Changes in dairy herd management, primarily herd expansion, have turned mycoplasma mastitis into an emerging disease. The U.S. National Animal Health Monitoring Service conducted a recent survey, which estimated during any single year, 20 percent of 500-plus-cow herds in the U.S. will have a positive mycoplasma mastitis bulk tank.
“This means mycoplasma mastitis is infecting about one-fifth to one-quarter of all large dairy herds annually,” says Dr. Larry Fox, professor at Washington State University. “That may be an underestimate because it can be difficult to diagnose.”
Research conducted by Fox showed the U.S. Pacific Northwest experienced a five-fold increase in clinical mycoplasma mastitis over a two- to three-year period in the mid-2000s. While this region appears to have plateaued, more recently, mycoplasma mastitis has also emerged in Canada, England, New Zealand and Belgium.
Mastitis is estimated to cost the U.S. dairy industry billions of dollars every year, and Fox says he thinks the same is true for dairy producers in Europe.
How do mastitis types differ?
The basic difference in categories of mastitis is their physiology or cell structure. Gram-positive mastitis bacteria have a thick cell wall. The cell wall’s membrane is highly selective about how and what move in and out. Gram-negative mastitis bacteria have a thin cell wall with two membranes that are somewhat selective, and the outer membrane protects the cell wall.
“Penicillin antibiotics need to attack the cell wall in order to be effective. For gram-negative infections, the penicillin-type antibiotics, also called beta lactams, cannot get to the cell wall,” describes Fox. “As a result, gram-positives tend to be more susceptible to penicillin antibiotics; gram-negatives are more resistant.”
Similar to gram-negative bacteria, mycoplasma do not have a cell wall; they only have a cell membrane, making them much more resistant to antibiotics because there's no cell wall to attack. Mycoplasma is very slow growing, which gives it an advantage because some antibiotics focus on the metabolism and reproduction aspects of the cells. Slow growing bacteria are not reproducing or metabolizing as fast, making them more resistant to antibiotics.
Cows with a gram-negative infection tend to get systemically sick and would be considered a “hotter,” more acute case of mastitis. Gram-positive mastitis infections tend to show abnormal milk and elevated somatic cell count. Mycoplasma symptoms are somewhere in between, and typically multiple quarters of the animal are infected.
When it comes to antibiotic treatment, Fox recommends treating gram-positive infections, but says antibiotics are not a solution for mycoplasma or gram-negative infections. When antibiotics aren’t used, some advocate for frequent milk-outs, which is designed to evacuate the toxins causing the clinical signs of mastitis from the mammary gland.
Closely monitor your bulk tank diagnostic test results to identify problem areas, and then work with your herd veterinarian to develop a mastitis control program that fits your specific operation.
Mycoplasma mastitis is unique
Mycoplasma is an unusual organism. Not only does it cause mastitis, but it affects other body sites in the cow. Often when a mycoplasma mastitis outbreak occurs, it will be accompanied by cases of pneumonia, lameness, arthritis, metritis or, in some cases, vaginitis.
“A cow infected with mycoplasma may have concurrent mastitis and another form of mycoplasma disease. In other words, if you have 10 cows, three could have mycoplasma mastitis, three have pneumonia and four have arthritis. That's more typical of a mycoplasma problem,” Fox says.
Mycoplasma is considered a contagious agent. It isn't only transmitted in the milking parlor, but it's also transmitted from nose-to-nose contact between cows. The nasal area is the site of mycoplasma colonization, including the nares, nose and nostril area. It'll pass from cow to cow or calf to calf with the reservoir being the infected cows.
To limit the spread of mycoplasma mastitis, strict milking time procedures are required, including:
- Disinfection of the teats prior to milking
- Use of a single service towel to clean the cow's teats off
- Use of latex gloves that can be easily cleaned between milkings
- Use of a post-milking teat disinfection procedure
- Milking units should be flushed with either a large volume of water or with disinfectant
“Every dairyman at every milking at every second of the day should be applying a strict milking time hygiene procedure because the cost of doing it is minimal relative to the risk of infection,” says Fox.
Mycoplasma risk factors
A key risk factor for mycoplasma mastitis is the introduction of new animals. According to Fox, although definitive research studies are not complete, data indicates a strong correlation from newly introduced cattle that are either clinically infected or subclinically infected with the mycoplasma agent.
Another risk factor is stress. Fox noted his recent research shows evidence that corticoids, a steroid hormone produced by the adrenal cortex, become elevated during stressful events like parturition or bringing new cattle in that upset the social order.
“Weather can cause a tremendous change, or anything that creates a stress event can render the animal more susceptible to the clinical disease,” says Fox. “The cow may be carrying the mycoplasma agent and looks fine with no clinical disease signs. Then she undergoes a stressful event or a series of stressful events, and she’ll erupt with clinical manifestations, which also means she's now shedding a lot more of the mycoplasma agent, having gone from a subclinical infection to a full-blown clinical infection.”
Both the introduction of new animals and stress are not completely avoidable, but there are ways to become more vigilant and manage the situation. For example, when buying new animals, consider gradually introducing them into the herd. Dairy managers should weigh how much time and effort he or she wants to spend trying to minimize stress to the animal versus the risk of introducing a problem. It can’t be eliminated, but it’s a risk-reward situation and stress management is possible.
Why is mycoplasma mastitis underestimated?
A producer's previous experience plays a huge role in how he or she perceives the threat of mycoplasma mastitis.
“If they haven't had a problem with mycoplasma, they don't perceive it as a risk or a threat,” Fox says. “However, those herd owners who have had a mycoplasma problem – they've been burned once and they are very serious about protecting themselves against a second burn.”
Outbreaks can vary. Some are major outbreaks, infecting 5 percent to 10 percent of the herd. Because mycoplasma mastitis is so slow growing, it can seem like trying to solve a problem in slow motion. Producers will start isolating clinical cases and culling cows. Meanwhile, new cases start popping up.
“Our data suggests that eventually producers get a handle on it, depending on how many cows are infected,” Fox says. “When herds are in an outbreak situation, most herds will use a search and destroy approach. They’ll identify the positive animals and remove them as quickly as possible. It can be quite costly, but it’s effective.”
However, some dairies develop a mycoplasma-positive string and isolate those animals and milk them last. If they're clinical, they're shedding high cell counts and you can't sell the milk. However, there can be cases of spontaneous cure, so producers may try to work through an outbreak using that approach.
“A lot of operators test all cows at parturition and all cows that are clinical. If cows come up positive with mycoplasma mastitis, they ask no questions and immediately cull the cow,” he says. “They do a search and destroy through their routine testing and, therefore, never really experience a mycoplasma outbreak. Especially if you’ve been burned, you’ll have a lot less tolerance than if you've not been.” 
Dr. Christina Boss is the senior global product manager for Thermo Fisher Scientific.
PHOTO: Not only does mycoplasma cause mastitis, but it also affects other body sites in the cow, causing pneumonia, lameness, arthritis or metritis. Photo provided by Larry Fox.
