Immunohistochemistry of a lung lesion showing Mycoplasma reactivity in the center of the abscess, field case.
Photo credit: Ricardo Rosenbusch, DVM, PhD
Editor’s note: Last in a Mycoplasma bovis roundtable series.
Eye-balling a sick feedlot animal with pneumonia and saying for sure that it has a Mycoplasma bovis disease is a tenuous diagnosis at best. It may very well have an M. bovis infection, but most likely it also has bovine viral diarrhea virus (BVDV) or other pathogens that are at work.
Diagnosing M. bovis
Diagnosing an M. bovis infection isn’t easy. “To me, the biggest challenge to the producer and the veterinarian is detecting early symptoms of Mycoplasma bovis,” says Ricardo Rosenbusch, DVM, PhD, Iowa State University. “With other diseases, you get early symptoms, and then you can have relatively early intervention. With Mycoplasma bovis, you get a call because someone has crippled calves. Those calves have had a pneumonia for 15 days and an arthritis for three or four days. They’ve probably been treated, and you’re coming in for the second treatment. The chances of success are going to be relatively low because of late intervention.”
Rosenbusch has been searching for early signs that might indicate an M. bovis problem. “Is there a cluster of early signs you can look for? Veterinarians who deal with veal operations look for conjunctivitis as a reliable early sign. They look at calves coming in at 7 or 8 days of age. If they break with conjunctivitis, they start worrying about Mycoplasma bovis. They put them in an early almost-metaphylactic treatment.”
On the other hand, Rosenbusch notes, the feedlot and stocker operators seem to rely on a moist cough as a symptom. “They don’t see enough conjunctivitis as a symptom because they have problems with flies and most calves already have conjunctivitis. They’ve learned to associate a moist cough presentation at a week after arrival with Mycoplasma bovis. I wish we had better science.”
Rosenbusch notes that in a study where calves were infected with a known strain of Mycoplasma bovis and transported to a feedlot, there was a major outbreak in the feedlot. Every dead animal was necropsied. “We had a significant mortality of 7 percent. We observed every animal, and we knew exactly on which day the infection was started. Every strain of Mycoplasma bovis that came out of that outbreak had the same fingerprint as the one we put in.”
However, the cowboys could not find the Mycoplasma bovis animals. “They were correctly pulling out the Mannheimia hemolytica cattle and they were being treated, but the M. bovis animals were not pulled,” Rosenbusch says. “Febrile responses reached 103° but were never high enough to cause a severe enough depression for the cowboy to pull the animal.”
Within this consolidated lung, there are many raised, white-yellow, sharply demarcated foci of caseous necrosis. Cheesy exudate may be expressed from a cross-section of lung. These foci of necrosis are often 2-10mm in diameter but occasionally coalesce to form large areas of necrosis.
Photo credit: Dan Grooms, DVM, PhD