The never-ending parasite dilemma

Although complex diseases, viruses and infections are often the health main concern cattlemen worry about, sometimes the simplest and smallest organisms that have been a nuisance since the dawn of time can wreak havoc on a herd of cattle without the producer even realizing it.

Dr. John Gilliam, DVM, and associate professor of food animal production medicine and field services at Oklahoma State University Veterinary Medical Hospital, said understanding parasite treatment and the worms that can plague cattle is critical to developing healthy herds.

First off, it is imperative to know a parasite’s lifecycle. According to Gilliam, worms are inside the cow, they produce eggs that are passed into feces and out onto pasture and those eggs hatch and go through a series of developmental stages on grass until they get to what is known as the L3 stage, which is the stage that is infective to the grazing animals when they consume forage. Cattle pick up microscopic parasites as they graze and the larvae go through a few more stages inside the animal until they become a mature adult and start producing eggs that cause the clinical effects in the animal. Additionally, some of the infective larvae can actually cause clinical impacts on production or disease even before they reach adulthood.

Trichostrongyles are a common group of parasites found in cattle and include: Ostertagia ostertagi or brown stomach worm, Haemonchus placei or barber-pole worm, Cooperia oncophora and Cooperia punctata. Less significant parasites include: Nematodirus helvetianus, Oesophagostomum radiatum or nodular worm.

Parasites are usually in a moisture droplet on forage and because they go through so many stages on grass, the pasture environment plays a key role in their lifecycle. Warm temperatures and having enough moisture are all critical to completion of each stage. If it is too dry, hot or cold, it can reduce the number of infective larvae that are on a pasture for animals to pick up.

The prepatent period, or lag in time, from ingestion of L3 larvae to shedding of new eggs, can also have an effect on treating parasites. Gilliam said this period of time varies depending on the type of parasite and the age of the host animal.

“The period is typically about four to six weeks in adult cattle because they typically develop some resistance and immunity to parasites so that immune suppression makes it take longer for parasites to complete their life cycle,” Gilliam said. “In young animals, the prepatent period can occur more quickly because those animals don’t have any developed immunity so about three to four weeks is when we start to see eggs being shed in their feces. When we start thinking about developing deworming strategies, we need to consider the prepatent period and add that on to our deworming interval. It may also impact the products we choose to use and at what time.”

Treatment and tests

Anthelmintics are medications used to kills parasites inside an animal, however Gilliam said no anthelmintic is 100% effective. Some surviving parasites are not killed by the medication because they can carry resistant genes. Those parasites will either have the resistance by chance or have gone through mutations in their genetics and have developed mechanisms to resist deworming products. Additionally, repeated exposure to anthelmintics over time increases the proportion of resistant parasites in the population.

“We actually select those parasite populations for resistance, because we kill all the ones that don’t have the resistant genes,” Gilliam explained. “Over time, they become a bigger and bigger part of the parasite population. This is how we have gotten to the point of having resistance with sheep and goats. Fortunately for cattle producers, the parasites in cattle are slower to develop that resistance than parasites in sheep and goats so we aren’t at the same point as them but we do have some of the same issues.”

Many beef producers might be wondering how they can be sure their anthelmintic is effective or not. Gilliam said resistance is out there, and it appears to be increasing to a certain degree so we can no longer just assume that our anthelmintic strategies are effectual. Testing is the only way to detect and monitor resistance.

The most accurate, albeit least practical option, is to do a postmortem worm count. For this procedure, an animal is sacrificed and necropsied to count the parasites in its gastrointestinal tract. Most cattlemen are not dealing with parasite resistance to the degree that they would need to sacrifice an animal, so the less radical option of a fecal egg count reduction test is probably more suitable for most. This test can be done by taking a fecal sample and counting the parasite eggs per gram of fecal material. Once the sample is taken, the animals are dewormed and cattlemen will wait two to three weeks and obtain another fecal sample and count the eggs in that sample to determine the percent reduction in eggs.

“We’re looking for 90% reduction or more to know that there is no significant resistance in that parasite population,” Gilliam said. “If you’re going to do this at a herd level, it is recommended to take around 20 samples because there is a significant variation in egg shedding in individual animals. We can’t just go out and sample two animals in a herd and base a herd recommendation on those two animals.”

However, there are some limitations as far as sampling and laboratory error with the fecal egg count reduction test, so results might not be 100% accurate. Additionally, some anthelmintic treatments can suppress egg production without killing the parasites, but parasites could remain alive in the animal and presumably could recover and start producing eggs again at some point. Furthermore, this test does not determine the species of parasite present because all of the eggs look the same. It can be cost and labor intensive as well.

Slowing down resistance

With more and more resistance developing to anthelmintics, the question is what are the options? There are no new products on the market or any coming soon to reverse the building resistance threat, however, several methods have shown progress for slowing down resistance percentages in parasites. One option is refugia, which means to only treat some of the parasites so select pressure of treatment is not applied to all the parasites in the population. This can be accomplished by leaving some animals untreated.

“The question is how do we decide who to leave untreated,” Gilliam said. “For sheep and goats, we use the Famacha scoring system to look at the animal’s mucus membranes to determine its paleness on the scoring system. Pale or white means they are suffering from anemia and heavily parasitized and need to be treated. If the mucus membranes are pink then we don’t treat them and whatever parasites they do have aren’t being selected for resistance and can contribute to the overall population in a pasture and keep the resistant parasites diluted.”

However, it becomes more difficult to determine which cattle are parasitized because the primary parasites in cattle do not cause anemia like in sheep and goats.

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“We do know that there are significant differences in the level of parasites in individual animals and there is evidence that about 80% of the parasites live in about 20% of the animals, but it’s hard to figure which ones they are,” he added.

Gilliam suggests using adult beef cows for a source of refugia because there are few cases of clinical disease from parasites in adult cattle because they often develop some resistance. Some data suggests dewormed cows produce more milk, however other studies show no effect on performance whether or not cows were dewormed. Additionally, there is a lot of evidence that deworming young animals improves growth and performance.

Gilliam noted other tips cattlemen can try to fight parasite resistance in their herds. The first is combination therapy using multiple classes of drugs on cattle and in effect getting the benefits of each in treating parasites. Genetics has also been known to play a role in parasite problems. Gilliam said Braham-influenced cattle are known to have more resistance than non-Braham-influenced cattle, so incorporating some cross-breeding of this breed could be a welcome addition to the genetic pool if parasites are overwhelming a herd. Another way to prevent cattle from consuming as many parasites to begin with is to adjust grazing management.

“Typically the parasites stay in the lower few inches of forage, so if we avoid over grazing we can reduce the number of parasite they are picking up in the pasture,” Gilliam said.

Finally, understanding the best drug administration and dosage to give is fundamental in managing parasites. Gilliam said although many cattlemen prefer pour-on administration because it is easy and convenient, it might not be the most effective delivery method of medication.

“It’s concerning with resistance because it tends to produce lower and more variable drug concentration at the parasitic level,” he said. “Both of those issues tend to select for resistance.”

Additionally, cattle are known to lick the medication off each other’s backs and weather can wash it away before it starts to work. Regardless of which method is used, administering the correct dosage is key to the deworming process.

“We don’t ever want to dose them for the average animal because most likely we are under-dosing half the animals,” Gilliam explained. “Always error on the side of giving too much rather than not enough.”

Lacey Newlin can be reached at 620-227-1871 or [email protected].