Efficacy of a temperature-sensitive Mycoplasma synoviae live vaccine.

A temperature-sensitive clone of Mycoplasma synoviae (MS), MS-H, derived from the chemical mutagenesis of the Australian field isolate 86079/7NS, was investigated for efficacy as a live vaccine. Titers of MS-H vaccine ranged between 1.16 x 10(8) and 8.4 x 10(8) color changing units/ml when incubated at 33 C and were consistently > or = 10(2) lower when incubated at 39.5 C. Laboratory-produced MS-H vaccine protected 8 out of 10 specific-pathogen-free Webster white leghorn chickens against a combined experimental challenge of aerosols of the virulent wild MS strain 88064/FP and T-strain infectious bronchitis vaccine administered intratracheally. Laboratory and commercially produced vaccines were compared for efficacy against thoracic air sac challenge, with both achieving similar levels of air sac protection. Air sac lesion incidences of five air sacs with lesions out of 32 and four out of 32 were seen in groups vaccinated with laboratory and commercial vaccines, respectively, compared with 13 out of 20 in nonvaccinated specific-pathogen-free hybrid white leghorn chickens. An attempt to determine the dose response of the commercial vaccine was conducted with 0.5, 2, and 4 times the standard dose. Air sac lesion incidences of 16 air sacs with lesions out of 32 in the 0.5 times, 11 out of 32 in the 2 times, and 1 out of 32 in the 4 times dose groups were observed.