Does vaccinating calves against <i>Mycoplasmopsis bovis</i> (formerly <i>Mycoplasma bovis</i> ) reduce clinical signs of bovine respiratory disease?

Mrs Smith, a dairy farmer, is experiencing an outbreak of bovine respiratory disease in her calves. The affected animals exhibit severe clinical signs, with high morbidity and mortality. Many of the affected calves present with increased respiratory rate, nasal discharge, persistent coughing and head tilt. Postmortem examination of one affected calf reveals suppurative and caseonecrotic bronchopneumonia associated with unilateral suppurative otitis media – findings typically consistent with Mycoplasmopsis bovis (formerly known as Mycoplasma bovis) infection. As the attending veterinarian, you are now considering whether vaccination against M bovis would be a justified and potentially effective strategy for this particular farm. In [calves at risk of M bovis infection], does [vaccination versus no vaccination] reduce [clinical signs of disease]? The search strategy can be viewed at https://bestbetsforvets.org/bet/597, and is also available as a supplement to this article on Vet Record's website at https://bvajournals.onlinelibrary.wiley.com/toc/20427670/2026/198/1 Although some other papers were found that focused on M bovis, they involved previous challenges with other pathogens, such as bovine herpesvirus or bovine viral diarrhoea virus, or the administration of other vaccines, which may have altered the immune response capacity of some calves. As such, these papers were excluded from the evidence evaluation. Search last performed: 10 November 2025 Paper 1: Field evaluation of a Mycoplasma bovis bacterin in young dairy calves1 Patient group: A total of 373 Holstein heifer calves born in three dairy herds in Florida with a history of M bovis infection were randomly assigned to receive either M bovis bacterin (n=184) or a placebo (n=185), administered subcutaneously at three, 14 and 35 days of age. Study type: Randomised controlled trial. Outcomes: Calves were followed from three to 90 days of age. During that time, all treatment for clinical signs of respiratory disease (fever, increased respiratory rate/effort, coughing, nasal discharge) was recorded by farm personnel. Any calves that died or were euthanased underwent postmortem examination, and samples were taken for histopathology and bacterial culture to determine the cause of death. M bovis-associated morbidity and mortality were then calculated. In addition, nasal swabs and blood samples were collected weekly from a subset of 100 calves (50 from the vaccinated group and 50 from the control group) until eight weeks of age, and again at 90 days of age. The serum samples were analysed for M bovis-specific IgA, IgM, IgG1 and IgG2 using ELISAs. Meanwhile, the swabs were cultured to detect nasal colonisation with M bovis. Key results: Overall mortality, M bovis-associated mortality and M bovis-associated morbidity did not differ significantly between vaccinated and control calves. Similarly, the average age at which vaccinated and control calves first received treatment for signs of respiratory disease was not significantly different. The proportion of calves with M bovis-positive nasal swabs at each time point did not differ significantly between the vaccinated and control groups. However, while no significant differences in the serum levels of M bovis-specific IgA, IgM or IgG2 were found between vaccinated and control calves, IgG1 levels were higher in vaccinated calves than in control calves (P<0.05). Study weaknesses: No specific measurement parameters were described for the clinical parameters assessed (eg, temperature, respiratory rate/effort, coughing, nasal discharge). Furthermore, although the investigators trained farm staff to use a standardised protocol based on veterinarian-defined case criteria, there was no verification of how well this protocol was followed across the three herds. Some points raised in the discussion indicate that elements of the study design may have contributed to the observed lack of vaccine efficacy. For example, calves were vaccinated at a very young age, which may have limited their immune response. The prevalence of M bovis within the study herds also suggests that many calves were likely infected before vaccine-induced immunity could develop, and the high challenge load in these herds may have further compromised vaccine efficacy. Paper 2: Attenuated Mycoplasma bovis strains provide protection against virulent infection in calves2 Patient group: Twenty female calves aged between five and six months were purchased from a market in Zaoyang, Hubei, China. After acclimatisation to the experimental housing unit, the calves were divided into four groups: P150, P180, positive control and negative control. Each calf in the P150 and P180 groups was immunised with 109 colony-forming units (CFU) of attenuated M bovis strain P150 or P180, respectively, via the nasal cavity, while calves in the two control groups received a placebo. Forty-six days later, the calves in the P150, P180 and positive control groups were challenged with 1010 CFU of virulent M bovis via intratracheal injection once daily for three consecutive days; the negative control group received a placebo. Study type: The study design was not explicitly stated, but it is likely to have been a controlled trial. Outcomes: The calves' rectal temperatures and general clinical condition were monitored daily for the 46 days between immunisation and challenge and for 25 days after challenge. Clinical signs, including nasal discharge, respiratory rate/effort, cough and joint lesions, were scored on a four-point scale (0: normal to 3: most severe condition). In addition, blood samples were collected at various time points to quantify leukocyte counts, neutrophil ratios and serum levels of IgG, IgA, IFN-β and TNF-α. Nasal swabs were also obtained at the same time points and cultured to detect M bovis. Furthermore, calves were weighed on the day of challenge and 25 days later, and their daily weight gain was calculated. Key results: Mild clinical signs were observed in the two immunised groups (P150 and P180), which first appeared around 14 days after challenge. Clinical signs appeared approximately seven days earlier in the positive control group, with these calves having significantly higher clinical scores than those in the immunised groups (P<0.01), indicating more severe disease manifestations. Differences were also noted in rectal temperature profiles among the groups: temperatures increased by more than 1 °C in the positive control group and by approximately 0.7 °C in the P180 group, whereas they remained relatively stable in the P150 and negative control groups. However, the paper does not state whether these temperature variations were tested for statistical significance. When compared with the other three groups, the calves in the positive control group had statistically significant weight loss (–105 g/day) (P<0.01). Daily weight gain did not differ significantly between the negative control and immunised groups. Study weaknesses: The study design was not explicitly stated, and it is unclear whether the calves were randomly assigned to the treatment groups. It is also unclear whether the outcomes were assessed blind. As such, the possibility of observer bias cannot be ruled out. The number of calves included in the study was very small (five per group), and no justification was provided for this sample size. Furthermore, although calves were all females of a similar age, no information was provided on their breed or weight at enrolment. Differences in these characteristics could therefore have confounded the analysis. Inconsistencies in the reported significance levels also complicate the interpretation of the results. A P value of less than 0.05 was reported as significant for the analyses comparing neutrophil and cytokine changes, while a P value of less than 0.01 was reported as significant for the analyses comparing antibody levels and clinical parameters. The paper does not explain the rationale for these differing significance levels. The ‘Evaluating The Evidence’ section of Vet Record aims to answer specific clinical questions using a systematic approach to identify and succinctly summarise the relevant evidence from the scientific literature. The shortcomings of this evidence are also taken into account, thereby enabling vets to incorporate the best available evidence from the literature when making clinical decisions. Please contact us at [email protected] if you have an article you would like us to consider for publication in this section. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.