Editor’s note: Third in a mastitis series.
Contagious mastitis problems are common on dairies, but it’s how you and your clients manage them that will determine whether you or the pathogens have the upper hand. The three most common contagious pathogens seen on dairies are Staphylococcus aureus, Streptococcus agalactia and Mycoplasma bovis. Staph. aureus is also the prevalent pathogen in beef cow mastitis.
Staphylococcus aureus continues to be the most prevalent pathogen on U.S. dairies. It has decreased worldwide as well as in the U.S. in the last 20 years with the advent of control programs targeted at contagious mastitis, but it’s still present in essentially all herds, says Meg Cattell, DVM, MS, Dipl. ABVP, Dairy Research and Technology, Loveland, Colo.
“Mycoplasma may be as prevalent as Staph. aureus but we don’t always test for it because it requires special culture medium,” she says. “Awareness of it being a major mastitis pathogen has spread over the last 5-10 years, and it’s being found in herds that never knew they had it. Streptococcus agalactia is decreasing dramatically because it’s very easy to eradicate once it’s identified in the herd.”
And though it’s not usually thought of as a contagious pathogen, Streptococcus uberis, technically an environmental pathogen, can act like a contagious pathogen, says Cattell. “Contagious pathogens are bugs that can colonize teat skin and other mucus membranes and can be spread cow-to-cow with milking equipment. Strep. uberis can colonize teat skin and teat ends and it behaves like a contagious pathogen but it also survives in the environment.”
The spread of infection
The three common contagious pathogens are spread cow-to-cow. “A big discriminator between contagious and environmental pathogens is that contagious pathogens cannot survive in the environment,” says Cattell. “Environmental pathogens can also live on the cow and be spread cow-to-cow, but contagious pathogens have to be spread cow-to-cow.”
“The cow is the reservoir and anything that moves milk from cow-to-cow, which is primarily procedures at milking time, will spread pathogens,” says Mark Wustenberg, DVM, Kilchis Dairy Herd Services, Bay City, Ore. “I would say in the vast majority of cases where infectious organisms have become a real problem, something in the basic mastitis hygiene that we have known since the 1950s is either broken down or not working consistently.” Wustenberg says that includes proper teat dipping, using separate towels for cows, dry treating and other basic hygienic measures.
“Staph. aureus will grow in all of the mucus membranes of the cow, so in herds where they’re feeding sick cow milk to baby calves or allowing calves to suckle it’s been found that you can swab it out of the mouths of baby heifers,” says Cattell.
“That is why calves shouldn’t be allowed to suckle each other because Staph. aureus is found over the exposed skin surfaces of animals, can colonize the teat skin and eventually winds up in the udder.”
Mycoplasma almost always comes into an expanding herd by purchased heifers from any part of the country. “I think we’re finding situations where our perception was that Mycoplasma was a problem in big herds such as in California,” says Wustenberg, “but I don’t think that’s the case anymore – it can come from small herds, too.”
Cattell believes that Mycoplasma can enter herds through heifers that are infected by drinking sick cow milk. She says Mycoplasma can be found in either colostrum or sick cow milk that’s not pasteurized, but pasteurizing sick cow milk can kill Mycoplasma quickly because it’s very heat intolerant.
She adds that baby calves get infected, then get infections such as joint infections, ear infections and increased respiratory disease that spreads from calf to calf by aerosolized droplets, and they become carriers.
“When they freshen they can spread Mycoplasma with a nasal discharge, can develop metritis and spread it in the metritic discharge and even inoculate their own teats. If they have it in the udder, they can shed it in extremely high levels in fresh heifer colostrum.”
Bulk tank screening
The best way to monitor herds and mastitis-causing pathogens is to do routine bulk tank culturing. Wustenberg has his own lab and he uses a general blood agar media to pick out the different types of Staph. aureus. He suggests to consider using differential media for Strep. identification because it is a little harder to pick out.
Cattell does monthly bulk tank culturing, and on large dairies, weekly culturing for bacteria counts and somatic cell counts to differentiate manure contamination or wash problems. “If the two go up together, it’s probably a cow problem, and from there you can start to do semi-quantitative speciation of the bugs to differentiate between cow and environmental problems.”
In a herd with a control program, if there is a positive bulk tank test, Cattell takes drip line samples (Practice Tips, Bovine Veterinarian, July-August 1998) from all pens, identifies which pen the animal(s) is in, then cultures all animals in that pen. “That’s how we would cost-effectively identify an individual animal or subgroup of animals that are shedding contagious pathogens.”
Wustenberg’s strategy with larger herds is to figure out what the new infection rate is and what is causing it. “No matter what you do, selective culling or treatment, if you don’t first get the new infection rate under control, you’re creating infections as fast as you’re resolving them and you don’t get the problem handled.”
Wustenberg says if you’re going to screen all fresh cows for mastitis, you need a plan. “I’m not a big fan of doing a lot of whole herd culturing where we are not going to do anything effective with the information,” he says. “If the client is not willing to commit to following through with actions such as more aggressive culling, based on the screening information, what’s the point? If he finds a problem cow as she’s freshening and he’ll get rid of her or treat her, then it’s a good idea.”
Wustenberg says instead he prefers to screen fresh cows with the CMT paddle then take the cows with positive quarters and culture them. “We’re using a pre-screening technique which has the milkers looking for strongly abnormal milk. In the majority of herds, that type of screening gets you where you need to be without spending a lot of money on culturing.”
He pulls samples on all clinical cows but may not run all individual cow cultures. He instead freezes the samples, then at a regular visit goes through the records and decides if he wants to run the cultures. He might also culture cows based on SCC activity. “We may pick a representative group of cows that have recently converted from low to high somatic cell counts. This gives an indication of the type of organism responsible for the current new infection rate.”
When bulk tank and individual cow cultures indicate a contagious mastitis problem, Wustenberg first goes back to the basics and looks at the milking machine and equipment to make sure there isn’t a problem there. He says often problems exist with basic mastitis control practices. For example, it’s common to find inconsistencies in the post-milking teat dipping prcedures. Skin condition can also be a big factor in how well dips control infectious organisms on the teat.
“There’s a misconception with milkers that if you spray dip on a cow, no matter where it hits her it disinfects the entire barrel of the teat, but that doesn’t happen,” he says. “Dairies are buying the teat dip and using it, but it’s not going where it’s supposed to.” In herds with that type of problem, he likes to have the dairy go back to using dip cups for post-milking teat dipping.
Cattell agrees. “The number one method of reducing cow-to-cow spread of Staph. aureus is post dipping,” she says. “That may seem counterintuitive because you may think you want to disinfect the teats before you put the milking unit on to prevent spread to the unit, but what you’re really trying to do is kill the bugs that are growing in the milk film that’s left on the teat and goes all the way up the side of the teat. Staph. loves to grow there, so you need the residual activity of a disinfectant to kill of the bacteria on the outside of the teat, and you’ll cut down on the total colonization of all cows on the farm to prevent cow-to-cow spread from the milking machine.”
Second to teat dipping problems, Wustenberg says poor milking procedures which spread milk from cow to cow contribute to the problem.
Cattell adds that, especially in the sick pen, hands are an easy way to spread pathogens cow-to-cow because secretion, loaded with organisms, is stripped onto an employee’s hands, then those hands go to the next quarter or the next immunosuppressed cow. She recommends in those situations that sick pen employees wear gloves and/or disinfect their hands between cows.
The general recommendation for bringing new cows into the herd, says Wustenberg, is that you do not mingle purchased cows, even heifers, until they’ve been quarantined to ensure the cattle are vaccinated. Regarding mastitis, sometimes you have to wait until heifers freshen before you can do individual milk sampling. “Good bulk tank screening techniques for contagious pathogens are the cheapest and best way over time to determine whether we’ve got the introduction and the potential for pathogen spread.”
Cattell says that keeping Mycoplasma out of your clients’ herds comes down to biosecurity. “If you are purchasing lactating mature cows, which hopefully isn’t often, you should culture the animal at least once prior to the purchase, but if not, after purchase isolate the animal in the sick pen and milk it while you’re waiting for the culture results,” says Cattell.
For purchased heifers where a milk sample is not available, she suggests to make the sale contingent upon the animals being clear upon freshening. “The risky thing is exposing your own culture negative animals to those fresh heifers that are shedding a lot of Mycoplasma. I saw a heifer shed enough Mycoplasma into a 48,000-pound tank load of milk, and when sampled, the Mycoplasma was deemed too numerous to count from the tank in one day after plating.”
Treatment of contagious mastitis requires knowing which pathogens are in the herd by doing screenings and monitoring, sensitivity testing and making informed decisions based on what products are available. “Anyone who is selling a teat tube or prescribing a therapy better be practicing good medicine,” says Cattell. “This is the one place we can really be doing good, prudent drug use that’s beneficial for the client, is done correctly and is cost-effective.”
Streptococcus agalactia can be treated with over-the-counter products, using on-label therapy, and reculturing at the end of therapy to make sure the animal is clear of the infection. “If the dairyman is doing any kind of dry cow treatment, Strep. ag doesn’t persist between lactation, but it’s so infectious that it does have a tendency to reinfect rapidly,” says Wustenberg. “You’ll see cows that I think probably get cleared up in the dry period and sometime during the next lactation they get infected again.”
Staphylococcus aureus cows can be isolated from others in the herd, culled or treated with antibiotics, usually with a lesser cure rate. “With Staph. aureus there’s still some discussion as to whether it’s really treatable or not,” says Wustenberg. “My experience has been that if I stick closely to what we’re supposed to be doing with medications, I have found almost always that it’s not practical to treat those cows. In many of those herds if you follow those treated cows back into milk, you’ll find they’re still infected.”
There is no treatment for Mycoplasma-infected cows and they need to be culled.
To stay on top of contagious mastitis you have to treat, prevent, then control. “I never recommend initiating treatment before control and prevention measures are in place,” says Cattell. “The biggest part of control is pen and milking hygiene. Once problems occur, monitoring is critical, then using a selective treatment. It all depends on knowing what pathogen you have.”
Wustenberg adds that there are really only a few basic things needed for contagious mastitis control: “We need good dry cow treatment with a good medicine, effecive post-milking teat dip procedures with an effective product, minimizing cow-to-cow contact of milk, a reasonably well-functioning milking machine and good teat skin health.”
Prudent use of mastitis antibiotics
Antibiotic resistance and the prudent use of antibiotics by veterinarians and producers are taking center stage in discussions around the country. So where does antibiotic use for mastitis on the dairy fit into this discussion?
Meg Cattell, DVM, MS, says that dry cow therapy is the only place veterinarians routinely and with medical reason administer antibiotics to all animals regardless of known infection status. “It’s at a dose that tends to be of a fairly optimal duration which is one of the questions of prudent drug use,” she says. “It stays above MICs, in general, long enough to eliminate most subclinical infections and prevent newer subclinical infections, without being a low level long term exposure that may be related to the development of resistance.
“However, if we use the same dry cow therapy over and over and use the same active ingredient in the lactating therapies for clinical mastitis, we definitely have a chance of seeing resistant strains. Those animals that freshen with a subclinical infection are very likely to have a new infection that developed in the presence of some residue of the dry cow therapy because most infections develop during the first week or two after dry off, or they went through dry cow therapy without responding.”
Cattell says a general recommendation is that if you’re going to put a tube of antibiotic into each quarter of every cow once a year, then take the time to select an antibiotic that may be appropriate given the species and resistance pattern of the typical Gram+ mastitis pathogens on the farm, and that it’s worthwhile to select a lactating therapy that’s different from the dry cow therapy.
“We need to be truly making medical decisions when we’re prescribing drugs and not doing combination therapy we know nothing about or using blanket or broad-spectrum therapy.”
Pirlimycin and prudent drug use
Cattell says extended pirlimycin therapy for S. aureus can be prudent drug use. “There are people whose first intuitive response is that extended antibiotic therapy for an incurable disease is not prudent drug use, but we have a list of reasons why it is prudent drug use,” she says. Those reasons are:
1. The pathogens and their drug sensitivities are known.
2. The duration above MIC that is needed is known.
3. Antibiotics are used for local infections and are not exposing gut pathogens.
4. Testing is done after treatment to assess efficacy.
5. Treatment prevents shedding of bacteria to other animals and into human food products.
Fly transmission of Staphylococcus aureus
Not only is Staphylococcus aureus transmitted between cows and milking equipment, work done by Steve Nickerson, PhD, Louisiana State University, and others, demonstrated that Staph. aureus is shown to colonize horn flies, and the teats of uninfected heifers exposed to those horn flies resulted in intramammary infection. They also found that uncolonized flies allowed to feed on teat scabs of naturally infected heifers also became colonized with Staph. aureus, therefore fly control on dairy heifers in herds with a known Staph. aureus problem is recommended as a method to help prevent these infections.