Watch Out For Screwworm

The U.S. Department of Agriculture's Animal and Plant Health Inspection Service is alerting veterinarians and others involved in the livestock industry in certain areas of the United States to be on the lookout for screwworm.

On February 27, 17 horses were imported into the United States from Argentina through APHIS' animal quarantine center in Miami. Two of the horses were shipped to Georgia, five to California, one to Pennsylvania, one to Texas, and eight to Florida.

"We are asking veterinarians in private practice in those states where the horses were shipped to be on the lookout," said Alfonso Torres, deputy administrator for APHIS' veterinary services program, a part of USDA's marketing and regulatory programs. Suspect cases should be reported to the USDA/APHIS area veterinarian in charge or to the state veterinarian.

On March 2, a private practitioner found one of the horses in West Palm Beach, Florida, to have screwworm larvae. The National Veterinary Services Laboratories in Ames, Iowa, confirmed that the larvae were screwworms on March 4. The horse and the premises were treated March 3; the horse received a second treatment March 6.

NVSL reported that the screwworm larvae were at least 24 hours from maturity when they were collected on March 2; thus it is unlikely that any larvae dropped from the wound on or before March 2. The premises were thoroughly treated March 3 to ensure that any larvae that might have exited the wound were killed.

Veterinary services conducted foreign animal disease investigations of the other horses in this shipment and found that only the one horse in Florida had been infested. Veterinary services is conducting intensive surveillance in the West Palm Beach area.

Screwworm is the common name of a pest native to the tropical areas of North, South, and Central America that causes extensive damage to domestic livestock and other warm blooded animals. The larvae of these pests feed on the raw flesh of the host animal.

Animals are infested when the screwworm eggs hatch in the wound of an animal and the larvae feed on the animal's flesh. Practitioners should be on the lookout for:

  • Wounds that may become infested with maggots. Wounds commonly infested include those caused by feeding ticks, 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. In cases, the openings in the skin may be small with extensive pockets of screwworm larvae beneath.
  • Bloody discharge from the infested wounds
  • Malodor
  • Discomfort
  • Decreased feed intake
  • Decreased milk production
  • Seclusion from rest of herd or flock. Animals may seek shady or secluded areas to lie down.

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. The U.S. livestock industry could suffer $750 million in production losses annually if this pest were reintroduced to the United States.

Current information on animal diseases and suspected outbreaks is also available on the Internet. Point your Web browser to to reach the APHIS home page. A factsheet about screwworm is available at

About the Author

Stephanie L. Church, Editor-in-Chief

Stephanie L. Church, Editor-in-Chief, received a B.A. in Journalism and Equestrian Studies from Averett College in Danville, Virginia. A Pony Club and 4-H graduate, her background is in eventing, and she is schooling her recently retired Thoroughbred racehorse, Happy, toward a career in that discipline. She also enjoys traveling, photography, cycling, and cooking in her free time.

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