Mastitis pathogens

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Editor’s note: second in a mastitis series

Almost all dairies deal with mastitis problems from one degree to another, but the problems and the pathogens that cause them are not all alike. Depending on the particular pathogen(s) that are present and the dynamics of your dairy client’s environment and management, controlling and/or preventing mastitis can take several different avenues.

“One has to think about what organisms you are looking for before you even start talking about bulk tank vs. clinical samples,” says Page Dinsmore, DVM, Colorado State University. “In the bulk tank, we look for three basic mastitis pathogens, Streptococcus agalactiae, Staphylococcus aureus and Mycoplasma bovis. It’s crucial to identify them because depending on what you find, there are very different ways of managing these organisms.”

“It’s most important to identify organisms when there’s a herd outbreak,” adds Steve Nickerson, PhD, Louisiana State University. “Many of the outbreaks are caused by Streptococcus agalactiae. That organism can be successfully eradicated, so it’s important to know if that’s what is causing the outbreak.”

Dinsmore adds that mastitis microbiology is fairly simple, with a few exceptions, such as Mycoplasma which takes special procedures. “Every producer, regardless of size or geography, should know the pattern of organisms on the dairy in order to proceed with management of clinical cases and to be able to identify when an outbreak is occurring.”

“It’s important to identify pathogens because different pathogens behave differently,” says Karen Jacobsen, DVM, MS, University of Georgia. “I like labs that will quantitate the different bacteria and I wish more diagnostic labs would adopt those procedures. Many will identify which bacteria are present, but not count them.” Jacobsen says at the end of the year she takes those numbers from the lab and puts them on an Excel spreadsheet, plotting each organism on a graph to show clients how the organisms increase or decrease in the herd over time.

Nickerson agrees but says unfortunately many laboratories don’t have the capabilities or knowledge to thoroughly identify mastitis organisms. “If a veterinarian is going to the trouble of looking at bacteria and somatic cell counts, I think it would be worth the extra effort or money to get a species identification where possible.”

Indicators of change

Bulk tank tests can be great indicators of change in management procedures, milking equipment operation and environmental changes. Jacobsen notes that one organism she tracks with routine monitoring is Corynebacterium bovis. “It’s kind of a sentinel organism to let you know whether or not you’re doing a good job because C. bovis does not colonize the udder, per se, it just colonizes the streak canal. If you have a high count it’s an indicator that something has gone wrong with your post-dipping program, such as a change in teat dips or improper dipping.”

Nickerson says one of his clients was suddenly having a lot of clinicals in the fresh cows from Staph. organisms, and looking back at the dry cows they found Staph. there, too. “The producer quit using dry cow therapy to save money, and it caught up with him. At least he kept good records so we could look back at what had changed.”

Another of his clients cultured a lot of Klebsiella, an environmental organism, from cows with mastitis. Sampling a load of new green sawdust that was full of Klebsiella indicated where it came from. “At that point you know it’s not Staph. or Strep., it’s an environmental pathogen that’s causing the problem. In those cases it’s necessary to know what the bug is.”

Each of these organisms has a source and there is generally a common way of preventing their spread, but you have to look at what kind of pre-dip and post-dip is being used, bedding and other sources of contamination. “You can’t fight a battle if you don’t know who you’re fighting,” says Jim Brett, DVM, Macon County Animal Hospital, Montezuma, Ga. “By looking at the type of bugs you get, you can look at sources, then at prevention. Is it a human problem, a cow problem, an equipment problem or a combination? If you find you have an equipment problem, quit running around culturing individual cows. Break the system down and see where the problem is.”

Start culturing

When faced with a mastitis problem on a client’s dairy, the first step is to assess whether it’s an acute problem that needs to be investigated, an outbreak of clinical cases or a high somatic cell count. “Unfortunately, many times you are called out because the client is in trouble with high somatic cell counts that are close to the legal limit,” says Brett. “The first thing I do is take multiple bulk tank samples and look at the farm size. If it’s a large farm, maybe only half or two-thirds of the herd is in the bulk tank, so you have to make sure you get everyone cultured by culturing other tanks as well. That’ll help determine if it’s a cow or an equipment problem.”

Starting a culturing program on a herd can involve weekly bulk tank testing until a pattern of organisms is identified. Jacobsen says for an initial problem in a herd, culturing on a weekly basis or culturing bulk tanks for different groups of animals within the herd can help you identify those patterns.

Brett says when a client’s herd has a mastitis problem he will take daily samples for three to seven days to make sure he has all the information he needs. He then sends the samples to a diagnostic lab and within four to five days he has organism identification and counts. “If I have a herd problem and identify that it’s Staph. aureus or contagious, then I have to back up and look at the individual cow,” he says. He does this by culturing different strings of cows to identify certain sections of the barns that have problems, or on smaller dairies he uses the California Mastitis Test paddles to identify individual cows.

When Brett starts a monitoring program, he likes to culture weekly for four to six weeks, then according to what he finds and the size of the farm, every two to four weeks after that.

“I also encourage clients to sample any clinical cases they have,” says Brett. “The results we get are not for that cow; it’s for us to monitor and look at preventatives and historical data.” Some of Brett’s clients take a sample from every clinical cow, freeze it, then send in a group of samples to the lab once a week. “I get a group sample that I can look at and compare to the data. Then we can identify the contagious cattle and if the client has a low level of Mycoplasma or Staph. aureus, we can check the individual to see if she’s worth keeping, or we might decide to cull her and get her out of the system.”

Routine monitoring

Probably the most valuable tool you can use in the fight against mastitis is good, routine monitoring of the herd. But to get accurate, consistent results, either the herd veterinarian or a designated employee or employees on the dairy should be the ones who take the samples. “It’s extremely important to have one person who is designated, trained and responsible for taking samples,” says Nickerson. “People may think it’s not a significant job, but you wouldn’t believe the contamination we can get in samples. Many times the problem is when the designated person is gone or on vacation and someone else takes the samples who isn’t trained. It’s very important to have a certain individual responsible for taking sanitary samples.”

Brett has found that identifying a person to collect samples can be rewarding for both him and the employee. “If you can find someone who likes a little bit of a challenge and can be your culture person, you give them two vials for samples, mark them and use one for HyMast and put one in the freezer for culture. That employee usually enjoys this job and will write it down in the records, keep a list of positive and negatives and have it ready to show you when you come out to the farm.”

But all of the routine monitoring in the world won’t tell you anything if you don’t keep good records and use those records to assess your situation. “Records are quite important,” says Dinsmore. “If you don’t have a way to retrieve the information, you won’t have a way of knowing where you’ve come and what you’ve accomplished, or what trouble you’ve gotten into. You have to be able to track things and it’s just as important to keep track of culture results, patterns of pathogens and bulk tank samples as it is to keep track of milk production. You can’t manage something if you can’t count it.”

“I used to think you didn’t need bulk tank tests every month in a closed herd,” says Jacobsen. “But one thing I have found is most of our herds are not closed, and bulk tank tests are a good way to let you know if you’ve brought in a cow with Mycoplasma or Strep. ag if you didn’t have it before.”

Nickerson agrees. “Dairy farmers should culture any new animals that are brought into the herd, whether it’s replacement cows or replacement heifers. They can be buying or adding significant problems if you put a Strep. or Staph. cow is put in the herd.”

One tool that has been helpful in mastitis identification and treatment has been the HyMast test by Pharmacia & Upjohn. Dinsmore says the test can indicate whether you have a Gram– or a Gram+ organism in a milk sample. “You should treat the Gram– coliform group differently than the Gram+ Strep. and Staph. group,” he says. Once you have the problem segregated into those basic groups you can use the information to make different treatment recommendations. “But if you usually treat all mastitis the same, whether it’s Gram– or Gram+, and your client uses a commercial mastitis tube on all clinical cases, you’re wasting your money.”

Brett agrees. “I’ve been amazed at the savings to a producer by not tubing or giving antibiotics to every cow. If you see that she’s Gram–, you know antibiotics aren’t going to help anyway.”

Dinsmore says with the E. coli mastitis vaccines, what used to be severe coliform cases are now quite a bit milder and may be harder to distinguish from the Gram+ types, so the HyMast test may be more useful in those vaccinated herds to differentiate mastitis organisms.

Keeping clients on a routine monitoring program can be difficult if all seems to be going well. Some of Brett’s clients frequently monitor even though they may not need to. Others get complacent and want to stop. “You have to tell these clients, ‘no,’” he says. “They’re still bringing heifers in, purchasing cows and  have some biosecurity problems. They must keep checking when they’re doing business like that.”

Brett summarizes his bulk tank analyses in a one-page report for clients. He keeps it short and gives them an idea of how things are now compared to the last report or last year, better or worse and recommendations for improvement.

“Fifteen years ago when the legal limit was a million SCCs, the average was probably around 600,000. Now the average in my county is around 300,000 or less and I have herds consistently below 200,000. By keeping track of what’s going on, my clients do a good job and have learned how to tweak the system when it needs it.”


Types of mastitis-causing pathogens

Karen Jacobsen, DVM, MS, offers the following chart on mastitis-causing pathogens, their source and means of spreading infection.

TYPE: Staphylococcus (coagulase +) aureus, hyicus
SOURCE: Infected udders, teat lesions, udder skin, etc.
SPREADBY: Cow to cow by contaminated udder wash rags, teat cups, hands, etc.

TYPE: Staphylococcus, spp. (coagulase -) epidermidis, micrococcus, etc.
SOURCE: Normal inhabitant of udder skin
SPREADBY: Poor udder preparation, milking wet udders and teats

TYPE: Streptococcus agalactiae (causes high somatic cell counts)
SOURCE: Infected udders
MEANSOFSPREAD: Cow to cow by contaminated udder wash rags, teat cups, hands, etc.

TYPE: Streptococcus non-ags uberis, faecalis, dysgalactiae
SOURCE: Numerous locations on the cow: hair, lips, vagina, feces, bedding, muddy lots, etc.
MEANSOFSPREAD: Environment to cow by wet, dirty lots and bedding, milking wet teats, poor udder preparation.

TYPE; Corynebacterium bovis
SOURCE:Teat canal
MEANSOFSPREAD:Inhabits the teat canal. Appears in tank milk when cows are not pre-stripped.

TYPE: Coliforms Escherichia coli, Klebsiella, etc.
SOURCE: Manure, bedding, especially sawdust
MEANSOFSPREAD:Same as Strep. non-ags

TYPE: Arcanobacterium pyogenes
SOURCE: Moist environment, cracked liners, water hoses, refrigerators, drug contamination
MEANSOFSPREAD: Common sequel to lacerated teat sphincter. Carried by flies.

TYPE: Mycoplasma bovis, californicum
SOURCE: Infected cows, udder shedders, secretions in calves and older heifers
MEANSOFSPREAD:Infusion procedures in dry and lactating cows; cow-to-cow spread via milking machine and hands.

TYPE: Nocardia
SOURCE: Soil initially, secondarily contaminated treatment materials, hands and sponges
MEANSOFSPREAD:Contamination of syringes and cannulas in infusion; secondarily by hands

TYPE: Prototheca
SOURCE: Water, feces, flies, rotted materials, contaminated drugs
MEANSOFSPREAD: Originates in environment and initially spread by contamination in infusion. Becomes contagious cow-to-cow as it gains momentum.

TYPES: Miscellaneous Bacillus, Pseudomonas, etc.
SOURCE: Hoses, dirty water, milk manure, bedding, etc.
MEANSOFSPREAD:Same as Strep. non-ags

What are the lab tests?

There’s a variety of lab tests you can have done to interpret mastitis problems from bulk tank samples. Jim Brett, DVM, offers these definitions:

Who should be tested?

Brett offers parameters for those farms that can benefit from bulk tank testing. He notes that goals are set by the individual dairy farmer with veterinary assistance, and that no two farms are alike, but if the farm is above these general parameters, it is a good candidate for testing.

  • Standard plate count: greater than 10,000
  • Lab pasteurization count: greater than 500
  • Coli count: greater than 100
  • SCC: greater than 300,000
  • Presence of Staph. aureus, Strep. ag or Mycoplasma in herd
  • P.I. count: greater than 50,000

Standard plate count (SPC)

Bacteria making up the sum total called the SPC are divided into two major groups for veterinary purposes, those capable of causing herd mastitis problems and those not usually associated with mastitis. Bacteria capable of causing common herd mastitis problems, Streptococcus agalactiae and hemolytic Staphylococcus come almost exclusively from within the udder of infected cows. Their presence in bulk milk means infected cows were milked into the bulk tank. Other types of bacteria more frequently represent contamination from external sources into the milk.

Excessive SPCs resulting from mastitis-causing organisms warrant appropriate mastitis treatment and prevention programs. Excessive SPCs from sources outside the udder do not warrant cow treatments, but indicate need for improved hygiene of cows or milking equipment. It should be recognized that contamination, especially from external surfaces that contact teats, substantial enough to show up as a major portion of an excessive SPC in most cases, will contribute to an excessive incidence of mastitis.

Lab pasteurized counts (LPC)

The LPC is the bacteria count of milk after it has been heated to 145° for 30 minutes in the lab. The procedure kills all of the usual mastitis-causing bacteria that would have originated as external contamination of milk. Generally speaking, high LPCs reflect an inadequate milking equipment wash-up procedure. Poorly cleaned inflations and claws or inadequate pre-milking hygiene are situations that could result in both increased LPCs and increased incidence of mastitis.

Coliform count (Coli)

The coliform count is a bacteria count conducted with selective media that allows only Gram negative bacteria of the coliform group to grow and be counted. High coliform counts indicate fecal contamination of milk or milking equipment. Mastitis cows seldom if ever contribute to coliform counts. On the other hand, milking wet, dirty udders or using dirty milking equipment causes elevated coli counts and is conducive to coliform mastitis. Coliform counts in the hundreds are consistent with poor milking hygiene. Coliform counts in the thousands suggest that incubation on milking equipment is occurring.

Somatic cell counts (SCC)

Somatic cells are normally found in the milk in low numbers. When found in high numbers, they are most likely in response to bacterial infections. Their numbers are approximated by such tests as the California Mastitis Test (CMT) and the Wisconsin Mastitis Test (WMT) or counted more precisely by the Coulter Counters or Fossmatic electronic counters.

The SCC of bulk tank milk is a very useful monitor of herd udder health if used on a regular basis. Upward or downward trends in mastitis in SCC can be correlated with progress or lack of progress in controlling mastitis in a herd. Absolute numbers of somatic cells in bulk milk are useful in assessing whether mastitis is a problem in a herd. The combination of SCC and SPC by species enables the most informed appraisal of herd udder health.

Preliminary incubation (P.I.) count

For the P.I. count the raw milk sample is incubated at 55° F or 18 hours. Bacterial results below 100,000 per ml are acceptable, but the goal should be 50,000 or less per ml. Many results will be less than 10,000 just like the SPC, if sanitation is good.

Causes of high P.I. counts include dirty cows, poor udder washing practices, slow cooling or temperatures above 40° F, failure to thoroughly clean equipment twice each day, and neglecting to sanitize equipment before use.

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