Screwworms: when the larvae don't care about boundaries

Maggots. The very word invokes revulsion in many people. For veterinary professionals, the term conjures sights, smells, and, yes, sounds. The munching of maggots crawling inside a patient echoes in our memories.

When a maggot case arrives in a hospital, some professionals move toward the patient, and others find reasons to walk in the opposite direction. Myiasis, or fly-strike, is common in debilitated animals with infected or necrotic tissue.

Beyond manual removal, it is common to ensure the eradication of these distasteful dipterans from our patients with medications such as nitenpyram. An alternative is any of the isoxazoline drugs. The onsets of action for nitenpyram and isoxazolines are similar, with the obvious benefit of extended duration with isoxazolines of a month or more.¹

If you are wondering which to use based on label use, both choices have extralabel benefits with on-label dosing. If you have not heard of this, don’t be hard on your pharmaceutical representatives for not telling you—they can’t. The FDA has strict rules against extralabel promotion.^1,2

Pharmaceutical technical team members can only answer direct extralabel questions. Thus, unless you specifically ask, “Can I use your product to kill maggots?” company representatives cannot tell you. The 24-hour emergency hospital where this author was medical director regularly dealt with myiasis cases, and unless funds or patient logistics were an issue, the health care team regularly utilized isoxazolines.

Any pet on monthly isoxazolines will not be affected by myiasis. In some situations of debilitation, this could become a disadvantage.

Using maggot infestation as an advantage

Photo: Judy Gallagher/iNaturalist.org.png

Is there ever an advantage to being riddled with maggots? Yes, but this is not a reason to stop prescribing year-round isoxazolines for your indoor and outdoor patients. In current cases with myiasis, those baby dipterans are often helping the animal.

A common offender is Lucilia sericata, the common green bottle fly. Parasitologists, forensic entomologists, and some doctors are fascinated by these insects. L sericata often recognizes an individual’s death before anyone else. Within minutes of a person’s or animal’s demise, these flies will lay eggs upon the corpse.³

This egg-laying timing allows scientists to use maggots to help determine the time of death in some forensic investigations. The maggots’ larval instar, or stage, can help scientists know how long ago the fly eggs were laid.³

Although far less common today, doctors have used L sericata as a surgical maggot since the early 1800s. This myiasis leads to improved tissue regeneration and lower bacterial levels within patients. Wounds of injured Civil War soldiers who were left in the field were occasionally noted to be unusually bright and clean, and the effect was given the term Angel’s Glow. With the discovery of penicillin not occurring until the next century, these neglected soldiers in the field often had better survival rates than those with bandaged wounds in the medical tent who developed gangrene.^4-6

Civil War Angel’s Glow did not arise from maggots, per se. The greenish-blue bioluminescence arose from a nematode-associated bacterium that infected the wound. The maggots managed the cleanliness. Patient survival was a team effort: the maggots' removal of necrotic tissue and the bacterium’s antimicrobial exotoxin helped save lives. Until a 2001 high school science project elucidated the bacterial role, the flies took the credit.⁶

Secondary myiasis typically is not painful because these maggots stop eating at the edges of healthy tissue. This is common for the calliphorid flies endemic to the United States. Calliphorids bear the common name of blow flies.

Although medical professionals and some injured soldiers of the Civil War had positive experiences with the flies, most sheep producers detest them because of their proclivity for laying eggs in damp wool. Any skin disease, such as infection, can quickly lead to unwanted myiasis.

What makes screwworms different

L sericata only provides secondary myiasis, meaning that tissue must be damaged or necrotic for maggots to feed. Primary myiasis caused by the New World screwworm (NWS), Cochliomyia hominivorax, was eradicated in the US as of 1966, except for a brief invasion in 2016. It is currently endemic in Cuba, Haiti, the Dominican Republic, and countries in South America.⁷

NWS has recently spread to Costa Rica, Nicaragua, Honduras, Guatemala, and now Mexico for the first time in 3 decades. The US Department of Agriculture is expecting reinvasion of NWS in the US. A family member of Calliphoridae, NWS generates primary myiasis—its larvae eat healthy tissue of any warm-blooded animal. Their victims include cattle, sheep, pigs, dogs, cats, and humans.⁸

Since primary myiasis eats living tissue, there is no pain-free Angel’s Glow here. These maggots induce excruciating tissue destruction with potentially fatal sequelae for their victims. If this foreign animal disease arrives here, we will witness an onslaught of myiasis cases like we haven’t seen before. Take a nap on the backyard hammock next to your dog, and you may both awaken with C hominivorax eggs laid upon you. Within 24 hours, the maggots emerge from their eggs and begin to devour healthy flesh.^4-6,9

In late 2024, the US canceled all Mexican cattle imports, due to NWS being identified in Chiapas, Mexico.⁷ Despite this measure, concerns remain that the northern movement of NWS will not be stoppable.

The USDA best describes the etymology of the name screwworm:

“The name screwworm refers to the maggots’ feeding behavior as they burrow (screw) into the wound, feeding as they go like a screw being driven into wood. Maggots cause extensive damage by tearing at the hosts’ tissue with sharp mouth hooks. The wound can become larger and deepen as more maggots hatch and feed on living tissue. As a result, NWS can cause serious, often deadly damage to the animal.”⁷

Although there is no certainty NWS will arrive in the US, the risk is realistic and high. If this eventuality occurs, we will have myiasis cases where the maggots do not benefit the animal. The flies neither need nor search for debilitated or recently dead animals. Screwworms prefer their meat grade A.

Managing screwworm risk

Figure 1. Posterior end of L sericata sampled from a canine myiasis case, 2024.

What can we do now? First, continue to prescribe year-round isoxazolines for your indoor and outdoor patients. Secondly, be vigilant. Beyond the patient’s travel history and clinical presentation, several key features help define the maggot. Start at the posterior end of the maggot where breathing occurs. (Figure 1).

Both C hominivorax and L sericata boast Calliphorid-type spiracles (the external breathing apparatus), which look like 3 lines pointed toward a button (Figure 2). To differentiate these 2 species, look for C hominivorax’s pigmented tracheal trunks, visible on the dorsal aspect of the maggots.

Figure 2. Close-up of L sericata spiracles.

NWS has prominent hooks on its anterior end— imagine a vampire maggot. Many maggots will have these hooks, but they are large on NWS (Figure 3).

Finally, if you suspect NWS, report it. Take samples and contact your local health department and state veterinarian.

Figure 3. The anterior end of L sericata, demonstrating the diminutive hooks, in contrast to C hominivorax’s larger hooks.

Christopher Lee, DVM, MPH, DACVPM, is a preventive medicine specialist who practices in emergency medicine. He also is an item writer for the North American Veterinary Licensing Examination.

REFERENCES

1. Cutolo AA, Perier N, Menz I, Thyssen P, Silva FO, Beugnet F. Efficacy of afoxolaner (NexGard) on the treatment of myiasis caused by the New World screwworm fly Cochliomyia hominivorax (Diptera: Calliphoridae) in naturally infested dogs. Vet Parasitol Reg Stud Reports. 2021;24:100569. doi:10.1016/j. vprsr.2021.100569
2. Correia TR, Scott FB, Verocai GG, et al. Larvicidal efficacy of nitenpyram on the treatment of myiasis caused by Cochliomyia hominivorax (Diptera: Calliphoridae) in dogs, Vet Parasitol. 2010;173(1-2):169-172. doi:10.1016/j.vetpar.2010.06.019
3. Ricciuti E. How blow flies compete (or not) for decomposition duties. Entomology Today. October 27, 2020. Accessed February 24, 2025. https://entomologytoday.org/2020/10/27/ how-blow-flies-compete-decomposition-duties/
4. Miller GL. Historical natural history: insects and the Civil War. Montana State University. Accessed February 24, 2025. https:// www.montana.edu/historybug/civilwar2/flies.html
5. Horobin AJ, Shakesheff KM, Woodrow S, Robinson C, Pritchard DI. Maggots and wound healing: an investigation of the effects of secretions from Lucilia sericata larvae upon interactions between human dermal fibroblasts and extracellular matrix components. Br J Dermatol. 2003;148(5):923–933. doi:10.1046/j.1365-2133.2003.05314.x
6. Lyons J. “Glowing” Wounds: why Civil War soldiers had injuries that shone. War History Online. October 19, 2018. Accessed February 24, 2025. https://www.warhistoryonline.com/ instant-articles/glowing-wounds-civil-war.html
7. New World screwworm. US Department of Agriculture. February 6, 2025. Accessed February 13, 2025. https:// www.aphis.usda.gov/livestock-poultry-disease/cattle/ticks/ screwworm
8. Mexico News Daily Staff Report. US suspends Mexican cattle imports after flesh-eating worm detected in Chiapas. Mexico News Daily. November 28, 2024. Accessed February 24, 2025. https://mexiconewsdaily.com/news/mexico-cattle-screwworm/
9. Companion Animal Parasite Council. Cochliomyia hominivorax. May 9, 2017. Accessed February 24, 2025. https://capcvet.org/ guidelines/cochliomyia-hominivorax/