screwworm_myiasis

Last Updated: January 2006

SCRW_F0106

Screwworm

What is screwworm and

what causes it?

Screwworms are the larvae (mag-

gots) of a certain ﬂ y species that feed

on living tissue of animals. Although

these ﬂ ies have been eradicated from

the United States, Mexico and Cen-

tral America, they can still be found

in some countries of South America,

the Caribbean, Southeast Asia, India

and Africa.

One of the concerns with this para-

site is that it will be reintroduced into

the United States from an infested

animal. The larvae cause great dam-

age to the hides of animals as well as

have the potential to cause death in

aﬀ ected animals.

What animals get screwworm?

Screwworms can infest all warm

blooded animals, including birds.

Most cases occur in cattle, goats and

sheep.

How can my animal get

screwworm?

Screwworms (vector) are transmit-

ted when a female ﬂ y lays her eggs

on a superﬁ cial wound of an animal.

Screwworms can infest a wide variety

of wounds (e.g. tick bites, dehorn-

ing or branding wounds, wire cuts,

and other injuries). Navels of new-

born mammals are a common site

for screwworm infestation. Wounds

infested by screwworms often attract

other female screwworms and multi-

ple infestations are common. Screw-

worm ﬂ ies can travel from over 12

miles.

How does screwworm affect

my animal?

The screwworm eggs laid in the

wound hatch into larval screwworm,

which feed on the living tissue of the

animal. Screwworm infestations are

diﬃ cult to detect at ﬁ rst. Slight move-

ment might be seen inside a wound,

but the larvae feed and the wound

gradually enlarges and deepens.

A blood tinged ﬂ uid with a distinct

odor may seep from the wound. As

many as 200 vertically oriented lar-

vae may be packed deep inside the

wound.

Infested animals usually separate

from the herd and lie down in shady

areas. Discomfort, decreased appe-

tite and lowered milk production are

common. Untreated animals may die

in 7 to 14 days from toxicity or sec-

ondary infections.

Can I get screwworm?

Yes. Humans can get screwworm

(vector) in the same manner as ani-

mals – by ﬂ ies depositing larvae on

an open wound.

Who should I contact, if I

suspect screwworm?

In Animals – Contact your

veterinarian immediately.

Screwworms are not currently

found in the U.S.; suspicion of disease

requires immediate attention.

In Humans – Contact your

physician immediately.

How can I protect my

animals from screwworm?

Animals imported from countries

where the screwworm ﬂ ies exist

must be carefully examined by vet-

erinarians upon entry into the United

States. Wounds should be closely

checked for any screwworm eggs or

larvae that could be there.

Observe your animal for signs or

the presence of screwworms. Screw-

worm eggs are creamy and white and

deposited in a shingle-like manner

on or near the edges of superﬁ cial

wounds. The larvae are cylindrical with

one pointed end and one blunt end

and have rings of dark brown spines

around the body. Female screwworm

ﬂ ies are larger than a houseﬂ y with

a dark blue to blue-green body and

reddish-orange head.

How can I protect myself

from screwworm?

While traveling in countries that

have screwworms, pay careful atten-

tion to any wounds to help protect

yourself from screwworm infestation.

For More Information

CFSPH Technical Fact Sheets. Screw-

worm Myiasis at http://www.cfsph.

iastate.edu/DiseaseInfo/

Screwworms

are speciﬁ c ﬂ y larvae

that can damage the hides

and skin of livestock.

Photos show the screwworm ﬂ y (top) and its

various lifee stages (bottom) from North Carolina

State University and Dr. Sanderson.