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Published June 16, 2021
By Donal O'Toole
Most food animal veterinarians are familiar with mycoplasmosis in cattle. On the
diagnostic side, we most commonly see it in weaned beef cattle, as well as in young
calves. It generally presents as a subacute-chronic polymicrobial pneumonia in weaned
cattle, commonly after 1 or more rounds of unavailing antibiotic treatment. In some
it is associated with polyarthritis and/or otitis media. There was disagreement in
the past about the importance of Mycoplasma bovis in bovine respiratory disease (BRD)
complex. The consensus is that, except in very young calves where it can be a primary
pathogen, its role is to complicate infection due to the common conventional bacterial
BRD culprits, and extend the clinical course of pneumonia. Gross and histological
features of longstanding pulmonary mycoplasmosis in cattle are often characteristic.
The anteroventral portions of affected lungs contain multiple distinctive 1 mm - >1
cm white caseonecrotic nodules (IMAGE 1). It is common to find Mannheimia haemolytica,
Pasteurella multocida, Histophilus somni and/or Trueperella pyogenes, in addition
to M. bovis, in such lesions. Outbreaks involving M. bovis can be frustrating to control.
The organism resists many commonly-used antimicrobial agents, due it now having a
bacterial cell wall – the target for many antibiotics. Many aspects of the ecology
of M. bovis remain poorly understood.1
IMAGE 1: Typical small discreet white caseonecrotic foci in sliced lung of backgrounded bovine
calf that died of anteroventral bronchopneumonia. Tissue positive by PCR for Mycoplasma
Laboratory diagnosis at the WSVL of bovine mycoplasmosis is based on PCR detection, fining caseonecrotic lesions in lungs, and, in some cases, immunohistochemistry. Unless you ask us to do so or we suspect we are dealing with an unusual strain of M. bovis, we generally don’t attempt culture. Isolation requires special growth media, time, and technical expertise. For that reason it is not a popular hobby among laboratory personnel. M. bovis has fastidious growth requirements and does not remain viable for long in autolyzed samples. Commercial vaccines are generally ineffective in spite of M. bovis having highly immunogenic surface lipoproteins (phase- and size-variable membrane surface lipoprotein antigens; Vsps).2 The high spontaneous rate of Vsp phenotypic switching due to DNA re-arrangements makes M. bovis a moving immunological target and allows it survive for extended periods in the host animal.
Starting around 1999, North American producers and veterinarians began to recognize M. bovis-associated pneumonia in bison. Unlike in cattle, clinical cases in bison affected older animals, often 3 or more years.3 Another difference is that some M. bovis isolates from bison can cause pulmonary disease in experimentally challenged bison yet not in cattle.4 Some M. bovis strains are unique to bison.5 The number of peer-published accounts of outbreaks is limited, but bison producers reported high mortality rates, some of them approaching 30%. Such rates are unusual in feedlots where BRD outbreaks involve M. bovis. A recent producer survey found that common signs in affected bison were dyspnea, coughing, reluctance to move, and swollen joints,3 The syndrome can be recurrent over successive hears in affected herds. Many infected bison with overt pneumonia either die or require euthanasia. Some herd owners report rejection of bison carcasses at slaughter due to severe lung lesions in survivors, indicating that some do survive but with extensive pulmonary damage. The stigma associated with mycoplasmosis in bison operations results in some producers being reluctant to investigate or discuss suspected outbreaks. We diagnosed several episodes in Wyoming bison herds in 2020 and 2021. Lesions included multiple small caseonecrotic foci that are similar to those seen in cattle. Bison may have dramatically large abscesses containing up to 7 liters of purulent or semisolid exudate, some with extensive fibrous adhesions (IMAGE 2). If you find lesions like this in bison, particularly when multiple animals are affected and responses to antibiotics were disappointing, M. bovis is a possibility you should consider.
IMAGE 2: Large abscess in lung of a thin yearling bull bison. Several large abscesses were present, some resulting in pleural adhesions. Tissue was positive by PCR for Mycoplasma bovis. Trueperella pyogenes and Pasteurella multocida were also present in affected lung. Image: Dr. Tim Dawson.
Serology for M. bovis is of limited value when assessing the status of individual bison with pneumonia, but can be helpful at a herd level. Clinically healthy bison in unaffected herds can carry the organism in their upper respiratory tracts. Few commercial vaccines exist for M. bovis. The current practice among bison owners is to use autogenous products. Several such vaccines are used for problem herds. Their efficacy is unknown. Generally they contain several strains isolated from bison that died with M. bovis pneumonia. If you are dealing with a confirmed outbreak of mycoplasmosis in bison and wish to pursue an autogenous vaccine, please let WSVL’s bacteriology unit know so that its folks can attempt isolation.
Drs. Sondgeroth and Malmberg are particularly interested in mycoplasmosis and wish to pursue it both diagnostically and as a research target. This is due in part to a recent collaborative investigation they did with Wyoming Game and Fish personnel of a die off in pronghorn involving Mycoplasma bovis.6
If you chose that B, you are likely to be an excellent pathologist. But we’ll pass on any preserves you made recently.
1. Calcutt MJ et al.: 2018, Gap analysis of Mycoplasma bovis disease, diagnosis and
control: an aid to identify future development requirements. Transbound Emerg Dis.
65 Suppl 1:91-109. doi: 10.1111/tbed.12860. Epub 2018 Mar 27.
2. Perez-Casal J et al: 2017, Status of the development of a vaccine against Mycoplasma bovis. Vaccine 2017, 35, 2902–29073. 3. Bras AL et al.: 2017, Clinical presentation, prevalence, and risk factors associated with Mycoplasma bovis-associated disease in farmed bison (Bison bison) herds in western Canada. J Am Vet Med Assoc. 250(10):1167-1175.
4. Register KB et al.: 2018, Relative virulence in bison and cattle of bison-associated genotypes of Mycoplasma bovis.
Vet Microbiol. 2018 Aug;222:55-63. doi: 10.1016/j.vetmic.2018.06.020. Epub 2018 Jun 30
5. Register KB et al,: 2019, Comparison of multilocus sequence types found among North American isolates of Mycoplasma bovis from cattle, bison, and deer, 2007-2017. J Vet Diagn Invest. 2019 Nov;31(6):899-904. doi: 10.1177/1040638719874848. Epub 2019 Sep 11
6. Malmberg JL et al.: 2020, Mycoplasma bovis infections in free-ranging pronghorn, Wyoming, USA. Emerg Infect Dis. 26(12):2807-2814. doi: 10.3201/eid2612.191375 .