Resistance remains the biggest challenge with parasite control in horses. Each year, the efficacy of dewormers declines.
“We used to be able to control different life stages of each parasite and a wide range of different parasite species with a single treatment, but that’s not true anymore,” said Martin Nielsen, DVM, PhD, DACVM. “Each product still has a place, but we no longer have the broad spectrum control there once was.”
The issue of resistance is compounded by the lack of development of new products. Pharmaceutical companies have not developed new deworming medication classes for use in horses since 1981. Nielsen, an associate professor at the University of Kentucky’s Gluck Equine Research Center, said that it is concerning that the same products are on the market today that were there three decades ago.
While pharmaceutical companies have not provided new medical solutions, researchers are committed to finding new solutions for parasite control. In the article that follows, Nielsen highlights cutting-edge research that might bring promising methods for control in the future.
Traditional deworming regimens recommended rotating drug classes to slow parasite resistance, but rotation has been found to have no effect on the development of resistance. Research in Australian and New Zealand sheep flocks shows that using a combination of dewormers can be more effective and sustainable than rotational deworming, and there are a slew of combination products on the market there.
“Veterinarians are stuck up against a wall without an effective treatment, especially for strongyles and ascarids,” Nielsen said. “I receive a lot of calls from veterinarians who ask about using products in combination.”
Nielsen is leading a study that is evaluating how horses with multiple-drug-resistant parasites respond to treatment from combination dewormers. The study's second year was recently completed, and it will continue into its third year in 2018.
“Hopefully we can be at the forefront of combination deworming in horses,” he said. “We’re hoping to learn how well this approach will work in the short and long term, and I think it will be a very important approach.”
In organic agriculture practices, using naturally occurring biological controls is an important part of pest control. Organic growers use naturally occurring bacteria to keep beetles and other arthropods from feeding on plants. Through a project Nielsen calls “Let the Germs Get the Worms,” he is applying this concept to parasite control in the equine population.
“This study is examining members of the <i>Bacillus</i> family, which produce a group of proteins with lethal effects on certain groups of parasitic worms,” he said.
Nielsen was the first researcher at the University of Kentucky to use crowdfunding to secure financial support. The initial investment jumpstarted the project and provided traction to achieve USDA funding that went to four different institutions to study the concept in different livestock species.
Determining a formulation of the bacterial product that can make it intact to the large intestine is key. “These are naturally occurring bacteria, not ones that would infect a horse, people or other animals,” he explained.
Fecal Egg Counts
Deworming protocols based on fecal egg counts have been recommended for decades. Unfortunately, the results from the 2015 National Animal Health Monitoring System (NAHMS) survey show that there is still a long way to go to get people to adopt the recommendations of deworming based on a fecal egg count.
“A low percentage of respondents reported that they regularly use fecal egg counts,” he said. “There are still a lot of places that don't adopt recommendations based on parasite fecal egg counts, and this could be for a number of reasons. We will be learning about those in 2018 as the NAHMS reports are released.”
Nielsen added that further education is still needed to understand and interpret egg count results.
“In my experience, some people who really get into fecal egg counts do more than recommended, and this sometimes can create new challenges,” he said.
For example, young horses are rarely going to have negative egg counts, because it’s natural for that population to have strongyle and ascarid counts into the thousands.
“That sometimes makes people uncomfortable because they would rather have a zero, a clean slate. But that's just not reality,” he said. “A horse may have a 1,000 or 2,000 eggs-per-gram count, but how does it look? If the horse looks great and has no clinical signs, it doesn’t have a problem. It only has parasites, which is normal, especially for young horses.”
Temperature and rain play a significant role in parasite transmission, and the duration of the grazing season varies widely geographically. In some areas, there are two distinct parasite transmission cycles—in the spring and fall—with less favorable conditions for parasite transmission in between. In other areas, there is one short parasite transmission peak over the summer.
“Veterinarians and horse owners are tired of hearing there is no one-size-fits-all solution for equine parasite control,” Nielsen said.
Designing a customized treatment might soon be less complicated.
“A computer model has been developed to simulate the strongyle parasite lifecycle. The model makes use of weather station data, which can be imported from any weather station in the world. We typically base our simulations on 10 years’ worth of weather data for a given geographic region,” he said.
The model is able to create an output with a percentage of parasite eggs that hatch and develop to the infective stage in different months of the year, and charts can be printed out for different regions of the world.
“In a future scenario, charts can be made available by state or region, so that a veterinarian and barn owner can customize treatment,” he said. “There are a lot of nuances to the project, but it's turning out to be very interesting.”
A World-Wide Perspective
While parasite resistance is a significant concern for veterinarians and researchers, Nielsen said the biggest challenge in parasite control is striking a balance between too little and too much use of anthelmintic products.
In Denmark, his native country, and other countries in the European Union, dewormers are regulated on a prescription-only basis. Only veterinarians can prescribe these products, and horse owners and facility managers are unable to buy dewormers over the counter. This ruling was introduced because of concerns regarding drug resistance.
“This approach has worked really great in relation to the intensity and frequency of treatment, and it has encouraged veterinarians to get much more involved with parasite control," he said.
The approach might have delayed development of resistance, but it has increased the population of large strongyles (bloodworms), which definitely is an unwanted negative side effect.
“Bloodworms are endemic in Danish and Swedish horse populations, and they are causing significant disease in some horses,” he said.
Conversely, in the United States, horses don't tend to be affected by large strongyles due to the frequent anthelmintic treatments. However, this comes at the cost of very high levels of resistance in small strongyle and ascarid parasites.
“We have to keep in mind that parasite disease only occurs very rarely in horses,” noted Nielsen. “For example, disease caused by small strongyle parasites is rarely reported here in Kentucky. What we do see is tapeworm- and ascarid-associated diseases, but in a very small proportion of horses. As long as we keep large strongyles out of the picture, which we currently do well, we are unlikely to see many cases of severe parasitic disease.”
Parasite resistance is an issue that isn’t going away, but researchers such as Nielsen are working hard to find effective alternative treatments. Stay tuned throughout 2018 and into the future as the research projects are continued, concluded and released for publication.