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Has Mycoplasma bovis jumped the species barrier — again?

Vet Advice with Dr. Ron Clarke

Mycoplasma bovis is an important bacterial pathogen associated with chronic pneumonia and arthritis in feedlot animals, mastitis in dairy herds and middle ear infections in calves. Over the last two decades, Mycoplasma bovis emerged as a cause of troubling respiratory disease and arthritis in feedlot cattle and extended into young dairy and veal calves. A significant proportion of these animals are eventually euthanized because of the chronic nature of the disease. Lesions often include severe pneumonia with extensive involvement of lung tissue (up to 80 per cent).

Mycoplasma’s role in pneumonia of free-ranging ungulates in Wyoming is under investigation. Over a three-month period in early 2019, approximately 60 free-ranging pronghorn with signs of respiratory disease died in northeast Wyoming. A consistent finding in submitted carcasses resembled severe pneumonia characteristic of the type caused by M. bovis. PCR and immunohistochemical tests indicated that a distinct strain of M. bovis caused fatal pneumonia in this group of pronghorns. A retrospective survey by PCR and immunohistochemical analysis of lung tissue from 20 pronghorns that died with and without pneumonia between 2007-18 yielded negative results, indicating M. bovis infection in antelope is probably a new and emerging syndrome.

Where it came from and where it might lead is an open question. Reports from wildlife officers, local ranchers and disease investigators indicate up to 500 animals have perished.

The bacterium Mycoplasma bovis is an economically important pathogen of cattle that contributes to the complex nature of bovine respiratory disease. In addition to causing respiratory disease, the bacterium causes a range of clinical conditions in beef and dairy cattle worldwide.

In a 2006 study, M. bovis could be isolated in 85 per cent of cattle with acute fibrinous pneumonia and 98 per cent of cattle with chronic pneumonia (Gagea et al., 2006).

Not surprisingly, at about the same time M. bovis emerged in the cattle populations, outbreaks of polyarthritis and pneumonia were noted in bison herds. In 2002, an outbreak of severe M. bovis-associated pneumonia with arthritis appeared in a Sask­atchewan bison herd. During this period there were also anecdotal reports of mycoplasma pneumonia with high morbidity and mortality with subsequent decreases in herd fertility in large Midwestern American bison herds. (Dr. Murray Woodbury, University of Sask­atchewan). Causative organisms identified in these outbreaks were nearly identical to M. bovis strains known to cause disease in cattle. Woodbury noted at the time, “It is reasonable to suspect that published and unpublished reports represent the tip of the proverbial iceberg representing mycoplasma disease in bison herds.”

Chronic, asymptomatic infection with occasional shedding of organisms is possible with M. bovis. This appears to be important to the transmission of M. bovis organisms between individuals, and especially to the maintenance of infection within a herd and exposure of naive populations (Maunsell et al., 2011).

Mycoplasma organisms are susceptible to the effects of drying and sunlight but survive for relatively long periods outside the host in cool, humid conditions. M. bovis persists for months in recycled sand bedding and has been found in cooling ponds and dirt lots on dairies (Bray et al., 1997). There is a lack of information about the role of environmental reservoirs and fomites in maintaining or spreading infection among or between herds.

Bison herds are most likely infected through the introduction of asymptomatic carriers of M. bovis organisms. It is probable that the stress of transportation, introduction or mixing of new animals in the herd, and other stress-related factors causes carrier animals to shed organisms in respiratory secretions, resulting in transmission via aerosols, nose-to-nose contact, or indirectly through contamination of feed, water or farm equipment.

There are several implications of Mycoplasma bovis infections in antelope:

1. Despite increased recognition of its role in economic loss in the cattle industry, M. bovis remains a clinical challenge because of a common carrier state in clinically healthy animals, variable disease expression, intermittent shedding and the lack of rapid accurate diagnostic assays. (Mycoplasma bovis Infection in Free Ranging Pronghorn, Center for Disease Control and Prevention)
2. Clinical disease is not considered necessary to maintain M. bovis in populations, and M. bovis is commonly detected in asymptomatic adult feedlot cattle. If a reservoir of infection exists, it can be assumed populations of free-ranging antelope are under attack by a deadly and relentless pathogen.
3. Mycoplasma jumped the species barrier in wild ruminants previously. Disastrous results followed. In the early 2000s, M. bovis caused several high-mortality epizootics in bison in North America (case-fatality rate 45 per cent). These events raised concern about emergent virulent strains, and research began to characterize isolates from different host species.
4. An important difference between outbreaks of mycoplasma in bison and cattle is that in bison, few or no co-infecting bacterial or viral pathogens are consistently detected. Although M. bovis virulence factors are poorly defined, evasion of an animal’s immune response is implicated in maintaining chronic infection. One study found that that up to 79 per cent of bison herds in Western Canada have at least one seropositive animal and that nearly 80 per cent of herds with seropositive animals had no history of M. bovis disease, suggesting non-clinical carriers are common. Many questions remain about M. bovis in antelope.
5. Until its recent discovery in antelope, documented cases of M. bovis in free-ranging ruminants were uncommon.

The emerging situation in antelope appears serious. Approximately a one-half to one million pronghorn exist in North America. Herds commonly overlap with range cattle and ranched bison. Are they threatened?

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]). 

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Dr. Ron Clarke

Dr. Ron Clarke prepares this column on behalf of the Western Canadian Association of Bovine Practitioners. Suggestions for future articles can be sent to Canadian Cattlemen ([email protected]) or WCABP ([email protected]).

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