Recently, the diagnosis of Mycoplasma bovis in previously disease-free countries has increased the spot-light on this pathogen. In the UK Veterinary Investigation Diagnosis Analysis (VIDA) data, there has been a steady increase in diagnosable submissions being attributed to M. bovis. In the period from January 2006 to December 2017, there were 1,102 diagnoses of M. bovis associated with respiratory disease, mastitis and arthritis. Of these diagnoses 86.4 percent were associated with respiratory disease. The proportion of respiratory cases being attributed to M. bovis has increased steadily with the organism being identified in 7.5 percent of cases where a diagnosis was reached.
The full economic cost of M. bovis in cattle is difficult to determine. In the US, one report estimated costs to the US beef industry of $32 million per year as a result of loss of weight gain and carcass value, and $108 million per year to the US dairy industry as a result of M. bovis mastitis. The absence of accurate prevalence figures makes economic analysis difficult, although it is clear that the costs of disease include reduced production, drugs and labour for treatment, death and culling losses as well as the financial impacts of implementation of diagnostic and control measures. Costs per case are typically high, relative to other pathogens. M. bovis can also contribute significantly to antimicrobial usage on-farm and was highlighted as a specific focus area for the Responsible Use of Medicines in Agriculture (RUMA) Targets Task Force Report.
The organism
Mycoplasmas belong to the class Mollicutes and are distinguished by their lack of a cell wall. They are generally host-specific with 13 species having been identified in cattle, although not all are implicated in disease. The ability of M. bovis to produce biofilms and also change their surface proteins allows it to colonise and persist on mucosal surfaces and enables it to evade host immune responses.
M. bovis has been recognised as a pathogen in cattle for over 50 years, and in experimental infection studies it has been demonstrated to cause mastitis, respiratory disease and arthritis. In naturally occurring infections, M. bovis can be isolated in pure culture in cases of mastitis, arthritis, tenosynovitis, abortion, keratoconjunctivitis and pneumonia. In calves, M. bovis is the predominant pathogen isolated from the middle ear of animals with otitis media. Across both experimental and natural infections, variable disease expression is a key feature, which along with the limited sensitivity of some diagnostic methodologies has resulted in a number of knowledge gaps about this pathogen.
Transmission
The organism can be carried asymptomatically, and the introduction of these subclinical animals is thought to be the primary means by which naïve herds become infected. Once present in a herd, M. bovis can be readily transmitted from infected to uninfected cattle. Transmission is reliant on close contact with aerosol being believed to be the main route of spread. In calves, infection can also occur via maternal contact and also infected milk. Fomite-mediated transmission has been demonstrated to occur and, whilst mycoplasmas are susceptible to desiccation and sunlight, M. bovis can survive for long periods in protected environments with the greatest survival in cool, humid conditions.
Diagnosis
Rapid and accurate diagnosis of M. bovis infections is compromised by the low sensitivity and some cases specificity of available tests and further complicated by subclinical infections and intermittent shedding.
Serology can be applied for surveillance or as part of a biosecurity protocol for buying in stock. For detection in clinical samples, culture can be used although it requires specialised media and extended incubation. The sensitivity of milk culture for diagnosis of Mycoplasma has been reported as being around 50 percent for bulk milk tank samples, and can drop to below 30 percent in individual cows with sub-clinical infections. Given the limitations of culture, PCR is the method of choice for Mycoplasma detection (Figure 1).
Treatment
Due to its lack of a cell wall, M. bovis possesses a natural resistance to penicillins and cephalosporins. In addition to this, its folic acid-independent metabolism gives it natural resistance to sulphonamides-trimethoprim. Theoretically effective products include macrolides, oxytetracyclines, fluoroquinolones and florfenicol with a number of products containing these active ingredients now licensed for treatment of M. bovis. However, the nature of the pathogen, including its ability to create biofilms, can reduce clinical response to treatment. The key to treatment success is early intervention, which requires client education to rapidly identify affected animals.
Prevention
The development of vaccines for M. bovis is challenging given its ability to change its antigenic make up and its relationship with its host. Autogenous vaccines have been used on some farms in the UK and we are now able to import a commercial M. bovis vaccine from the USA under the Special Import Certification system; however, the epidemiology of the disease can present additional difficulties when implementing vaccination programmes, with infections being acquired at a young age and the pathogen spreading rapidly after introduction.
In dairy herds, the best route for control and prevention is the maintenance of a closed herd. Buying from herds of known disease status is always recommended although it must be remembered that herds frequently do not know their Mycoplasma status. On units where the disease has been identified, control measures are aimed at reducing exposure of calves to the organism; avoid feeding whole milk to calves and if it is being fed it should be pasteurised before doing so. Batch pasteurisation of milk at 65°C for 10 minutes, 70°C for 3 minutes or high temperature (72°C) short-time pasteurisation will inactivate M. bovis.
Steps should be taken to minimise spread between calves, in particular good hygiene of the feeding equipment (Figure 2). Calves with clinical Mycoplasma shed very large numbers of organisms, so isolation of affected animals can help reduce the spread. In rearing units, instituting all-in all-out systems and avoiding the mixing of age groups can help control the disease. Pens should be disinfected between calves using an appropriate disinfectant. M. bovis is sensitive to heat as well as chlorine, chlorhexidine and iodine-based disinfectants.
The impact of M. bovis can be reduced by ensuring that all non-specific factors related to health are addressed; good colostrum management, nutrition and control of diseases such as bovine viral diarrhoea can all help limit the impact of M. bovis. Appropriate vaccination programs should be in place for respiratory viruses, as controlling other pathogens can help reduce the risk of M. bovis coinfections.
Conclusions
The management of M. bovis on-farm can present significant challenges. It is important that farmers are aware of the impact of the disease and are able to identify its clinical presentations rapidly. It is clear for farms that are unaffected the emphasis should be on preventing introduction through good biosecurity and stringent controls on purchasing stock. For herds where Mycoplasma is already present, steps should be taken to minimise spread and reduce the impact of the disease.
