Beef producers turn to veterinarians to diagnose disease and determine the best treatments, but when it comes to parasite control, many use a “one-size-fits-all” approach. This is not surprising, as availability of effective, economical, broad-spectrum deworming products — particularly since the introduction of ivermectin and others in the “macrocyclic lactone” class of anthelmintics in the 1980s — allowed easy treatment and dramatic improvements in cattle health and productivity.

However, as parasite research has progressed, veterinarians and parasitologists have learned that a wide range of factors, including the local environment, grazing practices, nutrition, cattle type and types of parasites present in a herd all come into play in determining the “best” parasite-control program for a particular cattle operation. Effective parasite control, and particularly cost-effective parasite control, is not a matter of simply treating all cattle with a particular product at the same time each year.

We are also facing the issue of emerging resistance to dewormers among parasites — a global problem in sheep, goats and other livestock and one that is now beginning to affect cattle production in North America. The development of anthelmintic resistance in parasites, says Colorado State University  (CSU) veterinarian Lora R. Ballweber, DVM, MS, DACVM, follows a model of population genetics involving survival of the fittest. None of the available anthelmintics, used at safe, approved doses, kills every parasite present in an animal. Those few worms with the genetic ability to survive treatment will reproduce and pass their genetic resistance on to their offspring. Over time, with continued exposure to the same or similar dewormers, the percentage of resistant worms in the population increases, and the dewormer becomes less effective.

Researchers have found populations of barber pole worms (Haemonchus contortus), large stomach worms (Haemonchus placei) and small intestinal worms of the Cooperia genus with resistance to avermectins and other common classes of anthelmintics. A 2001 study estimated the economic impact of resistance in nematode parasites in U.S. cattle at over $2 billion.

So, how can cow-calf, stocker and feedyard operators work to ensure continued efficacy and economic rewards from their parasite-control programs? The first step for many should be to include their veterinarian in assessing the parasite status in their operation. From there, they can develop an appropriate control plan, monitor its success and adjust accordingly. A key component in that process, Ballweber says, involves diagnosing which parasites actually are present in the herd.

Since the introduction of generic, over-the counter anthelmintics, relatively few producers involve their veterinarians in assessing parasite populations in their herds, such as by conducting fecal egg counts. Fewer still use methods such as fecal egg-count reduction (FECR) tests, in which egg counts before and after treatment serve as a measure of the effectiveness of a treatment program. Almost none use diagnostics to identify which specific types of parasites are present in cattle, and which types might be emerging, increasing in numbers or surviving treatments.

Ballweber and her colleague Ashley K. McGrew, DVM, PhD, are using a new polymerase chain reaction (PCR) test for parasite diagnosis at the CSU Veterinary Diagnostic Laboratory. The two veterinarians wrote about the test and its applications in the September 2014 issue of Bovine Veterinarian magazine.

The CSU lab offers a “3+2” PCR test used to amplify DNA in order to detect the five major types of ruminant worms: Ostertagia, Haemonchus, Trichostrongylus, Cooperia and Oesophagostomum.  The “3” portion test, McGrew explains, detects the three most important genera (Ostertagia, Haemonchus and Cooperia) — those that are either of great economic significance or those that have commonly been associated with resistance issues.  The detection of Trichostrongylus and Oesophagostomum can be added on as the “2” portion of the test. 

In running this test, laboratory workers first isolate eggs from individual fecal samples.  Eggs from up to five individuals can be pooled for use in a single composite PCR reaction.  The DNA is then extracted from the eggs, and the PCR is carried out using genera-specific primers. 

The PCR test provides an alternative to the larval-culture method which takes two to three weeks to grow and identify parasites from fecal samples. The PCR test is much quicker and less labor-intensive, with accurate results available in as little as two to three days. Ballweber points out, however, that the test identifies which parasites are present but not how many parasites or the percentage of each genus. Researchers are exploring options for adding a quantitative component to the test, but currently, the time and labor involved would be cost-prohibitive.  A typical fecal egg-count test provides an estimate of the numbers of strongyle parasites present but does not identify them.

Putting diagnosis to work

So how can producers use parasite diagnosis to improve their management and treatment decisions? Ballweber and McGrew say the test can have short-term and long-term applications at different production stages.

Cow-calf producers can use diagnosis, along with fecal egg counts, to establish a baseline record of the numbers and types of parasites present in the herd. Subsequently, they can use the test for surveillance, evaluating the effects of their management systems on parasite populations, emergence of new parasites or possibly the development of resistance in particular types of worms.

If resistance is suspected, PCR testing before and after treating a group of cattle, in conjunction with the FECR test, could identify which type or types of parasites are not responding adequately to treatment, helping the veterinarian develop management plans for addressing the problem.

In stocker operations, producers routinely bring in calves from multiple sources and locations, resulting in significant risk of introducing new parasites, or possibly anthelmintic-resistant parasites, into pastures. If stocker operators suspect the presence of resistant worms, cooperation with their veterinarian on diagnostics and analysis can provide solutions. McGrew and Ballweber provide an example involving a large Southeast stocker operation with a longstanding parasite-control program including fecal egg counts. In 2013, the operation reported apparent treatment failures. Previously, post-treatment egg counts had been negligible, but that year they were finding significant numbers of eggs 10 to 14 days after anthelmintic treatment.  The operation’s managers placed a group of the cattle into a drylot in an effort to monitor treatment results and made the decision to send samples to the CSU Veterinary Diagnostic Laboratory for PCR analysis. 

The results showed that Haemonchus and Cooperia were both present in the herd post-treatment.  Whereas Cooperia had been noted in this herd before, the presence of Haemonchus was new and surprising.  Some calves also had some combination of Trichostrongylus, Ostertagia and Oesophagostomum. Based on these results, the operation, with assistance from its veterinarian, switched from treating cattle for parasites in the chute upon arrival to a feed-through program using a different anthelmintic for the cattle on grass. 

Larry Smith, DVM, who runs Larry Smith Research & Development in Lodi, Wis., was involved in that case and has seen other instances of resistance in stocker cattle. Smith has managed large numbers of stocker cattle in Wisconsin for decades, on his own farm and on client operations. These calves ship from the Southeast, typically coming off winter grass for summer grazing in Wisconsin. Smith and his clients traditionally dewormed the calves prior to turnout and twice more during the grazing season to optimize weight gains. A few years ago, he noted insufficient response to treatment on a client’s operation. Smith and parasitologist Lou Gasbarre, PhD, who then was employed with the USDA, purchased a sample of the animals, treated them with various dewormers and necropsied  the animals two weeks later. They found Haemonchus and Cooperia with resistance to avermectins and benzmidazoles.  Working with suppliers in the Southeast, Smith initiated a pre-shipment treatment using an injectible macrocyclic lactone and oral levamisole.  Pre- and post-treatment egg counts, and post-treatment PCR testing confirmed the combination provides efficacy against the worms in question.

Smith recommends stocker operators work with their veterinarians to monitor parasite populations. Use Fecal egg count reduction tests to monitor treatment efficacy, and PCR to identify the genus of parasites that survive treatment. Depending on seasonal conditions, he says, treatment failure can reduce pasture gains by as much as 20 to 70 pounds per head.

In a feedyard setting, the development of drug resistance in parasites is less of a concern since cattle go directly to slaughter. However, as resistant populations grow, feedyards can experience less-than-satisfactory results when using their usual treatments upon arrival. Cattle feeders focus on getting cattle to gain weight quickly after arrival, Ballweber points out, and a persistent parasite infection interferes with that goal. If they find sets of cattle in which their anthelmintic treatment appears to fail, they can use results of the PCR test to help guide follow-up treatment decisions. If they suspect cattle from certain regions or ranches are arriving infected with anthelmintic-resistant parasites, they could collect some samples for pooled PCR testing upon arrival and test again two weeks later to evaluate the efficacy of treatment.

While PCR diagnosis can help guide parasite-control decisions in the short term, McGrew and Ballweber say some of the most important benefits could be realized over time, as diagnostic labs, the industry and individual producers gain better knowledge of parasite populations and resistance trends, and their relationships with different environments, production systems and management practices. In any case, parasite diagnostics, and parasite programs in general, offer an opportunity for producers to involve their veterinarians in economically important management decisions, especially when every extra pound of weaned calf is worth $2.50 or more.

How to submit samples

To submit samples for PCR testing, collect a minimum of 3 grams of feces from each animal, from the rectum or immediately after being passed.  Include egg counts of the individuals, if known.  Place each sample in a clean enclosed container, such as a sealable plastic bag, devoid of air.  If samples cannot be shipped immediately, temporary storage overnight in the refrigerator is acceptable.  Ship samples cold, but do not allow them to freeze or come into direct contact with ice.  Arrange for the samples to be received within 48 hours.  Please feel free to contact the CSU Veterinary Diagnostic Laboratory if you have any questions.  

Send samples to:

CSU Veterinary Diagnostic Laboratory

300 West Drake Road

Fort Collins, CO 80526

Phone: 970- 297-1281

Fax: 970-297-0320

Find this article and others about feeding for fertility and 2014 marketing strategies in the October digital edition of Drovers/CattleNetwork.