Erin Oberhaus, Neely Heidorn and Adriano Vatta

All grazing horses are susceptible to internal parasites or worms, and deworming is an important management practice to ensure the continued health and well-being of horses. An overburden of intestinal and stomach parasites can interfere with nutrient absorption, leading to weight loss. Additionally, poor hair coat, fever, diarrhea, decline in performance and abdominal pain known as colic can be attributed to an overburden of internal parasites.

Over the last several decades, the recommended approach to deworming has changed as resistance to available dewormers has increased. In years past, deworming was recommended at regular intervals (e.g., every two to three months); however, overuse of dewormers has led to resistance. A more individualized approach is now recommended based on the parasite egg-shedding status of a particular horse. Fecal egg counts (FEC) are a way to determine the egg-laying activity of strongyles (Strongylus spp.) and roundworms (Parascaris equorum), both of which are parasites found in the intestines of horses. Strongyles, including both large and small, are commonly found and present the greatest threat to an adult horse’s health. Large strongyles are particularly harmful as they can cause significant damage to the lining of blood vessels. Fortunately, the prevalence of large strongyles in the United States has been greatly reduced, and small strongyles (cyathostomins) now represent most parasite eggs seen on a fecal exam. Adult horses are largely unaffected by roundworms; however, roundworms do pose a significant threat to horses under 2 years of age.

A fecal egg count is made by examining a fresh fecal sample and counting the number of eggs per gram (epg) of feces. The shedding status is then determined based on the number of eggs. While FECs are a great way to assess the shedding status of a horse, they do not determine the risk of disease associated with parasites in that horse. In other words, high-shedding horses are not necessarily at a greater risk of parasite-associated disease. However, many of the eggs that are shed will develop into infective larvae in a pasture, and these pose a risk to other horses, who may be more susceptible to parasites. So, what exactly is the shedding pattern of groups of grazing horses in southern Louisiana?

Researchers at the LSU AgCenter and LSU School of Veterinary Medicine joined together to determine just how many strongyle eggs per gram of feces were being shed over the course of one year by horses residing in herds at the Doyle Chambers Central Research Station horse unit in Baton Rouge and the Reproductive Biology Center in St. Gabriel. Fecal samples were collected from 68 mares and geldings every two weeks for one year (2021-2022). The horses were divided into two groups: those that were dewormed with ivermectin only in spring and fall, and those that received dewormer when individual FECs were greater than 500 epg. One of the goals of deworming horses anytime FEC exceeded 500 epg was to determine the amount of time if took for the eggs to reappear after deworming. One of the first indications of dewormer resistance is a shortened egg reappearance period following deworming with any of the three classes of dewormers. Ivermectin and moxidectin are the most widely used compounds in equine management, and the average egg reappearance periods in susceptible parasites are eight weeks and 10-12 weeks, respectively.

From the yearlong study, it was determined that the average FEC remained below 1,400 epg for horses at both farms and appeared to be highest during the summer months (July to September) and lowest during the winter (Figures 1 and 2). The egg reappearance period after deworming with ivermectin was approximately six weeks in the summer and 10 weeks in the fall for the horses at the Central Research Station, and six weeks in both summer and fall for horses residing at the farm in St. Gabriel.

Resistance to ivermectin (for example, Durvet Ivermectin Paste) and moxidectin (for example, Quest Gel) has progressed more slowly in horses than to compounds such as fenbendazole (for example, Panacur) and oxibendazole (for example, Anthelcide EQ); however, evidence of resistance to ivermectin and moxidectin has now emerged, and owners are now tasked with using dewormers more judiciously. With few new deworming compounds on the horizon, owners are now encouraged to deworm all horses only once or twice per year with additional deworming as deemed necessary for high-shedding horses. It is worth noting that all horses in the study were apparently healthy with no evidence of parasite-related disease. Fecal egg counts as high as 3,800 epg were recorded for some horses that persisted throughout the summer. This raises the question as to whether higher parasite loads should be left alone in the absence of clinical symptoms. Of course, special considerations may be needed for horses who have evidence of parasite-related illness or decline in performance.

One interesting finding of the study was the prevalence of tapeworms in the herds year-round. Fecal egg counts do not accurately assess the degree to which a horse is infected with tapeworm infection, but if their eggs are detected on fecal examination, this indicates that a horse is infected. Only one compound, praziquantel, is approved for killing tapeworms in horses. In the present study, tapeworms reemerged fairly soon after deworming, which was attributed to their long life cycle and the inability of any dewormer to target all developmental stages. These findings did, however, shed light on the year-round tapeworm population in grazing horses in Louisiana. In extreme cases, tapeworm burdens can contribute to certain types of digestive upset (colic), such as an obstruction of the intestines. Therefore, it is recommended to include praziquantel as a compound in at least one yearly deworming. Parasite management of the horses residing at the research stations now includes deworming with a praziquantel-containing dewormer during the fall.

Individualized deworming protocols, as opposed to treatment of all the horses on a farm at defined regular intervals, are gaining acceptance but require knowledge of seasonal egg shedding patterns and individual horse susceptibility to parasite infection. However, in an industry survey distributed by LSU researchers and extension professionals, 73.4% of respondents indicated that they do not rely on fecal egg counts to determine their deworming practices and outdated practices such as deworming every other month are still prevalent. Fifty-five percent of respondents also indicated that they were not concerned with their horses' current parasite load and that they strongly believe current deworming products are effective. The goal of parasite management in horses and other grazing livestock is not to eliminate parasites but to reduce the risk of disease associated with a parasite burden while preventing further resistance due to overuse of the products. It is apparent that there is a gap in the acceptance of new recommendations for deworming and additional education is warranted. Over-the-counter deworming products range from $10 to $20 per treatment. Owners who reduce the number of dewormings to twice per year can save an average of $60 per horse per year and, more importantly, prevent further contribution to dewormer resistance.

Erin Oberhaus is an associate professor, and Neely Heidorn is the state equine extension specialist and an associate professor in the AgCenter School of Animal Sciences. Adriano Vatta is an associate professor of parasitology at the LSU School of Veterinary Medicine.

This article appears in the fall 2024 edition of Louisiana Agriculture magazine.

Horses graze in a pasture at the LSU AgCenter Doyle Chambers Central Research Station in Baton Rouge. Photo by Olivia McClure

Erin Oberhause places a bridle on a horse at the LSU AgCenter Doyle Chambers Central Research Station in Baton Rouge. Photo by Olivia McClure

A horse grazes at the LSU AgCenter Doyle Chambers Central Research Station in Baton Rouge. Photo by Olivia McClure