Dairy cows: chronic mastitis and infection control 

Cows with chronic mastitis are unlikely to recover and should be culled because they’re likely to pass bacteria onto other cows. Culling them prevents the infection from spreading, and this protects healthy young cows, which are the future of your herd.  

Note that on its own, culling isn’t a way of controlling contagious mastitis. The spread of mastitis within the herd also needs to be controlled to stop more cows from becoming chronic cases.  

Antibiotic DCT won’t cure all existing infections, and cure rates are lower for older cows with chronic infections. Consider culling cows that have had:  

• Three or more cases of clinical mastitis in a single lactation  
• Persistent high SCCs in two consecutive lactations, despite treatment with antibiotic DCT in the dry period in between  

Other factors to consider before deciding to cull high-cell-count cows include:  

• The type of infection – for example, treatment of Streptococcus agalactiae is likely to be successful, while treatment of Staphylococcus aureus infection probably won’t in chronic cases  
• The risk of mastitis spreading to other cows in the herd  
• Their impact on the BMSCC and, consequently, on milk payment  
• The number of chronic cows that could be culled  
• Genetic merit  
• Milk yield  
• Fertility and general health status  
• The sourcing and cost of replacement cows  

Before culling, always check the cow’s history. Cows that have had a very high cell count in only one lactation are candidates for antibiotic DCT.  

With your vet, think about monitoring the success of mastitis treatments, using cow SCC in the months after treatment as a measure. You should aim for under 20% of animals in your herd to have an SCC equal to or over 200,000. Consider culling chronically high-cell-count cows.   

Don’t use antibiotic DCT in cows due to be culled in the next couple of months. If cows have been treated, note the withholding period.

You can reduce the risk to the rest of the herd from persistently infected quarters by simply stopping milking them.    

If there have been three or more clinical cases in a single quarter, you have the option of drying off that quarter either for this lactation or permanently. This gives the quarter a prolonged dry period that may improve the chance of a cure by the next lactation.   

Make sure you clearly identify cows and dried off quarters so that all milkers know not to milk that quarter.  

Segregating infected cows (and quarters) away from herd mates is critical to managing infectious diseases. Although difficult to implement in some herds, it’s often effective. The aim is to try to keep cows that are more likely to be infected with a major pathogen away from healthy cows.   

Segregation could mean the physical removal of infected cows out of the main milking groups and the creation of an infected group that’s milked last. This significantly reduces the risk of infection spreading through the herd, including clinical cases and cows that have subclinical mastitis infection.  

Segregation can also be virtual, using separate clusters for infected cows. Bear in mind that:  

• The segregation system must be practical, e.g. group size related to the number of units in the parlour  
• The milking plant will need to be decontaminated after milking infected cows last, so use a hot wash after every milking  

Be aware that purchased cows could still be under the withhold period for a previous treatment. Test their milk for antibiotic residues before adding it to the bulk tank.  

While many people consider major diseases such as TB, BVD and IBR when buying cows, they often overlook the fact that bought-in cows and heifers may be infected with mastitis pathogens. Infected cows are a possible risk to other cows in your herd, and while it can’t be totally avoided, this risk can be minimised by taking some biosecurity measures.  

If you can’t avoid buying in cows, you can reduce the risk of bringing mastitis into your herd by:  

• Checking the individual cow SCC records of the cow’s original herd before purchasing any animals from that herd – ideally, at least bimonthly records for the previous six months. For example, only buy a cow that has never had an SCC of over 100,000/ml during her life  
• If buying a cow in early lactation, check whether she received antibiotic DCT and teat sealant last time she was dried off and find out the dates these were given. If antibiotic DCT was administered, find out why it was used  

Milk any bought-in cows last until they’ve had three negative CMT tests on consecutive days.