By Robert Fears

Healthy animals can handle a few worms, but don't let those numbers grow!

When we are struggling to pay for the feed required to keep cattle alive during drought, any opportunity to cut other input costs is welcome. An area to explore for cost cutting is internal parasite control.

When we speak of internal parasites in this article, we are referring to worms that infest the intestinal tract. Dr. Tom Craig, Texas A&M University veterinary medicine, discussed gastrointestinal worms at a recent session of the 2012 Annual South Texas Beef Cattle Short Course in San Antonio. Here are some of his key messages:

How many is too many?

"The seriousness of intestinal worm (nematode) threat to animal health depends upon parasite numbers," says Craig. "High numbers of nematodes may be detrimental to cattle, but low numbers won't do any damage to healthy animals.

"A few worms in the digestive tract may even trigger an immune response. A small number of worms may cause adverse effects to cattle nutritionally stressed or stressed from other causes. We are playing a numbers game when dealing with internal parasites."

During the past 50 years, very little disease or sickness has been caused by gastrointestinal worms, largely due to the effective dewormers on the market.

The most common symptoms of worm infestations in cattle are reduction in growth and weight loss. Economic thresholds for use of dewormers vary with size, class and breed of cattle.

You can't compare a 5-year-old cow with a 5-month-old-calf. If we treat the 2 classes of cattle for worms in the same way, we will have some problems.

Adult gastrointestinal worms are found in the cow's abomasum (fourth stomach), large intestine or small intestine.

Each nematode species lives and deposits eggs in a different place within these 3 organs.

Eggs are passed from the animal in its feces, where they hatch into larvae. The larvae feed on bacteria in the feces and go through various growth stages (instars) until they develop into the fourth stage, known as the infective larvae.

Moisture is required for these growth stages to occur and survive, because they are vulnerable to heat kill between the egg and infective larvae stages. Cold weather only slows development, unless it is prolonged. Extended periods cause larvae to dry out and die.

Once larvae reach the infective stage, they can survive in fecal pats for months, as long as pats retain some moisture. When conditions are right, infective larvae move out of the fecal pats onto blades of grass where they are picked up by cattle while grazing.

The larvae migrate away from fecal pats and to the upper parts of the grass blades, increasing the chance they will be consumed by cattle. The higher concentration of larvae, however, will be near the ground and close to the fecal pats. Overgrazing increases opportunities for cattle to become worm-infected, because they are eating close to the ground. During drought, cattle graze closer to fecal pats — that may be the only place green grass is available.

Fall is a good time of year for worm activity. There are still some warm days, but severe summer heat is over and normally there is precipitation. About 2 inches of rain is needed within a month to provide enough moisture for infective larvae to move from fecal pats onto vegetation. Larvae move up and down the grass leaf in as little moisture as a dew drop. With a few exceptions, different gastrointestinal worm genera go through the same life cycles.

The characters

"The most common nematode in Texas that occurs as far south as somewhere between Kingsville and Raymondville is the brown stomach worm (Ostertagia ostergi)," says Craig. "This worm does not do well in the lower portions of South Texas, because it can't stand the heat. It is a temperate zone organism having come from Europe with the early importation of cattle."

The brown stomach worm has adapted well to most U.S. climates and is active in pastures from October until May. If its larvae are on blades of grass as late in the spring as mid-May, they will cook to death by the sun.

They are winter parasites whose larvae are ingested by cattle during February, March and April. The ingested larvae enter a cow's stomach and crawl into the glands, where the larvae life cycle stops.

In September, when the weather is cooler and there is moisture, they restart the life cycle by producing eggs, so the worms can be transmitted to new hosts during the following autumn and winter.

Brown stomach worms do their greatest damage by injuring tissue as they exit the stomach glands. The damage prevents the glands from secreting enzymes that help cattle digest roughage. This action causes them to feel full and to lose their appetite, resulting in weight loss. Brown stomach worms infect young animals, as well as mature cattle.

The barberpole worm is a tropical species and feels at home in Texas, since most of the state is tropical.

Their infective larvae stages coincide with warm season grass growth. The name comes from the fact that the worm is red and white striped. The red comes from the tremendous amount of blood sucked from an animal, and the white layers on the females contain eggs. Each female lays 5,000 to 6,000 eggs every day, which allows barberpole worms to infest a pasture quickly with a large number of larvae.

Young cattle are more susceptible to the barberpole worm than mature animals. Mature cattle slough off most of the ingested worms not letting them multiply. They pass enough eggs to allow their calves to ingest small numbers of infective larvae to build immunity. The most susceptible types of cattle are stockers and purchased heifers, or young cattle.

Cooperia, commonly called coopers worm, is a small intestine worm. There are 2 species of Cooperia that cause problems in Texas. One is a warm season parasite (Cooperia punctata) and the other (Cooperia oncophora) is a cool season organism.

These parasites are more prevalent in cattle less than a year in age, because older cattle have built up immunity. Twenty thousand or more Cooperia in a calf's intestine cause the animal to have diarrhea. Cooperia are usually not a problem in a cow-calf operation, but they can be of economical importance in dairies and young stocker cattle.

Do we treat for parasites during the drought?

Due to many influences, gastrointestinal worms behave differently on each ranch and in each local area, so there is no common answer as to how and when control programs should be administered.

The most reliable way to determine if a dewormer should be used is to take fecal samples for egg counts during the time of year when eggs are expected to be present.

Since 20 percent of the animals produce 80 percent of the worms, it is important to sample at least 20 cow pats to ensure that a representative sample is taken.

Submit samples to your veterinarian or a qualified laboratory for egg counts. It is best to take fecal samples from freshly weaned calves, since they are the class of animals that are most likely to be adversely affected. Consider deworming if egg counts are medium to high.

"It is important to continue monitoring for the presence of internal parasites during drought," says Craig. "In addition to egg counts, the monitoring program should include crumbling dry fecal pats into a bucket of water to see if any wiggling larvae are observed. When worm larvae are observed or known to be present, cattle should be treated with a dewormer at the optimum periods. If parasites are present during drought, then their populations will explode when rains do occur."

Destocking and leaving pastures vacant because of the drought will cause worms to die. The concern is when restocking occurs and cattle are brought onto the ranch from other locations.

To avoid re-infesting pastures with worms, treat the incoming cattle with a dewormer and put them in a dry lot for 15 days. Then examine the feces for eggs. Fifteen days will give the animals enough time to shed any worm eggs that are in their digestive system and to make sure the treated worms are not survivors of the dewormer treatment.

In a dry lot, there is no way for infective larvae to be re-ingested, allowing the cattle to be worm-free when put on pasture.

Deworming expense can be reduced or eliminated as long as egg counts are showing that gastrointestinal worms are not a threat to animal health.

"A Numbers Game" is from the April 2012 issue of The Cattleman magazine.