By Patti Wilson Contributing Editor
We sometimes think that events happening far away won’t affect us; we are immune by distance. Occasionally, we are proven ignorant of our vulnerability, this time by the lowly and disgusting New World screwworm (NWS).
In late November 2024, a new NWS discovery sparked such a scenario when one lone cow was found infected in the southern Mexico state of Chiapas. She turned up at an inspection point near the Guatemalan border. Indeed, last year Central America saw an uptick in movement northward by the NWS. This prompted the U.S. Animal Health and Plant Inspection Service (AHPIS) to step up efforts in that area to control the pest.
APHIS considers the threat “severe” and restricted the importation of animals “originating from or transecting Mexico effective immediately and pending further information from Mexican veterinary authorities of the size and scope of the infestation.” As of this writing, the ban on U.S. imports of cattle was still in place.
How Does This Affect Us?
At a time when U.S. cattle numbers are at their lowest level in 75 years, it has acutely affected the markets in this country. Already-high cattle prices for grass and feeder cattle at U.S. auction markets have spiked even higher since the Nov. 22, 2024, APHIS announcement.
According to CattleFax, Mexican cattle imports will often hover between 1 million and 1.45 million per year, the bulk being fed, of course, in Texas and New Mexico, though they make their way into several other states. A 10-year rolling average of Mexican imports is 1.17 million head, according to the USDA. It is estimated that Mexican imports make up an eventual 3 percent of U.S. fed cattle slaughter.
About New World Screwworm
What was your reaction when you heard about this lone cow on the southern Mexico border that had some screwworm problem? Mine was, honestly, “So what?” My response was of one who had never seen or heard of New World screwworm. Lucky are we who live in a cold climate. NWS has been a problem in the United States since the 1930s, discovered and eradicated several times over the years. Always turning up in southern areas (so far), we are not immune to infestation from the nasty beast.
In an effort to be more educational, I considered running a photograph of the NWS. Suffice it to say this is not something anyone wants to see and might elicit nightmares among the young. I opted for an old-time cattle inspection photo.
Here’s the big deal: In 1933, Texas suffered an NWS outbreak that killed 180,000 cattle in less than half the Texas counties. According to the USDA, Dr. Emory Cushing, USDA scientist, was one of the frontline fighters responsible for containing NWS. He helped develop the Sterile Insect Technique, eventually ending the Texas nightmare. One critical element was developing a way to raise large numbers of NWS flies for research without using live animals. A laboratory at Menard, Texas, headed by Dr. Raymond Bushland, developed a diet of ground meat, beef blood, water and a small amount of preservative. By 1958, the USDA was capable of producing 50 million lab-grown sterile flies per week, which were turned into the wild. They, in turn, mate with NWS females, producing no offspring. It is a method that is still used effectively. This natural control method vastly increases the ability to stay ahead of outbreaks internationally, as well.
Although the United States was declared officially NWS-free in 1966, a subsequent outbreak occurred in 2016 in Florida that triggered an Agricultural State of Emergency. In September of that year, National Key Deer Refuge Rangers noted an increasing death rate among deer. By October, they realized what they thought were normal deer injuries were NWS infestations disguised as horribly infected wounds. Other local animals also suffered before the outbreak was contained. The refuge housed just shy of 1,000 Key deer prior to the outbreak. The NWS killed 132. The 2016 Florida outbreak was ended by use of the Sterile Insect Technique.
The last outbreak of NWS in the United States was in 1982 in Texas.
Eradication Continues
U.S. government researchers have developed new strains of sterile NWS males that are more effective in competing with native males. Mexico has built its own production plant, capable of turning out 500 million flies per week, aiding in control of the pest. This sterile male eradication technique has been employed in the Darian Gap, between Panama and Columbia, the same area we hear about that is used heavily by the throngs of immigrants that has poured into the United States. Additionally, immigrants themselves are capable of spreading the parasite.
The USDA estimates the U.S. livestock industry benefits by over $900 million per year as a result of the eradication of NWS. Bear in mind this number is pre-inflation and pre-market boom. Mexico and Central America have benefitted economically in the billions of dollars. Gains realized from reduction and eradication have had far-reaching effects and have required the cooperation of federal and state governments, individual producers and their alliances, as well as scientists.
Life Cycle
What about the parasite itself? NWS mature flies lay their eggs in exposed flesh of warm blooded, live animals. Another favorite place is the nasal cavity of newborns. When the wound is disturbed, they burrow or “screw” into the flesh. Unfortunate humans are most likely to be affected in the nose or sinus cavity. The pests cause severe tissue damage and continue burrowing, even to the point of death to their host. Feeding screwworms enlarge the wound and attract additional female flies, which deposit more and more eggs, exacerbating the problem. This phase of the cycle lasts three to seven days after hatching. The larvae then fall to the ground where they tunnel into the soil and develop into pupae with a brown, dry shell. Seven days later, adults hatch, starting the cycle over. Females are ready to mate within five days, and can produce up to 3,000 eggs. She will breed only once, making the Sterile Insect Technique a dependable, if not quick, asset for control. Their life cycle is 21 days. Adult flies are about twice the size of house flies.
Tropical Florida and the southern tier states’ climate somewhat mimic that of NWS’s endemic home of Panama and Columbia, a most likely area for a return of the parasite. This makes cooperation with these countries, and especially Mexico, imperative. The progress made through scientific research and unwavering efforts from APHIS protect our entire livestock industry as well as wildlife. The National Agricultural Library shares a world of information on this and other topics, for further information.
And so, the battle continues. It is one that may never end.
