Tiny mycoplasma bovis bacteria major cause of pneumonia

The bacterium mycoplasma bovis is responsible for significant illness and death in beef and dairy cattle. 


They are the smallest of all known bacteria and lack a cell wall, which is an important target for many antibiotics. 


Several clinical diseases are associated with infection, including pneumonia and less commonly, arthritis, udder and inner ear infections. 


Airways in the front lung lobes fill with dead cells, pus and bacteria, which prevents normal air movements and leads to impaired breathing and clinical pneumonia.


Mycoplasma is transmitted through infected milk from cow to calf. Weaned calves acquire infection through direct contact and possibly aerosolized means. 


Half of the healthy calves that arrive in feedlots are infected with M. bovis, and almost all become infected after two weeks. 


However, a tricky feature of this bacterium is that cattle can be infected without any illness. Simply giving calves mycoplasma will not induce the disease. They have to be affected by other factors to get the disease. 


Healthy calves that stay on their home turf are much less likely to develop pneumonia compared to calves that are shipped, mixed and put in feedlot pens. 


The reasons why feedlot calves develop mycoplasma pneumonia are poorly understood. 


Some researchers have identified associations between mixing different aged cattle, which might increase social stress as the pecking orders are worked out. 


Prophylactic antibiotic treatment may also increase the risk. 


Mycoplasma pneumonia is a relatively new player to the bovine respiratory disease game, having been first described in 1961. 


Perhaps a more deadly strain emerged or maybe producers, feedlot staff, veterinarians and pathologists have just become better at recognizing it. 


Acute cases of mycoplasma pneumonia are indistinguishable from other bacteria and viruses that cause pneumonia in feedlot calves. 


Signs of illness include coughing, weight loss, fever, flared nostrils, increased respiratory rate and effort. 


One major difference is that mycoplasma also infects joints, leading to lameness and arthritis. 


Calves with mycoplasma pneumonia do not respond to antibiotic treatment and go on to develop chronic pneumonia. 


Not only are these calves repeatedly treated, but many are euthanized for welfare reasons. As a result, mycoplasma pneumonia hurts the bottom line. 


One study has estimated that the disease results in $32 million in annual losses in the United States. 


Mycoplasma can be eradicated from herds, but this may not be practical in most settings because a large percentage of animals may be carriers. 


However, some dairy herds have successfully eliminated mycoplasma, so it can be done. 


Researchers at the University of Saskatchewan’s Vaccine and Infectious Disease Organization are developing a new type of vaccine. While traditional vaccines have used killed virus, it is questionable whether or not these vaccines are effective. 


In the first stage of their work, the researchers found a way to reliably recreate the disease for experiments. They discovered that infection with bovine herpes virus, which is the causative virus of IBR, followed by inoculation with mycoplasma, reliably created a chronic, progressive pneumonia that is similar to mycoplasma pneumonia in feedlots. 


In the same study, they found that BVD infection does not lead to mycoplasma pneumonia. 


This work is important because it sets the stage for future vaccine trials in which mycoplasma pneumonia can be reliably induced and the protective effects of vaccine evaluated. 


With no effective treatments, efficacious vaccines may be our best hope at controlling this significant cause of disease in calves. 


Dr. Jamie Rothenburger is a veterinary pathology resident at the Western College of Veterinary Medicine, University of Saskatchewan. Twitter: @DrJamieR_Vet


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