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DURING an equine respiratory disease outbreak, the modified-live intranasal vaccines available against strangles (Streptococcus equi subspecies equi) and equine influenza virus (EIV) are commonly used due to their ability to induce a rapid onset of mucosal immunity. However, horses displaying clinical signs of respiratory disease are routinely tested using quantitative PCR (qPCR) analysis of nasal secretions to help determine the causative pathogen(s), and these qPCR assays do not distinguish between a modified-live vaccine pathogen and the wild-type pathogen. Thus, if a horse that had recently received a modified-live intranasal vaccine subsequently develops signs of respiratory disease it would be unclear whether the qPCR detection of that pathogen in nasal secretions is from the vaccine or represents true infection. Therefore, the objective of this study was to provide a timeline for how long after vaccination will a horse continue to shed modified-live vaccine S. equi subspecies equi and EIV.
A total of 23 adult horses with no intranasal vaccine history were selected from the Center for Equine Health, School of Veterinary Medicine, University of California at Davis and randomly assigned to one of two groups (12 horses in S. equi vaccine group; 11 horses in EIV vaccine group). Groups were housed according to gender in large paddocks of 3–5 horses each, with central water and food areas. Study horses ranged from 4 to 15 years of age (median=10 years), with a total of 13 geldings and 10 mares. Most horses belonged either to the Quarter Horse or Thoroughbred breed. Horses were vaccinated annually against eastern/western equine encephalitis virus, tetanus, West Nile virus, rabies virus, equine herpesvirus type 1/herpesvirus type 4 and EIV. Horses had not been vaccinated for over 12 months at the time of study commencement. Prior to study commencement, each study horse was deemed to be healthy based on a physical evaluation, …