New World Screwworm

I have seen a few patients with cutaneous botfly larvae (proper medical terminology being furuncular myiasis) in my career. They are extremely gross, but relatively easy to deal with. However, I had never seen (or even heard of) their much nastier cousins: the New World Screwworm. 

The scientific name of the New World Screwworm is Cochliomyia hominivorax. Consider that name. Hominis meaning man. Vorax meaning consuming. This is a man consuming parasite. That might be all you need to know.

This is one of the many human maladies I knew nothing about. I must have skipped that day of medical school. Honestly, considering its emergence in areas where I was planning to travel, I think I was happier in my ignorance. The adult fly lays its larvae in open wounds of warm blooded mammals (including humans), and the larvae burrow or “screw” deeper into the flesh. Unlike the botfly, which causes a localized problem, the screwworm maggots are capable of causing severe tissue damage, and even mortality. 

In order for larvae to be deposited into the skin, a wound of some sort is required. In endemic areas, this often occurs in patients with chronic conditions, such as peripheral artery disease or pressure wounds. However, it can also occur after trauma, which is probably the more likely route of infection in returned travellers. 

To start, patients will present very similarly to botfly myiasis. There will be a red inflamed area that will look like a furuncle, but which has a central pore that will intermittently exude a serosanguinous discharge (actually the feces of the larvae). These will progress to painful, non-healing ulcers, with relatively significant surrounding tissue damage. The key to the diagnosis is recent travel (previously Caribbean and South America, but I would now also consider Central America as a risk factor). 

Treatment focuses on mechanical removal of the larvae. Unlike the single botfly larvae that can often be removed in the emergency department, these infestations often involve large numbers of larvae with lots of tissue destruction, and so may require plastic surgery consultation. It is also important to consider secondary bacterial infection and the potential need for antibiotics. 

The real reason that this bug caught my eye was the fascinating control program that has been incredibly successful (until recently). COPEG – United States Commission for the Eradication and Prevention of Screwworm – is a program co-administered by Panama and the United States, with the goal of keeping the screwworm, which is endemic in South America, out of Panama and therefore the rest of the Americas. This program currently drops approximately 20 million sterilized screwworm flies every week by plane over the Darien Gap. This program works because female screwworms only mate a single time in their life, and therefore if a female mates with a sterile male there will never be any offspring. The program keeps the environment saturated with sterile male screwworms, essentially ensuring that the screwworms cannot propagate. 

There was a time when screwworm was rampant across the southern United States, costing millions of dollars a year in lost livestock. They were eradicated in the States, and slowly pushed back through central America to the current (or perhaps past) dividing line in Panama. This program has been incredibly successful for decades. Unfortunately, after being completely eradicated from central America in the 1990s, the parasite has undergone a significant resurgence, with many cases and multiple human mortalities being reported from Costa Rica and Nicaragua. (Taylor 2025) There are also cases in livestock in Mexico and Guatemala. Our local infectious disease experts in Toronto have seen numerous cases in returned travellers from Costa Rica, and so this now needs to be on everyone’s radar. It is hypothesized that significant human migration across the Darien Gap might be contributing to the spread of the screwworm. 

Unfortunately, this is yet another medical issue that could be made dramatically worse by ridiculous politics. Among the many CDC employees fired by the Trump administration were the specialists at the Epidemic Intelligence Service that specifically managed public health threats like the screwworm. (Taylor 2025)

Some other images

I have not seen this personally, and there are very few medical images for new world screwworm available, so I thought I would include a few images of the more tame cousins, just for reference. (If you want to be truly disgusted, you can google veterinary images of new world screwwworm).

From the CDC: https://www.cdc.gov/dpdx/resources/pdf/benchaids/NWS-Bench-Aid-2024-508.pdf

Related FOAMed

LITFL Furuncular myiasis

Core Ultrasound Ultrasound of the week 22: maggots by ultrasound

If you want more about screwworms, I thought this long form piece in the Atlantic was an interesting read. 

CDC: Clinical Overview of New World Screwworm Myiasis

References

Bernhardt V, Finkelmeier F, Verhoff MA, Amendt J. Myiasis in humans-a global case report evaluation and literature analysis. Parasitol Res. 2019 Feb;118(2):389-397. doi: 10.1007/s00436-018-6145-7. Epub 2018 Nov 19. Erratum in: Parasitol Res. 2019 Feb;118(2):723. doi: 10.1007/s00436-018-6187-x. PMID: 30456490

Reichard RE. Area-wide biological control of disease vectors and agents affecting wildlife. Rev Sci Tech. 2002 Apr;21(1):179-85. doi: 10.20506/rst.21.1.1325. PMID: 11974628

Taylor L. New World screwworm: Nicaragua confirms 30 human cases of flesh eating parasite. BMJ. 2025 Feb 19;388:r355. doi: 10.1136/bmj.r355. PMID: 39971344Zhang S. America’s Never-Ending Battle Against Flesh-Eating Worms. The Atlantic. May 26, 2020. Available at: https://www.theatlantic.com/science/archive/2020/05/flesh-eating-worms-disease-containment-america-panama/611026/