Duration of protective immunity induced by Mycoplasma gallisepticum strain ts-304 vaccine in chickens
Introduction
Infection of chickens with Mycoplasma gallisepticum (MG) causes chronic respiratory disease, characterised by tracheitis and airsacculitis (Whithear, 1993, 1996). This results in reduced productivity in chicken flocks, from reduced growth rates, weight gains, hatchability and egg production, and losses due to carcass condemnation (Evans et al., 2005; Kleven et al., 1998). Mycoplasmoses remain a constant threat to poultry producers, so effective preventative control is crucial. Live attenuated vaccines are widely used, and have been proven to be safe, highly efficacious and capable of conferring lifelong protection by induction of sustained antibody responses, presumably through antigen persistence within the host (Whithear, 1996). Three live attenuated M. gallisepticum vaccines, F strain, strain 6/85 and strain ts-11, are commercially available and have been shown to have a high degree of efficacy in controlling the dissemination of this pathogen in chickens (Carpenter et al., 1981; Evans and Hafez, 1992; Whithear et al., 1990a, b). The live attenuated temperature sensitive vaccine strain ts-11 (Vaxsafe MG, Bioproperties Pty. Ltd., Glenorie, NSW, Australia) has been used to control, and in some cases even eradicate, disease caused by infection with wild-type M. gallisepticum in commercial layer and breeder flocks (Kleven et al., 1998; Whithear, 1996). It is administered by eye drop, stimulates a detectable, although variable, systemic antibody response (Noormohammadi et al., 2002), and persists in the upper respiratory tract for the life of the flock, inducing long-lived immunity (Whithear, 1996). Colonisation of the respiratory tract epithelium by M. gallisepticum has been shown to be mediated, in part, by the primary cytadhesin GapA (Goh et al., 1998; Keeler et al., 1996). Previous work in our laboratory identified two ts-11 variants within Vaxsafe MG (strain ts-11), one with an intact and fully functional gapA gene and a second with a 20 bp sequence duplication that resulted in a frameshift within the gapA gene (Kanci et al., 2004). The variant with a functional gapA gene was cloned from Vaxsafe MG (strain ts-11) and designated strain ts-304. Our recent studies have established that strain ts-304 is a safe and efficacious live attenuated vaccine. In these recent studies, administration of a single dose of MG ts-304 to 3-week-old chickens was found to protect birds challenged with the M. gallisepticum wild type strain Ap3AS by aerosol at 4 weeks after vaccination. A dose as low as 105.5 colour changing units (CCU) was capable of inducing similar protection to a 40 fold lower dose of the current commercial Vaxsafe MG (ts-11) vaccine (Kanci Condello et al., 2020).
The duration of protective immunity induced by vaccination with the 6/85 and ts-11 strain vaccines has been investigated previously in chickens (Noormohammadi and Whithear, 2019). This study showed that both these vaccines induced protection against challenge with virulent M. gallisepticum up to 36 weeks after vaccination. Other studies have shown that the ts-11 strain can protect up to 40 weeks after vaccination (Whithear, 1996). However, no published studies have investigated whether the duration of immunity afforded by a single dose of a live attenuated M. gallisepticum vaccine is sufficient to provide protection for the whole life (60 weeks) of long-lived layer or breeder birds. The objective of the study reported here was to determine the duration of immunity induced by a single dose of MG ts-304 to 3-week-old chickens by assessing their protection against experimental exposure to an aerosol challenge with the virulent M. gallisepticum strain Ap3AS at 40, 48 and 57 weeks after vaccination.
Section snippets
Mycoplasma gallisepticum strains
The MG ts-304 vaccine used in this study was a freeze-dried product obtained from Bioproperties Pty. Ltd. (Glenorie, NSW, Australia) and was used as recommended by the manufacturer. For vaccination, the MG ts-304 vaccine was reconstituted in Marek’s Disease vaccine sterile diluent (Merial Select Inc., Gainesville, Georgia, USA), then diluted to 106.0 CCU per 0.03 mL dose in the same diluent immediately prior to inoculation. For challenge, an ampoule of the virulent M. gallisepticum strain
Results
All chickens appeared healthy at the commencement of the study. No signs of respiratory distress or any other disease were observed throughout the experiment. None of chickens in the unvaccinated negative control group (Group D) had detectable levels of antibodies against M. gallisepticum throughout the experiment. At 2 weeks after vaccination, 20/30 (66.7 %) of birds in Group A (vaccinated and challenged) and 23/30 (76.7 %) of birds in Group B (vaccinated only) had positive RSA test scores (≥
Discussion
This study has shown that vaccination of 3-week-old SPF chickens with a single dose of 106.0 CCU of MG ts-304 was able to induce a protective immune response that persisted until at least 57 weeks after vaccination. The single dose of the vaccine also elicited a serum antibody response that was detectable in a proportion of vaccinated chickens by rapid serum agglutination test and ELISA for up to 57 weeks after vaccination, with no significant reduction over time. This persistent antibody
Declaration of Competing Interest
Gregory J. Underwood is an employee of Bioproperties Pty. Ltd. The University of Melbourne licenses the M. gallisepticum ts-11 and ts-304 vaccines to Bioproperties Pty. Ltd. and, as employees of the university involved in the creation of ts-304, AKC, PKS, PFM and GFB are entitled to a share of any royalties generated from this license. The other authors have no potential conflicts of interest (financial, professional or personal) related to the research reported here.
Acknowledgements
The authors gratefully acknowledge the assistance of June Daly and Angela Chircop for care of the animals, Denise O’Rourke and Ling Zhu for the serological analyses, and Faye Docherty and Paul John Benham for assistance with histopathological processing. The investigators would also like to acknowledge the assistance of the staff of Bioproperties Pty. Ltd. - Petrina Young, Fabian Carter, Daniel Andrews, Hilda Joubert, Julie Brogestam and Gamini Jayawardane.
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