New World Screwworm: What Science Still Needs to Answer

Introduction

The recent northward spread of New World Screwworm (Cochliomyia hominivorax) across Central America and into Mexico has renewed a meaningful global concern. I am receiving several emails about this topic every day, hearing more fear, cautions, and stories. Given that this is one of the most economically devastating parasites affecting livestock, this makes sense. However the scientist in me came up with a question this morning – have we ever looked at the microbiome side of the story?

We know the traditional control strategies have relied on surveillance, wound management, insecticides, and then the highly successful Sterile Insect Technique (SIT). I did a quick literature search and tried to find a read on possible advances in microbiome science for the question in my mind:

Could microbial ecology play a role in future screwworm prevention and control strategies?

Before I take you to the questions living in my mind rent-free, let’s have a quick tour on screwworm and where we are with it now, as mid of June 2026.

What Is the Screwworm and Why Is It Back?

Many fly species feed their larvae primarily on necrotic tissue. New World screwworm larvae invade and consume living tissue, causing severe wound enlargement, secondary infections, production losses, and in severe cases, inevitable end… death.

A gravid female lays up to 400 eggs in an open wound, a navel, a tick bite, anything with exposed flesh. The larvae hatch within 12 to 24 hours and burrow in. Left untreated, the wound expands rapidly, secondary infestations compound the damage, and the animal deteriorates fast.

The U.S. eradicated the screwworm from its territory in 1966 through one of the most successful biological control programs in history: the Sterile Insect Technique (SIT), managed by USDA-APHIS and COPEG in Panama. But eradication is not the same as extinction. The parasite persists in South America, and in late 2024 and 2025, confirmed cases appeared further north, triggering an emergency USDA response. Currently, multiple confirmed cases in sheep, cattle, dogs and goats were reported in Texas and New Mexico. All confirmations placed as of June 2026. You can read the CDC’s 4 June 2026 article here. Other states are also taking precautions. Florida temporarily banned the rescue and shelter dogs’ entrance from affected regions. For North Carolina, animals entering from infested zones must comply with NWS standardized animal movement guidance. Health officials in Colorado, Oklahoma, New Mexico, and California have expanded agricultural surveillance.

In August 2025, the U.S. Department of Health and Human Services issued a declaration allowing FDA to authorize emergency use of animal drugs for New World screwworm control. This was followed by a series of unprecedented regulatory actions, including conditional approvals and Emergency Use Authorizations (EUAs) for products intended to prevent or treat screwworm infestations in livestock and companion animals.

So the scientific community is actively asking what better control tools might look like. And that's where microbiome science enters the conversation, because most of the current therapies focus on killing larvae after infestation occurs. Far less has been directed toward understanding why certain wounds become attractive to screwworm flies, how wound microbial communities influence infestation risk, whether wound microbiome modulation could reduce attraction or reinfestation, and whether microbial ecology could complement existing prevention programs.

Biological Relationship Between New World Screwworm and Microbes

One of the most recent studies directly examining the screwworm gut microbiome came from the USDA Agricultural Research Service in May 2025, led by Arp and colleagues in the G3 journal. It wasn't looking at biotics as a tool. It was asking a more immediate operational question: does tetracycline, used in transgenic male-only rearing programs for SIT, disrupt the fly's gut microbiome in ways that could compromise the fitness of sterile males released in the field?

SIT is a biological pest control method where scientists mass-rear target insects, sterilize them using radiation, and release them in large numbers into the wild. When sterile males mate with wild females, no viable offspring are produced. Repeat this at scale, over time, and the wild population collapses.

The answer was reassuring for SIT program operators. Tetracycline does shift midgut microbial communities during active exposure. But once it's removed, the fly's gut microbiome reverts rapidly to its baseline community structure. Biological performance including mating competitiveness was not meaningfully harmed. So the transgenic Tet-Off rearing approach appears safe from a microbiome-disruption standpoint.

What this tells us practically: C. hominivorax has a structured gut microbiome that can be perturbed and that rebounds. It's not a sterile gut. It's a microbially active system. And that matters for thinking about biotics-based control strategies

The relationship between insects and microorganisms is increasingly recognized as a critical component of public health including development, reproduction and environmental adaptation. We already know that for mosquitoes, tsetse flies, termites, and several agricultural pests, their microbial symbionts contribute to nutrition, immune regulation, vector competence and even mating behavior.

One of these studies published in 2022, had researchers from the USDA and collaborating institutions. They’ve conducted one of the first comprehensive investigations of the microbiome of New World screwworm populations. They were collected from both wild infestations and sterile insect production facilities, and the results have shown a significant difference between wild and mass-reared populations. It was easy to guess but now we are sure that wild flies exhibit greater microbial diversity. The authors of this study concluded that microbial communities may influence screwworm fitness and potentially affect sterile insect program performance.

This finding, alone, brings a scientific concept to the surface clearly: The screwworm is not simply an insect parasite but a biological system that continuously interacts with microbial communities!

Is The Missing Piece Wound Microbiome?

Adult screwworm flies are highly attracted to wounds. I am hoping you are not reading this article/post during your breakfast or lunch time (warning!) but here is the reality: that odors emitted from wounds play a critical role in attracting gravid females seeking oviposition sites. On the other hand, interestingly, many of these ‘attractive’ odors are not produced directly by the host animal. They come from the microorganisms colonized in the damaged tissue. If there is a wound, and there is bacteria around, they generate volatile organic compounds (VOCs) as a hobby. These VOCs are usually indole, phenolic compounds, sulfur-containing metabolites, and also ammonia-derived compounds and products of protein decomposition.

When the chemical signals change, they attract blowflies and screwworm flies since they are heavenly odors and signals for these flies. Experimental studies have also demonstrated that bacterial communities associated with wounds can generate ‘semiochemicals’, which are basically capable of attracting screwworm adults.

Well… my two questions at that point:

• could altering wound microbial communities reduce the chemical signals that attract screwworm flies?

• does the host animal's microbiome influence susceptibility to screwworm infestation in the first place?

At present these remain unanswered, but my mind does not stop to think around it!

In a wide concept, that could work. For years, when I’ve applied a consortium of probiotic microorganism as a liquid product on the environment or surface -into an animal barn or trash bin- to degrade the VOCs, it works pretty successfully. It breaks VOCs down, that possibly changes the signals, and the number of flies decreases significantly. But… how does this concept apply on a wound, on a living biological tissue?

There is a review article published in 2024, concludes that probiotics could provide benefits in the prevention and adjuvant treatment of a wide range of wounds, especially the ones that are complicated by microbial infections.

Okay, there is no published study that has demonstrated that any type of biotics prevent specifically “New World Screwworm” infestations or eliminate established myiasis. However, the absence of evidence should not be confused with evidence against the concept, right? If we know that probiotics can influence wound healing through multiple mechanisms, this might be something that we need to focus asap!

The above mentioned research shows the potential with specifically six related paths, including competitive exclusion of pathogenic bacteria, biofilm disruption, immune modulation, anti-inflammatory activity, promotion of angiogenesis and enhanced epithelial repair. And we already have enough evidence that show the beneficial effects of topical and oral probiotics in both human and animal wound-healing models. Species such as L. plantarum, L. rhamnosus, L. casei, and Bifidobacterium spp. have shown particular promise but my scientist instincts tell me that there are way more others that we have not worked on yet and could be the main brick on this building!

To summarize this part of the thoughts, I can say from a veterinary perspective that, improved wound healing could theoretically reduce the duration during which animals remain vulnerable to screwworm oviposition. This remains a plausible... but currently another untested hypothesis.

How About The Potential of Other Biotics?

Among all microbiome-based approaches, postbiotics may offer particularly interesting opportunities for future investigation. Postbiotics, the bioactive metabolites and structural components of microbial cells, include short-chain fatty acids, bacteriocins, and heat-killed cell preparations. Their gem is that they deliver ‘microbial-derived bioactive compounds’. They bring organic acids, antimicrobial peptides, cell wall fragments, exopolysaccharides, and fermentation metabolites to the game, and this provides biological activity without viable microorganisms.

Recent research suggests that postbiotics can support wound healing by reducing pathogenic bacterial growth, modulating inflammation, enhancing tissue repair processes, and improving local immune responses. Adding other pros to that, such as greater formulation stability, improved shelf life, lower regulatory complexity, and reduced concerns regarding viability during storage, puts postbiotics in a unique practical position that needs more research attention.

Prebiotics work by selectively feeding beneficial resident microbial populations. In the context of host skin and wound health, prebiotic applications to support beneficial skin commensals have been explored in human wound care. The animal parallel is early-stage but emerging. Again, no screwworm-specific data exists.

Synbiotics combining both live organisms and their preferred substrates have shown the most consistent effects on gut microbiome recovery in companion animal studies. Whether this translates to a wound-health benefit that influences ectoparasite susceptibility is an open question.

One Health: Why This Goes Beyond the Animal

Screwworm myiasis affects livestock economics directly. An infested animal cannot be transported, cannot be sold, requires immediate veterinary intervention, and in severe cases is lost. In deer and wildlife populations, the ecological impact of an uncontrolled outbreak compounds quickly. In a One Health context, the screwworm also poses a human health risk, with cases of human myiasis documented globally, mostly in regions where the parasite is endemic.

A microbiome-informed approach to screwworm management would fit naturally within the One Health framework. If biotics can improve SIT program quality by restoring gut microbiome fitness in mass-reared sterile males, that improves eradication efficiency. If host microbiome support reduces wound attractiveness to gravid females, that adds a complementary layer to chemical and biological control. These are not competing approaches. They are just additive ones!

AMR is also a real consideration here. Screwworm-infested wounds frequently acquire secondary bacterial infections. Current treatment protocols rely on larval removal, wound management, macrocylic lactones, insecticides and newly authorized FDA treatment options. If microbiome-supportive approaches could reduce secondary bacterial load in wounds without adding antibiotic pressure, that would be a meaningful AMR mitigation contribution.

What This Means for Veterinary Practice and Product Development

If you're a veterinarian currently managing cases in or near the outbreak zones, there's no approved biotic protocol for screwworm. The standard of care remains immediate wound treatment, removal of larvae, and topical/systemic insecticide use per USDA guidance. What microbiome science can offer right now is a supporting lens, not a replacement tool.

For product developers and animal health companies, the opportunity is real and early. Skin and wound microbiome support in livestock and companion animals is a growing category with defensible formulation rationale. You don't need screwworm-specific clinical evidence to develop a science-based wound microbiome support product for general use. The screwworm angle is a hypothesis layer on top of something that already has an evidence base in wound healing.

For researchers, this is a wide-open field. The medfly probiotic-SIT model provides a clear research template. A pilot study characterizing the C. hominivorax gut microbiome under standard vs. probiotic-supplemented rearing conditions is feasible, affordable, and would be genuinely novel.

My Take

The screwworm-microbiome connection is real in principle and almost completely unstudied in practice. That gap is frustrating, but it's also an invitation. We're at the point in the evidence curve where the hypothesis is credible, the adjacent science is strong, and the first person to do the work will be defining the field.

I've spent 20+ years building products at the intersection of microbiology and animal health. And the pattern I keep seeing is that the biggest breakthroughs come from asking the question one step earlier than everyone else. The screwworm question is that question right now.

Dr. Nihan Marun,

DVM, PhD Microbiology

PS: If you're a researcher actively working on screwworm biology and you've thought about the microbiome angle, I'd genuinely love to connect. The field needs the conversation. Let's have it.

This information is for educational and scientific discussion purposes and does not constitute veterinary advice. Probiotic and biotic applications for screwworm control are currently in hypothesis phase. No approved biotic product or protocol exists for screwworm prevention or treatment as of June 2026.

SOURCES:

1. Arp, A. P., Quintero, G., Sagel, A., Gonzales Batista, R., Phillips, P. L., & Hickner, P. V. (2022). The microbiome of wild and mass-reared New World screwworm, Cochliomyia hominivorax. Scientific Reports, 12(1), Article 1042. https://doi.org/10.1038/s41598-022-04828-5

2. Arp, A. P., Tietjen, M., Sagel, A., Vasquez, M., Quintero, G., Bodine, D. M., Saelao, P., & Hickner, P. V. (2025). Tetracycline has no long-term effects on gut physiology and microbiome of the New World screwworm, Cochliomyia hominivorax, which has positive implications for transgenic male-only rearing systems. G3: Genes, Genomes, Genetics, 15(5), Article jkaf058. https://doi.org/10.1093/g3journal/jkaf058

3. Bădăluță, V. A., et al. (2024). Probiotics in wound healing. International Journal of Molecular Sciences, 25(11), Article 5723. https://doi.org/10.3390/ijms25115723

4. Centers for Disease Control and Prevention. (2026, June 4). New World screwworm outbreak. https://www.cdc.gov/new-world-screwworm/situation-summary/index.html

5. Haytham, H., Kamel, C., Wafa, D., et al. (2024). Probiotic consortium modulating the gut microbiota composition and function of sterile Mediterranean fruit flies. Scientific Reports, 14(1), Article 1058. https://doi.org/10.1038/s41598-023-50679-z

6. Hickner, P. V., Mittapalli, O., Biedler, J., et al. (2020). Physiological and molecular correlates of the screwworm fly attraction to wound and animal odors. Scientific Reports, 10, 20771. https://doi.org/10.1038/s41598-020-77541-w

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