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6 criteria for justifiable antibiotic usage

Progressive Dairyman Editor Audrey Schmitz Published on 12 January 2018
bottle and syringe

Mastitis is the most common bacterial disease on most dairy farms. According to Pamela Ruegg, a veterinarian for Michigan State University, 65 to 85 percent of all antibiotic handlings on-farm are used to treat and prevent mastitis.

Many cases are treated symptomatically without any knowledge of pathogen type and are usually treated by farm workers without veterinary supervision.



In Ruegg’s presentation at the American Association of Bovine Practitioners 50th Annual Conference, she spoke about the use of antimicrobials to treat non-severe clinical mastitis and ways to help farmers and veterinarians develop protocols that ensure responsible and justifiable antibiotic usage.

According to Ruegg, when antibiotics are routinely used to treat mastitis without determining the type of pathogen, approximately 35 to 60 percent of antimicrobial treatments will not benefit the cow. In an era where use of antibiotics on farms is increasingly controversial, this is difficult to justify.

Following are the six criteria Ruegg gave for justifiable antibiotic usage:

1. Local veterinarians should develop and assess protocols

Local veterinarians need to be intimately involved in the development and assessment of antimicrobial usage protocols.

Ruegg’s areas for veterinary involvement include:


  • Supervising on-farm culture programs and reviewing diagnoses
  • Developing protocols for chronic cases and identifying the cows who won’t benefit from antimicrobial therapy
  • Detection and management of culture negative cases that are bacterially clear
  • Deciding whether to use a narrow or broad-spectrum antimicrobial
  • Determining the appropriate duration of antibiotic treatment
  • Deciding when to change treatments
  • Assessing the outcome of antimicrobial treatment

These are all great areas for veterinary participation and ways farmers can change their practices in order to justify antimicrobial usage, Ruegg said.

2. Cows should be examined before antibiotics are used

Cows are typically first examined for non-severe clinical mastitis when the milker detects a swollen quarter or flakes in the milk. However, further steps should be involved to review the medical history of the cow before treatment.

Ruegg said antibiotics are rarely useful on cows with these histories:

  • Previous diagnosis of an unmanageable pathogen
  • Three or more previous treatments for clinical mastitis
  • More than four months of SCC greater than 200,000 cells per milliliter
  • Any chronic clinical or subclinical mastitis in a previous lactation

If a farmer decides to still attempt treatment, Ruegg highly advised diagnosing the pathogen type.

3. There should be reasonable belief that a bacterial infection is present

Mastitis detection is usually based on inflammation response to an infection. Normally a farmer doesn’t know when the infection actually occurred or if it is still active. When milk from a cow with clinical mastitis is cultured to identify the bacteria causing the infection, Ruegg said no bacterial growth is found in 30 to 40 percent of cases. In these “no growth” cases, the cow’s immune system was able to cure the infection by the time the milk sample was collected. In general, antimicrobials are used to treat rapidly multiplying bacterial infections. Therefore, there is no need to use antibiotics when no bacteria is present, and this could be 30 to 40 percent of cases in a herd, Ruegg said.

4. Narrow spectrum drugs should be the first choice

Narrow-spectrum drugs are used against a specific type of organism, either gram-positive or gram-negative. In contrast, a broad-spectrum drug is effective against both types of organisms.


Because of the emerging issue of bacterial resistance, the World Health Organization has classified all drugs and antibiotics around the world based on their medical importance. They want to protect the most critical drugs that are used to treat critical diseases in humans, Ruegg said. The top category is called “highest priority critical drugs for human health” and includes third- and fourth-generation cephalosporins. In North America, the only drug farmers have available to treat intramammary mastitis is third- and fourth-generation cephalosporins.

When thinking about responsible drug usage, it is hard to justify the five-day treatment of a third-generation cephalosporin for a mild culture negative case of mastitis, Ruegg said. Therefore, it is important to know the type of organism and reserve such broad-spectrum drugs only for cases of mastitis that will benefit.

5. Duration should be as short as possible to result in effective cure

There is no evidence of increased effectiveness with extended duration treatment for many mastitis pathogens, Ruegg said. So making a decision to use an extended therapy based on symptomatic signs without knowing the pathogen and knowing if it will benefit, not only is an inappropriate use of antimicrobials, but also is not an appropriate use of a farmer’s dollar. Without knowing the pathogen type, it is difficult to recommend long-duration therapy for most mastitis cases.

6. Extra-label use should be avoided when on-label use is possible

Extra-label usage refers to any use of a drug not specifically listed on the drug's label, and it is only permitted under the supervision of a local veterinarian who has a valid veterinary-client-patient relationship (VCPR).

In the U.S., only intramammary antibiotics are approved by the FDA to treat mastitis. Therefore, the use of any injectable product (anything given in the muscle, under the skin or intravenously) to treat mastitis is considered extra-label usage and must be given based on the guidance of the veterinarian who holds the VCPR for the farm.

To ensure responsible drug usage, farmers need to read and understand the labels on all products, Ruegg said. The labels describe the illnesses the products are approved to treat, the dosages that should be given and the withholding periods for meat and milk. If drugs are used in a manner not described on the label, then the withholding periods for both meat and milk are not applicable, elevating the residue risk dramatically.

The goal of mastitis treatment should be to reserve antibiotics for infections that are most likely to benefit from treatment. In particular, knowing what pathogen you are dealing with can help farmers make the best treatment or management decision for each case of mastitis.  end mark

Audrey Schmitz
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