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Selecting Dewormers
Which ones do we pick and how do we use them?
With respect to the slow (same dewormer all year) vs. fast (alternating dewormers) rotation deworming schedules, there is now evidence that both can select for resistance in small strongyles. Small strongyles are the most significant parasite in horses and contribute to unthriftiness, anemia and hypoproteinemia. In addition, small strongyle larvae encyst in the walls of the colon and evade treatment with most dewormers, only to emerge at a later time and cause significant damage to the gut wall. If small strongyles develop resistance to our available dewormers, horses will be at great risk.
It is important to determine the true effectiveness of each dewormer. One way of doing this is determining the reduction in FEC following treatment with a dewormer. The degree to which the eggs are reduced reflects the drug’s ability to kill the resident adult female parasite population capable of producing eggs. The FEC prior to treatment with the dewormer is compared with the FEC taken 10-14 days after deworming. The percentage of reduction is calculated using a formula referred to as the Fecal Egg Count Reduction Test (FECRT). It is reported as a percent and the accepted values vary with the class of drug (dewormer) being tested. For example, for a benzimidazole to be considered effective, a 90% reduction or better is acceptable; however, for moxidectin, the % reduction must be 98% or more. Reasons for a lesser degree of effectiveness of the drug could be due to improper dosing, individual horse bioavailability issues or the development of resistant parasites. FECRT gives us the ability to monitor for the emergence of parasite resistance.
It is also important to understand the concept of Egg Reappearance Period (ERP). Certain dewormers have the ability to suppress the parasite’s ability to produce eggs longer that others. The macrocyclic lactones, ivermectin and moxidectin, are able to suppress egg production for 8 and 12 weeks, respectively. This is in contrast to the benzimidazoles (fenbendazole, oxibendazole) and pyrantels which only suppress egg production for approximately 4 weeks. Both the FECRT and ERP are important indices of the effectiveness of the dewormer in a horse. It is through these indices that we can identify horses that are not responding to dewormer or may be harboring resistant parasites.
How else are parasites controlled?
The horse’s immune system also plays a role in whether or not parasites are a major concern. The strongyle contamination potential (SCP) is used to determine a horse’s innate ability to limit strongyle infection. SCP is determined by repeating the FEC four weeks after the ERP of the previously administered dewormer. At this time, the effects of the dewormer should be completely gone and the innate ability of the horses' immune system to limit egg production is apparent. SCP is graded as low (<150 epg), medium (150 - 500 epg) and high (>500 epg). Horses with a low SCP have a better functioning immune system with respect to limiting internal parasite populations.
Probably the most straightforward way to control parasite resistance is to minimize pasture contamination. Manure removal is best but impractical under many situations. Dragging of pastures should only be done when the environment is least favorable for larval development, i.e., extreme heat and lack of moisture. Pastures should never be drug after October 1st.
Introducing new horses into the herd can introduce resistant parasites to a previously sensitive population of parasites. Farms need to take precautions to prevent this from happening. This is done by performing FEC on all new horses. Based on results of the FEC, a horse may need a larvicidal treatment or simply one dose of a macrocyclic lactone, but the only way to know is to perform the FEC before and after treatment and calculate the FECRT. New arrivals should be quarantined for two weeks until the FECRT is performed and it is known that the EPG has been effectively reduced by the chosen dewormer.
Typical appearance of horse with heavy small strongyle infection and high EPG.
Some General Recommendations:
As a general recommendation, all horses regardless of age, body condition, etc., should have a FEC performed in late March/early April in the northern hemisphere. This is the time when infective larvae are low on the pastures due to the previous colder winter temperatures (<45F).
Horses with FEC greater than 500 epg need a larvicidal treatment with five days of double dose fenbendazole or moxidectin (note that moxidectin has contraindications - please read the label!) These horses especially need to be monitored with frequent FECs as they are the pasture contaminators. After the larvicidal treatment, another FEC should be done at 2 weeks to determine the FECRT. Another FEC should be done based on the ERP of the chosen dewormer (12 weeks after treatment for moxidectin, 4-8 weeks after treatment for fenbendazole). Subsequent deworming should be performed based on the ERP.
Horses with EPGs >150 epg should be dewormed with ivermectin and another FEC should be done 2 weeks after treatment to determine the FECRT. After this evaluation, FEC are repeated at intervals according to the ERP of the prior dewormer. Subsequent deworming is done in individuals with FEC that exceed 150 epg. Within this program, fall deworming should include a macrocyclic lactone/praziquantel combination to target Gasterophilus spp. (bots) and Anoplocephala spp. (tapeworms).
Horses with EPGs < 150 epg may not need to be dewormed or alternatively, one dose of ivermectin could be given and the SCP determined. These horses may only need two dewormings/year. As above, a fall deworming should include a macrocyclic lactone/praziquantel combination to target Gasterophilus spp. (bots) and Anoplocephala spp. (tapeworms).
Remember to protect your pastures and identify those horses with high FEC before they contaminate it for all of their pasture mates.
What are the benefits of this approach?
The benefits of this approach to deworming may at first seem to be outweighed by the effort to collect fecal samples and the cost of testing. However, these approaches over time will greatly reduce the use of dewormers and thus reduce overall costs by only focusing on deworming those horses that truly need deworming. Another benefit is to identify those horses that are at higher risk of parasite-associated disease and individually address their needs. In addition, for those that are concerned about the repeated use of chemicals, some of which are known to occasionally result in adverse effects, minimizing the use of dewormers may be very attractive.
In conclusion, we at TOVS hope that this information has cleared up much of the confusion out there regarding deworming. As you can see, there is no general recommendation for all horses because each horse must be treated as an individual. Remember that this only applies to adult horses, foals are dewormed differently. Remember to always read the label and instructions on every dewormer. This is overlooked but is very critical and could mean the difference between life and death or a very serious adverse reaction.
Parasite season is upon us! Please call Dr. Woerner at 540-338-7081 to discuss how to incorporate these new concepts of deworming in your horses.
Parasitology By Dr. Tania Woerner
February 2010
As you may have heard, there are major shifts in the way dewormers are being used in adult horses. The reason that we are changing our ideas about how to use dewormers is that equine parasites are developing resistance to many of the major drug classes of dewormers. This is of great concern to equine parasitologists and veterinarians. Unfortunately there are no new drugs on the horizon and there is no simple way to address this. The key is understanding the relationship between the parasite, host and environment. By understanding this relationship we can design more effective and less frequent deworming protocols that will minimize the development of resistance.
Fecal Egg Counts (FEC)
What are they and how are they used? Rational application of routine quantitative Fecal Egg Counts (FEC) are needed to differentiate horses that need deworming from those that do not. FEC are performed at TOVS on a fresh fecal ball or equivalent amount of loose manure using the quantitative McMasters fecal test. The eggs that are counted are strongyle-type eggs, characteristic oblong shaped with an inner cell mass.
Typical strongyle-type egg as seen under microscope
Typically FEC in adult horses <150 epg are considered low, 150-500 epg - medium and >500 epg - high. Horses with low FEC are at lower risk of parasite-associated colic and do not significantly contaminate the environment with eggs. Horses with medium FEC are more at risk of colic and contribute to the pasture contamination problem. Horses with high FEC are definitely at risk of parasite-associated colic and significantly contribute to pasture contamination. In addition, individual horse variability in susceptibility to parasites exists, and some horses will have higher FEC than their pasture-mates under identical conditions. The goal is to identify the more susceptible horses with higher FEC and focus on deworming these horses effectively. This changes the old-fashioned way of deworming all horses in the herd simultaneously with the same product.
