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Research – for Dec. 6, 2010

Background

The Mycoplasma bovis bacterium is involved in the bovine respiratory disease complex (BRD) as well as the chronic pneumonia and polyarthritis syndrome (CPPS). These diseases are responsible for 25-40 per cent of feedlot calf mortality, and are a leading cause of death loss in high-risk fall-placed feedlot calves in Canada. There are several theories as to why CPPS has increased in prevalence.

The preventive use of long-acting antimicrobials to suppress other lung pathogens such as Mannheimia haemolytica and Pasteurella multocida may create an opportunity for M. bovis to take over and cause illness.

Rather than remaining in a sick pen for a few days, animals with bovine respiratory disease are often returned to their home pen immediately after being treated. This means that their temperature is often not taken again to determine whether they have responded to the treatment.

Viral infections suppress the immune system, so the risk of BRD and CPPS may be higher when calves are also fighting viruses.

Prevent or react? New antimicrobial products have helped feedlots react to the BRD and CPPS concerns that plagued them a few years ago. In the long term, preventing these problems will require a better understanding of the organism. To this end, the Beef Cattle Research Council (BCRC), Alberta Beef Producers (ABP), and Ontario Cattlemen’s Association (OCA) funded several M. bovis research projects. Dr. Murray Jelinski (University of Saskatchewan) studied different strains of M. bovis collected from feedlots across Western Canada. Dr. Jeff Caswell and Ken Bateman (University of Guelph) studied the spread of M. bovis in newly arrived feedlot calves. Dr. Steve Hendrick (University of Saskatchewan) examined whether different respiratory disease treatment strategies helped reduce the incidence of CPPS.

What they did

All three studies involved fall-weaned calves sourced at auction marts and managed according to standard feedlot health protocols. In addition, fluid samples were collected from the nose and lung of cattle at varying times after arrival, and joint and lung tissue samples were collected from cattle that died of CPPS. Samples were cultured for M. bovis, and bacterial DNA were examined to determine whether the same strains of M. bovis were found in healthy cattle as in those that became sick or died of CPPS. Dr. Hendrick also examined whether treating all animals with oxytetracycline on arrival reduced the incidence of respiratory disease, and whether the way in which sick animals were managed affected the incidence of CPPS.

What they learned

M. bovis is relatively rare on arrival, but it spreads rapidly. All three studies found that fewer than 10 per cent of calves carried M. bovis when they arrived at the feedlot, but over 60 per cent were infected within 90 days of arrival. Finding M. bovis in the nose did not accurately predict whether it would also be found in the lung, and finding M. bovis at feedlot arrival did not accurately predict whether the animal would develop respiratory disease later on. Many cattle that did carry M. bovis were able to clear the infection without becoming ill. On the other hand, M. bovis was found in close to 90 per cent of the lungs and joints of calves that died of CPPS.

No particular strain of M. bovis seems to be most harmful. These three studies found between 13 and 58 different strains of M. bovis that were grouped into clusters based on genetic similarity. None of the three studies could pinpoint particular strains or clusters that were associated with disease. The relative proportions of the different clusters were similar in infected noses, lungs, joints, healthy, sick and dead cattle.

Preventive and disease treatment strategies

Preventive oxytetracycline treatment on arrival reduced the number of calves that were treated for BRD by 30 per cent. Rates of BRD retreatment, arthritis treatment, and mortality were the same in calves treated with one subcutaneous dose (six ml) of florfenicol and returned to the pen as in calves that were given a three-ml intramuscular dose, kept in a sick pen for 48 hours, and treated with another three ml before returning to their home pen.

What it means

The preventive use of long-acting antimicrobials did not increase the risk of BRD and CPPS.

Returning animals to their home pen immediately after treatment did not increase the risk of BRD and CPPS.

Calves can successfully clear an M. bovis infection without becoming ill. This implies that Mycoplasma may cause more problems in stressed calves, particularly those that have not been appropriately immunized against common feedlot diseases before arriving at the feedlot.

The OCA is continuing to support research to identify the specifiM. bovis genes that cause illness. Research to develop an effective vaccine against M. bovis is also being funded by ABP. These efforts will help to refine management strategies to prevent M. bovis-associated BRD and CPPS in newly weaned, fall-placed feedlot calves.

ReynoldBergenisthesciencedirectoroftheBeefCattle ResearchCouncil.

About the author

Contributor

Dr. Reynold Bergen is the science director of the Beef Cattle Research Council.

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