When screwworm larvae were found on the prepuce of a four-year-old Thoroughbred gelding in Florida in March, it prompted a quick response.
A private practitioner in Florida examined the horse in West Palm Beach on March 2, discovered minor discharge from the prepuce but no swelling, and discerned that the lesion had a bad odor. Between 50 and 100 larvae were detected in the area of the distal portion of the penis. The practitioner called USDA-APHIS, which dispatched a foreign animal disease diagnostician. Samples taken from the lesion were submitted to the National Veterinary Services Laboratories on March 4.
The gelding was part of a shipment of 17 horses from Argentina that were imported through Miami on Feb 27. The destinations were Georgia (two horses), California (five horses), Pennsylvania (one horse), Texas (one horse), and Florida (eight horses). The other 16 horses were located, examined, and determined to be free of any larvae.
The USDA-APHIS area veterinarian in charge or the state veterinarian should be notified if existence of the parasite is suspected.
APHIS Veterinary Services has developed a draft protocol revising import procedures for horses from countries known to have screwworm. Call (800) 940-6524 or (301) 734-8073, or e-mail, emocusda [dot] gov.
Sentinel animals were placed around the West Palm Beach premises that housed the infested horse. The horse had been in a one-eighth acre paddock, isolated on three sides. The horse and paddock were treated March 3, and the horse received a second treatment March 6. It was released from quarantine March 15. Horses in adjacent paddocks were examined, and no wounds were discovered on those animals.
The National Veterinary Services Laboratories reported that the larvae in the infested horse were in the third instar stage, at least 24 hours from maturity. Mature larvae drop from the wound and pupate in the soil for approximately one week. Then it takes approximately one more week for the flies to become sexually active.
The screwworm is found in subtropical and tropical climates. In the 1930s there was a serious outbreak in the southern United States, with more than 1.3 million cases of infestation and the death of more than 200,000 animals. The United States has been free of screwworms since 1966.
In the early 1950s the USDA Agricultural Research Service developed a method of eradication using sterile flies to cause the species to breed itself out of existence. APHIS has worked with Central America by establishing and maintaining a permanent sterile fly barrier at the Darien Gap between Panama and Colombia. A sterile-fly rearing facility to be established in Panama in about three years will help to reduce the risk of reinfestation of the United States through accidental release of fertile flies.