Elsevier

Veterinary Microbiology

Volume 251, December 2020, 108883
Veterinary Microbiology

Duration of protective immunity induced by Mycoplasma gallisepticum strain ts-304 vaccine in chickens

https://doi.org/10.1016/j.vetmic.2020.108883Get rights and content

Highlights

  • A GapA positive clone recovered from Vaxsafe MG (strain ts-11) and designated MG strain ts-304 can colonise chickens.

  • MG strain ts-304 is a safe and effective vaccine that can protect chickens against aerosol challenge with the M. gallisepticum wild-type strain Ap3AS at 40, 48 and 57 weeks after vaccination.

  • A single dose of the MG ts-304 vaccine was capable of conferring solid protection against disease induced by experimental aerosol challenge in chickens and significant protection against colonisation by the challenge strain for at least 57 weeks after vaccination.

Abstract

Mycoplasma gallisepticum (MG) is an important pathogen of poultry worldwide, causing chronic respiratory disease in chickens and turkeys. MG ts-304 is a GapA positive clone recovered from Vaxsafe MG (strain ts-11) that has been shown to be safe in chickens when delivered by the eye drop route to 3-week-old specific-pathogen-free chickens and to confer protection against challenge at 4 weeks after vaccination, as measured by tracheal mucosal thickness and air sac lesion scores. In this study, specific pathogen-free chickens (SPF) were vaccinated with a single dose of the MG ts-304 vaccine (106.0 colour changing units) at 3 weeks of age and experimentally challenged by aerosol with the virulent M. gallisepticum strain Ap3AS at 40, 48 and 57 weeks after vaccination. There were no significant differences in tracheal mucosal thickness 2 weeks after challenge between chickens challenged at the three time points, or between the vaccinated birds after challenge and unvaccinated/unchallenged control birds. Thus there was clear evidence that the immunity conferred by vaccination with the MG ts-304 vaccine resulted in significant protection against tracheitis in chickens that extended to, but was highly likely to exceed, 57 weeks after vaccination and that similar long term protective immunity could be expected to be conferred by a vaccine dose lower than that used in this study.

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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  • Cited by (0)

    1

    Deceased.

    2

    These authors contributed equally to this work.

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