Winter Kills the Fly. It Doesn't Reach What's Already in the Soil.

Dr. Samantha Holeck did not sound like someone bracing for a short fight.

The New Mexico state veterinarian was supposed to brief Grant County livestock owners in person on Thursday evening. Severe weather pushed the meeting online, and the weather turned out to be the subject.

"This is not a sprint. It's a marathon," Holeck said. "This is something that we're going to be dealing with for a few years. It may come and go as the weather changes and as the conditions change, with the rain season and whatnot for these flies. The goal here is to try to stay ahead of it as best we can."

Behind that sentence sits a case count that has been climbing quietly through the fall. New Mexico's first detection was a dog in Lea County in June. Then a horse in Mimbres in September. Two more on Oct. 15, a calf and a horse, both in Grant County. A cow in Otero County on Wednesday. A calf in Hidalgo County announced Friday afternoon.

The Grant County cases landed close to home in more than one sense. Holeck said the Silver City animal was found inside the surveillance zone already drawn around the Mimbres case, which had the effect of pushing the original infested zone outward rather than opening a second one. She described the conditions around Silver City in terms a livestock owner would recognize from a pasture walk: rain, warm temperatures, and vegetation still green in late October.

"This particular part of the state is the most vulnerable in the whole state, given the rainfall, the temperatures," she said. "We've got a lot of vegetation there we don't normally see this time of year. It has created the ideal habitat for this fly, unfortunately. I suspect we may not be done with new cases down here. I'm hoping for cold weather."

Then she took the hope apart.

Adult screwworm flies are fragile in the cold — temperatures below 40 degrees work against them. The stage that survives winter is not the adult. Mature larvae drop off the host, work their way into the ground and pupate, and they can hold that state for as long as six weeks.

"If it gets below freezing, it can kill the fly but not the pupae," Holeck said. "The pupae that are in the soil waiting to hatch can delay that hatching. We would probably need a week straight of freezing or below-freezing weather to get the ground cold enough to kill the pupae, and that's not something we see very commonly in the southern part of New Mexico."

Anyone watching the Texas map should read that twice. The counties named in current Texas Animal Health Commission orders — parts of Brewster, Jeff Davis, Pecos, Reeves, Val Verde, Crockett and Terrell — sit at latitudes that make a week of hard, ground-penetrating freeze a rare event, not a scheduled one. A blue norther that drops air temperatures into the twenties for two nights does the visible work: it knocks down adults and slows flight. It does not necessarily chill the soil to the depth where pupae are waiting.

What the program is actually counting on is the fly supply. Holeck said sterile male releases around Mimbres started with ground chambers and switched to aerial distribution on Oct. 5, with flights on Mondays and Thursdays.

She walked through the production map behind those flights: a Chiapas, Mexico facility opened in July, producing 40 million sterile flies a week and expected to reach nearly 100 million by the end of the year, and the Metapa plant opened in June. "Some of those flies from Metapa are the ones being distributed in our cases here in New Mexico," she said.

The Texas facility at Moore Air Base near Edinburg is due to come online in April at 100 million flies a week, building toward 300 million within a year. Holeck put the total in the frame that matters: "That'll actually put us up to the 500 million flies a week we needed to eradicate it from the country before," she said, referring to the campaigns of the 1950s and 1960s.

Until then, the rules do the work. Animals leaving an infested zone need an inspection and a movement certificate from a livestock inspector, veterinarian or county extension agent. Certificates run five days, or three if the animals are headed straight to slaughter, and Holeck asked for at least 48 hours of notice so the state veterinarian's office can approve the paperwork. Horses that cross a zone repeatedly for ranch work can be written for a longer window.

Her ask of owners was simpler than any of that. Look at the animals, and call even when you are not sure.

"We would rather have 1,000 samples that are negative than miss one that is positive," she said.

For Texas, the practical version of that advice runs through the TAHC reporting line, 800-550-8242, and through the same inspection habit: ears, eyes, nose, mouth, and any wound that has started to smell wrong. Winter may thin the population at the margins. It will not close the file.