New World screwworm: What veterinary professionals need to know

New World screwworm is back in the headlines, which means it is also back on the radar for veterinarians across the US. In this episode of Vet Watch from dvm360, host Christopher Lee, DVM, MPH, DACVPM, DACVM (parasitology), welcomes Kathryn E. Reif, PhD, MSPH, Bailey Goodwin Endowed Professor of Parasitology at Auburn University College of Veterinary Medicine, to separate science from panic on one of the most talked-about emerging animal health threats.

The conversation covers the parasite's temperature-driven spread across the US, current documented cases in cattle, sheep, goats, and dogs, and the looming risk to wildlife. Reif also explains the sterile insect technique behind past eradication successes, the race to scale sterile fly production, FDA-authorized and conditionally approved treatments, and the real-world impact on beef prices and food safety.

Below is a partial transcript, edited lightly for clarity

Christopher Lee, DVM, MPH, DACVPM, DACVM (Parasitology): Which animals are at risk? Is this mainly a cattle thing, or should pet owners, shelters, horse owners, or even wildlife teams be paying attention too?

Kathryn E. Reif, PhD, MSPH: We hear about it most commonly in the context of cattle, because that's probably the species where it has the potential for the greatest economic impact. Just in the cattle/Texas economy alone, they estimate that the impact of New World screwworm annually to the Texas cattle economy could be close to $2 billion per year, 1.8 or 1.9 billion I think, is what the experts are estimating, so we oftentimes hear about it in the context of cattle.

But it's important to note that these flies are not picky, and as long as it is a living, warm-blooded host, it is susceptible. So, the cases that we've already heard about, or have been documented in the US so far, we have 29 cases that have been documented, about 17, if my counting is correct, are cattle, but we also have cases in sheep, we have cases in goats, and we even have cases in a couple dogs. So, really, again, any warm-blooded living creature is a potential, with an open wound, is a potential host for this really terrible fly, and even humans aren't immune.

There have been over 2000 cases of humans being infested with New World screwworm in Mexico, and last year we actually had a human case. A gentleman that was reportedly infested with New World screwworm, he went to a clinic in Maryland. They found the maggots in the back of his head, but he had had a recent travel history to El Salvador, which is a country that is endemic for New World screwworm. So pretty much any living warm-blooded creature is at risk for this.

Lee: Katie, you make me want to stay indoors.

Reif: Well, the good thing is they do have certain temperature requirements, and for those people loving to live in air conditioning, you probably won't get infested indoors.

Lee: Now, speaking of that environment, how far can this theoretically spread in the United States. How fast?

Reif: Yeah, so this fly is really temperature dependent. It really thrives in temperatures like really, really thrives high-risk regions in areas where the temperature is over 80 degrees Fahrenheit, and for most of the US, and right now we're experiencing this crazy heat wave, we can exceed 80 degrees Fahrenheit in that summertime season.

It starts to do less well when we get between like 60 and 75 degrees Fahrenheit, and it doesn't do very well at all under 60 degrees Fahrenheit, so really the threat of where we may see this fly is quite see in the US is quite seasonal. So many areas in Texas, and maybe even some areas in the southernmost regions of Florida, like Florida Keys, for example, they may have kind of year-round suitable habitat for this fly, but then as we go further northward, it becomes less suitable on a year-round basis, but seasonally certain states or regions may be able to support this fly, should it be introduced, for many months.

The fly can overwinter in its middle life stage, so to speak, in its pupa life stage in the environment, kind of in that top layer, layer of soil for about three months, for about three months, unless that temperature persistently stays below about 48-49 degrees Fahrenheit, so it does have a little bit of cold tolerance in that pupil stage, but the adult flies and the larvae and those wounds really require that warmer weather. So we can say anywhere from about 75 to 95 is really that moderate to high risk temperature with humidity being about 30-70%, so kind of a wider acceptable humidity range.