Mycoplasma bovis is a bacterial pathogen recognized globally as a major source of disease in cattle. Its widespread prevalence causes substantial financial losses to the livestock industry worldwide. These costs result from reduced production, expensive treatment failures, and the necessary culling of persistently infected animals. As a primary agent in complex and chronic bovine diseases, M. bovis challenges animal health across beef, dairy, and calf-rearing operations.
Understanding the Pathogen
The unique biology of Mycoplasma bovis makes managing infections difficult. This bacterium lacks a rigid cell wall, distinguishing it from most other bacteria. This structural absence renders an entire class of commonly used antimicrobial drugs, specifically beta-lactam antibiotics like penicillin, ineffective. The lack of a cell wall also makes the organism highly pleomorphic, meaning it can change shape, and it is considered one of the smallest living cells in nature.
The bacterium evades the host’s immune system by frequently changing its surface proteins, a process called antigenic variation. This makes it difficult for the animal’s immune response to mount an effective defense. Furthermore, M. bovis can invade and survive inside host cells, including immune cells, contributing to its persistence. M. bovis is not considered a zoonotic pathogen and poses no known risk to human health through meat or milk consumption.
Clinical Manifestations in Livestock
M. bovis causes a constellation of chronic diseases in cattle that are often unresponsive to initial treatment.
One common presentation in dairy cows is mastitis, an inflammation of the udder. This highly contagious form typically affects multiple quarters, resulting in a severe, watery secretion rather than thick pus. The chronic nature and poor response to therapy often necessitate culling infected cows to prevent spread within the milking herd.
In calves and feedlot cattle, the organism contributes significantly to the Bovine Respiratory Disease (BRD) complex. M. bovis pneumonia is characterized by chronic, destructive lung lesions. Affected animals may show subtle signs like low-grade fever, coughing, and decreased appetite, but the irreversible lung damage leads to poor weight gain and long-term health issues.
A third distinct manifestation is polyarthritis, involving inflammation of multiple joints and causing severe lameness. This joint infection often occurs secondary to respiratory disease when the pathogen spreads through the bloodstream. M. bovis can also cause otitis media, a middle ear infection presenting as a characteristic head tilt or drooping ear. These varied clinical signs make diagnosis challenging across age groups and production systems.
Transmission and Persistence
M. bovis spreads primarily through direct and close contact between animals. Transmission occurs via respiratory secretions expelled by coughing or sneezing, allowing aerosol spread in confined housing. Infected milk and colostrum are also significant routes of vertical transmission from dam to calf, especially when calves are fed unpasteurized milk from infected cows.
Control is complicated by “carrier animals,” which harbor and shed the bacteria without showing clinical signs. These subclinically infected animals can shed the pathogen for months, silently maintaining the infection within the herd. Introducing a carrier animal is the most common way the infection enters a previously uninfected herd.
The pathogen persists due to its ability to form biofilms, protective layers that shield the bacteria from the immune response and antibiotics. Survival inside immune cells facilitates the spread of M. bovis from the initial infection site, such as the respiratory tract, to distant sites like the joints or udder. This combination of carriers and internal persistence makes eradication extremely difficult once a herd is infected.
Management and Control Strategies
Managing M. bovis infection is complex due to the pathogen’s intrinsic resistance to many antimicrobials. Since it lacks a cell wall, antibiotics targeting that structure are ineffective. Treatment options are limited to specific classes like macrolides and fluoroquinolones. Even when effective antibiotics are used, chronic lesions in the lungs or joints often prevent the drug from reaching the infection site in sufficient concentration.
Prevention and biosecurity are the main focus of control programs. Strict biosecurity protocols prevent pathogen introduction by isolating and testing all incoming cattle. Within an infected herd, isolating sick animals and rigorous hygiene practices limit spread. Dairy operations must heat-treat colostrum and pasteurize waste milk fed to calves to eliminate the pathogen.
Management involves segregating or culling known carriers, especially those with chronic mastitis, to reduce the source of infection. Vaccination is also used, though commercially licensed vaccines are not universally available or fully effective. Producers may utilize autogenous vaccines, custom-made from the specific M. bovis strain found on their farm, as part of a comprehensive disease management plan.