Editor’s note: First in a Mycoplasma bovis series
Mycoplasma bovis infections permeate all segments of the cattle industry from calves to adult cows, dairy to beef and veal operations to feedlots. “People in a dairy context have a different definition of Mycoplasma problems from people in a beef context,” says Ricardo Rosenbusch, DVM, PhD, a Mycoplasma researcher at Iowa State University. “In a dairy context, more and more people are aware of the variety of Mycoplasmas involved in producing diseases. In the beef context, a Mycoplasma problem is Mycoplasma bovis.”
Mycoplasmas are bacteria that have no protective cell walls and minimal genetic material. They often elude their host’s immune systems to survive for long periods of time. The bacteria colonize the respiratory tract and other mucosal surfaces and infect the bloodstream. Mycoplasma bovis has been associated with a variety of bovine diseases, such as abortion and low fertility, arthritis, keratoconjunctivitis, mastitis, pneumonia and synovitis. This series will focus on Mycoplasma bovis and its contribution to otitis, arthritis and pneumonia. (For information on Mycoplasma mastitis, see Bovine Veterinarian September and October 2001.)
Not all Mycoplasmas are as important economically as M. bovis. “The dairy industry is concerned, for example, with a chronic cough in 2- and 3-month-old dairy calves,” Rosenbusch adds. “That could be a Mycoplasma dispar. That’s a very frequent type of presentation, but it’s not a medical emergency.”
Mycoplasma bovis infections in calves, stockers and feedlot cattle can run the gamut from ear infections to arthritis and pneumonia.
Other species of Mycoplasma that can infect cattle include: M. dispar, M. bovirhinis, M. alkalescens, M. arginini, M. canis, M. bovigenitalium and M. californicum.
An “all-purpose pathogen,” Mycoplasma organisms can attach to the mucosal layer of the respiratory epithelium and reside in the respiratory tract as an opportunist and work synergistically with other pathogens in respiratory disease. Because the organism has an affinity for membrane surfaces, it can readily colonize the synovia in one or several joints and lead to septic joints and arthritis problems. In calves and even lightweight stockers, ear infections due to Mycoplasma bovis can cause ear droop, excessive discharge from the eye, facial paralysis, head tilt and purulent material in the external ear canal.
It’s believed Mycoplasma bovis can be transmitted by a variety of ways. Rosenbusch says it can be transmitted through the udder to a calf. “We’ve had a situation where the only way we could explain how Mycoplasma bovis got into a clean herd was the purchase of a pregnant heifer that must have transmitted, through colostrum, to a single calf, and then nose-to-nose and calf-to-calf from there, in areas where calves were concentrated in small spaces.”
In an extended type of cow-calf operation, where the cows and calves are distributed over large acreages, an outbreak may be more difficult. “You can have Mycoplasma bovis transmission and not necessarily have an outbreak,” says Rosenbusch.
In stocker operations and feedlots, Mycoplasma bovis is harbored in the respiratory tract. “The major way it’s going to be spread is through aerosols, direct nose-to-nose contact and spread of discharge from the nasal passages from one animal to another,” says Dan Grooms, DVM, PhD, Michigan State University.
“We don’t have well-documented information on respiratory transmission,” adds Rosenbusch. “What little information we have indicates that nose-to-nose contact is very important for massive transmission.”
He explains that massive transmission would occur in a stocker operation that receives calves from one or two order buyers, puts them together and then experiences a significant acute outbreak. An outbreak such as this would occur in the first two to four weeks after arrival with 80 percent to 90 percent of calves showing some kind of clinical signs and mortality up to 7 percent. (See stocker survey sidebar.)
“You can super-infect an animal that’s already infected with a strain of Mycoplasma bovis and we can isolate more than one field strain from a nose swab from a single animal,” says Rosenbusch. “I’ve never seen an animal with three strains, but I see no reason why they wouldn’t be there. I’ve seen many with one strain and a few with two strains. Even though it might be ubiquitous, having a Mycoplasma bovis circulating through a stocker operation is no guarantee that they are immune to a different strain coming in. A second strain could cause an outbreak.”
Ricardo Rosenbusch, DVM, PhD, says it appears that weather and stress are critical components of significant high-mortality outbreaks.