Criteria for selecting the right antibiotic

There are a lot of factors at play and all of them need to be 
considered in every case where antibiotics are an option

vaccinating a calf

Although a few very effective antibiotics have been removed from the marketplace over the years, the choice has never been greater.

The challenge is there are so many factors hinging on the outcome, so deciding which antibiotic to use — or if one is necessary at all — can be a very difficult decision. Also with the ever-present possibility of antibiotic resistance, we as veterinarians need to decide what antimicrobials to stay away from or to only use in clinical cases when a lower-class antibiotic is not efficacious. Hopefully after reading this article, the thought process will become clearer.

One article can’t clarify every possible combination, so you need to work with your veterinarian to decide on a strategy and the choices for at least the common diseases.

Reading the label is always beneficial as diseases for which clearance has been granted are written on the label. This alone gives you a start as to what types of diseases and subsequently what organ systems, the antibiotic will get into.

The majority of antibiotics, especially new ones, requires a prescription from your veterinarian and so you need to know the conditions it is used for, dosage, method of administration, precautions, withdrawal, etc.

The first decision is whether antibiotics are even necessary. If there is no bacterial infection present or expected in the future, antibiotics may be unnecessary (viral infections are one example). On large mature cattle or feedlot animals, withdrawal times definitely need to be considered. If a condition becomes chronic, slaughter may be an option as we don’t want to burden ourselves and the critter with a long slaughter withdrawal.

Other considerations will be the syringeability (especially important in winter), dosage amount, safety, means of administration (subcutaneous, oral, intravenous etc.), and the cost of the product per treatment day. Cost per day is really the way to truly compare treatment costs. The longer-acting products will cost more because they last longer. The upside is less labour and less stress on the cattle. (This may be nullified if other procedures or painkillers must be given on daily basis.)

The main questions we as veterinarians (and you as farmers) face are: What are the conditions, what organ system is primarily involved, and is causative bacteria likely? The answers to these questions determine the most appropriate first-, second-, and third-choice treatment options.

The best choice takes into account farmer preference, as well as past and current research. Veterinarians will even have different ‘favourites’ for specific conditions. There is almost never a single choice.

A few antibiotics are called broad spectrum, which means they work against a wide array of bacteria in different organ systems. The older sulphonimides as well as newer drugs like Nuflor, Resflor, or Excenel are fairly broad in their effect. Other drugs are very specific — for example, treatment of pneumonia. The macrolide antibiotics are a class of drugs which specifically get into the lungs. Drugs such as Zuprevo, Draxxin, Zactran, and Micotil are all macrolides, and are used primarily for bacterial pneumonia and only a few other things. All prescription antibiotics are fairly specific for pneumonia or only a few other conditions.

Veterinarians may also prescribe them for very specific things, such as seminal vesiculitis in young bulls. There will never be a label claim against these oddball infections. A veterinarian’s experience is invaluable in writing an extra-label prescription. If we do see resistance against one drug in an antibiotic class it is usually resistant to the other drugs in that class.

There are two big classes of bacteria — gram positive and gram negative. Clostridial infections such as blackleg or anthrax are caused by gram-positive organisms. We were always told at veterinary school: P for positive and P for penicillin. This older antibiotic is still quite effective against certain conditions and most veterinarians still use it. Diseases such as blackleg produce toxins and the animal succumbs quickly, so prevention in the form of vaccination is the only effective way to prevent this disease.

To be effective, the right drug must be administered at the right time and right dosage. Weight must be estimated correctly. These antibiotics have been formulated to be effective at the appropriate dosage. Twice as much as necessary will not be more effective and will only cost you more and result in an increased drug withdrawal time. The safety rule of thumb is that if you double the dosage, you double the slaughter withdrawal. Always keep that in mind.

I would be remiss here if I didn’t mention supplemental drugs, such as painkillers, anti-inflammatories, and appetite stimulants. For specific disease, they are often given in conjunction with antibiotics to quicken or improve the response. Again your veterinarian can advise what works best.

Selecting the appropriate medication for a specific disease takes some thought. The biggest step saver is recording what products you use (record either the active ingredient or trade name) and list the diseases it is effective against as well as the dosage. Have a first and second choice and your own specific farm SOP set up by the veterinarian. This will go a long ways to making sure the appropriate product is given especially by new workers.

As well, put up drug dosage charts (available for most products) by the chute, and list withdrawal times. Have epinephrine handy (in case you get a drug reaction) and have the appropriate syringes and needles. Remember a lot of products you use require a prescription and only use products approved for cattle unless you have a written prescription from your veterinarian. Refrigerate the appropriate products and protect others from freezing. Doing all these things should maximize effectiveness of the products.

The future will see more and more vaccines and immune stimulants used, but good management is required to minimize disease and antibiotic usage.

The bottom line is we will always need them to a certain degree, so selecting the right one and using it appropriately will yield the best results.

This article originally appeared in the August 1, 2016 issue of the Alberta Farmer Express

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



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