APHIS Factsheet

Veterinary Services May 2014

New World Screwworm

New World screwworms are fly larvae (maggots) 

that can infest livestock and other warmblooded 

animals, including people. They most often enter 

an animal through an open wound and feed on the 

animal’s living flesh. If not treated, infestations can 

be fatal. While New World screwworm (Cochliomyia 

hominivorax) has not been widely present in the United 

States since the 1960s, it is still found in most of South 

America and in five Caribbean countries. If this pest 

became established in the United States again, it could 

cause more than $1 billion in losses for our country’s 

livestock industry. 

The U.S. Department of Agriculture (USDA) works 

diligently to protect the health of our Nation’s livestock 

and keep New World screwworm from reentering the 

United States. You can help support these efforts by 

observing your animals and reporting any signs of a 

screwworm infestation to State or Federal animal 

health officials.

Description

The adult screwworm fly is about the size of a common 

housefly, or slightly larger, but different in color and 

appearance. The screwworm fly has orange eyes and 

a metallic dark blue to blue-green or grey body. It also 

has three dark stripes running down its back, with the 

middle stripe shorter than the outer two.

A female screwworm fly typically mates once in her 

lifetime and lays her eggs on or near an open wound 

or the mucous membranes of an animal’s nose, mouth, 

or ears. In her lifespan, the screwworm fly can produce 

thousands of offspring. The eggs hatch into larvae 

within a day and then feed on the animal’s tissue for 

5 to 7 days before maturing. The mature larvae then 

tunnel into the ground and emerge as adults, ready to 

mate and continue the cycle.

While they can fly much farther under ideal conditions, 

adult flies generally do not travel more than a couple 

of miles if there are suitable host animals in the area. 

New World screwworm is more likely to spread long 

distances when infested animals move to new areas 

and carry the pest there. 

Signs To Look For

Screwworms are typically found on warmblooded 

animals that have a draining or enlarging wound. 

Almost any type of wound can become infested 

with screwworm larvae, including those caused by 

feeding ticks, castration, dehorning, branding, shearing, 

barbed wire fences, and even shedding of antler velvet 

in deer. The navels of newborn mammals can also 

become infested.

It can be very difficult to see the early stages of screwworm 

larvae feeding in a wound. The most obvious sign is a 

change in the wound’s appearance—as larvae feed, 

the wound gradually enlarges and deepens. An infested 

wound also gives off an odor and some bloody discharge. 

Even if the actual wound on the skin is small, it could 

still have extensive pockets of screwworm larvae 

beneath it.

Infested livestock usually show signs of discomfort, 

and they may go off their feed and produce less milk. 

Typically, these animals will separate themselves from 

the rest of the flock or herd and seek shady or secluded 

areas to lie down. Infested animals that are not treated 

in 7 to 14 days may die. 

While human cases of New World screwworm are rare, 

they have occurred. People infested with screwworm 

usually have discomfort or itching at the wound site. If 

it affects the eyes, mouth, sinuses, or lungs, infestation 

may be incapacitating.

Treatment

Screwworm infestation in livestock is treated by topical 

application of a pesticide or by a pesticide dip to kill 

the larvae. Treated wounds should be inspected daily 

until completely healed. The examining veterinarian will 

remove larvae from the wound with tweezers, place 

them in alcohol, and submit them to USDA’s National 

Veterinary Services Laboratories (NVSL) for positive 

identification.

History of USDA’s Screwworm 

Eradication Program

The New World screwworm likely originated in South 

America and moved into North America by the early 

1800s. In the 1930s, screwworms were present in the 

Southeastern United States, causing livestock producers 

to lose millions of dollars annually.
In the 1950s, USDA’s Agricultural Research Service 

developed a new method to help eradicate the pest 

using a form of biological control, called the sterile 

insect technique. Infertile male flies are released in 

infested areas. When they mate with local females, 

no offspring result. With fewer fertile mates available 

in each succeeding generation, the fly, in essence, 

breeds itself out of existence.

USDA began using this technique in Florida in 1957 

and eradicated the flies from the entire Southeastern 

United States by 1959. The technique was next 

applied to the more extensively infested Southwest in 

1962. By 1966, self-sustaining screwworm populations 

were eliminated from the United States. However, there 

were still infestations in Mexico, and animals crossing 

the border were causing some reinfestations in the 

United States. USDA worked with Mexican officials 

to reduce the flies in that country. After 1982, there 

were no more reinfestations in the United States, and 

Mexico was officially declared free of screwworms 

in 1991. USDA then partnered with the countries of 

Central America to eradicate New World screwworms. 

Today, USDA and its partners maintain a permanent 

sterile fly barrier at the Darien Gap between Panama 

and Colombia to prevent the establishment of any 

screwworm flies that enter from South America. 

Keeping Screwworms Out 

of the United States

Keeping New World screwworm out of the United 

States is an ongoing process. In September 2007, 

screwworm larvae were found on a dog that had 

recently entered the United States from Trinidad and 

Tobago. An alert private veterinarian detected the larvae 

and submitted them to the NVSL for identification. State 

and Federal animal health authorities immediately took 

measures to track the dog’s movements from Florida 

to Mississippi, disinfect sites where the dog spent time, 

and prevent any spread of the pest.

In May 2010, a similar incident occurred when a private 

veterinarian found screwworm larvae on a dog that 

traveled from Venezuela to Florida. The Florida 

location was thoroughly treated to destroy any larvae 

that may have exited the wound.

In both cases, awareness by animal owners and 

veterinary practitioners was critical in preventing 

a damaging screwworm outbreak in our country. 

Cooperation among animal owners, veterinarians, 

and animal and human health officials continues to 

be fundamental in our work to protect the United 

States from a screwworm infestation. 

Report Suspicious Cases

If you suspect an animal may be infested with 

screwworm larvae, immediately contact State or 

Federal animal health authorities.

Learn More

For more information on New World screwworm and 

other animal pests and diseases, visit USDA’s Animal 

and Plant Health Inspection Service (APHIS) Web site 

at www.aphis.usda.gov. Click on “Animal Health” and 

then “Animal Disease Information.”

Or, contact us at:

Veterinary Services

USDA/APHIS

4700 River Road, Unit 41

Riverdale, MD 20737-1231

Phone: (301) 851-3595

Fax: (301) 734-7817

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

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