November 1, 2020

Dewormers for Beef and Dairy Cattle

By Mike Catangui, Ph. D

Parasitism is a form of biological relationship in nature where one organism (the parasite) benefits at the expense of another organism (the host). Internal parasites are, in affect, freeloaders that rob the host of valuable nutrients, energy and general well-being.

Cattle have several internal parasites (e.g., roundworms, tapeworms, flukes, protozoans, cattle grubs and fly maggots) that can reduce weight gain, suppress appetite, reduce milk production and weaken immune response.

Biology. The brown stomach worm (Ostertagia ostertagi) is perhaps the most important roundworm parasite of pastured cattle in the U.S. The general life cycle of the brown stomach worm is presented in Figure 1. Brown stomach worms are small with adult worms measuring only about 14 mm (0.6 inch) long (Bowman, 2009). Infective or the so-called L3 stage larvae [Fig. 2] are ingested by cattle while grazing. These L3 larvae then lodge and feed on the gastric gland tissues of the abomasum (fourth chamber of the stomach of a ruminant), then moult into L4 larvae. If pasture conditions outside are sensed by the parasites as favorable for further development, the L4 larvae moult into adult male or female brown stomach worms, then exit the gastric glands and migrate toward the surface of the abomasal mucosa. Male and female adult worms mate, the female worms start producing eggs, and then the eggs are excreted with the manure onto the pasture to start the cycle again [Fig. 1].

Fig. 1. Life cycle of the brown stomach worm (Ostertagia ostertagi).

Fig. 2. Infective stage (L3) of a helminth on a dew drop on pasture
grass (photo: The Science Review, 2010; New Zealand AgResearch).

It takes about 3 to 4 weeks from the time L3 larvae are ingested to the time that adult female worms start producing eggs. The L4 larvae in the abomasal gastric glands can “decide” to become inactive and delay their further development for several months to avoid harsh environmental conditions in the pasture, such as drought and high temperatures in the summer in southern states, and low temperatures in the winter in northern states. In addition, eggs and larvae already on the cow dung or soil in the pasture, can also overwinter under snow cover in northern states. The biology of the brown stomach worm in dairy cattle is not well-understood and may or may not be identical to its biology in beef cattle.

Economic impact. The impact of internal parasites on cattle physiology and production is well-documented (Myers and Taylor, 1989; Williams and Loyacano, 2001; Sanchez et al., 2004). In grazing stocker cattle, the increase in cattle average daily gain due to treatment with certain pour-on formulations of endectocide anthelmintics (deworming products) had been estimated to be up to 21 percent, or about 59 pounds between treated and untreated stocker cattle (Woodward, 2011). In dairy cattle, the difference between treated and untreated cows is about 1 pound of milk per cow per day (Sanchez et al., 2004).
In feeder cattle, there is evidence that internal parasites can reduce average daily gains by up to 7.4 percent (Nebraska Farmer, 2012). Thus, treatment with an anthelmintic is a standard practice for the processing of arriving cattle in major U.S. cattle feedlots. According to the USDA-APHIS (2012), over 90 percent of large (8,000 head or more) cattle feedlots in the U.S. deworm or treat their arriving cattle for parasites.
There are currently many available and relatively inexpensive dewormers available to cattle producers in the U.S. And in general, the economic benefit of controlling cattle internal parasites with an anthelmintic is much greater than its cost.