I bought the stuff to do it, and it’s buried under jackets in the back seat of my truck.

Beck melissa
Freelance Writer
Melissa Beck is a freelance writer based in Oklahoma.

I just haven’t had time to get the cows up and do it.”

Busy schedules and messy trucks aside, have you ever seen an image of gastrointestinal nematodes or “worms?” If you’re feeling brave, do a quick internet search for “scanning electron micrograph image of brown stomach worms” and you’ll get to see some of these alienlike enemies up close.

In the Southern states, where 40 percent of the cattle in the U.S. are located, internal parasites flourish and can cost the industry around $2.5 million dollars annually, due to reduced gains and reproductive performance.

Know your enemy

The parasites cattle producers should be familiar with are roundworms (nematodes), tapeworms (cestodes) and flukes (trematodes).


Roundworms are the most economically important of the internal parasites in the Southern states, and include medium or brown stomach worm or ostertagia and cooperia. Tapeworms aren’t as economically devastating as roundworms. Flukes can be a problem in conditions favorable to snail populations, such as access to ponds, creeks, ditches and other stagnant sources of water.

Figure 1 shows a simplified life cycle of the nematode. The adult nematode produces eggs inside the host animal, which contaminates the feces, and therefore, the pastures.

Life cycle: cattle parasites

The resulting larva molt twice before migrating onto moist grass (usually within 14 days) where they are consumed by grazing livestock, continuing the cycle. Within about 2 to 4 weeks of being ingested, they mature into reproductive adults.

An added disadvantage to producers is these larva can live up to one year on a pasture, and the medium stomach worm can go into a hibernation called hypobiosis, which begins in the spring, and the larvae don’t emerge until summer.

Additionally, ostertagia are complex creatures and have the ability, as immature larvae, to hang around in the stomach glands for up to six months as inhibited or arrested larvae. This ability to invade and leave the stomach glands is thought to be triggered by nutritional or hormonal factors or the weather.

A slow release of ostertagia from the stomach glands has less impact on the host animal than a rapid emergence of a large number of the larvae at once, which can cause severe symptoms.

According to data from USDA’s National Animal Health Monitoring Service (NAHMS), due to widespread use of popular pour-on avermectin macrocyclic lactones (dewormers), cooperia has emerged as the most prevalent internal parasite in U.S. cow-calf operations.

The avermectin class of dewormers aren’t as effective against cooperia as other classes of dewormers like benzimidazole.

Heavily stocked pastures typically have higher parasite burdens than pastures with lower stocking rates. Cattle in drylot situations are less likely to have high infestation of internal parasites as compared to grazing cattle.

Mature cows develop a degree of immunity to parasites in the lower gastrointestinal (GI) tract over time, however the brown stomach worm (Ostergaia ostertagi) has developed an ability to resist the host animal’s immune system.

How do you know there’s a problem?

Symptoms of ostertagia infection range wildly from type 1 clinical disease with diarrhea, anemia, decreased appetite, rapid weight loss and bottle jaw (swelling under the jaw); to type 2 ostertagia disease caused by a sudden flush of larvae emerging from the stomach glands and can result in death loss.

Calves infected with cooperia have been shown to have reduced feed intake, resulting in reduction in average daily gain compared to their noninfected counterparts.

The best way to treat internal parasites is a two-pronged management approach focused on reducing parasitism in the pastures and anthelmintic control in the other host – the cattle.

Jeremy Powell, an extension veterinarian at University of Arkansas says, “The amount of parasite burden on any given cattle operation will vary with stocking rate, season of the year, age of the animals and environmental conditions.

Internal parasites tend to produce more eggs when environmental conditions are favorable for their larvae, so parasite egg production and pasture larvae contamination generally peak during the spring months, and we refer to this phenomenon as the ‘spring rise’ effect.

Parasite larvae on pasture will be lowest during the hot, dry summer months or severely cold winter months. Parasite pressure will also be less under good management conditions and optimal stocking rates.”

Pasture management

Pasture management includes keeping the more susceptible young cattle on clean pastures that haven’t been grazed in the last 12 months. An excellent example is cool-season annual pastures seeded into a prepared seedbed. Always deworm cattle before moving them onto clean pastures, otherwise they will become immediately contaminated via the feces.

The least susceptible livestock, older cows, can graze the more contaminated pastures with less consequences. Well-managed cows on a positive plane of nutrition will develop immunity to parasites and suffer less from infestations than younger cattle with naive immunity.

Overgrazing or forcing cattle to graze closer to the ground allows them to pick up more larvae. It’s counterintuitive, but studies have shown that rotational grazing can increase contamination as compared to continuous grazing. This is probably due to the short period of rest and higher stocking rates made possible with this grazing strategy.

Anthelmintic control of parasites

Producers who have a regular deworming schedule account for 85 percent of operations in the U.S. and have fewer economic losses than the 10 percent who only deworm when cattle are symptomatic.

There are two main classes of anthelmintics on the market, the avermectin/milbemycins (ivermectin, dormectin, eprinomectin and moxidectin) and benzimidalozes (albendazole, fendbendazole and oxfendazole).

The advantage of avermectin/milbemycin products is the additional benefit of external parasite control and prolonged protection for days to weeks post-treatment.

Brad McGinley, county extension agent in Arkansas says, “The biggest mistake I see most cow-calf producers make is the overuse of a single class of dewormers. Many producers also tend to overuse pour-ons and generic dewormers.”

A strategy for control

Like most beef cattle management strategies, a parasite control program should be specific to your operation. Mature cows should be treated at least once per year, usually near calving. In overstocked herds, treating twice per year is recommended. On some operations, the environment and management may be such that the parasite load is low and the mature cows don’t require treatment at all. It’s important to have a clear concept of your herd’s needs and consult with your veterinarian if you’re not sure.

Bulls, however, tend to be more susceptible to parasites and should be treated twice per year. Due to the acquired immunity to internal parasites that mature cattle tend to have, the older benzimidazole-type products may be sufficient in managing internal parasites.

Powell says, “My general rule of thumb is to deworm mature cows at least one time per year, preferably just before ‘spring rise’ effect to minimize increased egg production and further larval contamination of pasture throughout the rest of the grazing season. Treating in late winter or early spring before the ‘spring rise’ would also be near calving for many producers.”

McGinley says, “Rotating pastures and avoiding overgrazing can help reduce exposure to internal parasites. Producers should also have a regular dewormer schedule that rotates dewormer classes and route of administration to help reduce resistance.”

Calves, on the other hand, should be treated at 3 to 4 months of age and at weaning, unless you’re selling at weaning. Calves in drylot can be treated once.

Clearly, a strategy to control internal parasites should be based on your specific operation and environmental conditions and, to be effective, the products have to leave the back seat of the truck.  end mark

PHOTO: Gastrointestinal nematodes, otherwise known as “worms,” are something the beef industry should not overlook. Photo by Melissa Beck.

Melissa Beck