By Stephen Smoot
After over a year of shepherding resources to deploy against dangerous strains of the avian flu, agriculture and health officials are now warning about a parasite whose infestation can lead to significant infestations in some birds and most mammals, including humans.
Last week, Reuters reported that Mexican officials have reported a 53 percent spike in instances of New World Screwworm infestations in livestock. Geographically speaking, the incidence of these continues to move northward toward the United States.
“The updated numbers, which have not been previously reported to the public, showed 649 currently active cases,” reported Reuters. They quoted Neal Wilkins, Chief Executive Officer of the conservation and cattle oriented East Foundation, as stating “that’s absolutely concerning. Having a 50 percent increase in month-over-month numbers, particularly in the extreme heat, means they haven’t gotten it under control.”
Also significant, the Maryland Department of Health last week announced a Centers for Disease Control confirmed case of a human infestation. In a release, they stated that officials investigated a “travel-associated New World screwworm (NWS) myiasis in a resident of Maryland who had returned from a country currently experiencing a NWS outbreak.”
In the same release, they sought to dampen public fears by explaining that “the Maryland resident has recovered, and the investigation confirmed there is no indication of transmission to any other individuals or animals. Currently, the risk to public and animal health in Maryland from this introduction is very low.”
According to the Farm Bureau, states in climatic temperate zones face a lower level risk to livestock, saying “the larvae cannot survive temperatures below 46 degrees Fahrenheit. However, a cold climate does not eliminate the threat.” It goes on to explain that “livestock and wildlife movement can transfer NWS to the northern regions of the country in the summer months where they can find hosts and spread.”
The flies “are able to travel as far as 12 miles to find suitable hosts” and “does not discriminate among warm-blooded hosts.”
“NWS is a devastating pest,” states a USDA release, which goes on to explain that “when NWS fly larvae (maggots) burrow into the flesh of a living animal, they cause serious, often deadly damage to the animal. NWS can infest livestock, pets, wildlife, occasionally birds, and in rare cases, people. It is not only a threat to our ranching community — but it is a threat to our food supply and our national security.” The fly breached the United States in the 1960s and was eradicated within the country, but not everywhere. Officials also quickly stamped out an outbreak on the Florida Keys in 2016.
States along and near the southern border face a higher risk. The most current outbreak started in 2023. As Reuters explains, “screwworms have advanced northward from Central America into Mexico and have inched closer to the U.S. American ranchers and livestock industry players have closely monitored the fly’s progress northward.”
Should the infestation jump the border, Texas alone could see a $1.8 billion loss with ramifications for the beef market in the United States and around the world.
The Trump Administration has moved decisively to confront the threat. Last May, the United States Customs and Border Patrol closed the southern border to imports of cattle, bison, and horses. On August 15, Brooke Rollins, United States Secretary of Agriculture, announced alongside Texas Governor Greg Abbott a five part plan.
This includes a close partnership between the USDA and the Mexican government, create a “barrier zone of vigilance” on the US/Mexican border, work with state and local officials on creating emergency response plans, “exploring all options” to eliminate NWS, and “innovate our way to eradication.” The goal is complete elimination of the species, and therefore, the threat to animal and human life.
As the Farm Bureau explains, the only effective tool developed thus far has been the Sterile Insect Technique. Female flies only mate once in a lifetime and the release of sexually active, but sterile, male flies into the environment helps to drive down numbers.
Rollins announced that the United States spent $21 million on a Mexico-based facility that has the capacity to produce and release 160 million sterile NWS males flies into the wild each week. This technique had proved successful in the Florida outbreak.
As the CDC states, infestations start when a female fly lays eggs in the wound or other opening that allows entry into the animal. The wound could be as small as an insect bite. She can lay up to 200 to 300 eggs at a single time and 3,000 total in a lifespan of 10 to 30 days from a single insemination.
Damage takes place over the next seven days as larvae hatch in the animal’s flesh and consume significant amounts of tissue before dropping off the animal and then metamorphosing into adult flies. Though not visible until the third day, larvae cause considerable pain to the host. Unlike most flies whose larvae remain at the surface and even have beneficial medical applications, NWS larvae burrow deep into the animal and threaten vital organs.
Experts differ on treatment. The CDC states that “the only way to treat NWS is to physically remove the larvae from the infested tissue. All suspected NWS larvae should be collected and placed into a leak-proof container with 70% ethanol.”
Conversely, the Farm Bureau advises that “Nitenpyram is highly effective for killing and expelling NWS larvae quickly.” It also recommends applying anti-parasitic medications such as lindane or ronnel.
The Farm Bureau also states that the still politically controversial Ivermectin “has been highly successful in treating and even preventing NWS infections. The CDC, however, states that “there are anecdotal reports of using ivermectin in off-label treatment of NWS infestations in humans. However, there are no controlled, double-blind studies measuring the impact of ivermectin use on myasis. (the name for the resulting infection from a parasitic maggot infestation of living tissue)”
Suspected sightings of NWS flies, larvae, or related infections on mammals, birds, or humans should be reported immediately to the West Virginia Department of Agriculture and/or the USDA to ensure rapid response and containment measures.
