Parasite Management: Stewardship Tips & Research Back »

2018 Animal Care Wednesday Webinars
Husbandry Practices in the Spotlight

Parasite management for cattle and sheep was the topic for the August 1st Animal Care Wednesday Webinar. Mike Hildreth, Professor of Biology and Microbiology with South Dakota State University, shared his expertise regarding research findings and ways to be a good steward when implementing parasite control methods on farms and ranches.

Parasites come in many forms and impact ruminants in different ways by targeting certain areas of an animal’s body. A brief review is shown in Figure 1.

Outline of bovine with internal organs and the parasites that infect each organ labeled.
Figure 1. Categorization of common parasites impacting cattle and the main organs targeted.


More lice species are showing some level of resistance to common products. One example occurred in South Dakota where a herd of Charolais cattle had avermectin resistant long-nose cattle lice in January 2018 despite several aggressive treatments. The herd had been treated with an avermectin pour-on mid-November. Additionally, they were treated with both an avermectin pour-on and injectable mid-December after still showing infestations. In January, many lice could still be seen throughout several animals is the herd despite the previous treatments.

Treatment options for lice are dependent on the use of pesticides in three main application types.

  • Sprays and dusts (fogs also): includes permethrins and malathion
  • Pour-ons: includes macrocyclic lactones (avermectins and milbemycins), permethrin, insect growth regulators (IGR)
  • Injectables (endectocides): includes macrocyclic lactones (avermectins and milbemycins)

Treatment recommendations are moving toward combination products that target lice in more than one stage of life, for example products like Clean-UpTM II that combines insecticide classes. Research testing different approaches to slow the development of pesticide resistance is difficult and require many years to complete, but there are statistical models that that can be used to evaluate the effects of different approaches. These models are showing that rotating insecticides and anthelmintic classes do not slow the development of resistance compared to using each class until it is not longer effective, but implementing combinations of insecticide and anthelmintics classes has been used to slow resistance to all of the classes.

Horn Flies

Another pest during summer months is horn flies (Haematobia irritans irritans). These pests aren’t just a nuisance, but they can have negative impacts on production and health of the cattle if not managed. Control or management options primarily include insecticides or physical removal of horn flies.

  • Sprays both high and low pressure application
  • Insecticide impregnated ear tags: organophosphates, pyrethroids, organochlorines, macrocyclic lactones
  • Bags and rubs of powders
  • Mineral blocks or feed additives: larvacides
  • Traps

With limited new classes of insecticides being developed, management will benefit from incorporating physical removal options. Physical removal options rely heavily on fly behavior as a means to trap insects on cattle. However, training cattle to walk through and utilize traps takes time and may be a large hurdle for some animals to overcome from a behavioral and stress perspective. One example of a trap is the Bruce Walk-in Horn Fly Trap that was constructed in the 1940s.

Roundworms (nematodes)

Worm challenges primarily focus on stomach worms and small intestinal worms, or the nematodes. Based on a previous survey of South Dakota cattle herds (Hildreth et al. (2007), the two primary species of intestinal parasites present in all calves and cows were coccida and strongyles. When addressing worm issues in ruminants, it is important to remember that each ruminant species animal has different levels of relative susceptibility, especially to the trichostrongyle nematodes. For example, goats and sheep are extremely to very susceptible while bison and cattle are moderately susceptible; cattle are the least susceptible to these worms. Additionally, differences in susceptibility is evident within individuals in the same herd or flock. Given these individualized susceptibility differences within a group, culling animals based on which are the highest worm egg shedders could become a potential management strategy.

Producers should remember the life-cycle of trichostrongyle nematodes when developing their deworming strategies. Other challenges to overcome are based on the fact that some species of trichostrongyles cause definite clinical signs (e.g. anemia, diarrhea, bottle jaw), while others simply have subclinical impacts (e.g. reduced appetite, weaning weights, reproductive performance, etc.). For example, worms in South Dakota stocker calves cost producers approximately 10-15 pounds per calf for every 100 days on pasture in weight gains (Mertz et al. (2005).

Treatment options are available and products have varying ranges of persistence in the animal’s body for protection. Persistence can range from one day to 120 days of protection. When choosing products, it is important to choose products that will protect the cattle during the key time period the parasites are present, or when the cattle are grazing. When looking at parasite management plans, remember a few key points. Fall deworming targets approximately 10% of the total parasites, but doesn’t impact the 90% that are on the pasture. Product choice would not require longer persistence. Spring deworming targets 90% of the parasite population and requires more persistence or multiple strategic treatments. Resistance to the different types of anthelmintics is a significant problem in sheep and goats, even in South Dakota. There are some evidences that anthelmintic resistance is now becoming established in cattle from South Dakota.

This article summarizes only a few of the main points Dr. Hildreth shared. Watch the full webinar to learn more about all the parasite research and case studies discussed, or contact Mike Hildreth for additional information.


  • Epperson, W.B., B.D. Kenzy, K. Mertz and M.B. Hildreth. 2001. A single pasture limited treatment approach to estimate productive loss from internal nematodes in grazing stocker cattle. J. Vet. Parasitol. 97:269-276.
  • Harmon, A.F., B.C. Lucas and M.B. Hildreth. 2009. PCR comparison of trichostrongyle genera present in South Dakota cattle with and without springtime deworming. Proc. S.D. Acad. Sci. 88:147-154.
  • Hildreth, M.B., W.B. Epperson, and K.J. Mertz 2007. Effect of longitude and latitude on fecal egg and oocyst counts in cow-calf beef herds from the United States Northern Great Plains. J. Vet. Parasitol.149(3-4):207-12.
  • Mertz, K.J., M.B. Hildreth, and W.B. Epperson. 2005. Assessment of the effect of gastrointestinal nematode infestation on weight gain in grazing beef cattle. JAVMA: 226(5)779-783.

Animal Care Wednesday Webinars

To listen to this and past webinars, visit the animal care resource website. For more information about upcoming Animal Care Wednesday Webinars, please contact Heidi Carroll.

Join Animal Care Wednesday Webinars each month on the first Wednesday at 11:00 A.M. CT. To join webinars, log in to the Zoom Meeting a few minutes prior to the start of the webinar.

Disclaimer: SDSU Extension does not endorse any product named in this article. Product names are simply used to provide examples for educational purposes.

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