies: Adults emerge in about a week. Females mate once and can fly up to 125 miles, making them highly mobile vectors of disease.

This cycle can repeat rapidly in warm climates. Multiple infestations are common with multiple stages of larvae in one wound.  Many flies will revisit the same wound in a “feeding frenzy” resulting in rapid tissue damage. Adult flies prefer warmer temperatures and have historically only been able to overwinter in southern Texas, Florida, California, Arizona and New Mexico. Adult fly activity decreases as temperatures fall below 70 F.  Washington has never been infested with NWS due to the Northern geographic location and cooler climate that greatly reduces the risk of NWS establishment. 

Not just livestock: Screwworms can affect pets, wildlife, and even people

While screwworms are often associated with cattle or livestock, it’s important to understand that they can infest any living warm-blooded animal:

  • Pets like dogs and cats
  • Wildlife including deer or raccoons
  • Humans, especially those with open wounds, surgical sites, or poor wound hygiene

In fact, a rare U.S. human case was reported in Maryland in August 2025 in a traveler returning from Central America. Travelers to affected regions (such as rural areas of Central and South America) should take extra care to protect any wounds, use insect repellent, and avoid environments where flies may gather. If you return from travel and develop unusual symptoms, experience a sensation of movement with foul discharge or a painful non-healing wound, seek emergency medical care and inform your doctor about your recent travel history.

Symptoms: What to look for in animals 

In animals, watch for:

  • Wounds that worsen instead of healing
  • Maggots visible in or around a wound
  • Foul-smelling and/or bloody discharge or tissue
  • Signs of pain, depression, or behavioral changes such as tail swishing or kicking or isolation away from the herd
  • Tissue destruction near ears, eyes,  mouth, genitals or umbilicus

The key sign: foul odor or rapid tissue damage in a wound that appears more severe than expected. 

 

What to do if you suspect screwworm:

  • Act quickly. Screwworm infestations can be fatal.
  • For pets or livestock: Contact a veterinarian immediately. Treatment involves:
    • Manual removal of larvae
    • Cleaning and dressing the wound
    • Veterinary prescribed pesticides to prevent infection and promote healing
  • If a screwworm case is suspected in a pet or human, it's important for doctors and veterinarians to handle discarded tissues (like used bandages or tissues from nose-blowing) with care, as fly eggs or larvae may be present and could develop if exposed to the environment. Proper disposal and sanitation protocols are essential to prevent potential spread, especially in clinical or animal care settings.
  • HHS Allows FDA Emergency Use of Animal Drugs to Combat New World Screwworm, Protect U.S. Food Supply - Aug. 19, 2025
  • Screwworm is considered a reportable disease in the U.S. Immediately report suspected cases to your state veterinarian, especially if there's any risk of exposure from travel or imported animals. Rapid response is critical to prevent the fly from becoming established.

Interventions for eradication

  • Sterile Insect Technique to be expanded to increase production and dispersal capacity. Same strategies now as before.
  • Robust Regulatory Controls to stop illegal cattle movement.
  • Field surveillance and stakeholder engagement for reporting and treatment. Basic surveillance, traps and lures to understand where the pest is.

Imported pets & rescue animals required documentation:

Adopting animals from outside the U.S. or bringing pets from abroad can pose a hidden risk of introducing screwworms and other exotic diseases. If you’re adopting or transporting a pet across borders, follow these requirements:

  • Veterinary inspection: Have your pet examined by a licensed veterinarian—preferably both in the country of origin and upon arrival in the U.S.
  • Health certificate: Ensure the animal has an official Certificate of Veterinary Inspection (CVI) or International Health Certificate within 5 days of import, documenting:
    • Proof of health
    • Required vaccinations (including rabies)
    • Verification Statement with the CVI that the animal is free of visible wounds or parasites
  • Quarantine required for 5 days after import: Some animals may need to be quarantined before or after arrival, depending on the country and species.
  • Monitor after arrival: Watch closely for signs of wounds, unusual behavior, or poor healing. If you notice anything suspicious, call your vet.
  • Remember that rescues in Washington import dogs from Mexico and you should not assume that they have domestic origins.

A brief history of screwworm in the U.S. 

  • Screwworm was once widespread across the southern U.S., devastating livestock populations.
  • Through the Sterile Insect Technique (SIT) billions of sterilized male flies were released to collapse breeding. This resulted in screwworm being eradicated from the U.S. by the 1980s.
  • It was later eradicated from Mexico, Central America, and parts of South America.
  • A 2016 outbreak in the Florida Keys was quickly contained in Key deer using the same technique.
  • In 2024–2025, new cases emerged in Southern Mexico and a single human case in Maryland, renewing concern among animal health officials.
  • June 18 2025 , the Five Prong Plan announced.
  • June 27, 2025, USDA hosted listening sessions to gather feedback on NWS technologies and tools.
  • August 15, 2025, USDA announced that there are plans for a new domestic production facility in TX producing up to 300 million flies weekly and up to $100 million for innovative technologies. 

Learn more: