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The importance of bovine parasite control has been known for several decades. Newer studies have shown that there is a significant resistance problem with the use of current anthelmintics (de-wormers) in beef cattle today. This requires that we address this issue with a scientific basis as opposed to the older method of simply giving an anthelmintic and assuming our cattle will be parasite free.
Although there are many products available, there are only three classes of anthelmintics currently available for food animals. All products within a class kill parasites by the same method and therefore have similar effects. When parasites develop resistance they develop resistance to the class and not to just one product. The most recently developed class is the Marolytic Lactones, containing avermectins and milbemycins. This class includes ivermectin products (Ivomec and generic ivermectins), dormectin (Dectomax), eprininomectin (Eprinex) and moxidectin (Cydectin). The first product (Ivomec) was introduced in the late 1970’s and the last (Cydectin) was introduced in the early 2000’s. It will be important later to understand that we have not had a new class of dewormers introduced in over 30 years. The second class of dewormers, benzamidazoles, often referred to as the white wormers contains oral products such as albendazole (Valbazen), oxbendazole (Synantic), thiabendazole (TBZ) and fenbendazole (Panacur or Safeguard). The third class, nicotinic agonists, contains levamasole (Levasol, Tramisol), morantel (Rumatel, Nematel) and pyrantel (Strongid).
As products became less expensive, producers often deworm for insurance without knowledge of the level of parasitism they have. Products are required to show high efficacies (> 90%) in order to receive label claims for parasite control for a given worm and are very effective when they are introduced. In 2004 we started seeing reports of resistance or possible resistance in the US cattle populations. The anthelmintics we use reduce the level of parasites but do not eliminate them. For example if Product A is 98% effective on Worm A then it kills 98% of the Worm A parasites. The problem is that the remaining 2% may be resistant to Product A and without competition they are able to grow and reproduce thus creating a population of resistant parasites. This concept is a bit oversimplified but it should give you an idea of how resistant parasite populations develop over time. In addition, increased frequency of deworming keeps the non-resistant parasites in check increasing the speed at which resistant parasites develop.
Another posible issue is under dosing. By under dosing we kill less parasites thus leaving more to develope resistance. Some producers have advocated using lower doses to cut costs. Not only does this decrease efficacy, but it is likely associated with developing resistance. By showing more parasites the product without killing them they are more likely to become resistant. Another way producers under dose is by using the same dose they did 30 years ago. The cows may be 500 pounds heavier than they were 30 years ago but if 10ml was good enough then it is good enough now. The third mistake I see is dosing for the average weight of the cattle. If you dose for an average then half the cattle (the ones above the average) are getting under dosed. Dewormers should always be dose for the heavy end of the cattle or dosed to each individual animal. 
It has been shown on numerous studies that parasites are very costly to our industry, thus it is important to address this issue and apply cost effective deworming programs. The first step is to determine if you have a resistance problem with your operation. In order to determine this, your veterinarian will need to perform parasite reduction studies. Fecal egg counts are performed before and after deworming and if you get a 90% reduction in egg counts the product is still effective. In order to perform these studies properly, individual animals need to be tested instead of pooled samples (samples combined from several animals), which are often misleading. We also need to test adequate numbers of animals to avoid misleading information because parasite eggs can be shed somewhat sporadically and it is typical for a small percentage of the animals to shed the majority of the eggs. If you have less than 20 animals in the group you need to test them all. If the group consists of more than 20 animals then testing 20 will do a very good job regardless of the size of the group. Due to the population dynamics of the parasites, this sample size should be the gold standard for fecal egg reduction tests regardless of whether your group size is 100 or 1000 head.
When the fecal egg reduction test shows efficacy of 90% or greater and you are not seeing resistance. However, one should keep assessing yearly in case resistance develops. If the study shows parasite resistance then you should consult with a veterinarian as to the best method to approach the problem.  The approach will likely depend on the level of resistance and the level of parasitism but solutions will involve rotational or combination deworming and/or management practices that decrease parasite exposure. At this point there is not a lot of consensus as to how to handle resistance problems, even amongst parasitologists. An individualized approach will be necessary, rather than the one size fits all approach commonly utilized in the past. The future also holds the promise of new products in other chemical classes that will likely be highly effective when introduced. However, we do not know at this time when something new will be available. When a new product in a new class of anthelmintics does become available, it will be expensive due to the very high cost of R & D and the length of time it takes to get a new product approved by the FDA.