The best offense is a strong defense

In a decade marked by the impacts of the coronavirus pandemic, followed by the outbreak of avian influenza in the U.S. dairy herd, American farmers and animal health professionals are watching with bated breath as the New World screwworm has inched closer to the country’s southern border.

Dr. Adis Dijab, of the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service, Dr. T.R. Lansford III, of the Texas Animal Health Commission, and Dr. Sonja Swiger, of Texas A&M University, spoke about NWS in a webinar hosted by the National Milk Producers Federation.

The trio discussed its status and measures being taken to keep it from crossing our border.

NWS is a parasitic fly that feeds on the flesh of warm-blooded mammals — including humans. The adult flies are attracted to wounds and body openings such as the ears, nose, mouth and eyes, where they lay their eggs. The eggs hatch into maggots, which eat live tissue, worsening the area where they were laid.

The parasite was eradicated from the U.S. decades ago, but it is present in parts of South America. NWS has been spreading north, through Central America and Mexico.

NWS is not in the U.S. yet, but the scientists agreed this is the time to prepare in the event it makes its way into the country.

“There are things to be thinking about to enhance management,” Lansford said. “We need to start having those conversations and increasing awareness, not only as dairy industry leaders, farm owners and managers, but also certainly at the level of your employees, talking about awareness of wounds, monitoring and treating those wounds promptly, not only for NWS, but in general. The practice of identifying those wounds and doing early treatment is critical.”

Areas to be on the lookout for include ears where tags have been applied, castration and dehorning wounds, infected hoof tissue and the navel of newborn calves.

Developing an understanding of the fly’s life cycle and biology is crucial for mounting a strong defense. Dijab said that the entire life cycle of a NWS can be completed in 21 days. The fly functions best in temperatures above 55 degrees, Dijab said, noting that temperatures below 40 degrees can kill the pest. The fly enjoys shade and does best when its food source is high in moisture, such as greenery and flesh.

“These flies are lazy,” Dijab said. “The males usually spend 70%-80% of the time grooming themselves and being in the shade. The females are more aggressive from a feeding standpoint; however, they’re not really good flyers. Some research shows they can fly probably 1-1.5 miles a day, depending on wind speeds. They will not leave a specific area if they have their meals and debris.”

Swiger said the flies’ behavior is a plus when it comes to protecting our nation’s agricultural sector.

“On their own, they’ll maybe fly 6-12 miles in their lifetime,” Swiger said. … “One of the main issues we have with the spreading of NWS has to do with the movement of animals that are pre-infested with larvae. Sometimes this is done without knowing because the larvae themselves are inside a body cavity. If there is a small wound, it would go unrecognized for a very long time.”

Besides domestic livestock movement, Swiger said wildlife presents an additional challenge.

When an infested animal carries NWS into an area, eggs hatch into larvae that feed on the host animal. Those larvae develop into adult flies, which will become spreaders themselves, searching for hosts where they can lay their eggs. The larvae tend to pupate outside of the host, dropping away and frequently burrowing into the ground.

Additionally, if an infected host animal dies, the larvae will not subsequently die. Movement of dead animals becomes another way to spread the NWS.

“The most important thing about early detection and reporting is to get proper treatment,” Swiger said. “Our goal is eradication. We need to report to do that job. Without early reports, we end up falling behind the game.”

In an effort to surveil the U.S. border for signs of NWS, traps are being used to monitor adult fly activity.

Flies that have been sterilized using cobalt radiation — a tool Swiger said is a must — are being released in an effort to prevent females from laying viable eggs.

Treating an animal infected with NWS is challenging, Swiger said, making prevention paramount.

“One of the most important things to do is routinely inspect livestock for maggots,” Swiger said. “In a dairy, you’re most likely going to interact with these animals more frequently than we see in many of our other commodity groups. It’s very important to treat wounds promptly. We want to make sure we’re preventing the fly’s ability to find the wound. If the wound does occur, we want to protect that wound from attracting a female to lay its eggs.”

Lansford said he believes vigilance, enhanced management and biosecurity are the best weapons available to keep NWS from impacting the U.S. livestock herd.

“Routine inspection of livestock to identify and treat wounds prior to any infestation and certainly treating early infestations is critical to mitigating the risk associated with NWS,” Lansford said. “One thing we’ve learned from our colleagues in South America — where they’ve always had this, where it’s endemic — is that good management equals good prevention. Their well-managed operations don’t have much of an impact from NWS.”