New World Screwworm

A Serious Livestock Threat—But Not a Food-Safety Threat

New World screwworm is a dangerous parasite that can severely injure livestock, wildlife, pets, and people. However, it is important to understand what the parasite does—and what it does not do.

New World screwworm does not make beef unsafe to eat. It is not a foodborne illness, it does not infest meat, and the United States food supply remains safe. Animals with suspected infestations are isolated, treated, and kept out of the food supply. Only a small number of cases have been confirmed in the United States compared with the size of the national livestock population.

This page is part of the Wessel Ranch Education section, where we explain issues affecting cattle producers and consumers using information from government agencies, universities, veterinarians, and agricultural organizations.

What Is New World Screwworm?

New World screwworm is the larval stage of the fly Cochliomyia hominivorax.

Unlike most maggots, which primarily feed on dead or decaying material, New World screwworm larvae feed on the living tissue of warm-blooded animals.

The adult female fly is attracted to wounds, blood, drainage, and natural body openings. She lays eggs along the edge of a wound. After hatching, the larvae burrow into the tissue and feed together.

Without treatment, the wound can rapidly become deeper and larger. Severe infestations can cause infection, weakness, permanent injury, and death.

The Most Important Fact for Consumers

Beef Remains Safe

New World screwworm is an animal-health problem—not a food-safety problem.

The parasite attacks the living tissue of an animal. It does not infest packaged beef, spread through properly inspected meat, or make beef from healthy animals dangerous to eat.

Animals with suspected infestations are reported, isolated, examined, and treated. They are not simply sent into the food supply. Federal inspection and animal-health safeguards are designed to identify affected animals and prevent unsuitable products from entering commerce.

Consumers can continue eating beef with confidence. A limited number of screwworm cases does not mean the beef supply is contaminated.

How the Life Cycle Works

The life cycle begins when an adult female fly finds a wound or body opening on a warm-blooded animal.

1. The female lays a mass of eggs along the edge of the wound.

2. The eggs can hatch within approximately one day.

3. The larvae burrow into living tissue and feed for several days.

4. Mature larvae fall from the animal and enter the soil.

5. They pupate in the ground before emerging as adult flies.

6. The adult flies mate and begin the cycle again.

Under favorable warm conditions, the entire cycle may be completed in only a few weeks.

A single female can lay hundreds of eggs at a time, which is why quick detection and treatment are so important.

What Animals Can Be Affected?

New World screwworm can infest nearly any warm-blooded animal, including:

• Cattle

• Sheep and goats

• Horses

• Hogs

• Dogs and cats

• Deer, elk, and other wildlife

• Birds

• People

Most reported infestations occur in animals, particularly livestock, wildlife, and pets.

The parasite does not spread from animal to animal like a contagious virus. An adult fly must lay eggs on a wound or vulnerable body opening.

What Wounds Attract Screwworm Flies?

The wound does not have to be large. Even a small injury may attract an adult fly.

Possible infestation sites include:

• Newborn calf navels

• Calving injuries

• Castration wounds

• Dehorning wounds

• Branding sites

• Ear-tagging or ear-notching wounds

• Surgical incisions

• Wire cuts and scratches

• Dog bites

• Tick bites

• Foot injuries

• The eyes, ears, nose, mouth, or reproductive openings

Newborn animals may be especially vulnerable because the healing umbilical area can remain moist and exposed.

Signs of a Possible Infestation

Livestock owners should watch for:

• A wound that becomes larger instead of healing

• Bloody or watery drainage

• A foul odor

• Visible maggots or egg masses

• Unusual pain or sensitivity

• Frequent licking, biting, rubbing, or kicking at an area

• Head shaking when the ears are affected

• Reduced appetite

• Isolation from the herd

• Weakness or depression

• Larvae around the eyes, nose, mouth, ears, navel, or reproductive openings

A wound may appear small at the surface while extending deeper underneath the skin.

Maggots in a wound do not automatically prove that New World screwworm is present. Official identification normally requires examination of collected larvae by trained animal-health personnel.

What Should a Rancher Do?

A suspected infestation should be treated as an animal-health emergency.

The animal should be contained and separated from other animals, and a veterinarian or state animal-health official should be contacted immediately.

The animal should not be transported unless officials provide instructions to do so. Mature larvae could fall from the wound, enter the soil, and continue the life cycle.

In Montana, suspected cases should be reported to a veterinarian or the Montana Department of Livestock at:

406-444-2976

Quick reporting protects the affected animal, neighboring livestock operations, wildlife, and the broader cattle industry.

How Is an Infested Animal Treated?

Treatment may include:

• Removing larvae from the wound

• Thoroughly cleaning the affected tissue

• Removing damaged or dead tissue when necessary

• Applying or administering an authorized treatment

• Treating secondary infections

• Rechecking the wound

• Repeating treatment if eggs or larvae remain

• Keeping the animal isolated until the infestation is eliminated

Several animal drugs have received conditional approval or emergency authorization for preventing or treating New World screwworm in certain species and classes of livestock.

Treatment decisions should be made with a veterinarian and must follow the product label, including any age, species, milk, or slaughter restrictions.

How Can Ranchers Reduce the Risk?

Livestock owners can reduce the risk by:

• Checking newborn navels closely

• Treating injuries promptly

• Monitoring recent castration, branding, tagging, and dehorning sites

• Controlling ticks and other parasites that create wounds

• Removing sharp wire, broken equipment, and other injury hazards

• Inspecting newly purchased animals before mixing them with the herd

• Knowing where incoming animals originated

• Following livestock movement restrictions

• Reporting suspicious wounds immediately

During an outbreak, animal-health officials may recommend postponing elective procedures that create fresh wounds.

Routine inspection and fast reporting are more useful than panicking or unnecessarily treating every healthy animal.

How Was Screwworm Eradicated Before?

New World screwworm was once regularly found in parts of the United States.

Scientists developed a control method known as the sterile insect technique. Screwworm flies are raised in large facilities, sterilized using radiation, and released into affected areas.

Female screwworm flies generally mate only once. When a female mates with a sterile male, her eggs do not produce offspring.

By releasing very large numbers of sterile flies, officials can gradually reduce the wild population until local reproduction stops.

This technique helped eliminate New World screwworm from the United States during the 1960s and later pushed it south through Mexico and Central America.

The sterile insect technique is not gene editing. The released flies are sterile and do not create a permanent genetically modified population.

Why Has It Returned?

New World screwworm remained established in parts of South America after it was eradicated from the United States, Mexico, and much of Central America.

A long-term control zone near the Darién Gap between Panama and Colombia helped prevent the parasite from moving north.

Beginning in the early 2020s, cases increased in Panama and spread north through Central America and Mexico. The outbreak eventually reached the United States, where a calf in Texas became the first confirmed domestically acquired animal case of the current outbreak in June 2026.

Federal and state agencies responded with:

• Animal movement controls

• Surveillance and fly trapping

• Investigation of suspected cases

• Treatment and quarantine

• Sterile-fly releases

• Expanded sterile-fly production

• Public education and reporting programs

The confirmed U.S. cases represent a very small portion of the national livestock population, and officials are working aggressively to prevent wider establishment.

What About Montana?

Montana’s cold winters make long-term survival more difficult for New World screwworm than in warmer southern regions.

However, Montana is not completely protected. An infested animal could be transported into the state during warm weather, and temporary local reproduction could occur if larvae entered the soil.

Montana has established additional import requirements for certain animals originating from areas affected by or considered at risk for New World screwworm.

Livestock owners should confirm current import requirements with the Montana Department of Livestock before bringing animals into the state, particularly from Texas, New Mexico, or an officially designated infested area.

Does Screwworm Change the Safety of Beef?

No. Beef remains safe to eat.

New World screwworm is a serious threat to the health and welfare of individual animals, but it does not mean that beef on grocery-store shelves or sold by responsible cattle producers is unsafe.

Affected animals are quarantined, treated, and monitored. Animals and meat are subject to federal and state safeguards, including inspection before meat enters commerce.

Screwworm does not infest meat, and it is not transmitted by eating beef.

The presence of a small number of confirmed animal cases should encourage awareness and strong animal-health practices—not fear of the beef supply.

Questions and Concerns

New World screwworm raises legitimate concerns regarding:

• Animal health and welfare

• Livestock deaths and production losses

• Wildlife populations

• Treatment costs

• Ranch labor

• Livestock movement

• Trade restrictions

• Sterile-fly production

• Public understanding

• Consumer confidence

These concerns are serious, but they should be discussed using accurate information.

Calling screwworm a dangerous livestock parasite is accurate. Claiming that it makes the American beef supply unsafe is not.

Our Perspective

At Wessel Ranch, we believe cattle-health threats should be taken seriously without creating unnecessary fear.

New World screwworm can cause severe suffering and economic damage if it is not detected and treated quickly. Ranchers, veterinarians, government agencies, wildlife officials, and livestock owners all have an important role in stopping its spread.

At the same time, consumers deserve clear information:

New World screwworm does not make beef dangerous to eat. The food supply remains safe, affected animals are isolated and treated, and the number of confirmed U.S. cases remains limited compared with the size of the cattle industry.

Awareness, inspection, reporting, and coordinated control—not panic—are the right response.

Share Your Opinion

New World screwworm affects ranchers, veterinarians, livestock owners, wildlife managers, and consumers.

We would like to know how learning about the outbreak affects your opinion of beef and the cattle industry.

Take our anonymous New World Screwworm Survey below and share your perspective.

Sources

• United States Department of Agriculture Animal and Plant Health Inspection Service

• Screwworm.gov Unified Government Response

• Centers for Disease Control and Prevention

• United States Food and Drug Administration

• Montana Department of Livestock

• Texas Animal Health Commission

• World Organisation for Animal Health

Educational Notice

This page is intended for educational purposes. New World screwworm case numbers, affected areas, livestock movement rules, treatments, and government response measures may change.

Wessel Ranch strives to provide accurate and current information, but livestock owners should consult their veterinarian, the Montana Department of Livestock, USDA, FDA, and other appropriate authorities for the latest official guidance.

New World screwworm is a serious animal-health threat, but it does not make properly inspected beef unsafe to eat.