by Deborah Jeanne Sergeant

An infestation of parasites can cause cattle stress and diminish their conditioning and vigor. It’s worthwhile for farmers to fight these pests. Sarah Potts, Ph.D. and Extension specialist in dairy and beef with University of Maryland, presented “Best Practices for Deworming Cattle” as recent webinar hosted by MidAtlantic Women in Agriculture.

Disease vectors, physical harm and significant irritations are among the health aspects of parasite infestations. They can also reduce feed intake and gains and cause poor reproductive performance, which can be especially devastating to farms raising dairy cattle and/or engaging in genetic programs.

Potts divided cattle parasites into external (ectoparasites) and internal (gastrointestinal). The former includes flies, lice, ticks and grubs; the latter include nematodes (like roundworms), cestodes (like tapeworms) and trematodes (like flukes).

While producers should take care to reduce parasite loads, Potts said livestock will never be 100% parasite free.

“Cattle will naturally carry a parasite load,” she said. “Our goal with parasite management is to manage the parasite load to minimize the impact on health and performance.”

Potts said that nematodes are significant GI parasites in cattle. Farmers should look for Haemonchus (H. placei) in the most susceptible population – calves – and observe if cattle exhibit clinical symptoms, like anemia. For Ostertagia (O. ostertagi/brown stomach worm), calves younger than 15 months are most susceptible. Cattle can exhibit rough coat, diarrhea, weight loss and greatly reduced feed intake.

“Cooperia (C. oncophora) affects the lining of the upper small intestine,” Potts said. “It is not a blood-sucking worm.” Large infestations become problematic. The clinical symptoms include weight loss and poor production but not anemia.

To manage GI parasites in livestock, Potts recommended a first step of good nutrition and husbandry. “Cattle in good health will be able to handle parasites more effectively with less intervention,” she said. Managing pasture properly can also help. Overgrazing can increase the likelihood of cattle becoming infected.

Next comes what Potts describes as “appropriate, responsible utilization of chemical aids, like topicals, injectables and drenches. These should be used to supplement animal and pasture management and should not be used as the only measure you’re taking to manage parasites on the farm.”

Benzimidazoles work by disrupting worms’ metabolism. They target broad spectrum activity and are effective against adults and eggs from nematodes and trematodes. Prolonged contact between the chemical and the worm is necessary for effectiveness.

Imidazothiazoles work by causing sustained muscle contractions of the worms, resulting in their paralysis. They are expelled in the feces. This class of dewormers target broad spectrum activity and affect adult and larval nematodes. They have no effect on eggs or arrested larvae and are ineffective against tapeworms and trematodes.

Macrocyclic lactones function to also cause paralysis, but they do it by affecting neurotransmission, resulting in paralysis. They target broad spectrum activity and are effective against mature and immature nematodes and insects. Macrocyclic lactones provide prolonged protection against new infestations and insects.

With capricious use of all these options since the 1950s – and no new dewormers introduced for the past 40 years – resistance has grown. Potts classifies resistance as “lower than expected dewormer effectiveness when other causes are ruled out, like user error.”

Potts believes that overuse and misuse of dewormers can both cause resistance and hold implications for animal health and farm economics. “You might be missing out on possible gains or wasting your money on something that’s not effective in your herd if you have a resistance problem,” she said.

Though she did not state the types, Potts said that in her research four common products were tested for effectiveness among 331 animals. The average efficacy was 62.4%. Cooperia and Haemonchus showed the higest prevalence of resistance. Ostertagia was suspected of developing resistance. The population included 17 groups, 14 farms, calves and yearlings.

“I’m not saying ‘Don’t use dewormers’ but we should use them judiciously,” Potts said.

Every cow on a farm will carry some parasites. “Older cattle will develop tolerance,” Potts said. “This is probably the more critical step. We have to get ourselves out of the mindset that every animal needs to be dewormed.” Instead, focus on those who really need deworming, such as cattle under 12 months which are most affected by high parasite loads.

Identifying who needs dewormer can be determined by fecal egg count. Farmers can collect samples from individual animals. Higher levels of eggs in the manure show higher GI parasite loads. While effective, “it can be tedious, time-consuming and expensive,” Potts said.

Leaving some cattle untreated can contribute to refugia: a population of worms unexposed to dewormer. “It helps to dilute the gene pool with worms that are susceptible,” Potts said. “That reduces selection pressure for worms that possess resistance traits.”

Maintaining refugia is important to reducing dewormer resistance and keeping dewormer effective. “That is the easiest approach: to leave a population of cattle untreated,” Potts added. “In a lot of cases, that would mean deworming calves but now cows.”

High-risk animals are calves younger than nine months, unthrifty animals and newly purchased animals. Moderate-risk cattle are yearlings under 16 months and cows in late pregnancy. Low-risk animals are mature cows not in late pregnancy.

To get a little more accurate, farmers can use animal ranking based upon performance and leave the top 10% to 20% untreated.

Animals new to the farm should be dewormed upon arrival and quarantined in a dirt lot for two to three weeks to prevent contaminating the pasture with potentially resistant parasites.

Dewormer selection should be made based upon what dewormers will work on that farm. “Resistance against one class of dewormers does not mean resistance exists against dewormers in another class,” Potts said. “Choose a dewormer with an application method most appropriate to your set-up.”

Rotating dewormers may seem a good way to avoid the development of resistance; however, Potts said the research says otherwise. Using dewormers from two different classes at the same time may help.

Weighing animals can help determine the correct dosage. “If you underdose the cattle, not only will you not get any results but you’ll be inadvertently selecting for dewormer-resistant parasites,” Potts said.

Potts said dewormer evaluation should include cost savings, prevention of further development of resistance and informing future deworming decisions and strategies. Evaluations are completed by performing fecal egg count reduction tests. The test examines the number of eggs in the manure before and after deworming. Collecting from a subset of animals in groups of 15 to 20 can save both time and money.

Collection should be performed directly from the rectum and placed into individual containers or sealed bags. “I know that sounds complicated but you’d put a shoulder-length sleeve on like you’re doing AI, and you’d get the sample,” Potts said. “Store samples in a cooler if it is hot outside until compositing them.” Storing at less than 40º prevents eggs from hatching. Before compositing the samples, they should be weighed and an equal amount of each should be sent in a single container to the lab via overnight shipping.

After 10 to 21 days, repeat the manure collection process again from the same cattle, depending upon the dewormer used. Benzimidazoles or imidazothiazoles require 10 to 14 days; ivermectin and avermectins require 14 to 17 days; and moxidectin requires 17 to 21 days.

To calculate the reduction percentage, subtract the post-treatment eggs per gram from the pre-treatment eggs per gram. Divide that figure by the pre-treatment eggs per gram. Multiply that figure by 100%.

Potts said reduction greater than 95% means it’s effective with no evidence of resistance. Results of 90% to 95% mean reduced efficacy with suspected resistance. Less than 90% is ineffective with resistance likely. Less than 70% is highly ineffective with resistance present.

The timing of deworming is typically in spring turnout and in autumn, but Potts said to avoid deworming strictly by the calendar.

“Consider performing a composite fecal egg count analysis for the high-risk groups of cattle at those times to help determine parasite load before deworming,” Potts said. “If the load is low, it may not be economically favorable to deworm.”

She added that resting pastures for four to five weeks with rotational grazing can help reduce parasite loads in pastures.