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We are all familiar with deworming and dewormers. There has recently been a renewed focus on parasites in the research community due the fact that we are seeing more horses with worms that have been on a regular deworming program. This has raised flags regarding the effectiveness of our current deworming protocols. For a long time, we have seen drug resistance in antibiotics. There are some bacteria which gain a genetic mutation rendering some antibiotics useless against them; we are now seeing a similar move in parasites. There are a growing number of regions, including B.C., in which we are seeing parasite resistance. These parasites are not killed by the majority of the dewormers currently in our arsenal. This has the potential to become a big problem. Unlike antibiotics, of which we have many, we only have 3 commercially available types of dewormer. Sure, there are plenty of brand names at the feed store, but they are all based around 3 different types of drugs: benzimidazoles (panacur), pyrantel pamoate (strongid), and macrocyclic lactones (ivermectin, avermectin and moxidectin). An important thing to remember is that many dewormers have identical active ingredients; for instance eqvalan, bimectin, zimermectin and panomectin are identical wormers. Macrocyclic lactones (Eqvalan and Quest) are the only class which remain consistently effective.
There isn’t just one kind of parasite out there, but luckily, there’s just one main group we’re having trouble controlling at this point. In mature horses, the parasite of main concern is small strongyles (cyathostomes). Clinically, they can cause a problem, not when they are full grown worms, but when they emerge from the larval stage where they are living in the wall of the large bowel. The horse ingests the infective larvae and they invade the intestinal wall. They then live in the mucosa and form fibrous cysts; this causes an inflammatory response in the tissues. They can remain dormant for months and in some cases years. The most damage is caused when the cyst wall ruptures and mucosal inflammation occurs. If a large number emerge at one time widespread inflammation of the bowel can occur. This can cause colic, weight loss and diarrhea, amongst other things.
The good news: The other main parasites seen in mature horses are still susceptible to our current dewormers. Large strongyles were previously of the most concern, their larval stage migrates through the vasculature of the intestine and can cut off blood supply to sections of the small intestine causing a serious colic. Large strongyles need 6 months from ingestion of larvae to shedding of eggs. That means twice yearly deworming will render these worms pretty well under control. Roundworms (parascaris equorum) are not often a problem in mature horses. This is because the horse develops a natural immunity to them after about a year and a half of age. They can definitely cause problems in youngsters, but they are not often found in mature horses, and that means that we don’t need to worry much about them when we’re designing a deworming protocol. Tapeworms (anoplocephala spp) are still a concern as they have the potential to cause colic, and yearly treatment is warranted. At this point, we have these worms under control, but that is not to say that it will stay that way forever. We need to continue to be diligent and ensure that our protocols are effective against them to make sure that it stays that way.
More good news: Worms aren’t problems in all horses, in fact, 20% of horses are responsible for 80% of the fecal egg shedding. What this means is that all horses do not need the same deworming protocol. If we can control the shedding from those with the highest fecal egg counts we’ll be doing an even better job at reducing environmental contamination and managing the parasite burden of the entire herd. How do you know which are the highest shedders? Well, that’s simple. We need to test them. We now have more effective ways of testing than before; new and more accurate methods are currently being developed. A quantitative fecal egg count (FEC) is the best way of doing that. Knowing how many eggs there are to start will then give us a point of comparison to refer back to when we recheck the fecal egg count after we deworm them. The timing of these evaluations is important. If we’re curious about whether our current dewormer is effective we compare the pre and post deworming FEC. We consider a dewormer to have been effective if the FEC is reduced 90-95% depending on the product used. When we want to evaluate the parasite burden in a given horse to determine if they are a heavy or a light shedder, we want to check it 4 weeks after fenbendazole or pyrantel and 2-3 months after ivermectin. If the horse has significant burdens then we classify them as a highly contaminative individual and they need a deworming program that is more rigorous than one that has a low initial fecal egg count.
There are additional ways to contribute to parasite control than simply using a dewormer. Managing the environment in which our horses live is a very effective way of reducing parasite burdens to which our horses are exposed. Keeping paddocks clean of manure by picking them once or twice a week is a good place to start. Pasture rotation is another way. Manure spreading can actually facilitate parasite transmission because it increases the area over which infective larvae may be spread. It can be effective at reducing transmission if the pasture is left empty for several weeks following manure spreading and during that time temperatures are high enough (>30 C) to kill the eggs. The Fraser Valley presents some unique challenges due to its climate, and the fact that the temperatures here tend to favour parasite transmission year round, versus climates that have extreme highs and lows. This makes it even more important to be diligent in our pasture management.
If you’re wondering whether you should worry, the answer is yes. Parasite resistance is something that should be on your radar. We should all start taking steps to lower the environmental parasite load and ensure that the dewormers we are using are effective. What you need to do to protect yourself and your horse for the future: environmental control of parasite populations; judicious use of dewormers, use fecal egg counts to determine your horses’ deworming protocol.
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