What do Mycoplasma bovis lesions look like? Rosenbusch explains that there are two types of Mycoplasma bovis strains. Fifty percent of the strains coming from the field will produce abscesses and 50 percent will not. With abscessing strains, the presentation in the lung will be a coagulative necrosis lesion that can be up to 5mm in diameter, never much larger than that. Histologically, the center has formed around a bronchus that has accumulated a lot of neutrophils. The epithelium is still normal. That whole area of accumulation of neutrophils expands and is surrounded by a band of mononuclear cells, monocytes and lymphocytes. There’s very little evidence of a fibrotic capsule; often there’s no capsule whatsoever. “It’s not truly an abscess like you would see with a Pasteurella multocida abscess.”
The lesions don’t grow beyond that even if the animals have been clinically affected for 40 to 50 days. Lesions tend to be cranial in distribution but can be in the diaphragmatic lobes also. Wherever there are small localized lesions, the surface of the lung will be elevated. “It has goose bumps all over it, which is something you do not see in M. hemolytica,” notes Rosenbusch. As you cut, you see relatively dry, white caseous lesions. “You almost always see that the septums of the lung are enlarged, and the histopathology to that is thrombosis of the lymphoid system of the lung. The septums are tremendously enlarged on histological examination. When you look at the lung, you can see the divisions between the lobes much more than in a normal lung or an M. hemolytica or a P. multocida lung.”
The lung from a Mycoplasma mycoides infection shows a yellow, stringy, well-demarcated limit to the lung lobules and a deep meat-red color of the tissue in between. “That’s very similar to what Mycoplasma bovis looks like,” says Rosenbusch.
These classic lesions are what Dan Grooms, DVM, PhD, Michigan State University, sees as well. “Maybe we misdiagnosed this, but at times, I’ll see that classic small abscess throughout the cranial-ventral distribution, and in the midst of that, we’ll see what looks like some have been coalescing together to form a bigger area of necrosis. More commonly, we see the miliary lesions throughout the lung.”
“When we first started looking at these lesions, we thought about tuberculosis,” says Rosenbusch. “Miliary is an OK term, but in abscesses, it’s not a caseous type of lesion. What’s missing is the fibrotic component. You don’t see bundles of fibroblasts and a lot of collagen.”
Mycoplasma bovis lesions are clearly different compared to M. dispar, says Rosenbusch. “I don’t think you would even need immunohistochemistry. Pathologists who have seen M. dispar or even M. hyopneumoniae in a pig lung, and then who have looked at an M. bovis lung, clearly know it’s a totally different species. Also, many pathologists who started using immunohistochemistry have dropped it to lower the cost to the consumer. They’re now doing diagnosis on histopathology only because their eyes are already trained. They can do a very good job and can be over 90 percent accurate with histopathology alone.”
Severe synovitis and arthritis of the stifle joint extending distally into the tendon sheath of the peroneus tertius.
Photo credit: Dan Grooms, DVM, PhD