Using dry treatment on only some cows is called selective dry cow therapy (SDCT). The concept of SDCT is not new but has gained new significance, especially in countries where authorities want to reduce the use of antibiotic drugs in situations where there isn’t good evidence they are needed.
The benefits of SDCT go beyond just meeting some regulatory requirement. Not treating all cows and quarters at dry-off also lowers costs. Farmers counter to me that not treating also entails some risk for the cow and the farm. That can be true but there are ways to manage that risk.
If farmers decide they want to consider SDCT, they will need to work out a plan of how to identify cows that don’t need to be treated. They need to find out if cows have infected quarters when they are dried off. There have been a few research studies looking at how to do that.
One recent study was done in New Zealand. New Zealand dairy production is pasture-based and seasonal so lots of cows are dried off in quite a short time at the end of the milking season. Any method used to find out if a cow doesn’t need to be dry treated needs to be quick and simple — specifically since it isn’t very practical to culture that many cows in the time available.
Researchers looked at somatic cell count (SCC) data, California mastitis test and electrical conductivity of milk. They compared each of those for their ability to predict if a quarter was infected. They used bacterial culture to determine if a quarter was really infected. Once they had all the data, they looked to see which test (or combination of tests) best predicted whether a quarter was infected and should be dry treated.
Not surprisingly, none of the tests used alone found all the infected cows. The best single test was SCC, followed by California Mastitis Test. The least predictive test was electrical conductivity. The best combination of tests was SCC and the California Mastitis Test used together. The researchers concluded that if SCC data was not available, then the California Mastitis Test was better than electrical conductivity. They measured electrical conductivity using a hand-held meter.
Other research into SDCT also supports that the cow’s history of clinical mastitis needs to be considered when making a decision to dry treat. Cows that have a recent history of even mild to moderate clinical mastitis should be dry treated.
When mastitis researchers investigate the effectiveness of dry cow treatments, they will look to see if dry treatments cure quarters that are already infected. Although none of the dry cow treatments available in Canada are approved for prevention, mastitis researchers will also look to see if the dry treatment also protects cows from becoming infected during the dry period. Prevention of new infections is the main reason that people use internal teat sealants. Over many studies, internal teat sealants have been shown to reduce the risk of new infections by 50 to 70%. That is why farmers who do not dry treat every quarter should use a teat sealant in every quarter.
Dr. Robert Tremblay is a veterinarian for Boehringer-Ingelheim and lives near Guelph.