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I recently observed the economic collapse of several dairies brought about by contagious mastitis. In each case, the herds lacked a contagious-mastitis-biosecurity program for purchased animals, and herd owners only identified the problem when the somatic cell count or standard plate count for bulk milk exceeded the legal limits for the Pasteurized Milk Ordinance. By that time, drastic measures were required.
In these types of situations, it’s important to focus on all major contagious mastitis pathogens — Staphylococcus aureus, Streptococcus agalactia and Mycoplasma sp — rather than trying to control just one. Here’s what you need to do.
1. Sample all cows. All lactating cows in the herd must be sampled on the same day. Use a lab with an excellent reputation for identifying mastitis pathogens. Check into its training and validation programs. Make sure it can handle an adequate volume of samples in a timely manner.
On the day of the initial test, make a list of all non-lactating animals. As these animals freshen, each should be sampled immediately. Time it so you receive lab results before a cow moves into the milking string or before culling is scheduled.
2. Examine protocols. Take a look at your milk-hygiene practices. Review milking procedures, including dip application, function of pulsation and detachers, milking-system inspection, as well as hygiene involving teats, hands and towels. It’s not unusual to find poor dip application, multi-use towels and dirty, cracked milker’s hands on a “well-managed” dairy with a contagious mastitis problem. Often, the equipment is not getting timely maintenance.
3. Evaluate animal groups. Consider how animals are grouped and evaluate the standard practices for each stage of lactation. Is milk from sick cows fed to heifer calves without pasteurization? If there is a contagious or chronic mastitis pen, how do animals move in and out? Are fresh cows housed with sick cows? What about incoming animals? How quickly does the herd identify animals with contagious mastitis and isolate them? Each time there’s a point of contact with potentially infected animals, clean animals are put at risk.
4. Respond to results. As soon as culture results are available, infected animals must be isolated or culled, depending on the organism. We recommend:
Cull animals with mycoplasma.
Treat animals with Strep. Agalactia.
Selective, extended treatment or segregation of animals with Staph aureus.
In no case, should animals with contagious mastitis be returned to a “clean” group without at least three consecutive negative cultures. Even this is risky. One mycoplasma case can grow to 20 within a week.
5. Test again. A follow-up whole herd culture should be done one month following initial lab results. Collect string samples, and then conduct individual-cow cultures of those strings that test positive.
On-going culture of all fresh cows and cows with clinical mastitis can allow you to identify cows that weren’t shedding bacteria at the time of the first two herd cultures. Bulk-tank milk should be sampled monthly to identify any change in status.
With this protocol in place, we have yet to observe a relapse of contagious mastitis within a recovering herd.
Marquerita B. Cattell is a consulting veterinarian in Windsor, Colo.
Learn from others’ mistakes
Recently, we were asked to examin records from two herds that lost the battle with contagious mastitis. Both were sampling only a subset of animals based on California Mastitis Test screening. Yet, when moderately infected Staph. aureus herds only culture cows with a CMT score of 2 or greater, research shows they will fail to identify about 25 percent of infected cows. Eventually, the herd will collapse due to loss of cows from the constant spread of infection from unidentified positive cows.