Detection of Modified-Live Equine Intranasal Vaccine Pathogens in Adult Horses Using Quantitative Polymerase Chain Reaction

The duration of detection of modified-live intranasal vaccine pathogens in nasal secretions by qPCR appears to be dependent on vaccine schedule and ambient temperature. Authors’ address: Department of Veterinary Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616; e-mail: charms@ucdavis.edu.

1. Introduction

Horses displaying signs of respiratory disease are routinely tested using real-time polymerase chain reaction (qPCR) analysis of nasal secretions. However, qPCR testing from a nasal swab does not distinguish between modified-live vaccine and wild-type pathogens. Thus, if a horse recently vaccinated intranasally subsequently develops signs of respiratory disease it would be unclear whether the qPCR detection of that specific pathogen is from the recently administered vaccine or represents true infection. Therefore, the objective of this study was to provide a timeline for how long after intranasal vaccination a horse will continue to shed modified-live S. equi subsp. equi (S. equi) and equine influenza virus (EIV).

2. Materials and Methods

Twenty-three adult horses were randomly assigned to one of two vaccine groups (S. equi = 12 horses; EIV = 11 horses), with two of the horses in each group to remain unvaccinated. All horses to be vaccinated received the vaccine in the left nostril at different periods according to the manufacturers’ recommendations. After each vaccination, both nostrils of every horse were swabbed daily for 5 days. The swabs were processed and analyzed via qPCR for the presence of S. equi and EIV.

3. Results and Discussion

For both vaccine pathogens, qPCR detection was limited to 2 days following first vaccination (S. equi and EIV) and 1 day at second vaccination 3 weeks later (S. equi). Following a boost vaccination at 6 months, qPCR detection lasted up to 5 days for both vaccine pathogens. The authors hypothesize that lower environmental temperatures during the 6 month revaccination period caused prolonged pathogen shedding times, possibly due to enhanced replication within the upper respiratory airways and impaired mucociliary clearance.

Acknowledgments

Conflict of Interest

This study was supported by Merial and a grant from The Center for Equine Health, University of California, School of Veterinary Medicine at Davis.

The vaccines were kindly provided by Zoetis and Merck Animal Health.