Featured Post from Louisiana State University

LSU Expert Christine Navarre on the Threat of New World Screwworms

Sep 3, 2025

3 min

Christine Navarre

The New World screwworm (NWS), also known as the primary screwworm, is the larvae of the fly Cochliomyia hominivorax. Unlike the larvae (maggots) of other flies that only feed on dead tissue, the NWS feeds on live tissue. This leads to more severe and potentially deadly consequences which threatens livestock and wildlife populations.


Prior to their eradication form the United States, NWS 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 NWS eradication. Other benefits of eradication and control are enhanced human and animal health and welfare and increased survival of endangered wild animal species.


The NWS 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 (COPEG) 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.


In 2016 NWS were 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 problem but this incident illustrates the importance of remaining vigilant.


In November, NWS was detected in Mexico near the Guatemala border. The USDA Animal and Plant Health Inspection Service (APHIS) has imposed immediate import restrictions on animal commodities from Mexico. They are also intensifying efforts to prevent the northward spread of NWS 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 NWS. Any suspected cases should be reported immediately.

Clinical Signs

NWS 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. 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. Animals infested with NWS may show the following signs:


  • Presence of maggots in wounds or body openings
  • Wounds with a foul odor, bloody drainage or white/cream-colored drainage (eggs)
  • Depression, reduced appetite, weight loss
  • Isolation and/or signs of discomfort, head shaking
  • Fever and other signs of secondary infection

Diagnosis and Reporting

Maggots found on animals showing the above clinical signs should be sent to a veterinarian or veterinary diagnostic lab for identification to distinguish NWS 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 very important to immediately report any NWS infestations to the Louisiana Department of Agriculture and Forestry. 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 with insecticides. Wound care is also important to speed healing and prevent reinfestation.


Prevention

Treatment of NWS can be difficult, and eradication is very costly, so prevention of infestations is essential. Adult NWS flies can travel up to 12 miles to lay eggs, and eggs can be transported by animals and people traveling from infested areas. 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 Wildlife and Fisheries.
  • Pay close attention to nasal passages and eyes for signs of larvae (maggot) infestation.
  • Seek veterinary advice for immediate treatment of open wounds, including dehorning and castration sites and preventive use of topical and systemic insecticides.
  • Review biosecurity plans with the farm or ranch veterinarian.

Original article by the LSU AgCenter here. 

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Christine Navarre

Christine Navarre

Professor & Extension Veterinarian, School of Animal Sciences

Dr. Navarre's clinical interests are in beef cattle preventive medicine.

Beef Cattle HealthFood-Animal WelfareVeterinaryAntimicrobial StewardshipLivestock Management and Production

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