New World Screwworm

new world screwworm parasites screwworm Jun 04, 2025

The New World Screwworm (NWS) is a fly larva (maggot) that aggressively eats living and dead tissue.

"So what?! Maggots eat stuff all the time and no one freaks out about that." That is true, but the reason this parasite is a big deal is because it eats living tissue. In other words, small wounds that would normally heal and not be a problem become a potentially fatal problem if they get infested with NWS larvae because the larvae eat and dig into the wound going deeper and deeper causing more and more damage and eventually death. For example, castration sites, dehorning wounds, new born navals, damaged vaginas from calving, small wounds from fighting,

"So what?! I just googled it, we don't even have it here in the US." That is true, it was declared eradicated in 1966 despite a few cases being diagnosed throughout the 1970's and 1980's. NWS was present in the Southern USA and pushed farther south through eradication efforts. As the efforts continued, the fly was pushed south of Panama and believed to be contained to South America. COPEG was established to hold the line in Panama. COPEG stands for Comisión Panamá – Estados Unidos para la Erradicación y Prevención del Gusano Barrenador del Ganado, which translates to the Panama–United States Commission for the Eradication and Prevention of Screwworm. This binational organization was established in 1994 through a cooperative agreement between the Government of Panama and the U.S. Department of Agriculture (USDA).

Our lesson would stop here if the parasite was still contained to South America. It's not. The parasite has aggressively made its way north of Panama and is traveling northward through Central America and has made it to Southern Mexico about 700 miles south of the southern boarder of the United States (as of June 2025). See map below:

As part of efforts to prevent the NWS from entering the USA, on May 11, 2025, U.S. Secretary of Agriculture Brooke L. Rollins announced the suspension of live cattle, horse, and bison imports through U.S. ports of entry along the southern border

"So what?! I just googled that I can give my animals ivermectin to treat it and prevent it." That is true, ivermectin can help, but that is not the solution that successfully eradicated the parasites from multiple countries in the 1960's to the 2000's. Nor is that the only technique needed to successfully treat the disease. More on that later. Also, long term, mass-treatment of ivermectin will cause increased intestinal parasite resistance. With non-targeted, mass treatment you will find yourself in a situation where you are trying to prevent the NWS while increasing the likelihood you will have intestinal worms you can't kill with a dewormer. The only technique that has been shown to successfully fight the parasite back is the Sterile Insect Technique (SIT). Without getting too nerdy, this is how it works: 1) Breed male flies in a lab, 2) sterilize the male flies, 3) Mass release sterile males , 4) This prevents the females from having babies (the larvae/maggots. Here is the problem: There is only one lab in the world that makes this fly. It is in Panama. To clarify, there is not a lab in the USA that is breeding sterile NWS flies. The fear is the outbreak of the NWS, as it moves north, would be so great it that the Sterile Fly Lab would not be able to respond fast enough and would not make enough male flies to re-eradicate this fly from the Southern USA, Mexico, and Central America.

"WTH?! How did this is happen?" There are multiple factors in play (not listed in order of importance):

Supply chain disruption. Related to COVID.
Lack of field inspection/surveillance. Combination of lack of funds and complacency.
Uncontrolled and overwhelming migration of people and animals northward.
The lifecycle of the fly changed from 21 days to 19 days. This allowed them to have babies faster.
Long standing control of NWS has led to loss of knowledge on how to diagnose, treat, and prevent the disease and decreased awareness by the public and even many veterinarians.

"What do I do?!" First of don't panic buy toilet paper or hand sanitizer. Remember, this is just a fly. Flies have been around for millions of years. This fly was first described in the 1800's so it has been around for a really long time and our species has made it this far.

What you need to know is:

If this parasite makes its way up to the USA, it will have an economic effect on and will cause disease in domestic pets, domestic food animals, exotic pets, native wildlife, exotic wildlife, and even humans.
There is a flesh eating maggot that is a big deal that might come back to the USA
How to recognize the New World Screwworm fly and larvae.
Who to notify if you suspect a New World Screwworm infestation on your property.
How to collect the maggot and get it identified.
There are ways to treat most animals infested with screwworms. Some animals will not be caught fast enough and not be able to be treated and will die from the parasite. All open wounds are susceptible
The only way to eradicate the disease is using the Sterile Fly Technique. You will not be able to ivermectin or fly spray your way out of this one.
At the very end of this presentation, I have posted picture of new world screwworm infestations. They are not for the faint of heart. WARNING.

"How do I prevent my animals from getting infested? I googled that I can just put ivermectin in my feed." Here are the most important things you can do to prevent NWS

The most important thing you can do is regularly see your animals, look for wounds, and treat wounds and know what an infestation looks like. Know what the fly looks like. Know what the wounds look like. Know what the parasite looks like. (WARNING. Graphic pictures are attached at the end of the article.)
You can apply topical fly sprays like Co-Ral, Catron IV, Permectrin II, and Ultra Boss (These are not the only ones. These are some examples.)
Dispose of deceased animals appropriately by burying or burning them.
Some have advocated long-term, mass treatment with ivermectin. I'm not convinced this is a good thing to do. I'm not even convinced it would be very helpful. Actually, I'm concerned the use of ivermectin in this manner will increase intestinal parasite resistance to ivermectin. We have already seen this and I believe mass treatment will make resistance worse. Meaning, we could find ourselves in a situation where we give ivermectin to an animal that has intestinal worms, which are a huge problem in Texas, and it doesn't kill the worms because the dewormer is no longer effective because the worms have been exposed to ivermectin so many times. This technique, might help a few animals and prevent them from getting the NWS, but there are far to many questions and variables in this situation for me to support it. Is the animal eating enough to be effective? What about other animals in your pasture or pens? What about your neighbor? What are we going to do if we increase GI parasite resistance? The only effective long-term option that has been shown to be effective is the Sterile Fly Technique. Everything else is just a weak band-aid. Now, this is not a hill I'm going to stand and die on. Meaning, I'm not going to take time out of my day to argue with people. The reality is that I will have business whether someone uses ivermectin or not. I will be called to treat NWS. I will be called to treat resistant parasites. I will be called to fill prescriptions and do services either way so my motivation is to recommend what I believe will be the most effective and best option.

"What do I do it I suspect I have the New World Screwworm on my animal or on my property?" This is an easy on. You need to tell your veterinarian. They will take it from there. They will notify everyone important and help coordinate the response. They will teach you what to do. If you need help, reach out to me and I will point you in the right direction.

"How can I treat an animal that is infested with New World Screwworm? " Here are the most important things you can do to treat an animal:

First, get your veterinarian involved so they can get the state and federal government involved to help you. You will not get in trouble. There will be a big coordinated approach to help you.
Administer Ivermectin (shorter duration of action) or Dormectin (longer duration of action). Ivermectin is said to last 10-14 days whereas Dormectin is said to last up to 21 days.
Apply a topical pesticide on wounds for flies like Co-Ral, Catron IV, Permectrin II, and Ultra Boss (These are not the only ones. These are some examples.)
Remove all of the live and dead NWS. Yes, all of them. Save some of the larvae and destroy the rest. Do not let them grow.
Cover and treat wounds when possible.
Stay vigilant and watch for more infestations.

"OK, I'm convinced this Sterile Fly Technique is the way to go, but with their only being one lab in the world that makes the sterile fly, there is no way they could respond to an outbreak. They don't have enough flies and wouldn't be able to respond." I am apprehensive about that too. There is only one fly lab in the world that is breeding the NWS fly. However, I believe the USA is one of the most prosperous nations in the world. I believe even if the state or federal governments do not respond appropriately that private individuals, especially entrepreneurs, will get involved and will be successful. Senator Cornyn has introduced legislation to get funds to build a sterile fly facility (May 2025) so the federal government is taking steps already. The technique used to produce sterile flies is not new. There will be no groundbreaking techniques needed to implement the Sterile Fly Technique, so there will not be any R&D. It will be simple implementation.

There are a lot of people talking about this right now. Get involved with the discussion to get the most up to date information. There is a webinar on June 9, 2025. I'll try to keep this area updated with new info.