Treating Mastitis

Treating mastitis with antibiotics

Antibiotics are a valuable tool for managing mastitis and other bacterial diseases in dairy cows. But, as with humans, overuse of antibiotics in animals can lead to a greater risk of bacteria developing resistance to specific antibiotics. By using antibiotics in a responsible, or more prudent way, we can minimise this risk.

What's the issue with antibiotics?

Internationally, there are concerns about the use of antibiotics in food-producing animals, especially those antibiotics that are critical to human health.

When bacteria develop resistance, this may:

• reduce the ability of antibiotics to cure infections
• require the use of different drugs
• increase the risk of spreading resistant bacteria to other animals, farms or humans.

Mastitis control, including dry cow therapy (DCT), accounts for about 85% of the antibiotics used on dairy farms. It’s vital to understand the most effective ways to prevent and treat this condition.

Lactating antibiotics and dry cow antibiotics are used for mastitis control. Preventative use of dry cow antibiotics (i.e. whole herd or blanket DCT), whereby every cow receives antibiotic therapy at the end of lactation, is no longer supported unless specifically recommended by the herd’s veterinarian as part of a herd animal health plan.

Managing antimicrobial use on NZ dairy farms

Responsible use of antibiotics is best achieved by farmers meeting regularly with their vet to develop a herd animal health plan. This may involve a milk quality consultation, to review mastitis and milk quality management.

It will involve a restricted veterinary medicine (RVM) review, to identify the types of products allowed on farm, and situations when these can be used. Many veterinarians are using a “traffic light” approach to support responsible use of antimicrobials on farm.

This may alter the types of products recommended to treat clinical cases of mastitis. This has altered the way we use antibiotics when we dry cows off. For more details, see the NZ Veterinary Association (NZVA) publication, Antibiotic judicious use guidelines.

Selecting cows for dry cow antibiotics

The gold standard for selection would be to culture every quarter of all animals, however, this isn’t a very practical option. There are a number of indirect tests that can predict the infection status of cows. The most common of these are:

• Somatic Cell Count (SCC)
• Rapid mastitis test (RMT)
• Conductivity

Not all cows or quarters are infected at dry off. Ideally, only quarters with a ‘major pathogen’ infection should receive antibiotic dry cow therapy. A study of 38 herds over four regions in NZ found that 91% of quarters were not infected with bacteria at the end of lactation. Of those that did present an infection, only 2.4% of quarters (~7.5% of cows) were shown to be infected with a major pathogen.

For information see Technote 14.

Protecting cows with low SCC

Low somatic cell count (SCC) cows are likely to be uninfected with significant or major pathogens, but they are at risk of becoming infected over the dry period. Internal teat sealant alone is the best option for these animals.

Even if a quarter is infected and it receives teat sealant alone, the majority of glands will cure.

When compared, the cure rates for infected glands that received teat sealant alone (i.e. 85-90%) were comparable to cows that received DCT+ITS (i.e. over 90%), for cows that had infections due to minor pathogens (CNS, Corynebacterium) or major pathogens (Strep uberis).

Recent DairyNZ research tested the impact of doing nothing, using internal teat sealant alone, DCT alone, or a combination of DCT and teat sealant. Cows that had herd test SCC below 250,000 cells/ml during the lactation were in the study.

Around 16% of cows in the “Do nothing” treatment group were diagnosed with clinical mastitis during the dry period, or within the first 30 days of the next lactation.

By comparison, all 3 treatment groups had only about 4% of cows diagnosed with clinical mastitis within the first 30 days of the next lactation.

Antibiotic use in other countries

In 2012, the Netherlands placed a complete ban on the use of whole herd dry cow antibiotics. This ban resulted in a 50% reduction in use of dry cow antibiotics over a three-year period. There was a 25% drop in clinical mastitis cases over the same three-year time frame, and the bulk milk SCC continues to decline.

Scandinavian countries have never used whole herd DCT. More recently, they require farmers to prove the presence of an infection before commencing treatment, using bacterial culture or DNA tests (PCR).

The UK has a target to reduce antibiotic use, across all animal industries. France and Belgium have a target of 25% reduction in use.