Parasites: An Ounce of Prevention is Worth a Pound of Cure

Many of us are familiar with the previous standard for deworming horses - a rotational system where a different class of dewormer is administered every 8 weeks like clockwork to all horses on the property. The problem with that, we now know, is that it increases parasite resistance, which in turn decreases the efficacy of those dewormers in the future. Let’s break down how this can happen: 

1. Dewormers are administered and susceptible parasites are successfully destroyed. 

2. The less susceptible (stronger) parasites are left behind, but have now encountered the dewormer and have the opportunity to form resistance to it.

3. Those less susceptible, now more resistant parasites continue to exist in the internal and external environments, and continue to multiply.

4. The same dewormer is administered again, but the parasites are now resistant and unaffected by it.

According to a recent article from PennState Extension**, “small strongyles have widespread resistance to benzimidazoles, moderate resistance to pyrimidines, and early indications of resistance to macrocyclic lactones. Ascarids have widespread resistance to macrocyclic lactones and early indications of resistance to both benzimidazoles and pyrimidines.” The article goes on to say that horses tend to develop immunity to ascarids (roundworms) while young, so small strongyles become the main parasite we worry about becoming resistant in our mature adult horses.

Table 1: Taken from University of Minnesota Extension

Fortunately, horse owners have several tools in their toolbox to reduce and prevent this resistance from building up on their farms in the first place. The proverbial “ounce of prevention” can include: 

1. Manure Management - You’ve probably noticed your horses avoiding grazing in certain areas in the pasture, specifically where they have pooped. This is a natural instinct to avoid parasites. When complete removal of manure from an area for composting is not practical, like a pasture for example, we tend to look towards harrowing or dragging to break up manure and spread it around. This can be beneficial in a dry, hot climate where temperatures have been high and rainfall low consistently for several weeks. This is more helpful for strongyles than roundworms, which can withstand the hot and dry conditions. However, in a moderately warm and moist environment, spreading the manure can be more harmful than helpful altogether. This can take away the horses’ ability to graze around the infected feces. The best case scenario is to be able to keep horses off of a pasture for several hot and dry weeks after harrowing or dragging.

2. Rotational Grazing - Moving horses from one pasture to another in general can be extremely beneficial for the horses as well as the pastures. A period of rest gives the grass time to recover and strengthen its root system before being grazed again, making it heartier overall. What you see on top of the ground in a pasture is typically reflective of what the root system beneath it looks like. Parasites also like to live in the bottom 2-4 inches of the grass, so being able to pull horses off of a pasture when they get down to about 4 inches can greatly reduce their exposure to parasites in the first place. The less parasites they ingest, the less there will be for you to have to get rid of. Stocking density comes into play here as well. It is important to make sure there is enough grass relative to the amount of horses it is expected to support.

3. Fecal Egg Counts (FECs) - These tests are a great way to estimate the current parasite load of your horse at a given time. It is important to ensure that the sample being tested is fresh, uncontaminated, and handled correctly. This test can help evaluate how effective a dewormer was when administered. It is important to note that this test is not perfect. It does not reflect adult worm load or larva, and has been known to miss tapeworm and pinworm infections as well. The main parasite a fecal egg count detects is strongyle eggs. Therefore, it is still recommended to deworm adult horses at least one to two times each year for the parasites that may not have been detected on tests, like tapeworms, pinworms, bots, etc. Fortunately, anthelmintic resistance has not been reported in these parasites as strongly as it has with small strongyles.

4. Knowledge - Understanding the life cycle of different parasites can be extremely beneficial to horse owners when they go to plan their parasite management strategies. Timing treatments and preventative methods to coincide with stages of the life cycle is important. You want to make sure you are conducting FECs, administering dewormer, spreading manure, and rotating pastures, all at the right time to minimize parasite burden. 

Finally, it's important to remember that just like with most other things, horses can vary significantly from one to the next when it comes to factors that influence deworming needs. Things like age, shedding rates, and natural immunity can be so different between individuals that it is virtually impossible to develop an effective one-size-fits-all plan for a farm. Make sure to involve your veterinarian in developing a plan for each individual horse. You can also contact your local extension office for further resources to help with pasture management, conducting fecal egg counts, and much more.

**https://extension.psu.edu/decoding-dewormers-types-resistance-concerns-and-use-for-horses