Rosenbusch compares M. bovis to bovine viral diarrhea virus (BVDV). “BVDV is an immunosuppressive agent, and when you do a necropsy, you don’t need to take a slice of the lymph nodes. You can just look at them – they are depleted. When I infect with Mycoplasma bovis, regardless of the route of administration, I always get the tonsils positive by culture. When I do immunohistochemistry and histopath on the tonsils, they are not depleted. In very young calves, I’ve seen a hint of partial depletion with a very high dose of Mycoplasma bovis. Mostly, it’s the mesenteric lymph nodes.”
Rosenbusch notes that occasionally researchers stumble onto one or two calves that are refractory to a Mycoplasma bovis challenge. “Is it genetically resistant to developing lesions? It may have very high titers of Mycoplasma bovis in the lungs, but no lesions. I do believe there are some genetically resistant animals out there.”
How long-lasting immunity to Mycoplasma is isn’t completely known. Borrowing heavily from other Mycoplasma models, Rosenbusch believes immunity following natural infection and vaccination may be only two to three months.
The interesting contrast with experiments done in Britain is that when they infected cattle with Mycoplasma bovis and then went in with a Brucella strain-19, no interaction or inhibition of the immune response occurred, notes Rosenbusch. “Yet when they did the same experiment and went in with M. dispar, they saw significant immunosuppression and reduction of the response to strain-19. Here’s a Mycoplasma that’s not highly pathogenic. Generally, there’s no mortality and it’s highly immunosuppressive. This is an area of a lot of contradiction.”
Beef cattle infections of Mycoplasma bovis can take many forms, from respiratory disease to otitis to arthritis. “Mycoplasma bovis strains are variable,” says Rosenbusch. “Some of them will destroy the tracheal escalator; the majority of them do not. Some go systemic and cause arthritis; many do not. Some cause a febrile reaction; others do not. From one phenotype, I can take a strain and define a set of pathogenicity markers, clinical phenotypes, for that strain.” He adds that the organism is currently being sequenced at the University of Missouri, which will provide useful genome information for one strain of Mycoplasma bovis.
Infection can cause several problems at the same time, though they may not be evident visually. An example would be a field infection where arthritis is predominant and there is no respiratory sign, but on necropsy, the lungs are infected. Others may have a complete pneumonic presentation and not a single animal has a clinical presentation of arthritis, which, according to Rosenbusch, is very common. “I’d say arthritis represents less than 20 percent of the total symptoms.”
Rosenbusch adds that all Mycoplasma bovis strains will cause mastitis. But when you get into the respiratory and systemic component, that’s where you see a difference. “If you have a strain that is not affecting the tracheal escalator, then how does it get down to the lung?” he asks. “It needs a heavy component of some other bovine respiratory disease (BRD) pathogen to make its route down.”
Some experiments Rosenbusch did with John Andrews, DVM, PhD, at Iowa State University showed that a hot strain of Hemophilus somnus introduced into the nose would induce severe pneumonia in almost 100 percent of the animals. “In animals where we had Mycoplasma bovis in the nose, a necropsy revealed Mycoplasma bovis in the lungs in four days,” says Rosenbusch. “The animal had both Hemophilus and Mycoplasma bovis in the lungs. The controls that were not infected had Mycoplasma bovis in the nose and that never got into the lungs. It was a strain that couldn’t make it down to the lung on its own, but it easily went down once the tracheal defenses were destroyed.”
This information is from a Bovine Veterinarian Mycoplasma bovis roundtable sponsored by Boehringer Ingelheim and moderated by Bob Glock, DVM, PhD.
D. L. Step, J, G. Kirkpatrick. (2001). Mycoplasma Infection in Cattle. I. Pneumonia – Arthritis Syndrome. Bovine Practitioner June.
R. Sprowls. (2001) Mycoplasmosis/BRD in Stocker and Feeder Calves. Academy of Veterinary Consultants Proceedings, Volume XXIX, No. 1, April.
The Kansas Mycoplasma Stocker Survey
A 2001 study by Kansas State University of stocker/backgrounders in Kansas asked producers about the following clinical syndrome: About two weeks after arrival, calves pulled for pneumonia were nonresponsive after two antibiotics were tried. About three weeks after arrival, arthritic calves were pulled, with progressive conditions.
Some of the key findings were:
The syndrome was reported across all sizes of operations, especially larger operations.
The syndrome occurred in all weights of cattle but more often lighter-weight cattle.
Outbreaks were more likely to occur when more loads were increased during the winter.
The syndrome was reported in cattle from all areas, but less likely to be found in home-raised calves or calves from the western U.S.
The likelihood of a problem increased as cattle were received from an increasing number of states.
Increased stress from castration or dehorning (upon arrival or delayed) may have increased the likelihood of having a problem.
Operations feeding native grass hay as a primary ration reported more problems.
For information on the complete study, visit www.beefstockerusa.org and click on “Management practices and non-responsive pneumonia and/or arthritis in Kansas stocker operations.”