Screwworm Scare

Cattle Producers on Alert for Parasitic Screwworms

Thought eradicated for 30 years, screwworms have been found in deer in Florida. Cattle producers are urged to be vigilant as the situation develops. (DTN/Progressive Farmer photo by Becky Mills)

Today's younger generation of cattle producers may be nonchalant about reports of parasitic screwworms in deer in Florida. But Phil Kaufman, professor with the University of Florida's entomology and nematology department, underscores how dangerous the situation could become if the flies continue to spread.

Larvae from the parasitic screwworm fly (Cochliomiya hominivorax) must consume the tissue of a live warm-blooded animal to develop. Adult females lay their eggs on livestock and wildlife with superficial wounds, explained Kaufman, a veterinarian entomologist.

"From a strictly scientific point of view, screwworm larvae are incredibly well-adapted parasites. This species was a constant menace to Florida's cattle industry up through about 1960, when it was eradicated from the state," he said.

Commonly known as the primary screwworm fly or New World screwworm fly, the insect threatens the health of warm-blooded animals and people in areas where it is well-established, said Jack Payne, UF senior vice president for agriculture and natural resources.

"To put it plainly, a full-blown screwworm infestation is a death sentence for the host animal," Payne said. "This pest can kill a previously healthy cow or bull in a matter of weeks if the problem isn't treated properly. It's that serious."

Payne urges all livestock and pet owners to educate themselves about the symptoms of screwworm infestation and seek veterinary care for animals exhibiting tell-tale indications such as open wounds that do not heal, running sores, listlessness, loss of appetite or sudden weight loss.

Beginning in the late 1950s, federal and state officials combatted the fly by releasing sterile male specimens. Because the female flies mate only once, a female that mates with a sterile male is unable to produce live offspring for the remainder of her lifetime. This practice, known as "sterile insect technique," can rapidly drive down local populations when used repeatedly. State and federal officials plan to use sterile insect technique to quell the current outbreak.

Adult female C. hominivorax flies are attracted by airborne chemicals associated with open wounds, so they often zero in on animals that have sustained minor cuts or scrapes, or have undergone procedures such as castration or dehorning. Similarly, the fly may infect newborn animals with healing wounds where the umbilical cord was severed.

After emerging from eggs, screwworm larvae burrow head-first into nearby tissue, anchoring themselves with a pair of hook-like projections and breathing through two openings called spiracles located in their posterior end. On average, the larvae feed five to seven days before emerging from the host's body, dropping to the ground and burrowing into leaf litter or soil to pupate and emerge as adults, which are typically metallic bluish-green, about 10 millimeters long, and have the same general structure as a house fly.

"In a worst-case scenario, New World screwworm larvae can kill a host animal within a couple of weeks' time," Kaufman said. "The hosts often succumb to secondary infections that set in because their immune systems have been weakened by the screwworm infestation."

Severe infestations where massive tissue damage has occurred may require that the host animal be euthanized, he said, but when cases are detected early and the host receives appropriate veterinary care, chances for recovery are excellent.