Serratia - What Should You Do If You Find it in Your Herd?

Several producers have called recently to find out about Serratia marcescens mastitis after culturing milk samples from individual cows and finding Serratia as the cause of mastitis. This blog addresses their questions, decisions, and our current understanding.
Origin of Serratia marcescens

It is difficult to know where Serratia comes from without testing environmental samples, utensils, milking equipment, or milk samples from additional cows. If Serratia is found only in one cow, further environmental testing may not be worthwhile. However, if the bug is found in several cows' milk it would be important to act quickly. Serratia, although not a common cause of mastitis, is present in the cow's environment - soil, plants, feed, and water. Serratia has also been found in bedding and on the milking facility floor, indicating it may be traced back to infected cows. Serratia may originate in bedding or have leaked onto bedding from infected cows. This means that good hygiene and biosecurity are very important to prevent transfer from infected to 'clean' cows. It also means that culturing new infections - either found through DHI SCC or when fore-stripping - is important to reduce the chance of spread of Serratia. Herd outbreaks of Serratia mastitis have occurred in herds where Serratia grew in bedding and/or teat dip. Poor udder cleanliness and damaged teat ends also appear to increase risk of spreading Serratia to 'clean' cows.

Diagnosis and Severity of Serratia Mastitis Infections
Most Serratia infections will be found after a new DHI test comes back with a high cow somatic cell count (SCC). Visible signs of mastitis may only be seen in one-half of Serratia cases, meaning that visible symptoms won't always be apparent. Changes to milk are usually mild - few flakes - but infections won't cure, regardless of treatment. This means that Serratia mastitis is not a good thing, and prevention, and diagnosis through milk culturing, are important to keep infections low.

When are Serratia Mastitis Infections Most Likely to Occur?
New infections can occur at any time during lactation and may also occur during the dry period. Cows with high milk production are not at greater risk than cows with low milk production. Serratia is an "equal opportunity" bug, although is more likely to infect that next cow milked after an infected cow, and cows with poor teat ends. Because Serratia, like many environmental mastitis bugs, may be spread from cow to cow, it is important to milk infected cows last or with a milking unit used only for chronic cows.

How likely is Serratia to Cure?
Serratia is resistant to most antibiotics. One study found cure rate was only 14 percent. Mastitis treatment for Serratia rarely cures infections and is not recommended. After antibiotic treatment, milk may look better, but is usually only temporary. Because cows with Serratia mastitis are not likely to cure, emphasis should be placed on identifying new infections quickly.

How Can You Prevent and Control Serratia Mastitis Infections?
Serratia, like most mastitis bugs, plays by the numbers game. The more Serratia bugs in the cow's environment, the more likely the next cow will become infected. Teat and teat end cleanliness at milking, careful fore-stripping to prevent back splash or stripping onto cow beds, removing hair from the udder at least every 6-months, use of pre-dipping before milking will all be beneficial. In addition, reducing contact of the cow's udder with bugs between milkings, by scraping back of cow beds (where the udder rests), and applying fresh bedding frequently, will be worth your time. In herd-wide problems, quick identification of Serratia source:  cows, bedding, or teat dip, is essential to reducing infection spread.

Teat Dip and Serratia
Teat dip does not usually contain Serratia marcescens. However, Serratia is commonly resistant to chlorhexidine-gluconate or quaternary ammonium disinfectants. Consider culturing your teat dip if Serratia is found in more than one cow, and especially if one of these disinfectants is used as germicide in your teat dip. Most mastitis laboratories will culture teat dips. As long as a representative sample is sent to the laboratory, Serratia should be easy to find if present in the teat dip. A negative finding means that Serratia is not present in the teat dip sample and a positive finding means that Serratia is in the teat dip and is likely a source for new Serratia infections.

Summary
Serratia infections are relatively uncommon and it was surprising to have two calls within a short-time frame. Routine culturing was helpful to these particular farmers in that it confirmed Serratia was present in their herds. Because both producers tend to culture most new infections, they knew Serratia was not common and wanted to learn more about how to deal with the mastitis. In one case, the producer had the teat dip cultured and the result was negative. He was concerned whether a negative result meant the teat dip was not the source of the infection or the chlorhexidine teat dip was not the likely source of the cow's infection. In the other herd, Serratia was found in a good cow without previous mastitis, symptoms were atypical for the herd, and treatment was started before culture results came back from the laboratory. The producer's veterinarian recommended extended therapy because symptoms were atypical for the herd and this protocol was previously successful in curing herd mastitis infections. After treatment, mastitis was still present in the cow's quarter. The producer called to get information on what the research suggests in terms of ability to cure Serratia mastitis and whether additional treatment might be successful.

What Decisions Can Be Aided By a New Understanding About Serratia?
1). Serratia cows should be milked last or with dedicated milking units to reduce the chance of spreading infections;
2). Serratia cows should be put on the 'to cull' list. Treatment and especially re-treatment is not recommended;
3). Carefully watch for Serratia mastitis by culturing new infections;
4). Culture fresh bedding and/or teat dip (if chlorhexidine or quaternary ammonium based) if more than one case of Serratia is found;
5). Add new knowledge gained about Serratia to your tool box for quicker and profitable future mastitis decisions.

Thanks to Dr. Frank Welcome for guidance about teat dip culturing and Cornell QMPS for their summary of Serratia Research.