About 40 percent of animals infected with M. bovis in the feedlot either die or are in the chronic pen and euthanized
Mycoplasma bovis is a bacterium that affects cattle and bison. In cattle, it can cause mastitis, arthritis and pneumonia. Not all infected animals get sick, however. Some just shed bacteria without signs.
Dr. Jennifer Davies, pathologist and director of the diagnostic lab at the University of Calgary, said it can affect all age groups of cattle in beef and dairy.
“In feedlots it is economically devastating, causing chronic disease that is poorly responsive to antibiotic therapy, with significant animal health and welfare consequences. About 40 percent of animals infected with M. bovis in the feedlot either die or are in the chronic pen and euthanized.
“Some respond to treatment after extensive antibiotic therapy but never do as well, with significantly reduced weight gains.”
If young calves consume milk from a cow with mastitis caused by M. bovis, they may develop middle ear infection with or without pneumonia. The affected calf may be dull with dropping ears or eyelids.
In respiratory disease, a number of viruses and bacteria play a role, leading to pneumonia in the feedlot, but M. bovis is the smallest living organism that can replicate.
“It doesn’t survive well in the environment. It lives on mucous membranes of cattle, particularly in the nasal cavities,” Davies said.
These bacteria are mainly spread by direct contact. In a cow-calf operation they come into the herd with an infected animal or via fence-line contact with infected animals. “Many cattle have this organism in their nasal cavity but no signs of disease. These carriers can bring it into a herd,” said Davies.
The ones that get sick may be vulnerable because their immune system is compromised.
As long as it stays in the nasal cavity, it doesn’t do much harm, though it might be spread to other animals. If it gets into the lungs, however, damage is done.
“Every time we take a breath, bacteria from the nasal cavity go down into the airways. But as long as there aren’t too many bacteria in the nasal cavity, the lung defences can handle it. If these defences are decreased for any reason, they can’t handle the normal amount of bacteria taken in with each breath.”
Davies said stress from transport and handling, in a feedlot setting, can weaken an animal’s immune system, allowing bacteria to multiply and get inhaled. There they can overwhelm normal defences.
M. bovis is good at evading the immune system. It is not responsive to most antibiotics so it can cause chronic pneumonia. Then the organisms enter the bloodstream from the lungs.
“Once they get into the blood they can go to other places in the body, but one place they typically end up is in the large joints, like the stifle,” said Davies.
“The animal gets arthritis when these bacteria set up shop in the joints. Resulting inflammation often includes the tendons and tissue around the joints, with severe chronic lameness, which also does not respond to treatment very well. It becomes an animal welfare issue, the animal is euthanized, and samples sent to the lab for a diagnosis.
“We try to prevent infection in a cow herd through biosecurity measures, but this is not an option in the feedlot. Best defence involves keeping calves healthy, with strong immune systems. … Antimicrobials are most likely to work if we can diagnose the disease early. By the time it becomes chronic, the damage is done.”
There are vaccines for many respiratory diseases and some for this one but they are not very effective for prevention.
Dr. Murray Jelinski of the Western College of Veterinary Medicine at the University of Saskatchewan said infection with M. bovis tends to occur later in the feeding period than the typical shipping fever pneumonias that occur within eight to 12 days after calves arrive at the feedlot.
“Mycoplasma cases occur 30 or more days after. If we sample cattle upon arrival, we generally find Mycoplasma bovis in five to 10 percent of those calves’ nasal passages. These are healthy cattle coming off the trailer and a small percent have Mycoplasma,” said Jelinski.
“But if we sample all those cattle again in two or three weeks, we find it in almost all of them. It starts spreading quickly in the pen.
“While other pathogens are causing pneumonia, we think Mycoplasma by itself doesn’t cause pneumonia. It waits until other bacteria damage the lungs. Then it insidiously grows.”