New World Screwworm: What Producers Around Here Should Know
You have probably heard about this by now. Here is what is actually known, what it means for an operation in northeast Colorado, and what we would ask you to watch for.
First, the name is misleading.
It is not a worm. It is a fly.
The New World screwworm is a parasitic fly whose larvae feed on living tissue of warm-blooded animals (including pets). That is what makes it different from the flies you already deal with. Most maggots go after dead tissue. These do not. They burrow into living flesh, and an untreated case gets worse very quickly.
It is also not a food safety problem. USDA has been clear about that. It is a pest, and the beef supply is not affected.
Where things stand
USDA confirmed the first case in US livestock on June 3, 2026, in a three week old calf in Zavala County, Texas. The larvae were in the animal's navel. That was the first confirmation in livestock in the US since the screwworm was eradicated from this country back in the 1960s.
Within about a week the count had gone to six confirmed animal cases, four cattle, one goat, and one dog. The dog was in Lea County, New Mexico.
USDA's own assessment is that the current risk to animals and people in the United States is very low, and detections so far have been isolated rather than widespread.
What is being done about it
The same thing that worked the first time: sterile flies, monitoring through fly traps, and increased animal movement restrictions.
The female mates once in her life. If she mates with a sterile male, she produces nothing, and over enough releases the population collapses. USDA has funded a new production facility in south Texas to increase the number of sterile flies available.
Alongside that there are quarantines, movement controls, and border controls. A suspected case triggers a hold order on premises within roughly twelve miles, and animals cannot move until they have been examined. Intensive monitoring around a bigger surrounding zone will begin.
What this means for you and your animals
Northeast Colorado is a long way from the southern border, and nothing we have seen suggests you should be alarmed.
The realistic exposure for a producer in this country is animals coming in from somewhere else. It could even be a pet animal. Anything you buy, haul in, or bring back from a sale in an affected region is worth a closer look than usual, especially if it has an open wound.
What can I do to for my animals
Your eyes and your nose are the best tool that you have. Check all of your animals frequently and thoroughly. You might smell it before you see it.
Screwworm usually gets into an animal through an open wound where the fly has laid her eggs. That includes the ordinary ones you create yourself: castration sites, dehorning, branding, ear tags sites, eyes and the navel of a newborn.
Watch for:
A wound that is getting bigger instead of healing
A wound that is draining, or that smells wrong or putrid
An animal biting or licking at a spot
An animal that is off, standing apart, not eating
Larvae or eggs in or around the nose, ears, genitalia, or a newborn's navel
An animal that is irritated and head shaking with large amounts of mucous or pus coming from the nose
If you think you have found it
Call us. Do not wait to be sure.
Early detection is the single most effective tool anybody has against this, and that is true whether it turns out to be screwworm or an ordinary wound infection. We would far rather look at twenty wounds that turn out to be nothing.
There are treatment options available now, and FDA has authorized products specifically for this. Which one is appropriate depends on the animal and the situation, which is a conversation to have with us rather than something to work out from a website.
Questions we are getting
Can I just feed ivermectin or put it in my fly sprayer? These are really not good ideas because they will not work to prevent screwworm infestations and you will create internal parasites and other flies that are resistant to ivermectin. We also don’t have any way to determine a meat or milk withdrawal with these types of applications.
Can I treat every animal with ivermectin or Dectomax injectable for prevention? This is also not a good idea and will lead to internal parasites and other flies that are resistant to these drugs. The best use of antiparasitic drugs is in a strategic deworming plan. These are two major tools for parasite control and widespread resistance could be devastating to the cattle industry.
Is my beef safe to eat? Yes. USDA has been explicit that this is not a food safety issue and the food supply is safe.
Can it affect people? Rarely, yes, it can infest people, which is why it gets lots of attention. Human cases are uncommon and the current risk in the United States is described by USDA as very low.
Has it been found in Colorado? Not as of this writing. Confirmed cases have been in Texas and New Mexico. This information changes, so check screwworm.gov or with us for the current picture.
What if I am buying animals out of Texas or New Mexico? Look them over carefully, particularly any open wounds, and be aware that movement restrictions can apply to animals from affected areas. Talk to us before you haul.
We already run fly control. Does that cover it? Your normal fly program is not designed for this and should not be treated as protection against it. Wound management and watching your animals are what matters here.
What happens if a case is confirmed near me? Premises within roughly twelve miles of a suspected case go under a hold order, and animals cannot move until they have been examined by approved personnel.
Where can I read the official information? USDA APHIS maintains screwworm.gov. That is the source we are watching and it is where the current case counts live.
Sources for the facts in this piece: USDA APHIS announcement of 3 June 2026, APHIS screwworm response pages, and Swine Health Information Center case tracking as of Aug 30 2026. Case counts and geography move. Anything published should carry a date and get checked before it goes live.
Comments