(12/09/24) BATON ROUGE, La. — As many know, typical fly larvae, or maggots, only feed on dead tissue. But that is not the case of the New World screwworm larvae, which feed on live tissue. LSU AgCenter extension veterinarian Christine Navarre said this can lead to more severe and potentially deadly consequences, which threaten livestock and wildlife populations.

Also known as primary screwworms, the New World screwworms are the larvae of the fly Cochliomyia hominivorax. Navarre, who also serves as Louisiana Beef Quality Assurance Program state coordinator, has been tracking its movements in South and Central America for years. She said that prior to their eradication from the United States, New World screwworms were a major economic burden to the production of livestock, especially in the in the southwestern U.S. and Florida.

“The U.S. Department of Agriculture estimates that the U.S. livestock industry saves approximately $900 million a year as a result of New World screwworm eradication,” she said. “Other benefits of eradication and control are enhanced human and animal health and welfare, and increased survival of endangered wild animal species.”

According to Navarre, the New World screwworm fly was eradicated from the U.S. in 1966 with the release of sterile male flies to control the population. This status is maintained through the Panama-U.S. Commission for the Eradication and Prevention of the Cattle Borer Worm, which releases millions of sterile flies weekly along the Panama-Colombia border to create a barrier preventing the northward spread of screwworms. Due to these efforts, it is now found primarily in tropical areas of South America and some Caribbean Islands, including Cuba.

“Today, freedom from New World screwworm provides approximately $1 billion in direct benefits to livestock producers and approximately $3.7 billion in benefits to the general economy annually,” Navarre said.

But in 2016, the New World screwworm was found in Key deer in the Florida Keys. The source of the outbreak was never determined. Rapid recognition of the problem and response with the release of sterile flies quickly eradicated the issue — but this incident illustrated the importance of remaining vigilant.

More recently, in November, New World screwworms were detected in Mexico near the Guatemala border. The USDA Animal and Plant Health Inspection Service imposed immediate import restrictions on animal commodities from Mexico. The agency also is intensifying efforts to prevent the northward spread of New World screwworms by collaborating with Mexican and Central American authorities and urging livestock producers along the southern U.S. border to monitor their livestock and pets for signs of New World screwworms. Navarre emphasized that any suspected cases should be reported immediately.

“The northward march of this parasite is extremely concerning, as it could have devastating impacts, particularly on livestock and wildlife,” she said.

Clinical signs

Navarre said New World screwworms can infest any warm-blooded animal including livestock, pets, wildlife, birds and occasionally humans.

“Common sites of infestation are any fresh or old wounds, warts, tumors, tick bites and antlers in shedding,” she said. “Wounds left from management procedures, such as dehorning, branding, ear tagging, tail docking and shearing can become infested. The eyes, nose, vulva and prepuce are also vulnerable as well as the umbilicus in newborn mammals.”

Navarre said animals infested with New World screwworms may show the following signs:

— Presence of maggots in wounds or body openings

— Wounds with a foul odor, bloody drainage or white- or cream-colored drainage (eggs)

— Depression, reduced appetite, weight loss

— Isolation or signs of discomfort such as head shaking

— Fever and other signs of secondary infection

Diagnosis and reporting

According to Navarre, maggots found on animals showing the above clinical signs should be sent to a veterinarian or veterinary diagnostic lab for identification to distinguish New World screwworm larvae from other more common fly larvae. In Louisiana, larvae can be sent to the Louisiana Animal Disease Diagnostic Laboratory (www.lsu.edu/vetmed/laddl). Larvae should be placed in 70% alcohol for submission to the diagnostic laboratory.

“It is crucial to immediately report any New World screwworm infestations to the Louisiana Department of Agriculture and Forestry,” Navarre said. “A reported case will not result in herd depopulation but will allow animal health officials to take steps to help you manage your animals and prevent spread. Early detection and rapid response are critical to controlling this parasite.”

Treatment

Immediate veterinary care should be sought to remove larvae and properly treat them with insecticides. Wound care is also important to speed healing and prevent reinfestation, Navarre said.

Prevention

According to Navarre, treatment of New World screwworms can be difficult, and eradication is very costly, so prevention of infestations is essential.

“Adult New World screwworm flies can travel up to 125 miles to lay eggs, and eggs can be transported by animals and people traveling from infested areas,” she said. “This necessitates constant vigilance to ensure that reintroduction into the U.S. does not occur.”

Preventative steps include:

— Regularly inspect livestock and pets for cuts, wounds, scabs and tick infestations.

— Closely monitor the umbilicus of newborn livestock, vulva of females and prepuce of males.

— Use insect repellant and wound dressings to prevent fly strike.

— Report any unusual wildlife or bird deaths to the Louisiana Department of Wildlife and Fisheries.

— Pay close attention to nasal passages and eyes for signs of larvae (maggot) infestation.

Navarre said to seek veterinary advice for immediate treatment of open wounds, including dehorning and castration sites, and preventively use topical and systemic insecticides. Also review biosecurity plans with the farm or ranch veterinarian.

For additional information:

New World Screwworm

Screwworm | National Invasive Species Information Center

State-of-the-Art USDA Facilities Keep Invasive Pests Out of the Country | USDA

Introduction STOP Screwworms: Selections from the Screwworm Eradication Collection

Mexico Notifies United States of New World Screwworm Detection | Animal and Plant Health Inspection Service

The New World screwworm is the larvae of the fly Cochliomyia hominivorax. It was recently detected in Mexico near the Guatemala border and could have devastating impacts on livestock and wildlife in the United States, said Christine Navarre, LSU AgCenter extension veterinarian. Adobe Stock photo