Screwworm

Screwworms are parasites that can cause great damage to domestic livestock and other warmblooded

animals. The larvae of this pest enter open wounds

of the host animal and feed on the raw flesh. Rare

cases of humans being infested with screwworm

have been reported.

The United States has been free of screwworm

since 1966. The U.S. livestock industry could suffer

$750 million in production losses annually if this pest

were reintroduced to the United States.

The Pest and Its Life Cycle

The screwworm is actually an insect that, in its

adult stage, is about twice the size of the common

housefly. It has orange eyes and a blue-grey or grey

body with three dark stripes running down its back.

After mating, the female screwworm fly lays her

eggs in the open wounds of livestock and other mammals. One female can lay up to 400 eggs at a time

and as many as 2,800 eggs during its 31-day lifespan. These eggs can hatch into larvae in as little as

12 hours.

Screwworm larvae grow by feeding on the flesh

of living animals and can grow to be over one-half

inch within 5 to 7 days after hatching. The full-grown

larvae then drop from the wound and tunnel into the

soil where they form protective cases to house the

pupae. The adult screwworm flies emerge from the

pupal case and are ready to mate again within 3 to 5

days.

Clinical Signs of Screwworm Infestation

Animals are infested when the eggs hatch in the

wound of an animal and the larvae feed on the 

animal’s flesh. Wounds that may become infested by

screwworm include those caused by feeding ticks,

bites of vampire bats, castration, dehorning, branding, shearing, wire cuts, sore mouth in sheep, and

shedding of the velvet in deer. Navels of newborn

mammals are a common site for screwworm infestation. It is very difficult to see early stages of screwworm larvae feeding in a wound; only slight movement may be observed. As the larvae feed, the

wound is gradually enlarged, becoming wider and

deeper.

Usually at this stage additional screwworm flies

have deposited eggs, resulting in multiple infestations. A bloody discharge often exudes from the

infested wounds, and a distinct odor may be 

detected. In some cases, the openings in the skin

may be small with extensive pockets of screwworm

larvae beneath.

Animals with screwworm infestations may die in 7

to 14 days if wounds are not treated to kill the larvae,

especially in cases of multiple infestations. As many

as 3,000 larvae may be found in a single wound.

Death results from toxicity and/or secondary infection.

Infested animals usually exhibit discomfort, may

go off feed, and produce less milk. Typically, these

animals also will separate themselves from the rest

of the flock or herd and seek shady or secluded

areas to lie down.

Control

A screwworm infestation is treated with topical

application of approved chemicals to kill the larvae.

Wounds should be treated until the wound is completely healed. The larvae should be removed from

the wounds with tweezers.

Treating wounds and spraying or dipping animals

with an approved organophosphate insecticide will

provide protection against screwworm for 7 to 10

days. However, the most effective way to control

screwworm infestation is through eradication.

History of the U.S. Screwworm

Eradication Program

As early as 1825, U.S. Western States were

reporting serious problems with screwworm. By the

1930’s, screwworm had spread to the southeastern

States, where livestock producers were losing $400

million annually.

In the early 1950’s, the U.S. Department of

Agriculture’s (USDA) Agricultural Research Service

developed a new method to help eradicate the pest

using a form of biological control. During the pupal

stage of the fly’s life cycle, the pupae are subject to

gamma radiation designed to leave the fly sexually

sterile. The flies are artificially raised in a production

plant (originally located in Mission, TX) and then

released by aircraft over infested areas. When the

sterile males mate with females of the native fly populations, no offspring result. With fewer fertile mates

available in each succeeding generation, the fly, in

essence, breeds itself out of existence.

This sterile insect technique was successfully tested

in a field trial on the Dutch island of Curaçao in 1954

and then used operationally in Florida in 1957. By

1959, screwworm had been eradicated from the

entire southeastern United States. The technique

Factsheet

Veterinary Services February 2002

APHIS
was next applied to the more extensively infested

Southwest in 1962. Self-sustaining screwworm populations were eliminated from the United States by

1966. A barrier zone of sterile flies was established

along the 2,000-mile U.S.-Mexico border to prevent

reinfestation from Mexico.

However, constant reinfestation from migrating

flies or larvae carried by animals and then transported

by people, remained a problem. Therefore, in 1972,

the U.S.-Mexico Joint Commission for the Eradication

of Screwworm was formed with the goal of eliminating

the pest from Mexico and pushing the sterile fly barrier to the Isthmus of Tehuantepec, just north of

Guatemala. A new sterile screwworm plant at Tuxtla

Gutierrez, Chiapas, Mexico, was dedicated in 1976,

replacing the former production plant in Mission, TX,

which closed in January 1981. As a result of the

Commission’s efforts, Mexico was officially declared

free of the pest in 1991.

APHIS has also been cooperating with Central

America to eradicate screwworm from those countries

and ultimately to establish and maintain a permanent

sterile fly barrier at the Darien Gap between Panama

and Colombia. The Darien Gap is the narrowest geographical region in southern Panama, stretching only

102 miles long. The region is mainly jungle and has

no roads, so there is minimal risk of infested animals

being transported by land from South America into

Panama.

In addition, as part of the program’s overall strategy, a new sterile fly-rearing facility in Panama, to be

established around 2003, will replace the existing one

in Mexico. Locating the new facility in Panama, an

area where screwworm has not been eradicated, will

reduce the risk of reinfestation of the United States

through accidental release of fertile flies.

To date, Belize, Guatemala, El Salvador,

Honduras, Nicaragua, and Costa Rica have been

declared free of screwworm. Currently, screwworm

program officials are focusing their efforts on 

eradicating the pest from Panama. Eradication activities include regulation of cattle movement, treatment

of wounds, and release of sterile flies.

Threat to U.S. Cattle Producers

Although the screwworm program has been very

successful, limited outbreaks have occurred. In 1997,

screwworm larvae were detected on a dog shipped to

San Antonio, TX, from a military base in Panama.

Fortunately, a quick-thinking private veterinary practitioner detected and collected the larvae. State and

Federal animal health authorities immediately took

appropriate measures to track the dog’s movement

into the United States, disinfect sites, and prevent the

spread of the pest.

In March of 2000, a similar incident occurred

when a private practitioner found screwworm on a

horse shipped to West Palm Beach, FL, from

Argentina. Since the screwworm larvae were at least

24 hours from maturity when they were collected, it is

unlikely that any larvae dropped from the wound. Still,

the premises were thoroughly treated to ensure that

any larvae that might have exited the wound were

destroyed.

Suspicious Cases

These incidents remind us that screwworm is still

a real threat to U.S. livestock. Veterinarians and livestock owners who suspect an animal may be infested

with screwworm larvae should immediately contact

State or Federal animal health authorities.

For more information, contact

USDA, APHIS, Veterinary Services

Emergency Programs

4700 River Road, Unit 41

Riverdale, MD 20737–1231

Telephone: (301) 734–8073

Fax: (301) 734–7817

Current information on animal diseases and suspected outbreaks is also available on the Internet.

Point your Web browser to http://www.aphis.usda.gov

to reach the APHIS home page.

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