29.08.2015
Armour and Ferguson-Noel (2015) Av Path 44:296–304
Mycoplasma gallisepticum, the causative agent of chronic respiratory disease in chickens and the most pathogenic of the avian mycoplasmas, is both horizontally and vertically transmitted. In order to maintain flocks free of M. gallisepticum, replacement stock should originate from an M. gallisepticum-free source; Vaccination has been used as an alternative control approach in situations where prevention of infection is not feasible or as a component in control programmes whose ultimate objective is M. gallisepticum eradication. In the United States, vaccination is primarily used to control M. gallisepticum in multiple-age commercial layer operations. The ts-11 vaccine is a live attenuated M. gallisepticum vaccine, which was developed by chemical mutagenesis of an Australian M. gallisepticum field isolate and selected for its temperature-sensitive (ts+ ) phenotype (growth at 33°C). The ts-11 vaccine strain is regarded to be of low or no virulence and has been demonstrated to provide protection against respiratory disease and egg production drops induced by virulent M. gallisepticum infection and to reduce vertical transmission of M. gallisepticum field strains. In 2007, a decision was taken to vaccinate a number of broiler breeder flocks in northeastern Georgia with ts-11 vaccine, to control an M. gallisepticum epidemic that began in the area in 2006. Between 2008 and 2011, M. gallisepticum was found to be the cause of severe respiratory disease in the broiler progeny of several ts-11-vaccinated breeder flocks from four companies. M. gallisepticum isolates from these broiler flocks and their parents were indistinguishable from ts-11 vaccine strain by all genotyping methods used and were therefore termed “ts-11-like”. Spikes in mortality and rising M. gallisepticum antibody titres in the breeder flocks were reported to precede clinical disease in the progeny by approximately 2 months. A pathogenicity study confirmed that, a ts-11-like M. gallisepticum isolate from one of the infected broiler flocks, was significantly more virulent than ts-11 vaccine. The epidemiology of the outbreaks, the genotyping results and the pathogenicity study findings indicated that an increase in virulence and vertical transmission of ts-11 vaccine had occurred. A study, led by Dr. Natalie K. Armour at the University of Georgia, has investigated the ability of ts-11 vaccine and ts-11-like M. gallisepticum isolates from a broiler flock and their ts-11-vaccinated parent flock to transmit via the egg. The results provide the first conclusive evidence of transovarian transmission of an isolate genotyped as ts-11 and indicate the existence of a spectrum of egg transmission potential and virulence for ts-11-like isolates.
See also: Can a live Mycoplasma vaccine be transmitted vertically ?