Screwworm is one of those words that can stop ranchers and wildlife biologists in their tracks.
These days it is more ancient history, something known only about by ranchers and wildlife biologists who did not sleep through that class in college.
In the 1950s and 60s screwworms caused serious harm to the Texas livestock industry and kept white-tailed deer numbers low.
It was not just here. States throughout the Southwest and Southeast felt the impact of screwworms, maggots that infest livestock, wildlife, pets and occasionally people through open wounds.
In Texas a half-century or more ago it was public enemy No. 1.
Research into screwworm eradication began in the 1930s. It would be three more decades before researchers found they could defeat the infected flies with sterile male flies. Throughout the country an air force of planes dropped cartoons of sterile flies for the infected flies to breed with, then die.
It worked, saving the cattle industry and sending Texas’ deer population sky-rocketing.
“I believe screwworms were declared eradicated in Texas around 1966. It appears the deer population was starting to grow rapidly in the early 1960s probably about the time screwworm eradication started. The eradication of the screwworm definitely had positive impacts on the deer population in Texas,” said Alan Cain, Texas Parks and Wildlife Department’s white-tailed deer program leader.
Although the census methods were archaic compared to today’s, old TPWD records showed the statewide deer herd topped at 500,000 prior to 1955. By 1956 it was up to 9,000,000 and well upwards of a million by the time statewide eradication of the screwworm was completed. Today the population is estimated at more than 4 million.
The problem is screwworm is back. Not in Texas, but in the Florida Everglades where it is impacting an already fragile Keys deer population. Word is spreading through the wildlife community like wildfire.
The good news is that agricultural and wildlife officials know how to deal with the disease, and if found they are not starting at the ground floor in an effort to rid it. It is known as a slow-moving disease because typically the infected flies will go no farther than they have to find a new host so there is hope it can be controlled in Florida.
That might prove difficult, because even livestock are more mobile than they were in the mid-1900s. Trailers full of cattle are constantly on the move across the country, and it would not be hard for a pair of flies to catch a ride. If it occurred, Chronic Wasting Disease could become an afterthought.
“Screwworms do pose a major threat to wildlife. The success of the screwworm eradication program is a major reason allowing the deer population to flourish. Let’s hope this is a freak thing and will be controlled quickly. I’m old enough to remember screwworms and they are no fun,” said Dr. Bob Dittmar, TPWD wildlife veterinarian.
Dittmar said it is not time to panic, but Texans should be diligent.
Screwworm and CWD are not the only concerns. Other real disease threats to deer include epizootic hemorrhagic disease and blue tongue, both of which are already found in Texas.
Dieoffs related to EHD have been reported in various states in recent years, but like in Texas most of the counts has been in the hundreds up to a few thousand.
EHD is another disease spread by flies, in this case the no see-ums, and is more active during periods of drought when herds are more concentrated.
Like EHD, blue tongue is a viral disease. Although they are separate diseases they are sometimes hard to tell apart.
“EHD and BT have been around Texas for a long time. Not only in wildlife but also in livestock. Almost all our deer (and pronghorn) are serologically positive, meaning they have been exposed at some time,” Dittmar said. “We do see localized mortality events related to those diseases occasionally. I’ve had a few reports recently.”
Dittmar, who grew up on a ranch during the eradication era of screwworms, said if he had to pick his poison between the deer diseases for the one he would least want to see in Texas, he was hard-pressed to come up with an answer. He said all three present different issues. Currently the state’s herds seem to be mostly immune from EHD and blue tongue. On the other hand old-timers know what screwworm can do, and it may take generations to figure out the final impact of CWD.
To reduce the impact or spread of any wildlife disease Dittmar said the best prescription is to control deer numbers.
“Diseases are more likely to be a problem when you concentrate animals, therefore keeping numbers at carrying capacity is the best way for landowners to reduce disease spread. Moving animals increases the chance of spreading disease to other areas,” he explained.
He said there are portions of the state, in some case ranchland and others urban settings, where numbers are dangerously high. Add to that the use of feeders and any outbreak could spread quickly.
“Feeding animals in concentration, that is the elephant in the room. That is a hard thing to talk about, especially in Texas,” Dittmar explained.
For now the first line of defense will be livestock producers who will go back to diligently inspecting herds. Hunters can do their part this season by keeping an eye out for deer dying of natural causes.