Why antimicrobials fail

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Calf-care specialist Sam Leadley encounters the same concerns from producers every winter: (1) “This antimicrobial just doesn’t work like it did last year” and (2) “I have changed drugs, but we still end up having to retreat calves for pneumonia.”

The solution doesn’t lie in changing antimicrobial products. Rather, it has to do with changing the management practices on your farm, says Leadley, who works in conjunction with Attica (N.Y.) Veterinary Associates.

Here are the top reasons why antimicrobials fail in calves.

The starving calf

No wonder the number of complaints about antimicrobial products goes up during the winter months, says Leadley. When the temperature drops below 60 degrees F, calves’ nutrient requirements start to go up. If you don’t increase the level of nutrients fed, a calf’s immune system will not have the fuel needed to mount an effective attack against invading pathogens.

And contrary to popular belief, a calf’s immune system is actually more important in fighting the disease than the antimicrobial drugs, explains Jim Reynolds, service chief at the University of California Veterinary Medicine Teaching and Research Center. Antimicrobial drugs are designed to knock down the total number of bacteria and slow down bacterial growth so that the immune system can gain the upper hand. Antimicrobials do not kill all of the bacteria present in the calf. They are merely a tool to help the immune system do its job. That’s why adequate nutrition is so important. (To learn more about calf nutrient needs, see “Pour on the calories … it pays,” from the December issue of Dairy Herd Management.)

Wrong diagnosis

Another reason why antimicrobials may appear ineffective stems from an incorrect diagnosis. Take, for example, pneumonia and septicemia. In both cases, calves will have rapid, shallow breathing accompanied by a fever, says Reynolds. People often assume it’s pneumonia and will treat accordingly.

Products designed to treat pneumonia target three specific bacteria that cause pneumonia. But if the calf has septicemia instead, those products will make little difference — and you could lose the calf as a result. 

That’s why protocols to diagnose and treat calves are so important. A properly-trained employee knows that to differentiate septicemia from pneumonia, you have to look farther. If a calf has septicemia, the blood vessels in the white portion of its eyes will be enlarged, and the calf won’t be able to get up. Antimicrobials are used to treat septicemia, but the treatment is off-label and requires a valid patient-client relationship.

Stopped treatment too soon

Following label directions — or your veterinarian’s instructions — would prevent a lot of perceived treatment failures on farm, stresses Leadley. But when the person in charge of calf care gets the directions second-hand, or a new bottle of antimicrobials is left on a desk with a note that says “The vet says to follow label instructions,” you may have set that employee up for failure. When an employee doesn’t know the exact protocol, or doesn’t have a chance to ask questions, he will use his best judgment. And that often leads people to stop treating a calf too soon. The logic, explains Leadley, is that if the calf looks better after three shots, it probably doesn’t really need the other two. And when the medication is expensive, some owners and managers opt to stop treatment as soon as the calf looks better — despite what the veterinarian said — just to save a little money.

If the label says one injection every day for five days, you need to give all five! When you don’t complete the treatment, you help develop drug-resistant bacteria. And then when you have to retreat that calf for the same problem, the treatment is less effective and you double your treatment cost. “The two most common reasons why calves have a relapse three to seven days after supposedly recovering from pneumonia is that the treatment was stopped too soon, or some injections were missed,” says Leadley.

Started treatment too late

When calves first get sick, the bacteria level is just enough to cause infection. If left unchecked with poor immune response or no treatment, the bacteria multiply inside the calf and start attacking internal organs, explains Reynolds. When that happens, it often leads to organ failure and death — even when the correct antimicrobial treatment is finally given. That’s because you started treatment too late. Bottom line: The sooner you recognize disease and start appropriate treatment, the better the result. 

Wrong drug

If “product A” works on “problem A,” then why not use it on “problem B” or even “problem C?” While some antimicrobial products do work against more than one pathogen, this fractured logic does not always apply.

Whenever you use an antimicrobial drug to treat calves, always select the one that has been proven to be efficacious against the pathogen at hand. Failure to do so often leads to costly retreating, or sometimes results in death because the antimicrobial product was not designed to combat the bug the calf was fighting. 

Always record treatment outcomes and review that information with your veterinarian. Doing so can help you fine-tune your treatment program to make sure that the right drug is used to treat the right bacteria — and that it yields the expected results.


When it comes to selecting the right antimicrobial for the job, efficacy is just part of the equation. Some antimicrobials may be efficacious for the problem at hand, but can cause damage in young calves, explains Jim Reynolds, service chief at the Veterinary Medicine Teaching and Research Center in Tulare, Calif. He recommends that you NOT use tetracycline, gentamycin or sulfa drugs in calves less than three weeks of age. They can lead to kidney damage and even death — especially if the calf is dehydrated.

 



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