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Livestock Medicines
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How Antimicrobials Work and When to Use Them

This Section Reviews the Following:

How do Antimicrobials Work?
Categorizing Antimicrobials Based on Human Importance
When are Antimicrobials Appropriate?
Different Uses of Antimicrobials

1. How do Antimicrobials Work?

Antimicrobials can function in two ways, they:

Kill bacteria

This is called a bactericidal antimicrobial
While this type of antimicrobial kills bacteria, it will only work properly when it reaches a certain level in the body. It is very important when using bactericidal antimicrobials that the proper dosage is used. If the drug never reaches the appropriate level (e.g. the wrong dose is selected), it will not kill bacteria. Some examples of bactericidal antimicrobial include penicillins and cephalosporins

Slow down or stop bacterial growth

This is called a bacteriostatic antimicrobial
This type of antimicrobial will interfere with the reproduction of bacteria and slow its growth. It will not kill the bacteria. After the first dose of a bacteriostatic antimicrobial, bacteria will remain at the same number but will not grow. If a steady level of a bacteriostatic antimicrobial is maintained, the bacterial levels will decrease as they will naturally die, or the body’s immune system will kill them. This is why it is important not to miss doses of bacteriostatic antimicrobial. Some examples of bacteriostatic antimicrobial include tetracycline and trimethoprim sulfa

Let’s view the impact of these antimicrobials on bacteria:

Important Note: Antimicrobials do not completely eliminate all of the disease-causing bacteria from the animal’s system. The immune system plays a major part in fighting bacterial disease by killing and removing the offending bacteria and repairing body damage caused during disease. Antimicrobials are a tool that prevents the immune system from becoming overwhelmed by bacteria.

2. Categorizing Antimicrobials Based on Human Importance

Antimicrobials are important for treating bacterial infections in both humans and animals. Some of these antimicrobials are essential for the treatment of serious, life-threatening infections in humans. Thus, to limit the development of resistance in life-saving antimicrobials, categories have been developed to highlight their importance in human health.

3. When are Antimicrobials Appropriate?

When deciding if an antimicrobial is necessary, it is important to make sure that you’ve correctly identified the cause of disease. This can be done by consulting with your veterinarian, who will work with you to look at the animal, evaluate their surroundings, inspect their feed, housing, and other management components and perform any diagnostic tests as necessary.

Work with your veterinarian to develop treatment protocols to determine which cases need antimicrobials!

When your veterinarian is choosing which antimicrobial medication to use, they will ensure:

The disease is correctly identified (the diagnosis is accurate)
An antimicrobial is needed
The correct antimicrobial is chosen, taking into consideration its categorization and effectiveness
The correct dosage will be provided
Proper withholding times will be met to minimize risk of residues in food

4. Different Uses of Antimicrobials

Antimicrobials, depending on the label, can be used in a variety of ways.

The three main uses are:

Therapeutic
Therapeutic antimicrobial use is the treatment of an animal showing clinical signs of illness. Therapeutic use involves a specific dose of antimicrobial used for a limited amount of time, in an amount calculated for an individual animal, flock, or herd to cure a particular disease or illness. An example of therapeutic antimicrobial use would be:
- Providing an antimicrobial to a nursery pig that has been diagnosed with pneumonia
Metaphylactic
Metaphylactic antimicrobial use refers to the administration of antimicrobials to an animal or group of animals after the diagnosis of clinical disease in part of the group. The ultimate goal is to prevent the spread of the infectious bacterial disease to other animals. An example metaphylactic of antimicrobial use would be:
- Providing antimicrobials to a group of veal calves when more than 15% of calves in the group have been individually treated for pneumonia. We know that a significant proportion of animals in the group are sick already, and we want to treat the whole group to try to limit further spread of disease and treat those animals that are infected but not yet showing clinical signs of disease
Prophylactic therapy
Prophylactic antimicrobial use refers to the administration of an antimicrobial to an animal or group of animals before clinical signs of disease to prevent the occurrence of disease or infection. An example of prophylactic antimicrobial use would be:
- Providing an antimicrobial to a dairy cow that is about to have surgery to correct a left displaced abomasum. The surgery is normally conducted in a “less-than-sanitary” environment, and antimicrobials are given prior to surgery to prevent post-surgical infections

Next Page | Section 1 | Roles and Responsibilities of Veterinarians and Producers

Introduction to Livestock Medicines

Regulations on Antimicrobial Use and Access

Antimicrobial Stewardship

Antimicrobial Stewardship in the Ontario Beef Industry

Antimicrobial Stewardship in the Ontario Veal Industry

Antimicrobial Stewardship in the Ontario Dairy Industry

Antimicrobial Stewardship in the Ontario Aquaculture Industry

Section 1 | An Introduction to Livestock Medicines