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A proper dosage of antibiotics is crucial for efficiency

Animal Health with Heather Smith Thomas

When treating cattle with antibiotics, dewormers and other medications, it is important to use the proper dosage — which is generally determined by the weight of the animal. Thus it is crucial to know, not guess the weight. Under-dosing may not give the desired results, and overdosing in some instances can be harmful. In the case of dewormers, under-dosing won’t kill the parasites and may simply lead to drug resistance.

Dr. Steve Hendrick, Coaldale Veterinary Clinic, a feedlot, dairy and cow-calf practice in Coaldale, Alta., says vaccines are not an issue. These are usually dosed at two or five millilitres per head, depending on the product; the purpose is to provide antigen to stimulate an immune response; it’s not weight-specific.

Dr. Nathan Erickson, assistant professor, large animal clinical sciences, at the Western College of Veterinary Medicine in Saskatoon, says some producers ask if they should give the same vaccine dose to a calf versus a mature cow, and the answer is yes. “With vaccine, it’s not about size of the dose; it’s the amount of antigen that is in that dose. Whether the animal is large or small it needs the same amount of antigen to stimulate immune response,” he explains.

Antibiotics and dewormers are a different story, regarding size of the animal, but for any injection (including vaccine), each animal needs to be given the dose specified on the label, administered at the proper site on the animal, and by proper route, listed on the label (subcutaneous, intramuscular or intranasal).

“If there is an option on the label (subQ or IM) for the injection, go with the SubQ route and dosage, if possible, because there is a smaller chance of creating fibrosis or scarring. If the label includes both routes as options, SubQ administration is always preferred over IM injections,” says Erickson.

When giving any injection, make sure the entire dose is deposited where it should go (deep into the muscle, or under the skin) and that none of it leaks back out — or the animal may not receive an effective dose.

“It helps if you use the proper size (length and diameter) needle for the animal,” he says. This will depend on the size of the animal, how thick the skin is, and the injection method.

When giving antibiotics there are several important considerations. “One reason it is important to always administer according to label directions, with proper dosage, is for withdrawal times. The withdrawal times established for that antibiotic are based on giving the correct dose, for the correct duration and by the correct method,” Erickson says. If you overdose or give a certain antibiotic more days than recommended, or by incorrect route, it may take longer for residues to be eliminated from the animal’s body.

“There are several reasons to not overdose or under-dose,” says Hendrick. “If you overdose, it’s costly. Under-dosing — not giving enough antibiotic or dewormer or delousing product — runs the risk of not being effective, and a chance for some of the more resistant parasites or pathogens to survive. There is a big push today to try to avoid development of resistant microbes or parasites. One of the main ways to perpetuate this problem is continuously under-dosing,” he says.

That doesn’t mean we should err on the side of overdose. We have to be smart about it because there are also disadvantages when overdosing. Besides the expense there may be adverse side effects for the animal if you give too much of a certain drug. Overuse of antibiotics in some situations may kill off the good bugs in the digestive tract, for instance, and lead to other problems.

To be most effective, dosage should be appropriate for the size and age of the animal.

“This means knowing the weight of the animal you are treating,” says Henrick. “I’ve had the opportunity to work in research herds and ranchers’ herds, running cattle through the chute, and we often wager bets on what a certain animal weighs before it’s on the scale. Producers might think they have cows that weigh about 1,200 pounds, when in reality they have some that weigh 1,600 to 1,800 pounds. The average might be about 1,200 or maybe 1,400 pounds, but some individuals weigh a lot more. When trying to estimate weight, people can easily be off by 200 pounds or more.”

There have been studies looking at this. One study in South Africa had health professionals and producers estimating cattle weights and in general the producers tended to under­estimate the weight of their animals. By using weight tapes or, even better, using a scale, you can improve accuracy of weight-based dosage and also become better at estimating weights.

“I know from experience looking at feedlot animals or going out to a ranch to treat calves, once you get them restrained or on the ground it’s tough sometimes to estimate their weight,” says Hendrick.

Some people are better at assessing weight than others, but it can be deceptive comparing animals that are short and stocky versus tall and leggy, or long-bodied or short-backed and thick.

The average of the herd is what producers often go by when running cattle through for delousing/deworming treatments and setting the dose gun for a certain dose. The problem with that is there’s often a swing of 100 to 200 pounds either way in a group, according to Henrick.

“The ideal situation is to have a scale at your squeeze chute, so you could dose each animal correctly,” he says. “This is impractical when treating animals out on the range, but a scale in your chute system is very helpful.”

When using a topical, pour-on product for deworming or delousing, follow label directions for dose, and make sure you apply it all the way from the withers to the tail-head. Some delousing products also recommend applying it along the neck and on the poll.

“Some producers seem to feel that as long as it hits the animal, it’s acceptable, but sometimes it doesn’t all get on the animal,” says Hendrick. “Putting it down the middle of the back is not always possible or practical, but if people knew how important this is, they might make a better effort to do this, rather than just squirting it on the side.”

Proper application will help control lice, especially biting lice that an injectable product won’t control since biting lice feed on skin and don’t suck blood.

Correct dosage is also important from a food safety standpoint, to avoid residues in the animal by the time it goes for harvest.

“When giving oral medication, or oral dewormers, keep in mind that there’s a chance the animal won’t swallow it,” says Hendrick. “Make sure you have the right technique, getting it over the tongue (and at the back of the mouth) and that the animal is actually swallowing and not wasting it. If they spit it out you’ve wasted your money and the animal is not treated. Proper restraint is important when medicating animals. This enables you to use proper technique, whether giving oral medication or an injection. For instance when giving a subcutaneous injection you want the animal restrained so you can make sure the entire dose gets deposited under the skin and doesn’t leak out.” You might think you are giving the proper dose, but if it doesn’t get there, you are under-dosing.

When treating sick calves that might be dehydrated, be aware of the dangers of overdosing with certain antibiotics and anti-inflammatories. “If the calf is severely dehydrated some drugs can be very hard on the kidneys and other organs. Since calves are so small, it doesn’t take much for them to become severely dehydrated if they have scours,” says Hendrick.

An overdose of anti-inflammatory medications tends to damage the kidneys if there’s not enough fluid to dilute them when they are being excreted in the urine. “A calf’s body generally contains a higher water content than an adult and they also dehydrate more readily,” he says. They would be more likely to suffer kidney damage with overdose of certain drugs.

Their metabolism is also a little different, thus calves may metabolize antibiotics more rapidly, which means that you also need to make sure you are not under-dosing with most other antibiotics. “In these situations, stick to the high end of the labelled dosage, and not under-dose, in calves. Most producers are okay with this because from a dose standpoint, even with a very expensive antibiotic, you are not giving very much to a 100-pound calf — compared with a 1,400-pound cow, dosing by weight.”

Some producers, especially with feedlot cattle, still give medications in feed.

“This is less specific regarding dose because it depends on an animal’s feed intake. Some animals get more of it than others. If the animal is already sick and off feed, that individual likely won’t get the proper dose. There are also some dewormers and fly control products that are designed to be put in mineral or salt blocks to feed on pasture, but consumption will be quite variable. Are cattle getting enough within that day to actually be effective or are they just getting a low dose while other individuals are getting an overdose? You have no control over dosage in this situation,” says Hendrick.

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