Mycoplasma bovis driven by inflammation and co-infection, not just exposure

New research is reshaping how farmers should think about Mycoplasma bovis in calves, suggesting that exposure alone is not what drives disease. It’s actually what happens after exposure that matters more.

Most calves exposed to Mycoplasma bovis never get sick. It’s the presence of stress and co-infection, especially inflammation in the respiratory tract, that appears to determine whether calves progress to clinical disease.

“By far the majority of those infected calves will appear clinically healthy,” said Dr. Jeff Caswell, Professor in the Department of Pathobiology at the Ontario Veterinary College. “So just because you detect Mycoplasma bovis doesn’t mean it’s responsible for the disease.”

For veal producers, that distinction matters. It shifts the focus away from trying to eliminate the pathogen — which is likely unrealistic — and more toward managing the conditions that allow it to cause damage.

Mycoplasma bovis is widespread in cattle populations and spreads quickly, especially when calves are commingled. According to Caswell, feedlot studies have shown that within two weeks of arrival, the majority of calves are affected. Transmission is primarily through nose-to-nose contact, milk or colostrum from infected cows, and movement and mixing of calves from multiple sources.

Unlike many bacteria, Mycoplasma bovis behaves differently in several important ways that make it difficult to control. It lacks a cell wall, making some common antibiotics ineffective, it doesn’t produce toxins (damage is caused by the calf’s immune response), and it can linger inside an animal for months. It’s a combination, Caswell noted, that explains why treatment outcomes are often inconsistent.

The most recognized form of Mycoplasma bovis is pneumonia, but not the typical acute respiratory disease producers might expect.

“This is always a chronic disease. Even if it appears suddenly, it’s been developing for at least two weeks,” he said.

The classic lesion — caseonecrotic bronchopneumonia — involves firm lung tissue in the front and lower lobes of the organ, and distinct nodules filled with thick, cheese-like material. These lesions are more than just diagnostic clues; they also help explain poor treatment response.

“If you give an injectable antibiotic, it’s plausible you’re not going to eradicate the bacterium from a lesion like that — you just can’t get antibiotics into that dead lung tissue,” he added.

Beyond pneumonia, producers should also watch for ear infections (head tilt, droopy ears) and joint infections (lameness, swelling). Joint infections in particular are difficult to treat and can significantly impact performance.

One of the most important findings that research has highlighted is the disconnect between infection and disease. Even in heavily exposed groups of calves, infection rates can reach 70 to 80 per cent of the animals, but only some actually develop clinical signs.

So what makes the difference? The answer appears to be inflammation and co-infection, according to Caswell.

Experimental work showed that calves exposed to Mycoplasma bovis alone developed fewer and milder lung lesions. But when an additional inflammatory stimulus — such as another bacterial infection — was present, disease severity increased significantly.

“It seems that one of the reasons animals progress to clinical disease may be that they have some other inflammatory stimulus at the same time,” Caswell said, adding that this aligns with field observations where respiratory disease is often complex and involves multiple pathogens and stressors.

These findings help explain why Mycoplasma bovis cases can be frustrating to manage on the farm. One of the biggest challenges is antibiotic resistance. Because the bacteria lack cell walls, some antibiotics like penicillin don’t work; at the same time, resistance to other antibiotic classes is common and variable.

Another major hurdle is physical barriers to treatment. Dead lung tissue and joint lesions limit drug penetration, and while calves may appear to respond, they can relapse after treatment stops. Together, these factors mean treatment is often only partially effective, especially in advanced cases.

Ultimately, Mycoplasma bovis is not just a respiratory pathogen, it’s a disease shaped by the interaction between infection, inflammation, and management. That’s why understanding it matters when it comes to keeping calves healthy; managing stress, co-infection, and overall calf health can have a greater impact on outcomes than just focusing on the pathogen itself.

For Caswell, the key message for producers is clear: you may not be able to keep your calves from being exposed, but you can influence whether that exposure turns into a problem for your herd.

Farmers should work with their vets to develop a protocol and/or treatment plan, and while eliminating Mycoplasma bovis is not realistic, here are tips for reducing its impact:

1. Focus on preventing co-infection. Because inflammation is a key trigger, it’s important to maintain strong overall calf health, manage respiratory disease risks, and minimize exposure to other pathogens as much as possible.
2. Manage commingling risk. Mixing calves from multiple sources increases exposure, so limit commingling wherever possible and if you are comingling, know that newly assembled groups of calves are at high risk.
3. Support immune function. Healthy calves are less likely to progress to disease. To keep them healthy, ensure proper colostrum intake, give them the best nutrition possible, reduce stress during transport and arrival on-farm, and provide good ventilation and housing.
4. Pay attention to milk management. Infected or subclinical cows can shed bacteria in milk. Pasteurization or acidification can reduce transfer risk to calves.
5. Identify and manage clinical cases early. Sick animals tend to shed a lot of the bacterium into their environment, so early treatment as well as keeping them away from healthy animals can help reduce the spread of infection.
6. Set realistic expectations. The goal should be management, not elimination, as eradicating Mycoplasma bovis is difficult or probably impossible, noted Caswell.