Hunting For A “Bad” Mycoplasma Bovis – for Sep. 6, 2010

Past research has shown a link between the bacteria, Mycoplasma bovis, and a condition of feedlot cattle known as chronic pneumonia and polyarthritis syndrome (CPPS). In some animals, an M. bovis infection progresses to CPPS, yet other animals respond to early treatment. Still others seem to be able to clear the bacteria on their own. A project recently completed by master’s student Rose Whelan at the University of Saskatchewan tested the hypothesis that a unique strain of M. bovis is associated with CPPS.

As the name implies, CPPS is a chronic disease of the lower respiratory tract characterized by arthritis in at least one joint, Whelan explains. Treatment and labour costs are high for feedlots because it can take weeks or months between the onset of the disease and death or recovery. Animal welfare becomes a concern when animals have to be treated numerous times.

At first, the animal shows symptoms associated with undifferentiated fever or undifferentiated respiratory disease, meaning that the animal has a fever, signs of respiratory disease, or general illness, but the pathogen or pathogens responsible have not been diagnosed. In cases of CPPS, pneumonia often persists despite treatment with an antimicrobial, and signs of arthritis — lameness, abnormal posture, reluctance to move — become noticeable if they hadn’t been observed before treatment.

M. bovis was first isolated from cattle in the 1970s, so it is a relatively new area of research. It is one of several known Mycoplasma species, which are incredibly small bacteria that don’t have cell walls and other cell structures to support life. Thus Mycoplasma bacteria are considered parasites because they require a host to provide the nutrients necessary for survival.

Several studies have found that M. bovis is one of the most common pathogens present in bovine respiratory disease.

It’s important to consult with your veterinarian when choosing an antimicrobial if M. bovis is suspected. Antimicrobials such as the common Beta-lactams (penicillin types) that target the cell wall of bacteria, have no effect on M. bovis, Whelan explains. It is also resistant to the sulfonamide group and may be gaining resistance to the tetracyclins and macrolides, though very few trials have suggested that these antimicrobials are suitable candidates for treating M. bovis pneumonia.

The antimicrobials with the greatest potential for treatment are the flouroquinolones and some of the new drugs that don’t fall into the above groups: tiamilin, valnemulin, lincomycin and florfenicol. Adding an anti-inflammatory appears to be more effective than treatment with an antimicrobial alone.

Vaccines would be the clearcut answer, says Whelan, not only to reduce the odds of M. bovis acquiring resistance to the antimicrobials that are showing good potential, but to save on the drug and labour costs of treating animals in the feedlot.

Though vaccines to guard against M. bovis infections have been available through the years, vaccine development is proving to be a huge challenge because the proteins on the outside of the M. bovis bacteria are always changing, says Whelan. Antibodies work like a lock and key with surface proteins, so vaccines are developed using those specific proteins to trigger an immune response. When the surface proteins change, the vaccine still triggers an immune response and the antibodies target the bacteria withthosespecifi proteins, buttheydon’tattackM. bovis bacteria with altered proteins.

IDENTIFYING STRAINS

There have been numerous attempts in the past to determine the prevalence of CPPS in North America, Europe and Australia, but not in Western Canada. The evidence to date suggests the prevalence of certain strains varies from one geographical region to the next, and maybe even from year to year within a region.

There are two current theories as to why M. bovis doesn’t always cause CPPS: the immune response of the individual animal and the virulence of the strain. The virulence of a strain is associated with its ability to attach to specifitissue cells so that it has access to nutrients for its survival and can cause the disease.

Whelan’s project sought to unlock the identity of a virulent strain that causes CPPS by studying three groups of calves.

NECROPSIED CALVES

Veterinarians assisted by forwarding lung and joint swabs from calves confirmed for CPPS from 11 feedlots in Alberta and Saskatchewan in 2006. The calves were sourced from across Western Canada.

Twenty-five of the 29 animals were positive for M. bovis. Three strains, A, B and C, were found in lung and joint samples.

Strain A was the most prevalent, with the presence of strainsBand beingsimilar. Multiplestrainswerefound in 38 per cent of the lung samples and 29 per cent of joint samples.

Commercial Calves

The next year at a commercial feedlot in Saskatchewan nasal swabs were collected from 56 calves on arrival and again when they were pulled and treated for respiratory disease within the first 90 days on feed.

Twenty-four of the 56 tested positive for M. bovis at arrival but 38 were positive when treated indicating that the presence of M. bovis increases with days on feed, most likely due to commingling with carrier animals, says Whelan.

Almost all— 99 per cent— of the M. bovis samples were strain A. The other one was strain B. None of the treated calves died from respiratory disease.

U of S calves

In 2008 nasal swabs and lower lung samples were taken from 36 calves at the U of S feedlot 14 days after arrival and again at 90 days. M. bovis was the second most common Mycoplasma species isolated at the nose and the lung in both tests. Seventeen head were M. bovis positive, and Whelan was surprised to find it turned up more often on the nasal swabs than the lung samples, where you would expect the disease to take hold. No new strains were identified in the lower lung samples.

Multiple Mycoplasma species were present on six per cent of nasal swabs and 11 per cent of the lung samples.

As in the commercial feedlot study, no strain was found in these calves, but 68 per cent were strain B and 32 per cent strain A.

Even though none of the calves at the university feedlot showed signs of respiratory infection, strain A (the most prevalent strain in the necropsied calves) was fairly common in these calves as well. This suggests the presence of M. bovis alone doesn’t necessarily mean an infection will occur.

Fortunately for these two feedlots, but unfortunately for the project, no full-blown cases of CPPS developed so Whelan could follow the progression of the disease.

Conclusion

Although strain was the only one found in calves necropsied for CPPS but not in the live cattle, Whelan says we can’t conclude it is the strain that causes CPPS because it wasn’t found in every calf that died of CPPS and it wasn’t the only strainofM. bovisintheCPPSmortalities. Indeed, strain was the least prominent of the three found in the necropsied group.

If there is a single fatal strain of M. bovis causing CPPS deaths in feedlots it remains a mystery.

This study did confirm the need to develop vaccines against more than one strain of M. bovis, however, and research to develop a vaccine continues at the Vaccine and Infectious Disease Organization on the U of S campus.

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