Among the more problematic pathogens affecting cattle, Mycoplasma bovis (M. bovis) stands out for its involvement in chronic infections. The interaction between the outer makeup of the microorganism and the animal’s immune response contributes to its insidious nature, making treatment as well as prevention difficult. Despite the challenges imposed by M. bovis, understanding its unique aspects of virulence helps in the implementation of approaches and interventions to mitigate the protracted effects.
M. bovis has a reputation for being pervasive, lending to the difficulty controlling it. There are multiple clinical syndromes attributable to it in dairy cattle including pneumonia, inner ear infections, lameness and mastitis. The organism can be found in the nasal passageways of the upper respiratory tract in healthy cattle. A debate exists over whether it is a primary or secondary pathogen capable of either inciting disease in the lungs or, rather, acting as an opportunist furthering the destruction of another infectious agent, usually a virus. Similarly, the distinction between categorizing it as a contagious or an environmental cause of infection in the udder is not clear.
Unlike bacteria, M. bovis lacks a true cell wall. This has implications on-farm when selecting antibiotics because the mode of action for some medications is directed against this outer structure. Without the proper target, drugs like penicillin, ampicillin and ceftiofur are ineffective therapeutics. A membrane encloses the organism instead. Attached to the exterior are variable surface proteins. Changes and alterations in the expression of these components help M. bovis inflict damage and avoid recognition by the host immune system.
Associated with the different disease presentations, affected cattle may display nasal discharge, cough, labored breathing, drooped ear, head tilt, head shaking, difficulty moving, swollen joints, inflamed quarters and abnormal milk. A classic feature of M. bovis is collections of inflammatory and cellular debris forming nodules that resemble abscesses seen scattered throughout infected tissues, especially lungs. Although special media is required, the organism can be detected by culture and through other advanced diagnostic methods in necropsy specimens or milk from lactating cows.
Cattle may be carriers without showing signs of illness. The means by which M. bovis becomes pathogenic, however, are not fully known. Factors such as predisposing or co-infections, inadequate natural host defenses, commingling and the influence of stressors are speculated to have an impact. Often, an individual animal or a group of cattle will appear sick and fail to respond to initial antimicrobial treatment because of the inherent resistance provided by the surface structure as well as the inability of the drug to penetrate through the necrotic material at the sites of infection. This delay of effective therapy contributes to the chronic nature of the disease.
An effective management plan starts with a strategy to evaluate potential introductions if considering purchasing from outside sources. With M. bovis being widespread, chances of dealing with it are elevated. While no reliable individual method exists for testing youngstock or non-lactating adult animals, questions about the herd history can be asked alongside monitoring of milking cows. No matter the age group of cattle being brought into an operation, the scope of the risk assessment should include the gestating cow at the initial source farm and both the protocols for her as well as the calf once she has freshened. The value would be to learn what control measures have been implemented:
- Have there been clinical signs associated with any of the syndromes that seemingly linger despite treatment?
- Is the bulk tank screened routinely for M. bovis with any positive results followed up by subsampling milking strings to narrow down and ultimately identify cows with mastitis caused by it?
- Are affected cows remaining in the herd segregated and milked last or culled?
- Is colostrum and non-salable milk appropriately pasteurized or are commercial substitutes used?
Furthering the limitations on antibiotic use, there are no effective antibiotics for M. bovis mastitis. With the lack of a positive outcome possible for these animals, it is best to remove them from the herd. Keen attention to clinical cases and use of information from milk testing along with prompt separation of infected animals from healthy cattle, and implementation of either pasteurization or utilization of replacers reduces the likelihood of spread. A review of these practices is the same if M. bovis is suspected within a closed herd.
When M. bovis is thought to be a problem, collaboration with a veterinarian is essential. Confirmation is based on several factors including the application of laboratory procedures, although antimicrobial selection is not based on sensitivity testing. The modified culture methods and slow-growing nature of the organism interfere with conducting the assay. The herd veterinarian’s knowledge can guide the decision based on understanding product label claims, distribution of drugs to sites of infection and modes of action needed to inhibit the pathogen. For pneumonia, inner ear infections and lameness, it is key to detect and intervene early with an appropriate medication to avoid it becoming chronic.
The goal of prevention focuses on good animal husbandry practices including cleanliness of housing and equipment along with adequate ventilation as well as maintaining cohorts of animals intact. Allowing a poor-doer, which may have already been infected over a long period of time in an older pen of cattle, to fall back to a younger group should be avoided. Optimized air quality with the correct flow and proper exchanges reduces aerosol transmission. Drafty conditions should also be avoided to limit the possibility of animals becoming chilled, leading to cold stress. The risk of transfer through fomites can be minimized through a well-maintained environment.
Vaccination may be another consideration with both commercial and autogenous options available. The custom-made vaccine route offers the potential to incorporate the strain variation of a herd as expressed through the genetic makeup of the variable surface proteins. Along with the viral and bacterial diseases typically addressed in protocols, a veterinarian can help fit the implementation of a product for M. bovis into the strategy for bovine respiratory disease (BRD) management. Calves are a focus with the modern approach of raising them at large ranches where multiple sources come in contact. Despite the application with BRD, this approach is not effective for M. bovis mastitis.
The dynamic interaction of animal host, pathogen and environment is influential in the development of disease caused by M. bovis. With special features that give it an innate ability to persist, the deleterious effects of longstanding infection slowly and continually erode the health of infected cattle. Prompt attention and judicious use of antibiotics offer the most potential for treatment success while the risk of exposure to herdmates can be decreased through a combination of prevention strategies in conjunction with removal of disease reservoirs. A well-executed M. bovis control program encompasses an awareness of the distinctive disease characteristics and timely interventions for lasting well-being and productivity.
