by Sally Colby

Dr. Ray Kaplan, parasitologist and professor at the University of Georgia College of Veterinary Medicine, has some stern words for cattlemen: “We need to start changing the ways we think about controlling worms. What we’re doing now is not only not working well, it’s heading us to potential disaster.”

Finding the right balance for managing internal parasites in beeves involves different thinking, including fewer treatments, the use of combination drugs, attention to route of administration and refugia-based strategies.

While deworming has typically involved treating every animal on the farm, refugia means leaving some animals untreated. This leaves a portion of the worm population that isn’t experiencing the drug – they’re in refuge from the drug. “The best way to think of refugia is on the day you’re going to treat the herd, which worms on your farm are not experiencing that drug?” Kaplan explained. “That comes from any animals left untreated and all the eggs and larvae on pasture. If we manage refugia, we can help maintain a majority of drug-susceptible worms. Failure to manage refugia is the single most important factor in the development of resistance, and the evidence for this is strong.”

Kaplan used an example to further explain how refugia work: In a group of four cows, one is quite wormy, one has a moderate worm burden and two have low worm populations. The worms are mostly susceptible (to deworming products), but a small percentage of the worms are resistant. Most of the eggs in feces shed by these cows will be killed by treatment, but resistant worms will survive.

“We treat all the cattle on pasture. Then after we treat, the only worms left behind are the resistant ones, which means all the eggs shed in feces after we treat are from resistant worms,” said Kaplan. “This means the next generation of larvae is going to be coming just from the resistant worms. So every time we treat, we’re increasing the percent of the worm population on the farm that has resistance, and over time, the resistant worms are predominant and the drugs stop working.”

However, given the same group of cows and only the cow with the most worms is treated, all the worms in the other cows serve as refugia. Kaplan said refugia includes all the worms, which means there are some resistant worms in the refugia as well, but the next generation of worms is going to look much like the population of worms prior to treatment. “There will be some resistant worms there, but most are susceptible,” he said. “That’s how refugia works – by diluting resistant worms and keeping them at bay in the overall population.”

The other component of refugia is pasture. If there’s good refugia, with a lot of eggs and larvae on pasture, those eggs and larvae will dilute the worms shed after treatment and the worms on pasture will be like they were prior to treatment. “However, if there is low refugia, there’s very little to dilute the resistant worms and the population will shift dramatically to become more resistant,” he said. “As the numbers of worms accumulate, the resistant worms will take over and dominate. The numbers of larvae on pasture will vary depending on pasture usage and time of year – sometimes there will be high refugia, other times there will be low.”

The easiest way to apply refugia in cattle is to not treat all animals, or what Kaplan referred to as selective non-treatment. “We’re leaving 10% to 20% of the herd untreated,” he said. “These could be the best-looking animals. If they look good, parasites clearly aren’t impacting them, and more than likely, treatment won’t be much benefit.”

Animals can be sent through a chute, identified via RFID and weighed. If the animal meets the criteria for a certain weight it goes into one pen; if it doesn’t meet the weight gain, it goes into a different pen. All animals not meeting the weight target are dewormed. Kaplan pointed out that it’s important to have high drug efficacy for refugia-based strategies, an aspect that is often overlooked.

How many refugia are enough, and how many animals should remain untreated? There’s no direct answer because it depends on the efficacy of the deworming products. “If you have 99.9% efficacy, you can leave 1% of your animals untreated and still get a tenfold dilution,” Kaplan said. “If you leave 10% untreated, treating 90%, you’ll get a hundredfold dilution. If you have effective dewormers that are giving you 90% and leave 10% untreated you won’t get resistance for a long time because of the resistant worms – with refugia. If efficacy was 95%, which sounds good, you’d have to leave one-third of the herd untreated to get the same tenfold dilution. The size of refugia is highly dependent on efficacy.”

There may be value to leaving the bottom half of animals, those which don’t appear to be showing signs of parasitism, untreated. “If we use a drug that has 99.9% effectiveness … [and] we treat just half the animals, because the drugs are so effective, the worms in treated animals are so low that after treatment, they make up only a tiny fraction of the total population,” said Kaplan. “If you can identify the highest egg shedders, you can get a significant dilution of resistant worms by treating only half the herd – if they can be identified.”

Although fecal egg counts sound like a good idea for monitoring parasite burden in a herd, not all cattle have the same parasite levels. “Worms are overrepresented in a small percentage of animals,” said Kaplan. “Just 10% of calves are shedding more than half of all the eggs. Grabbing a handful of fecals from a group is worthless and will be a misrepresentation of the true picture.” While egg counts are unreliable for determining whether cows need to be treated or not, they can be very useful when particular animals are suspected to have a worm problem as manifested by poor body condition.

Kaplan suggested determining whether products are working when apply refugia. A fecal egg count reduction (FECR) is the gold standard for measuring efficacy and is accurate for detecting resistance. Kaplan said in general, when anthelmintic drugs were first introduced, they showed 99% reduction in eggs. “Now, we consider greater than 95% adequate efficacy,” he said.