Current Strategies In Parasite Control In Virginia Beef Cattle

By Dee Whittier, D.V.M., M.S., and John F. Currin, D.V.M., Extension Veterinary Specialists, Virginia Tech

Many advances have been made in the field of livestock parasite control over the past few years. Because parasites decrease production, usually through decreased weight gain, advances in the control of parasites can have a direct economic impact on beef cattle operations. Parasites that affect cattle can be divided into two major categories, internal and external

Internal Parasites

Internal parasites which affect cattle include: roundworms, flukes, and tapeworms. Tapeworms are not considered to be of economic importance in cattle. Flukes are a problem in the Gulf Coast states and Pacific Northwest, but do not pose a major concern for Virginia cattle producers.

There are several d ifferent species of roundworms that can affect cattle in Virginia. Of these species, the one thought to be of most importance is Ostertagia, also known as the brown stomach worm. There are several aspects of this worm's life cycle that are important in des igning a complete deworming program. Cattle are most susceptible to this worm at less than 2 years of age. Most cows greater than 2 - 3 years of age have developed immunity to this worm and do not show an economical benefit to deworming: However, deworming o f cows can decrease the exposure of younger animals pastured in the same field.

Another important aspect of this roundworm's life cycle is its ability to go into hibernation in the abomasum, or true stomach, of cattle. This is commonly known as the inhibi ted larval stage of Ostertagia. This process can occur during the winter with these larvae maturing and developing into adult worms in the spring. While not common, large numbers of inhibited larvae can cause individual calves to show severe signs of paras itism, severe diarrhea, and rapid weight loss. This condition is known as Type II Ostertagiasis. Most but not all dewormers kill inhibited stage larvae

Deworming

Dewormers for beef cattle come in several forms including paste, injectable, drench, pour - o n, bolus, and as a feed or mineral additive. Products also have various lengths of activity and costs. See Table 1 for comparison

Strategic Deworming

Strategic Deworming involves developing a program with the goal of maximizing the economic benefit of deworming cattle while also removing the larvae from infected pastures. Animals have often been dewormed at the start of the grazing season and at the end, but this is insufficient. One deworming in the spring is not cost effective because it does not prevent a buildup of the worm burden later in the grazing season. Deworming in the fall may prevent the "sleeping" larvae from do ing dama ge the following spring; h owever, this is only the case if the right drug is used and cattle are kept off contaminated pastures following deworming

Newer deworming programs, based on current knowledge of the persistent activity of dewormers, provide for much greater benefits of deworming. Studies have shown that strategic deworming programs can provide 30 - 100 extra pounds of gain per grazing season. In order to be most effective, these programs should start when cattle are first turned on to pastures to graze in the spring, with subsequent dewormings depending on the length of persistent activity of the chosen dewormer (See Table 1). Studies have also shown that an adjusted strategic deworming program can be accomplished by deworming at turnout and midsum mer. The benefits from doing this type of deworming program are less, however, than for strategic deworming. Consult your veterinarian to help you design the optimal deworming program for your herd.

External Parasites

External p arasites which affect cattle include lice, warbles (grubs) and flies. Lice are most commonly a problem in late winter, affecting both younger animals and adult cows. The primary clinical signs of lice are severe itching and hair loss, primarily around the neck and tailhead. The entire life cycle of the louse is spent on the animal's body, making development of a control program easier. There are three stages of the louse's life cycle: ( a) nit (egg), ( b) larva, ( c) adult. All products kill both the larvae an d adult stages, but no products kill the nit. In order to completely eradicate lice from a herd of cattle, they must be treated with the product twice 2 weeks apart or treated with a product that has greater than 2 weeks persistent activity. Lice problems will typically clear up as temperatures rise in late spring and early summer, but they can cause decreases in body condition and milk production if severe enough

Grubs (warbles) are the larval stage of the heel fly that migrates from the animal's heel (w here the eggs are deposited by the adult fly in early summer) to the back of the animal. These larvae can cause damage to the hide of the animal and if treated during the wrong time of the year can cause paralysis due to their location near the spinal colu mn. Cattle should not be treated with grubicide between November 15 and March 1

Source:ncsu.edu