Controlling Worms in Cattle

by Nolan R. Hartwig, DVM, & Douglas T. Ensley, DVM, Department of Veterinary Diagnostic & Production Animal Medicine, Iowa State University

Most cattle have some worms or gastro-intestinal nematodes in their digestive tract. Because it is practically impossible to keep cattle, even adults, completely worm-free, what is the economic threshold of infection? Should cattle be wormed routinely? If so, when? What are the implications for beef quality assurance programs? These are important questions for both beef and dairy producers.

Much research has been done on the economic importance of parasites and strategic approaches to control. Recommendations applicable to various cattle management systems have resulted from this research. Knowledge of the major worm parasites of cattle and how they are transmitted will help producers work with their veterinarians to develop economically sound control strategies.

The most economically important gastro-intestinal nematodes are small, thread-like worms that are located in either the true stomach (abomasum) or intestinal track of cattle. The life cycles of the major species are similar. Three species are economically important in the Midwest. They are Ostertagia, Haemonchus, and Trichostrongyles.

Life Cycle

Worms are primarily a disease of pastured cattle. Economically significant levels of infection do not occur in beef and dairy cattle kept in dry lot. However, almost all cattle carry a few worms and can serve as a source of pasture contamination and infection of herd mates, especially animals younger than two years of age. Adult cattle are much more resistant to heavy infection than young animals.

Adult worms in the stomach or intestinal tract lay millions of microscopic-sized eggs that are passed in the feces. Under favorable pasture conditions, these eggs hatch and the worms mature through three larval stages, eventually becoming infective third stage larvae (L-3). L-3 larvae are ingested, invade the wall of the digestive tract, and become fourth stage larvae (l-4), which eventually mature into adult worms. In some worm species, especially Ostertagia, which is the most important nematode of cattle, the L-4 larvae may lie in the lining of the digestive tract in an “arrested” or hypobiotic state for long periods of time before becoming mature adults. In northern states this generally occurs during autumn and winter. It is a mechanism that nature uses to ensure the survival of the parasite. Arrested L-4 larvae do not mature until near calving, when they rapidly develop into egg-laying adults and heavily contaminate spring pastures. Heifers may develop clinical disease, including diarrhea, loss of weight and anemia. This phenomenon is referred to as type II Ostertagiasis. Young grazing calves are very susceptible to worm infection and may become heavily infected. This delayed maturation of L-4 larvae ensures heavy pasture contamination in the spring and is a major factor that must be considered when developing worm control strategies. In southern states, hypobiosis may occur during extremely warm and dry conditions during the summer. L-4 larvae mature and heavy pasture contamination occurs during the fall months. Control strategies must be tailored to the respective geographic area.

Under ideal conditions, worm eggs shed on pasture will mature to the L-3 stage in as little as three weeks. Rapid development and maturation is favored by warm, moist conditions. Thick, dense pasture growth is conducive to rapid maturation of worm larvae. In northern states, many larvae can survive winter as eggs, which hatch in the spring and become infective larvae. They will soon die out in the spring, however, unless ingested by grazing cattle. Pasture contamination is relatively low in the spring and increases dramatically in mid to late summer.

There is no economically significant transmission of worms between cattle and sheep. In fact, alternately grazing cattle and sheep on a pasture may help control nematodes in both species.

Diagnosis

Heavily parasitized calves are unhealthy in appearance and usually have a dry, dull hair coat. Growth and feed efficiency are significantly reduced. Anemia, diarrhea, weight loss and death may occur when infection is severe. Subtle losses, usually in the form of reduced weaning weight, are common in temperate areas of the northern United States. Weaning weight reductions of 40 pounds or more are common. Moderate levels of infection may cause reduced milk production in dairy herds. Nutritional requirements of developing heifers become difficult to meet and so-called nutritional infertility may be the long-term result of heavy worm infection in heifers. In northern states, symptoms of parasitism are usually most evident in late summer and early fall.

There is a tendency to regard all “poor-doers” as wormy cattle. Such problems are often due to infectious diseases, management and nutritional problems, or toxins. Accurate diagnosis is important to avoid costly, unnecessary worming of cattle. Veterinarians can microscopically analyze fecal samples for worm eggs using a procedure called fecal floatation. On a herd basis, fecal examinations of suspected wormy cattle are helpful, but do not reliably indicate the degree of parasitism of individual animals because egg production of adult worms is extremely variable. Careful examination of the herd, multiple fecal examinations, ration analysis, blood tests, and post mortem examination of any dead cattle all help determine the level of worm infection in a herd.

The economic effect of low levels of parasitism in cattle is difficult to determine. Researchers disagree about the importance of low levels of parasitism in adult cattle. Egg passed by adult cows can contaminate pastures, but the level of pasture contamination caused by calves and yearlings is more significant. Weather and pasture conditions can affect the risk of parasitism. All producers should work out a worm control program that fits their management situation. Management practices and strategic use of anhelmintics (wormers) are equally important for parasite control.

Feedlot Cattle

Most calves and yearlings coming into feedlots are of uncertain background. The worm burden is thus unknown. Because most replacements have been pastured, significant infection is likely. This infection can reduce gain and feed efficiency. Parasitism increases the risk of feedlot diseases such as shipping fever. Heavily parasitized animals do not respond well to treatment for respiratory and other feedlot diseases. Response to vaccines may be poor. Even though the level of worm infection decreases in the feedlot, it is generally cost effective to treat feedlot cattle with an anthlemintic upon arrival. Because of variable feed intake, it is advisable to individually treat replacements with either an injectable or oral anthelmintic. Wormers can be administered by injection or by oral administration of boluses, drenches, gels or past products. A variety of safe, effective anthelmintics are available. Treating calves on arrival for worms is not an undue stress. Re-treating cattle later in the feeding period is not necessary.

Beef Cow/Calf Herds

It may not be necessary to worm every adult cow each year in commercial cow/calf enterprises in the upper Midwest. However, even lightly parasitized cows can serve as a significant source of infection for nursing calves. Herds with ample pasture and good grazing conditions are less likely to have significant parasite problems than when drought or over-grazing occurs. Cow herd worming recommendations should be based on a total herd health and production medicine program. Most veterinarians recommend worming adult cows in the fall when pre-condition calves and pregnancy testing occurs. Replacement heifers often have significant levels of parasites and serve as a source of infection for the next year’s calf crop. If they have been pastured after worming in the fall, heifers should be wormed in the spring 4 to 5 weeks prior to breeding. In herds allowed to overgraze pastures, more frequent worming is advisable. Cattle do not pick up significant levels of worms when grazing cornstalks.

Some confusion exists about pasture rotation and worm control. Rapid pasture rotation (every few weeks) is good pasture and nutritional management, but does not provide parasite control. In northern latitudes, worming in mid-summer when calves are moved to a new pasture is often cost effective. This removes worms that are in the animals and allows them to graze relatively worm-free pastures. The ideal pasture to move them to is a regrown hay meadow. It will be free of worm larvae.

Continuous use of low-level anthelmintics in creep feed or similar formats may help reduce parasite infection, but should not replace individual treatment when calves are weaned and pre-conditioned.

Dairy Cattle

Adult dairy cows are resistant to infection with nematodes. Cows kept in dry lot or confinement conditions do not pick up worms and do not need routine worming. When cows graze pasture, the level of infection should be monitored. Infection levels rarely become significant, however.

Calves pastured during the spring and summer are highly susceptible to parasites. In a trial at a Midwestern university, dairy heifers were wormed three and six weeks after turnout to pasture in the spring. These heifers weighed an average of 80 pounds more in weight at the end of the grazing season than unwormed herd-mates grazed under identical conditions. Ideally, winter and spring born heifers should be wormed three and six weeks after turnout to pasture in the spring. This gets rid of larvae that have over-wintered on pasture and are being picked up by the calves. It is a good idea to do a third worming in mid-summer or early fall when heifers are moved to a different pasture. Yearling heifers should be wormed in the spring and again several weeks prior to breeding. Move heifers to fresh, lush pastures prior to breeding and worm at the same time to avoid heavy pasture contamination. Heifers held and bred in dry-lot situations do not need to be wormed as frequently, but should be wormed when moved from pasture to dry lot. Worm heifers about one month prior to calving, regardless of housing and management systems.

Lungworms

Lungworm infection can cause a persistent dry, husky cough and high susceptibility to pneumonia and other pulmonary diseases. Death can occur if infection is severe. Infection usually occurs in areas where pasture conditions are normally wet or swampy on a continuous basis. Lungworm infection is endemic in some areas and rarely seen in others. Several anthelmintics are highly effective against lungworms. Feedlot operators who purchase replacements of unknown background should use an anthelmintic that is effective against lungworms.

Anthelmintics

Several highly effective dewormers are available. Each anthelmintic has advantages and disadvantages. Efficacy against gastro-intestinal nematodes, lungworms, and ectoparasites such as grubs and lice should be considered when selecting an anthelmintic. It is generally advisable to administer these individually, rather than in feed, since dosage is critical. Follow label directions carefully and DO NOT ALTER THE RECOMMENDED DOSAGE. Anthelmintics have a “threshold” dosage below which efficacy is nearly zero. Do not compromise the recommended dosage, or results will be severely compromised.

Beef Quality Assurance Issues

Except when clinical disease exists, worms rarely cause carcass quality issues. Anthelmintics, however, should be administered according to label recommendations. Injectable products should only be given in the neck or shoulder region in order to avoid injection site lesions. Use clean, sharp, 16-gauge needles. Oral dosing with anthelmintics in the liquid or bolus form should be done carefully to avoid trauma to the throat region of calves.