Characterization of a ts-11–like Mycoplasma gallisepticum Isolate from Commercial Broiler Chickens

Abstract

Several commercial broiler flocks in northeastern Georgia that were the progeny of the same parent flock (Flock 40) were diagnosed as Mycoplasma gallisepticum (MG) positive by serology, culture, and PCR. Flock 40 had been vaccinated with ts-11 live MG vaccine. Several isolates were obtained from the MG-positive broiler flocks, and these isolates were indistinguishable from the ts-11 vaccine strain by the molecular strain differentiation methods used. A pathogenicity study was performed to compare the virulence of one of the isolates, K6216D, to the ts-11 vaccine strain. K6216D elicited a significantly stronger antibody response and significantly increased colonization of the tracheas and air sacs. K6216D also elicited significantly greater air sac and tracheal lesions than the ts-11 vaccine strain at 10 and 21 days postinoculation (P < or = 0.05). This is the first report of a field case of the apparent reversion to virulence and vertical transmission of the ts-11 vaccine.

Identification of Strain-Specific Sequences That Distinguish a Mycoplasma gallisepticum Vaccine Strain from Field Isolates

“…The field isolates were recovered from clinical submissions to the Poultry Diagnostic and Research Center (PDRC), University of Georgia, Athens, GA USA, during a period from October 2007 to February 2010. The isolates were genotyped as ts-11 strains based on multiple genetic analyses, including targeted sequencing and random amplified polymorphic DNA (RAPD) analysis (9). The clinical picture was obtained from history at submission and/or interviews with field veterinarians.…”

“…The F strain is a naturally attenuated field isolate that was first discovered in the 1950s, while ts-11 and 6/85 were commercially produced using serial passage or chemical mutagenesis (7). These vaccine strains exhibit various degrees of effectiveness and safety (9–11), and there is evidence that some of these M. gallisepticum vaccine strains can revert to virulence (9, 12, 13).…”

“…These vaccine strains exhibit various degrees of effectiveness and safety (9–11), and there is evidence that some of these M. gallisepticum vaccine strains can revert to virulence (9, 12, 13). Currently, the genetic basis behind the attenuation of the vaccine strains is not well understood.…”

“…In addition, strain differentiation among vaccine strains and natural isolates has proven complex (14–17), making it difficult to determine if avian mycoplasmosis infection in vaccinated flocks is due to infection with an M. gallisepticum strain type similar to the vaccine (18), reversion of the vaccine strain (9), or mixed infection with the vaccine and a related strain type (19). …”

“…M. gallisepticum isolates from the broilers and their parents were indistinguishable from the ts-11 vaccine strain by the genotyping methods used and were termed “ts-11-like” isolates (9). The epidemiology of the outbreaks, as well as genotyping and pathogenicity results, indicate that an increase in virulence and vertical transmission of ts-11 vaccine occurred and that the ts-11-like isolates were very likely revertants derived from ts-11 vaccine (9, 16, 32).…”