NEW WORLD 

SCREWWORM:

What

You 

Need 

To Know

Animal and Plant Health Inspection Service

Program Aid No. 2222

HOW TO SPOT AN INFESTATION

Infested deer with 

extensive tissue 

damage

Fly with egg 

mass on wound

Close-up 

female fly, eggs, 

and maggots 

on wound

Severe 

screwworm 

myiasis on 

dog’s neck

NWS flies 

attracted to an 

animal wound

• Any warm-blooded animal (including

birds and humans) with maggots in

wounds or other body openings (like

nose, ears, umbilicus, or genitalia)

that are draining or enlarging.

• Animals that may have recently

suffered from a wound or surgical

procedure. Wounds as small as tick

bites attract flies.

• Egg masses may be around or in

the wound; larvae may be visible

by the third day of infestation.

• Because they feed on live flesh,

NWS maggots may burrow deep

into wounds or openings, while

other species of maggots may

appear around the outer surface

of the wound.

• Screwworm infestations are very

painful. Animals may become

depressed, stop eating, and

separate themselves from other

animals or people.

• Secondary infection may occur

in an NWS-infested wound. Left

untreated, animals may die within

1 week of being infested.

NWS INFESTATION:

What To Look For

NWS FLIES AND MAGGOTS:

What They Look Like

The adult screwworm 

fly is about the size of 

a common housefly 

(or slightly larger), 

with orange eyes, 

a metallic blue or 

green body, and 

three dark stripes 

along its back. The 

name screwworm is 

thought to refer to the 

feeding behavior of 

the maggots as they 

burrow (screw) into 

the wound, feeding 

as they go like a 

screw being driven 

into wood. Maggots 

(larvae) cause 

extensive damage by 

tearing at the hosts’ 

tissue with sharp 

mouth hooks. The 

wound can quickly 

become enlarged and 

deepened as more 

maggots hatch and 

feed on living tissue. 

Mature larvae

Screwworm pupae

Magnified mature larvae

Front cover: (top) Screwworm larva with flesh-tearing mouth hooks; (bottom) sterile fly

Photo credits: Images in “NWS Flies and Maggots” are by COPEG. The third image in 

“How To Spot an Infestation” is by Winthorpe Marsden, Jamaican Ministry of Agriculture; 

the second and fourth images are by Samantha Gibbs, U.S. Fish and Wildlife Service. 

All other images in this brochure are by USDA employees.

USDA is an equal opportunity provider, employer, and lender.

Issued January 2018

REPORT IT

Immediately report any suspicious wounds, maggots, 

or infestations to a local accredited veterinarian, your State 

Animal Health Official, or a USDA veterinarian. 

Anyone may report suspected cases of screwworm.

While most reports will likely come through veterinarians 

and diagnostic laboratories, we need everyone to be on the 

lookout. This disease is serious and can have catastrophic 

impacts on U.S. agriculture and animal and public health. 

Catching it early and eliminating it quickly is vital. 

If you hear of or see 

anything you think 

could be NWS, 

we want to know 

about it. Report it

right away to your 

local accredited 

veterinarian, State 

Animal Health 

Official, or a USDA 

veterinarian.

CONTACTS

• STATE ANIMAL HEALTH OFFICIALS

www.usaha.org/upload/Federal%20and%20State%

20Health/STATE_ANIMAL_HEALTH_OFFICIALS.pdf

• USDA-APHIS, VETERINARY SERVICES

www.aphis.usda.gov/aphis/ourfocus/animalhealth/

contact-us/sa_area_offices/vs-area-offices

ATTENTION 

ACCREDITED 

VETERINARIANS!

You are legally required to report 

all diagnosed or suspected cases 

of a communicable disease to 

USDA and your State Animal 

Health Official. This requirement 

applies to any disease for which 

USDA has a control or eradication 

program in place and for any 

animal diseases not known to 

exist in the United States. (See 

9 CFR 161.4 f, h for more detail.) 

Report any suspected NWS cases to your 

State Animal Health Official or a USDA veterinarian!

United States Department of Agriculture

244-845_USDA.indd 1 2/8/18 

To learn more about NWS, go to:

www.aphis.usda.gov/aphis/ourfocus/animalhealth/

animal-disease-information/cattle-diseaseinformation/nws/new-world-screwworm
Sterilized NWS pupae released in infested area

New World Screwworm (NWS) 

myiasis is a serious disease

that can affect livestock, pets, wildlife,

and in rare cases, people.

It is a painful condition in which the larvae (maggots)

of the NWS fly (Cochliomyia hominivorax) burrow into 

the flesh of a living animal. This causes serious, often 

deadly damage to the animal. The economic impact 

of NWS is also severe. During the 20th century, the 

presence of NWS cost the U.S. livestock industry 

more than $100 million annually.1 

The flies spend most of their time in forests and 

other wooded areas, but will also seek host 

animals in pastures and fields if available. Open 

wounds attract the flies and are what they feed 

on and live in to survive. 

The U.S. Department of Agriculture (USDA) 

needs your help to keep NWS out of our 

country. Read on to learn more about NWS, 

what an infestation looks like, and where to 

report possible cases of it. Together, we 

can protect the United States from this 

horrific disease. 

INFESTATION:

What Happens?

Screwworm infestations begin when a female fly lays 

eggs on a wound or orifice of a live warm-blooded 

animal. Female flies are attracted to the odor of a wound 

or opening such as the nasal or eye openings, umbilicus 

of a newborn, or genitalia. Wounds as small as a tick 

bite may attract a female to feed. One female can lay 

200–300 eggs at a time and may lay up to 3,000 eggs 

during her 10- to 30-day lifespan.

Eggs hatch into larvae that burrow into the wound to 

feed on the living flesh. After about 7 days of feeding, 

larvae drop to the ground, burrow into the soil, and 

pupate. The adult screwworm fly emerges from the 

soil after 7–54 days depending on temperature and 

humidity. Female flies mate after 3 days, and males can 

mate within 24 hours of maturation. 

STERILE INSECT TECHNIQUE:

How Does It Work?

To eradicate NWS, sterilized pupae 

may be placed in chambers at strategic 

locations throughout an infested area. 

Sterile flies may also be dispersed from 

aircraft over larger areas. As male flies 

emerge from the chambers, they seek 

out mates. Because female screwworm 

flies mate just once in their 30-day 

lifespan, the only eggs she will lay are 

not viable and will not develop into 

maggots. The population ultimately 

dies out as more sterile screwworm 

flies are released. The population of 

fertile screwworm flies dies off naturally 

over a few lifecycles. 

In October 2016, USDA and the 

Florida Department of Agriculture and Consumer Services 

announced the detection of NWS. It was successfully 

eradicated by March 2017 using this same method to 

eliminate screwworm from the United States once again. 

This was the first local infestation in the United States 

in more than 30 years and the first infestation in 

Florida in over 50 years.

NWS: What’s the Impact?

Another incursion into the United States 

could cost millions of dollars from livestock 

losses, trade embargoes, and eradication 

work. Pets, livestock, wildlife, and 

even humans may suffer and die 

from screwworm myiasis. 

1

 Novy, J.E. 1991. Screwworm control and eradication in the Southern 

United States of America. Special Issue of World Animal Review FAO, 

pp. 18–27. www.fao.org/docrep/U4220T/u4220T0a.htm

Sterile fly

History of Eradication

An eradication program to remove NWS from the United States 

began in 1957. It used a biological control technique (sterilized 

insects) developed by USDA’s Agricultural Research Service. This 

method is an ecologically safe and proven way to eradicate NWS 

populations by taking advantage of the fly’s own biology. The ste

insect approach eradicated NWS from the United States in 1966

In a cooperative program, the Panama-United States Commissio

for the Eradication and Prevention of Screwworm (COPEG) 

maintains a permanent sterile fly barrier along the border of Pana

and Colombia to prevent the re-establishment of screwworms.

Credit: Illustrations by USDA, Animal and Plant Health Inspection Service; text adapted 

from pp. 218–219 in Atlas of Transboundary Animal Diseases by Peter J. Fernández and 

William R. White (Paris: World Organization for Animal Health, 2010). 

History of the Screwworm Eradication Program

Credit: Esri, HERE

Adult screwworm fly 

is attracted to smell 

of an open wound and 

lays her eggs on the 

edges of it.

After at least 

7 days, an adult 

fly emerges.

Within a few hours, the 

eggs hatch into larvae, 

which burrow into the 

wound to feed. This 

worsens the wound and 

attracts more flies, which 

lay more eggs.

The larvae feed for 

about a week and then 

fall off and burrow into 

the ground to pupate.

Life 

Cycle 

of 

NWS

> 136 wildlife cases

> 15% of endangered Key Deer died from screwworm infestation

> 9 domestic animal cases

> Over 17,000 animals inspected at checkpoint leaving surveillance zone

> More than 188 million sterilized pupae placed in 35 sites over 6 months

> Approximately $3.2 million in taxpayer dollars spent on eradication efforts

2016–2017 

NWS Outbreak: 

By the Numbers

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