New World Screwworm

Preventing an infestation of NWS is key. Treatment can be difficult, and eradication is expensive. NWS are attracted to the open wounds and mucus membranes where they lay their eggs, and they can be carried by both domestic and wild animals. We recommend taking steps to quickly identify and treat open wounds and keeping wildlife away from your livestock to the extent possible.

• Frequently inspect livestock and pay close attention to tick bites, cuts, and dehorning and/or castration sites. Immediately treat any open wounds, scratches, or scabs. Screwworms in wounds can be killed by application of effective treatments.
• Closely monitor newborn livestock and mothers. NWS often lay eggs on the navel areas of newborn livestock, and the vulva and perineum of female. On males, monitor the sheath/prepuce as well.
• Pay close attention to nasal passages and eyes for signs of larvae (maggot) infestation.
• Minimize access of wild animals (birds, feral hogs, etc.) to livestock to the greatest extent possible.
• Practice good biosecurity measures like frequently cleaning of clothing, boots, vehicles, and equipment on your farm or ranch.

Registering your farm with a premises ID is the first step in a biosecurity plan for your farm and will ensure state animal health officials can alert you if NWS is detected in your area.

If you suspect NWS, contact your veterinarian who will work with the Missouri Department of Agriculture.

Meat and poultry products are safe from New World Screwworm. If animals affected by New World screwworm (NWS) enter the food supply, your food is still safe. NWS is not transmitted through meat or poultry products. Animals used for human food production must pass inspection before slaughter and after slaughter to ensure food safety and humane handling requirements are met. An infestation or animal illness that makes meat unsafe for consumers will prevent the animal from entering the food supply. In the case of NWS, animals must be released from quarantine by USDA’s Animal and Plant Health Inspection Service (APHIS) before they are slaughtered, and the animals must meet food safety requirements enforced by USDA’s Food Safety and Inspection Service (FSIS). If the infestation is too severe or if the animal has other signs of harmful disease, it will not be allowed into food production. Animals that are given medications, chemicals, or topical treatments to treat NWS are held for testing, and FSIS will remove them from food production if it is determined that animals are not fit for human food because of the presence of any residues.

For more information visit:

• Are meat and poultry products safe from New World screwworm (NWS)?
• Slaughter of Livestock Affected with New World Screwworm

The risk of NWS to pets is minimal in the United States. The best way to prevent NWS is to pay close attention to your pet and seek care from a veterinarian if you notice symptoms.

Wounds as small as a tick bite can be infested with NWS, and they are more likely to be around the face and genitals.

Dogs entering the U.S. from a screwworm-affected country (which includes many countries in South America, Central America, and Mexico) must be inspected for screwworm within five days prior to entry to the U.S. and be screwworm-free or be held in quarantine until treated and screwworm-free. For more information, see Bring a Pet Dog into the United States.

• A Threat to Wildlife: Information for Hunters

For more information visit Missouri Department of Conservation.

While rare, New World Screwworm can affect humans. People who suspect they are infested with NWS should seek immediate medical treatment following the Centers for Disease Control and Prevention (CDC) guidelines. For information on how screwworms affect people or to report human disease, visit CDC at New World Screwworm Myiasis. For healthcare providers, visit Clinical Overview of New World Screwworm Myiasis. For all questions and concerns related to New World Screwworm and human health, contact the Missouri Department of Health and Senior Services at (573) 751-6113.

Historically, the screwworm was a big problem in several areas of the southern U.S. prior to its eradication in the 1960s. Effective eradication in the U.S. was accomplished in 1966, and in 1967, U.S. cases fell to less than 1,000, a 99% reduction. Incursions by the pest continued for several years, prompting a multinational coordinated eradication effort in the late 1960s to eradicate the fly to southern Mexico and later to the Darien Gap in Panama. North America has been considered screwworm-free since eradication, but active infestations occur in Jamaica, Cuba and across South America. In 2016, an outbreak was reported on Big Pine Key in southern Florida. It caused significant mortality in male Key deer, an endangered species. Eradication was achieved the following year, but not until substantial loss was suffered in the native deer population and at great financial cost. Until 2023, the Darien Gap quarantine line held with occasional incursions into areas west of the Panama Canal, about 25 occurrences per year. Since 2023, the New World Screwworm has been reestablished north of the Panama Canal. For information on the current outbreak visit New World Screwworm Outbreak in Central America.

This is not just a cattle problem. NWS can impact any warm-blooded mammals including swine, horses, sheep, goats, dogs and wildlife such as deer and birds.

Eradication is accomplished through surveillance, host treatment and quarantine, but the core to the successful eradication is using the sterile insect technique. The fly is produced in massive numbers, and during the pupal stages, the flies are subjected to a specific irradiation exposure, damaging their chromosomes and making them sterile.

This technique relies on sterilizing the flies without damaging their ability to fly, finding a nonsterile wild female and being accepted for matting. Female flies mate only once in their lifespan.

The sterilized males are released either through targeted ground operations or widespread aerial dispersal in massive numbers at regular intervals to overwhelm the wild female population with sterile males and suppress matings with wild males.

Initial eradication efforts in the southwest in the 1960s began with a 50-mile barrier zone at the Mexican border by releasing 200 sterile males per square mile per week.

Typical insecticide use on the New World screwworm in not as effective as with other insect pests due to its wide host range and occurrence on wildlife. Eradication is the best way to control screwworms. However, control/removal of the fly when found on hosts is crucial to reducing the local population so that the sterile fly program succeeds more quickly and reduces animal suffering.

Historically, ranchers have altered breeding dates to avoid birthing during the fly season. Common livestock management practices and events, such as castration, dehorning, branding and birthing in all animals and antler velvet shedding in deer, often results in infestations. Infestations can occur at the site of any recent wound, like a scrape, lesion or even a tick-feeding site.

Other wounds of common infestation include those from ear tagging or shearing, as well as mucous membranes and antler shedding in deer. Newborn mammals are prone to infestations at the healing umbilical cord site.

Early stages of infestation can be difficult to detect. Key signs of screwworm infestations can include wounds that appear not to be healing or have excessive drainage or odor. Animals may exhibit:

• Bloody or light-colored drainage from a cut or wound.
• A cut or wound that rapidly enlarges for seemingly no reason.
• White or cream-colored runny substance (the eggs) in and around a wound.
• Presence of fly larvae (maggots) in wounds.
• Irritated behavior and signs of pain.
• Head shaking.
• Reduced appetite.
• Fever and other signs of secondary infection.
• Foul odor or smell of decay.

If a wound on a living animal is found to have larvae in it, a veterinarian must be contacted. Find an accredited veterinarian or contact the Missouri Department of Agriculture at Animal.Health@mda.mo.gov or (573) 751-3377. The veterinarian is mandated by law to collect and submit the larvae for identification to the USDA’s National Veterinary Services Laboratory (NVSL).

Also, the animal should be immediately confined for wound care, and all larvae should be removed and placed into alcohol. After removing the larvae, treat the infestation with topical pesticides labeled for use on the animal to kill any larvae that may not have been removed and reduce the likelihood of re-infestation. The wound should be monitored until it fully heals. When dealing with livestock, all other animals in the herd should be checked for wounds and larvae.

Anyone who finds fly larvae infesting a living animal, called myiasis, must report this to the Missouri State Veterinarian at Animal.Health@mda.mo.gov or (573) 751-3377.