We only have three classes of chemical dewormers for controlling equine internal parasites. As you can see, resistance is reported to all three classes in horses around the world:
1. Benzimidazoles (e.g., fenbendazole). For controlling mature strongyles and higher, extended daily dosing for encysted third and fourth stage cyathostominae larvae. Resistance reported for cyathostomins, early indications of roundworm resistance.
2. Tetrahydropyrimidines (e.g., pyrantel). For the control of roundworms, large and small strongyles, and pinworms. Resistance reported for cyathostomines, early indications of roundworm resistance.
3. Macrocyclic lactones, (e.g., ivermectin and moxidectin). For the control of adult and migrating strongyle populations, roundworms, and stomach bots. Resistance reported for roundworms, early indications of cyathstomin resistance.
The main reason that these chemical dewormers are no longer as effective as they once were is rotation deworming. Maybe once upon a time horse owners were told that it is OK (and maybe even told it was a great idea) to routinely deworm horses, but the tides have turned. As a result of anthelmintic resistance, horses are now once again at risk for parasite-related diseases: colic, diarrhea, poor hair coat, ill thrift and poor performance, among others. Further, there are no new chemical dewormers currently headed to market in the foreseeable future.
Although this situation sounds pretty bleak, equine experts with parasite prowess and reasonable solutions to the chemical deworming conundrum currently facing the industry are working on solutions. Based on their experience and research, here are 7 tips for integrating responsible parasite control strategies in the new millennium:
1. Know Thy Major Internal Parasites
As noted by the American Association of Equine Practitioners in their Parasite Control Guidelines, the major internal parasites of concern are cyathostomins and the equine roundworm, Parascaris equorum, in adult and young horses, respectively. Resistance to both of these parasites is widespread. In the case of foals and young horses, large burdens can be life threatening, whereas disease due to small strongyles only occurs in adult horses with very high burdens.
To determine if a certain herd is harboring internal parasites resistant to chemical anthelmintics, perform a fecal egg count reduction test (FECRT). Simply collect fecal samples from six horses, usually the highest shedders, and count the number of eggs per gram of feces (EPG) for both strongyles and roundworms. Deworm the horses and determine the EPG 14 days later. Use the following calculation:
EPG (pre-treatment) – EPG (14 day post-treatment) / EPG (pre-treatment) x 100
FECRT results between 85 percent and 95 percent are suggestive of resistance, depending on the deworming product. See the AAEP’s Parasite Control Guidelines for additional details and caveats. Two important points to note regarding the FECRT are (1) this test is not advocated for testing individual horses and (2) this is the only method currently available for detecting resistance in equine parasites.
2. Ensure Owners Understand the Goal of Deworming
The goal of deworming is not to completely eliminate all internal parasites, and some owners might not be aware of this. Instead, the three main goals of deworming horses are:
- Minimizing the risk of parasite-related diseases;
- Controlling parasite egg shedding; and,
- Maintaining efficacious dewormers and avoiding further development of anthelmintic resistance to the best of our abilities.
3. Timing is (Almost) Everything
According to experts, horses need to be dewormed with properly timed treatments using effective anthelmintics that are administered at an appropriate time of the year. Healthy horses only need to be dewormed when the environmental conditions are conducive to egg and larval development and survival.
For example, strongyle eggs and developing larvae are rapidly killed in the winter months in the north and the summer months in the south simply by the adverse environmental conditions (i.e., cold and heat, respectively). Deworming is unnecessary and expedites resistance. Thus, at those times of the year, there is little benefit to deworming if the horse is not showing any evidence of parasite-related disease.
4. Use Fecal Egg Counts to Choose When to Deworm
Although the FECRT is only recommended for herds of horses, any individual horse can have a routine fecal egg count (FEC) performed. This simple test determines if deworming is necessary and identifies what product should be applied. While FECs might seem tedious based on historical methods, a novel smart phone app has streamlined the process. Ready to be released this year, the Parasight System provides onsite, quantitative FECs in less than 5 minutes and is appropriate for all sizes of operations. This new procedure therefore eliminates unnecessary deworming, saving the client money on chemical dewormers.
5. Manage the Environment
Horses can only be infected with internal parasites if they ingest the infective egg or larvae. Those eggs and larvae are passed in a horse’s feces; therefore, removing manure immediately after it is passed is an effective method of controlling internal parasites. Effective, but not really feasible save the operations with dedicated manure removal personnel or pasture vacuums. Routine removal of manure is achievable on most farms (even if it is not immediate) and is recommended. Collected manure and bedding should be composted. Pasture rotation is only effective under the right environmental conditions (e.g., in the very hot summer months in the south).
6. Support the Expansion of “Parasite Projects”
Over the past few years, Penn State University’s Equine Extension Team created a comprehensive management short course called, “Managing Equine Parasites Using a Whole Farm Approach.” The goal of the program is to empower owners to make appropriate and effective changes to their deworming practices. After participating in this course, horse owners were challenged to adopt at least two new practices to reduce parasite burdens on pastures (such as those mentioned above). The Pennsylvania Parasite Project continues in 2016 and 2017 and not only benefits horse owners in that state but also serves as a model for other states to adopt. This type of project will help horse owners evolve from using rotational deworming to employing modern and responsible practices
7. Keep Up to Date With New Developments
Nonchemical deworming strategies are being developed as an alternative for chemical dewormers (and so-called holistic and natural products that also have questionable efficacy). Examples include:
- Combination deworming that involves the simultaneous application of two dewormers from different chemical classes even if resistance to those products is evident. Do not try this at home quite yet, though. Studies are ongoing and data are not yet available;
- “Let the germs get the worms.” Using a bacterial protein to kill strongyle larvae; and,
- Ultrasounding foals to identify roundworms in their small intestines. This has already proven to be a reliable method and can easily be used on farms when foals are being scanned for Rhodococcus equi abscesses and pneumonia.