By Loretta Sorensen
It's been more than 50 years since Mycoplasma bovis was isolated in an American dairy cow. Although this well-known respiratory disease has not been at the forefront of beef cattle health management practices, it is increasingly being identified in U. S. cattle with chronic infections such as pneumonia.
"M. bovis doesn't behave like other types of bovine respiratory diseases," says Russ Daly, SDSU Extension veterinarian. "It doesn't cause acute, harsh and severe pneumonia. It manifests as more of a chronic, long standing illness that may affect an animal's health for a much longer period than other bacterial infections."
Mycoplasmas differ from other bacteria in that they are the smallest known organisms capable of dividing on their own. In addition, the organisms don't possess a cell wall.
"That's important because many common antibiotics, such as penicillin and anything derived from it (amoxicillin or ampicillin) and cephalosporins kill bacteria by inhibiting cell wall formation in new bacteria. So antibiotics such as these have no activity on Mycoplasma."
There are several mycoplasma species, but researchers working with M. bovis have found the bacteria in half of the cattle in their study, cattle with otherwise normal lungs. In studies of cattle with chronic pneumonia, M. bovis was identified in 98% of studied animals.
"M. bovis is widely distributed through feedlot cattle populations," Daly says. "They are generally rapidly colonized upon arrival at a feedyard through nose-to-nose contact with other calves."
It takes just 100 organisms to colonize the nasal passage of a calf through the nasal secretions of infected animals.
"Stress is one of the major factors that aid spread of Mycoplasma in calves," Daly says. "Stress in the form of weaning, long transportation, co-mingling and adverse weather conditions. All of those stressors increase the level of cortisol in a calf's bloodstream. This hormone has an inhibitory effect on immune cells that normally keep bad bacteria in check in the nasal passage."
In just 5 days, what begins as 100 million bovis organisms in a calf's nasal passage can explode to the magnitude of 100 million organisms.
"That's enough to start causing damage to the animal," Daly says. "Colonization extends to the windpipe and larger airways. Here, the bacteria directly damage the 'ciliated' lining of the airways. These cilia are hairlike projections from the respiratory lining cells that function to sweep out debris and pathogens from the respiratory tract before they can damage the lung tissue below. Now, Mycoplasma has an easy path to lung tissue itself."
Inflammation of lung tissue can lead to death in some areas of the tissue, resulting in increased respiration, cough, fever, nasal discharge and pneumonia.
"Depending on the bacterial load and severity of prolonged stress, Mycoplasma infection can take weeks or sometimes just days to bring on illness," Daly says. "Typically, the pace is moderate and it is several weeks before signs of infection appear."
Symptoms of M. bovis may also be and easily missed until an animal becomes seriously ill. Common symptoms include increased respiration; frequent, hacking cough; nasal discharge; fever; decreased appetite; and arthritis.
"What makes M. bovis so difficult to diagnose is that there may not be much for clinical signs in some animals," Daly says. "The bacteria mostly settle in the lungs, but occasionally it moves to joint tissue and animals become lame or develop swollen joints."
M. bovis spreads easily through coughing, nasal secretions and direct contact with infected animals. It can also be present on fences, feed bunks, water troughs and in milk.
"Vaccination with vaccines against other, typical respiratory pathogens is important to help suppress the disease," Daly says. "There is no blood test for the disease, so it can be tricky to diagnose. Steps to prevent cattle from contracting the disease include low-stress handling of calves, vaccination and an adequate backgrounding program." Reducing co-mingling of animals, crowding, cleaning water tanks, bunks and other equipment is also essential. Culling chronically ill cattle can lessen the chances that Mycoplasma carriers are present in the herd.
"Update herd health protocols and work with your veterinarian to verify a diagnosis," Daly says. "This can be a slow and insidious infection. It requires strategies to identify, treat and prevent Mycoplasma infection."
Sorensen is from Yankton, S.D.