Cattle Newsletter – September 2020

Topic: Strategic Deworming | Author: Ben Bennett, DVM

This month we wanted to cover the topic of strategic deworming. As we prepare to pregnancy test the cow herd this fall, it also gives us the opportunity to administer annual vaccinations and dewormers. Traditionally, many operations deworm their cattle twice a year; once in the spring when going to grass, and once in the fall as they come off grass. Today we would like to challenge that theory and suggest another option.


First, why is it important to assess our deworming strategy? As I am sure that many of you have heard, drug resistance in parasites is becoming a larger and larger issue. Here in the northern United States, we luckily do not battle it like in the south-east, largely due to the cold winters that we experience. That is an advantage, but as we discuss further, that can also work as a disadvantage once resistance develops. Additionally, deworming is a costly, out-of-pocket expense every year. Best to ensure that those dollars are being spent wisely and not wasted on animals that do not need treated.

Every time we administer a drug that is designed to kill an organism (i.e. a dewormer or an antibiotic), if 100% of the targeted organisms are not killed, then we are selecting resistance. In the case of dewormers, if we use an ivermectin based dewormer and do not kill 100% of the targeted parasites, the ones that survive are likely resistant to that selected dewormer. As those parasites reproduce, we are back to the same level of infective parasites, but a larger proportion of them are resistant to ivermectin.

Traditionally, when we have talked about “strategic deworming” it meant that we rotate the dewormer class that we use throughout the year. That way if resistance is built to one type of dewormer, we then use one with a different mechanism of action that those parasites are still susceptible to. The way that this look for many people is to use an ivermectin based (drug class: macrocyclic lactone) pour on in the spring; like Dectomax, Ivermectin, Eprinex, or Cydectin. Then they use an oral, white wormer (drug class: benzimidazole) in the fall; like SafeGuard, Valbazen, or Synanthic. This has worked for a while, but I am sure that you can see where this is going. We are now seeing parasites that are resistant to both drug classes and we are left scratching our heads on what to do next. There is one other drug class of dewormer that we can use, but with time, we will just have selected for parasites that are resistant to all 3.

So that is where management practices can come into play. The main practice that we can use to combat parasite resistance is referred to as “refugia”. This is the idea of maintaining a population of parasites that are not exposed to dewormers, and thus do not develop resistance. We are past the point of getting rid of parasites, so we must learn how to live with them. By maintaining that population of parasites with minimal exposure, they can then cross with the population that has been heavily exposed and thus is highly resistant. So, by crossing a population with low resistance with one with high resistance, a population of parasites with medium resistance will result. Over multiple generations of the parasite’s lifecycle, we can effectively take a highly resistant population back to a level where it can effectively be treated.

How do we know which group of cattle to not deworm and maintain that refugia in? That varies for every operation and depends on the goals that you have for your different groups of cattle. For groups of cattle that have a known goal of gaining as much weight by a certain date (i.e. calves and stockers), continuing to deworm and keeping their parasite burden low is a great practice. But in groups of cattle that are healthy, in good condition, and do not have an average daily gain goal, like mature cows, that is where we believe we can maintain refugia.

The practice that we encourage is to conduct regular fecal egg counts on your cattle. To do this, fresh manure samples are collected from the pasture of the group of cattle that you wish to test. Drop those samples off at the clinic and we conduct the testing in-house. Using set standards, we can determine the assumptive parasite burden of those individuals. We can then combine that objective measurement with a subjective assessment of your cattle to determine if deworming would be advantageous. If we decide to not deworm, the parasites in those cattle serve as the refugia that weakens the resistance of the parasites in the cattle that we treat.

Just like any laboratory test, there are limitations to a fecal egg count. The count can be falsely low if the parasites present are not shedding many eggs or are dormant. Conversely, the count can be falsely elevated if the parasites present are shedding an abnormally large number of eggs. The way that we account for those inaccuracies is by taking an average from multiple cows. By sampling enough individuals, we can be relatively certain that the result we obtain will accurately represent the average parasite burden of the individuals in the herd.

To wrap everything up, the practice of deworming every animal on the ranch twice a year, or even annually needs to be critically assessed. Not only is that a large, out-of-pocket expense, but it will eventually lead us down the road of high resistance in parasites with little place to go. Maintaining refugia in your cattle is one way that we can incorporate management strategies into combating parasites. Luckily for us, parasites cannot develop resistance to good management. By subjectively assessing your cow herd and conducting fecal egg counts we can determine which animals need to be treated and which we can use to maintain that beneficial refugia in your herd.

Benjamin P. Bennett, DVM
Veterinarian, Mountain Valley Veterinary Srvcs.

-Thanks for reading and as always, please let us know if you have questions or concerns about what we have discussed. –

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