Written collaboratively by Written collaboratively by Joe Darrington (former SDSU Extension Livestock Environment Associate) and Taylor Grussing.
Spring is coming and with calving season underway it is important to keep our eyes forward on to the next step in production. Grass turnout in the spring is the most common secondary benchmark of the year. With grass turnout comes exposure to parasites that have overwintered either in the pasture or in the cattle themselves. Use of dewormer compounds can significantly improve the average level of production; however, care must be taken to avoid a buildup of resistant populations. A short list of deworming drugs can be seen in Table 1. Important differentiations between drugs include the overall effect on the larval state of internal parasites, the effect on external parasites, and the route of administration. General recommendations regarding the use of these drugs should be discussed with your herd veterinarian and in compliance with the label claims of your product. The rest of this article will cover some types, symptoms and treatments of common parasites (Table 2) as well as general best management practices and ideas to be used on your farm or ranch for parasite control.
Table 1. Dewormer Drug Examples for Stomach & Intestinal Worms
|Drug Class||Active Drug||Effective Against Adults||Effective Against Larvae||Common Routes of Administration|
|Imidathiazole||Levamisole||Yes||Moderate||Pour-on, Oral Drench, Injectable|
|Avermectin||Ivermectin||Yes||Yes||Pour-on, Oral Drench, Injectable|
Table 2. Cattle Parasites of Primary Concern
|Parasite||Symptoms of Infestation||Treatment|
|Brown Stomach Worm
|Diarrhea, anemia, slow growth||Any of the above anti-parasitics.
*Cattle develop resistance to worm infestations slowly. Cow’s older than 3-4 years should not require maintenance dosing. Weaned calves and heifers are the most affected. Watch for resistance.
|Cooperia||Diarrhea, in-appetent, slow growth|
|Haemonchus||Anemia, slow growth|
|Trichostrongylus||Diarrhea, slow growth|
|Nematodirus||Diarrhea, slow growth|
|Parasite||Symptoms of Infestation||Treatment|
|Lice||Hair loss, scrapes, discomfort||2 to 3 topical treatments separated by 2-3 weeks with ivermectin (be careful if your herd has a history of grubs) or topical insecticide|
|Mange||Hair loss, ulcers, discomfort, secondary infections||Topical insecticides, *work with your veterinarian as mange organisms are reportable infestations in the US|
|Grubs||Warbles (larval heel flies) appear in the spring along the backs of animals. Painful.||In the Fall, systemic ivermectin or an organophosphate to kill migrating larva.
*Do not apply systemic insecticides in the winter or early spring, can cause paralysis and/or bloat
Route of Administration:
There are many ways that products can be administered to cattle with the most common methods including: topical pour-on (with or without systemic absorption), injectable, and oral drenches. For treating internal parasites, injectable products and oral drenches ensure delivery of the desired dose of drug. Pour-on products can be effectively absorbed systemically and provide a good dose; however, there is more variability, especially if the weather does not cooperate. The most important aspect to ensure is that each animal receives an adequate dose based on its body weight, regardless of the route of administration. Basing the dosage off of body weight helps attain the best efficacy, as under dosing will not eliminate all the parasites while promoting resistance, and overdosing can be harmful to the animals and an unnecessary expense.
Rotation of Product:
Resistant parasite populations develop over time from repeated use of the same deworming products. The more frequently a dewormer is used, the quicker that resistance will develop. Monitoring effectiveness of treatment can help you determine if and when switching products is necessary.
Monitoring Treatment Effect:
If you are concerned about how well your current deworming protocol works, you can work with your veterinarian to perform a fecal egg count reduction test (FECRT). This test consists of evaluating the baseline level of parasite egg shedding in your herd, applying your standard deworming strategy, and then rechecking the level of egg shedding in your herd. If a deworming compound reduces the fecal egg count by more than 95% the dewormer works well. If the reduction is less than 95% there is resistance in your herd. As a reminder to veterinarians, best practices indicate conducting the FECRT on paired samples from individuals in the herd.
Herd Populations Most at Risk:
Cattle build up resistance to parasite infestations slowly and younger animals are more at risk of clinical disease. The most important populations to manage for parasites are weaned calves, heifers, and 2nd calf cows. Older cows have had the opportunity to develop resistance and should not require annual or semiannual treatment in the absence of clinical signs, work with your herd veterinarian to figure out a parasite control strategy that fits your situation.
Other Management Strategies:
Pasture management through rotation, alternate species grazing, haying, and rotational tillage can significantly reduce the number of infective larvae on a pasture. Focused deworming of individuals showing clinical signs of parasitism (diarrhea, anorexia, high fecal egg counts, etc) rather than mass treatment of groups can be very effective in promoting overall herd performance, reducing the development of resistant parasites, all while reducing the level of contamination of pasture. By only treating animals that are most affected we leave a population of parasites in the less affected animals that have not been exposed to the anti-parasitic drug. This population of susceptible parasites in the animals and environment are called refugia, and can lengthen the time that anthelmintic drugs will be effective. However, if mass treating in the spring, the use of a persistent product (check labels for duration of residual effect) for parasite control should be used to decrease new fecal shedding and pasture contamination. This treatment should be done as late as possible in the spring and up to 4-6 weeks after grass turnout to help limit infestation rates in calves. As a last thought, maintaining appropriate stocking densities to prevent overgrazing can also help limit parasite exposure.
The best way to manage parasites in your herd will be unique to your situation. Hopefully this article has given you a better understanding of current issues and your management options. Please contact your herd veterinarian, SDSU extension cow-calf specialists, SDSU extension veterinarian, or Joe Darrington with questions.
- Smith, B.P., 2009, Large Animal Internal Medicine 4th Edition, Mosby Elsevier Publishing
- FDA CVM, 2015, List of Approved Animal Drugs.