Why mastitis treatments fail


A situation that is very frustrating for dairy farmers and veterinarians is when we have to deal with a failure of treatment of clinical mastitis cases and recurrent cases.

Usually, this is attributed to decreased efficacy of the drug used. However, many other factors will determine whether a cow with clinical mastitis will be cured.

Below are some factors that can lead to treatment failure:

Characteristics of bacteria

Mastitis caused by certain bacteria can be more difficult to treat when compared to others. Mastitis caused by Staphylococcus aureus and Klebsiella spp are examples.

One possible explanation is that those bacteria possess some mechanisms, such as the production of biofilm (similar to a “carapace”), that protect them from the antimicrobials.

For instance, Staphylococcus aureus can be localized deep inside the gland tissue, surrounded by scar tissue protecting it from getting in contact with the drug, and thus not get killed.

Choice of antimicrobial

It is important to know what type of organism is causing the infection as some antimicrobials have better action against Gram-positive bacteria (as Staphylococci and Streptococci) while others can be indicated against Gram-negative bacteria such as Escherichia coli.

The consequence of selecting the wrong and ineffective antimicrobial will be treatment failure and antimicrobial resistance.

Antimicrobial resistance

Antimicrobial resistance can be detected by in vitro antimicrobial sensitivity testing. We also have to remember that some organisms that are not bacteria such as yeast (Candida or Criptococcus spp) and algae (Prototheca spp), have a natural resistance to antimicrobials.

Time of treatment

“The sooner the better” can be questionable. Although prompt treatment of clinical mastitis after the first symptoms increases the chance of cure, it is again very important to know what type of organism is causing the infection.

Treatment decisions should be based on culture results — even basic ones as those from on-farm culture (no growth, Gram positive or Gram negative).

Dose, route, duration

Dose and route of administration and duration of treatment are crucial to follow the prescribed dose by the vet and recommended by the label.

Attention should be paid as some intramammary drugs require only a single administration per day, while others are active for only 12 hours, and therefore need to be administered twice a day in order to achieve complete cure.

Extended duration therapy should be reserved for those cases when we expect to have good results — it is unlikely to be effective for treatment of cows that have multiple repeated cases of clinical mastitis for instance.

History of mastitis

Treatments should be administered after the farmer has worked closely with their veterinarian, has reviewed the medical history of the cow and evaluated the chances for therapeutic success.

Chronic cows (or cows with a high somatic cell count for more than three months) or cows with a history of previous clinical mastitis that suddenly present a clinical flare-up are difficult to successfully treat.


Mastitis is a multi-factorial disease, which means that factors such as poor nutrition, inadequate vaccination, poor environment, and stress can contribute to immune suppression.

Consequently, dairy cows with debilitated immunity are more susceptible to any infection including those that affect the udder.

New infections

Precautions should be taken when treating cows to avoid introduction of bacteria through the intramammary injector which might result in new infections. Each teat must be cleaned and disinfected prior to infusion of treatment.

Dairy farmers should work closely with their herd veterinarian to help develop treatment protocols, provide oversight for appropriate drug use and monitor the success of treatment.


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