by Sally Colby
Cattle producers are concerned about a number of internal parasites, but Dr. John Gilleard, professor of parasitology and associate dean of research on the faculty of veterinary medicine at the University of Calgary, says the main concern is roundworms.
“They’re invisible stealers of profit,” said Gilleard, describing gastrointestinal roundworms. “The major impact with parasite burdens includes digestion and absorption of nutrients. One of the biggest impacts is appetite suppression.”
Gilleard says roundworms result in subclinical disease — animals don’t look sick or in bad condition, but they don’t gain weight efficiently. The estimated cost of parasitism the U.S. is about two billion dollars annually.
The life cycle of the roundworm is fairly simple: adults in the gastrointestinal tract produce eggs that are passed in the feces. The eggs hatch in the feces and release microscopic larvae. Larvae develop into the infective stages and are passed to the pasture; some onto blades of grass which are ingested by grazing cattle.
“Because a lot of the life cycle is in the environment outside the host, parasites cannot live outside the host (cattle),” said Gilleard. “These parasites are very dependent on climate. This leads to a lot of variability in different situations.”
Gilleard points out the fact that livestock have evolved with parasites, and it’s normal for grazing animals to have some level of parasite infection. The goal is not to eliminate parasites, which is impossible, but to keep levels low enough so they don’t impact productivity.
Parasite infection builds in animals over the grazing season. The weather between April and October is most conducive to parasite development in pastured cattle. If eggs go onto pasture in winter, there’s less chance for transmission.
Infection occurs in the cow-calf system and in stockers on pasture. Inadvertent selection for resistance happens on pasture and not in the feedlot. “Cattle go into feedlots with a parasite burden from pasture, but there’s no transmission in the feedlot,” said Gilleard. “It’s a dead end for parasites.”
Gilleard explains stocker cattle going to pasture for winter will have a parasite burden and produce parasite eggs in feces, but the eggs don’t mature. As temperatures warm in spring, cattle graze and ingest developing larvae. Egg output increases throughout the grazing season, with a big increase at the end of summer and into fall.
Another important factor in parasite management is that parasites are somewhat regional, so producers should be aware of the predominant parasite species in their area. It’s also important to understand the impact of various parasite species. Ostertagia ostertagi (brown stomach worm) is most damaging, but produces the fewest eggs per worm.
Gilleard says one of the most pressing issues in parasite management is resistance to available products. In cattle, because production is emphasized, the resistance issue is easy to overlook. But after several years of continuous anthelmintic use, the proportion of resistant worms in the pasture system gradually increases, resulting in a higher number of resistant parasites.
Because of resistance, Gilleard suggests that cattle producers change to a more targeted approach. “It’s a challenge to get producers to change practices simply because of this long-term problem,” he said. “Because parasite control is not well-targeted, we’re losing money. By improving parasite control, we should be able to improve profitability and at the same time, make it more sustainable for the long term.”
Unfortunately, there’s no single parasite management plan that works for every producer. Parasites are not evenly distributed across a pasture, so areas where cattle congregate tend to be higher in egg and larvae content. Gilleard suggests fecal egg counts (FEC) for roundworms to get an estimate of parasite burden on pasture. “If you do that on a regular basis, you’ll build an understanding of impacts overall,” he said. “The easiest way is to take fecal samples from 20 animals on pasture.” Fresh samples from the pasture, collected with a glove and turned inside out and tied, will maintain stability for several days.
Gilleard lists five factors for sustainable internal parasite management: correct dewormer, correct animal, correct dose, correct timing and checking effectiveness.
Using the correct dewormer is critical. Gilleard says there are two major drug classes: macrocyclic lactones (ivermection, doramectin, moxidection) that are effective against both internal roundworms and external parasites such as lice and warbles; and benzimidazoles (fenbendazole and albendazole), which are effective only against internal parasites.
Gilleard says the concept of using two dewormers, one from each class, at the same time is the currently recommended deworming method. “It maximizes effectiveness and gives the best control,” he said. “Equally important, it slows down the road to resistance. In using drugs with two different modes of action, one drug protects the other.” To determine which animals should be dewormed, Gilleard suggests cattlemen work with their veterinarian to develop a treatment plan.
The correct dose is critical — underdosing is risk factor for resistance. Giving too little product can lead to resistance because only the weakest parasites are killed, leaving the genetically stronger parasites to reproduce and cause infection. Dosage should be based on weight, ideally individual weight, but if that isn’t possible, dosage should be based on the heaviest individual in the group to prevent underdosing. Gilleard says the challenge with timing parasite control is that there’s a limitation because animals are processed at a certain time and that isn’t going to change just for parasite control.
Producers should not assume that fall treatment with ivermectin will provide good roundworm control. “In the past, people tended to think that way,” said Gilleard. “They’d treat with ivermectin in fall for ectoparasites and it also helps control roundworms. But that really isn’t the case. We should be thinking about using combinations (one product from each of the two classes) in situations when we think roundworm control is necessary.”
Some parasite larvae survive on pasture over winter and provide a source of infection in spring. “Even if fall treatment was completely clears out the parasites, in spring, those animals will pick up larvae and start contaminating the pasture again,” said Gilleard. “If you can treat in spring and early summer, that prevents further pasture contamination.” Gilleard stresses the fact that animals should only be treated when the risk factor suggests treatment will be beneficial. The effectiveness of a deworming product can be checked with fecal egg counts, and the herd veterinarian can suggest options for that.
“One of the reasons we think resistance is so widespread is because animals move around so much,” said Gilleard. “When you buy animals, you buy parasites with them. If you’re buying cattle with high levels of ivermectin-resistant parasites, that’s going to be a factor in introducing it to your farm.”
Parasites are not evenly distributed among the herd — about 80 percent of parasites are in 20 percent of the animals. Although all animals are treated the same, there’s a subset of animals that are contaminating the pastures. “The problem is we don’t know which ones they are,” said Gilleard. “Leaving some animals untreated actually reduces selection pressure of drug on parasites and slows the development of resistance. It’s a good practice to leave 10 to 20 percent of cattle untreated.” The most practical way to accomplish that is to not treat animals that are in the best condition.
New strategies for parasite management in beef cattle
by Sally Colby