Regardless of location, cattle and profits stand to benefit from a summer parasite treatment. During the summer in Northern climates, Ostertagia ostertagi are hard at work multiplying, contaminating pastures and making a home in cattle — causing havoc throughout the season. In Southern states, Ostertagia larvae go into a stage of arrested development during the hot, dry months, waiting for favorable conditions when they emerge and leave their mark on cattle and profits.

This is why Dr. Frank Hurtig, director, Merial Veterinary Services, recommends cattle producers in all geographic areas include a summer treatment in their parasite control program. “An effective summer parasite control treatment is designed to remove all stages of Ostertagia ostertagi in cattle,” he says. “This also helps prevent pasture contamination and future infections.”

Ostertagia ostertagi, or the brown stomach worm, is considered the most economically important parasite in beef cattle. In Northern climates, pasture contamination of infective Ostertagia larvae increases dramatically beginning in midsummer and continues through late summer and fall. These high levels of infective larvae on the pasture are especially important because young cattle are susceptible to Ostertagia infections during this time. Nursing calves and stocker cattle ingest the infective larvae, which then migrate to the gastric glands of the abomasum. There the larvae grow to adulthood in about three weeks. The young adults then emerge from the lining of the abomasum — causing damage every step of the way.

Dr. Hurtig says the key factor driving the impact of the brown stomach worm is how many larvae the cattle pick up, which is a result of calves and cows depositing worm eggs on the pasture. “When cattle get progressively more infected, clinical signs will vary from decreased appetite to clinical ostertagiasis, which can be profuse diarrhea, rapid weight loss, bottle jaw, increased mortality, anemia or just generally poor condition,”2 he explains. “In Northern states, these costly effects can be prevented if calves and cows are treated for all stages of Ostertagia infections during the summer, before the larvae can continue their life cycle.”

Arrested development
In Southern climates, the majority of pasture contamination occurs during the winter months because brown stomach worm larvae don’t survive well on pastures during hot weather. The larvae are ingested during the spring and early summer and then go into a stage of arrested development in the stomach lining. They stay this way throughout the summer and then emerge en masse during September and October.

Dr. Tom Craig, parasitologist and professor, Texas A&M University, says the parasite’s ability to go into arrested development is why it is important in Southern states to treat cattle during the summer. “In the South, it makes sense to use a treatment during the summer to kill this parasite in the cattle before it emerges from its arrested state. Even at low levels, once the Ostertagia larvae emerge, they can cause appetite suppression, which can go unnoticed,” he explains. “Instead, we just have an animal that is not eating, therefore, it is not growing or producing as much milk, which results in a chain of production losses that can affect that animal’s profitability and health for years.”

  A summer treatment can help prevent future losses by helping reduce pasture contamination. “Not only are we helping to clean up the animal that has already ingested the parasite, but with a summer treatment, we also help to prevent future contaminations by reducing the parasite load transmitted to the pasture,” Dr. Craig says. “This is especially important with Ostertagia because it survives on the pasture through the winter and into the next spring and grazing season.”

Dr. Craig adds that drought does not necessarily mean that cattle are protected from the effects of parasites. “During a drought, the grass is shorter. Therefore, cattle are grazing closer to the ground, and they will likely graze areas they normally wouldn’t, such as close to dung pats,” he explains. “Cattle also are likely to have a marginal diet during drought, which means their ability to tolerate parasites could be lower also. It is possible that fewer parasites than usual can cause problems during drought.”

“Parasite control has been proven to have the greatest effect of any pharmaceutical technology on both cow/calf and stocker producers’ bottom line,” he says. “However, a parasite control program is only as effective as the timing of the application and the quality of the product used. "

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