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Q fever in sheep: Long-term control of infection by vaccination of gimmers
Bauer, B.; Janowetz, B.; Turowski...
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Introduction: Small ruminants are regarded as a major source of human infection with Coxiella burnetii (Cb). In summer 2012 a Q fever outbreak was observed in a sheep flock with 800 adult ewes. End of 2012 we got involved in this case in order to implement a control program. At that time, we assumed that most of the sheep were already infected or immune. Yearly primary vaccination of gimmers (replacement ewes) with CoxevacTM (Ceva Santé Animale) has been introduced as a long-term measure. The vaccine is not licensed for sheep; therefore, a monitoring of vaccination was implemented. This case-report describes the long-term effect of a primary vaccination of gimmers until 2020.
Methods: Gimmers were vaccinated twice three weeks apart (primary vaccination), no further revaccination was performed.
The following groups were ear-tagged for control purposes, animals were randomly selected:
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20-30 vaccinated gimmers each year (VG13, 14, 15 etc.),
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As a positive control 30 ewes were vaccinated in 2013 (VE13).
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Each year 30 unvaccinated gimmers were selected into a group of sentinels (S). Additionally, gimmers before first vaccination were included. After seroconversion animals were removed from the group.
Direct pathogen monitoring was performed by PCR-testing of vaginal swabs collected hours after parturition and nasal swabs. Indirect monitoring was based on blood samples from sentinels which were tested for seroconversion.
The immune response (PhI/PhII-antibodies, IFN-γ-Recall Assay (RA)) was assessed before and after primary vaccination. The ratio of titers (PhI, PhII) or IFN-γ-reactivity was assessed as n-fold (nx) increase. Quantitative PCR, PhI- and PhII-antibody tests and IFN-γ-RA were performed as previously described (Böttcher et al., 2013; Boettcher et al., 2017).
Animals which had been primary vaccinated in 2013 (n=6 i.e. VG13 and VE13), 2014 (n=3), 2015 (n=10), 2016 (n=11) and 2017 (n=10) were once revaccinated in 2018 and the immune response was assessed before and 3 weeks after re-vaccination.
Data were analysed by MedCalc Statistical Software ver- sion 19.1.3 (MedCalc Software bv, Ostend, Belgium; https:// www.medcalc.org; 2019). Groups were compared by Kruskal- Wallis-test.
Results: From November 2012 until February 2014 the rate of positive vaginal and nasal swabs was 78/268 and 67/263, respectively. The mean pathogen load in positive samples was 102,6 and 101,6 Cb per vaginal and nasal swab, respectively. Thereafter swabs were tested negative, the numbers of analysed vaginal/nasal swabs per year were: 2014: 69/62, 2015: 68/39, 2016: 105/42, 2017: 158/40, 2018: 86/40, 2019: 49/- and 2020(Jan): 97/-.
Percentages of annual seroconversion (PhII-titer >100) in sentinels were: 2013:16.8% (n=95); 2014: 25% (n=52); 2015: 5,6% (n=53); 2016: 1,3% (n=76); 2017: 2,6% (n=77); 2018: 0% (n=105) and 2019: 0% (n=95).
Groups VE13 and VG13 responded well after vaccination. E.g., VG13 showed an n-fold increase of PhI-, PhII-titers and IFN-γ-reactivity of 116x, 168x and 6x, respectively. In contrast, gimmers born after shedding had ceased (VG15-19) showed only weak immune responses after vaccination. The n-fold increase of PhI-, PhII-titers and IFN-γ-reactivity e.g. in VG16 was only 1x, 2,5x, 2,7x, respectively.
Available animals in groups VE13, VG13-17 were revaccinated once in 2018. In all groups a similar strong n-fold increase of PhI-, PhII-titers and IFN-γ-reactivity was observed after revaccination (e.g., VG16: PhI-titer (11x), PhII-titer (38x) IFN-γ (33x - in this case IFN-γ was determined as pg/ml).
Conclusions: Two years after an outbreak C. burnetii was still detected in vaginal and nasal swabs, however, pathogen load was very low. Although seroconversion in sentinels was even detected until 2017, it did not result in shedding at parturition. Vaccination preferentially boostered an existing immunity, because gimmers born since 2015 showed only a weak immune response after vaccination. However, despite this weak immune response these animals showed a strong immune response after a single revaccination even three years after primary vaccination. Consequently, primary vaccination of gimmers is a cost-efficient long-term measure to control C. burnetii in sheep. We do not know if it protects from infection, however, at least in case of urgency revaccination of such a sheep flock would rapidly increase the herd-level immunity.
Acknowledgements: This study was financially supported by the Free State of Bavaria and the Bavarian Joint Founding Scheme for the Control and Eradication of contagious Livestock Diseases.
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Affiliation of the authors at the time of publication
University of Veterinary Medicine Hanover, Foundation, Clinic for Swine and Small Ruminants, Hanover, Germany;
Bavarian Animal Health Service, Poing, Germany;
State Veterinary Office, Karlstadt, Germany.
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