New World Screwworm Update

November 2024 brought a shocking announcement as Mexico confirmed a positive case of the New World Screwworm (NWS), a long eradicated pest that was detrimental to the US cattle herd for decades prior to the 1960s.  Learning that the parasitic fly was once again infecting cattle through Central America has been disconcerting as it continues to makes its path northward to the US.

The US government was quick to stop live cattle imports from Mexico in an effort to protect American cattle herds after learning of the outbreak, and rightly so.  With herd numbers the lowest they have been in 75 years, the devastating parasite could wreak absolute havoc on the cattle industry and market.  

But closing the borders to Mexican cattle came with its own drawbacks, its consequences resulting in low beef supply and increased prices.  In an effort to both protect the US cattle herd but also continue trading with Mexico to meet American beef needs, newly appointed Agriculture Secretary Brooke Rollins lifted the ban on live cattle imports on a month-by-month basis beginning in February of 2025.

In the proceeding six months, no NWS has been detected connected with any Mexican imports.  Still, possibility of the parasite crossing into the US is a very real concern as confirmed cases continue to move northward.  Reports indicate that the flies have been detected in farms as far north as Oaxaca and Veracruz, a mere 700 miles from the US-Mexico border.  

While most eyes have been on Texas to watch for the first NWS to come into the country, it was Maryland that made an official finding and announcement of the detected parasite.  Instead of being found in cattle, however, this fly had infected a human host.  The screwworm hitched a ride from El Salvador, a country currently on the infested list.  Fortunately the traveler was treated and the parasite eradicated before any transmission had occurred.  But the infection was an abrupt cue for everyone involved to remain vigilant in monitoring for the NWS. 

The NWS hoards had been kept at bay, held at the Panama-Colombia border for decades by the continual release of sterile male flies from a factory established in Panama.  As the female flies would mate with the sterile males, they were unable to lay any viable eggs.  Unfortunately, the invisible border was breached in 2022 and the spread northward began.  

The cause of the breach is unclear, but many attribute it to the effects of the covid pandemic.  During the worldwide disruption, supply chain issues involving the Panama fly factory interrupted the consistent release of sterile flies.  Cattle inspections disrupted during the pandemic throughout Central America could also carry the blame of the eventual outbreak that has led to the current infestation.  

2024 saw the parasite’s most rapid spread and it moved from Panama through Costa Rica, Nicaragua, Honduras, Guatemala and Mexico.  Proceeding the 2022 breach, less than ten outbreaks had been seen a year.  Since then, a total of 788 outbreaks have occurred across five countries, resulting in 3847 cases among livestock, spreading 920 miles from the point of origin.  Researchers estimate the current spread of the NWS is 0.7 to 1.2 miles a day.

Farmers, ranchers, cattlemen, and industry advocates alike have been leading the call for action against the parasite, looking for solutions before the problem is within the US borders.  Recognizing the potential fallout, USDA calculated that an outbreak could cause a total economic loss of more than $10.6 billion.

The US is ramping up their efforts to combat the infestation heading for their borders.  Returning to the same technique that once eradicated the parasite, the government is once again in the midst of a NWS sterilization program.  The ultimate goal of the program is to have millions of sterilized male flies ready to be released to breed the hoard of infesting female NWS.  

While the program is already underway, expanding the program to multiple facilities is a priority for the US government.  A new sterile-fly production facility is planned to take shape at an Air Force base in Edinburg, Texas with plans for it to produce 300 million sterile flies weekly.

In addition to the sterile insect technique program, the US is also working to increase border monitoring with “tick raiders,” specially trained dogs trained to detect screwworm infestations in livestock.  

With the NWS parasite eradicated for more than 50 years, it had become a pest of the past.  But with its resurgence, the memories of those long years have quickly come back.  Government officials, producers, and consumers can only hope that the efforts being made are enough to keep the devastating parasite from entering the US cattle herd.  

At the current rate, the devastating fly will be crossing the Texas border in just over two years.  But if there is anything I know about those people that advocate, protect, and promote this industry, it is that they won’t give in, give up, or stop their work until they are assured that our herd is safe.  We may still be in the early stages of this reinfestation, but I’m putting my money on the man with the swatter.  One way or another, he is going to get those pesky flies.