As you manage cattle on grass, you are also managing gastrointestinal parasites. Helminth (ieHaemonchus, Ostertagia, Cooperia) life cycles includes eggs and first through third stage larvae residing on the grass. Fourth and fifth larval forms live inside cattle, where they consume blood. L4s and L5s divert blood (nutrients) from weight gain and milk production. Diagnosis is made by performing fecal egg counts (FEC); but it’s a biological system, the egg count is not linearly related to the number of egg laying L5s in cattle. Worm burden is estimated by analyzing body condition score, average daily gain and/or average daily milk production, and measured FEC, while taking into account grass conditions. Practicing Integrated Pest Management allows for strategic anthelmintic treatment.

Efficacy of macrocyclic lactones, ML, (Ivomec, Ivermection, Dectomax, Cydectin, Eprinex) reported in 2013 was 55% for the injectable and 52% for the pour-on formulations by researchers from Wisconsin and Nevada (Bliss, Moore and Kvansnicka, Summary of Fecal Worm Egg Count Reduction Trials in Merck Animal Health National Data Base). Producer miss-use contributes to the poor efficacy. Not dosing to the animal’s weight and poor injection technique under doses the animal. Skin is naturally designed to prevent transfer of chemical agents. Pour-on MLs, relying on uptake through the hair follicle, are impeded by excessive hair, dirt or mud. Directions for use indicate the product is to be applied across the topline in a long line, exposing as many follicles as possible to a slow wicking. Cattle self-grooming or grooming their treated herd mates may help achieve therapeutic levels by oral ingestion of the anthelmintic; however, erratic and unpredictable dosing results.

The same researchers reported Benzimidazole efficacy (oral products including SafeGuard, Panacur) as 99% in 2013. Delivered into the gastrointestinal tract, they bypass absorption problems. ‘White dewormers’ are not as easy to administer, and therefore not popular to use; so resistance has not occurred.

Parasites, not cattle receiving the drug, develop resistance. Resistance occurs when helminths survive the anthelmintic dose which would normally kill that same species and stage, passing this genetic ability to their offspring. Anthelmintic use is not creating new super worms. Susceptible worms are killed by the anthelmintic. Resistant ones continue to breed resistant genetics.

Develop measurable deworming protocols with your veterinarian. Determine efficacy of the herd’s treatment by measuring FEC prior to and two weeks after treatment. Therapeutic efficacy is achieved when a 90-95% reduction in FEC occurs. Examine the protocol when treatment fails to meet this level of efficacy:

  • Correct all dosing issues.
  • Consider additional lab diagnosis to identify the resistant helminth species in your herd.
  • Switch anthelmintic classes, to one with a different mode of action.
  • Evaluate options for decreasing pasture contamination by resistant eggs and larvae.

Veterinary parasitologists agree, using refugia maintains populations of susceptible helminths. Refugia, helminth larvae stages not exposed to anthelmintic at the time of treatment, provides a susceptible gene pool for resistant worms to breed with. Cattle develop an age-dependent immunity to helminths. Older cattle do not have to be continuously dewormed, and not treating them creates a source of refugia on your farm. Most susceptible to deleterious effects of worms, young stock need effective anthelmintics. Small ruminant producers successfully utilize refugia; it is reasonable to expect the same benefit for cattle.