Virulence of recent isolates of Mycoplasma synoviae in turkeys.

Systemic Mycoplasma synoviae (MS) infection was induced experimentally in commercial turkeys with recent MS isolates (K4822D and K4774J) from turkey breeder flocks that exhibited no clinical signs typical of MS infection except for a low incidence of swollen footpads. The virulence of each strain was compared by evaluating gross and microscopic lesions, serologic responses, and MS isolation rates at 10 and 21 days postchallenge and by comparing these results with those obtained from a known virulent isolate (K1968), another previously characterized field isolate (K4463B), and unchallenged controls. All strains induced lesions typical of infectious synovitis but showed distinct differences in the extent of the gross and microscopic lesions and in the isolation rates from the tissues in turkeys. K1968 induced the most extensive lesions in hock and stifle joints and footpads, but strains K4822D, K4774J, and K4463B all induced synovitis and were similar in virulence for synovial tissues. Very mild respiratory lesions were induced by all of the strains studied. All strains yielded strong positive serologic responses. We concluded that these recent field isolates, although able to induce synovitis, are less virulent for turkeys than a known virulent strain. Nevertheless, under severe experimental challenge, these strains have the capability of causing lesions that may be incompatible with economical turkey production.