Evaluation of Needle-free Injection Devices for Intramuscular Vaccination in Horses

Needle-free injection devices have been approved for the delivery of biologics with inherently low immunogenicity, such as plasmid DNA vaccines; however, no studies have described their use in equine patients. This article compares the use of two such devices (VitaJet-3 and Biojector2000) at typical vaccination sites in a cohort of six horses. After identifying the optimal device and vaccination site, a second cohort of five horses was used to document the biologic activity of a DNA plasmid vector delivered with the selected injector. Injector characteristics, including the amount of intramuscular drug deposition, residual skin dose, and pain responses, were evaluated following vaccination, with colored saline in the pectoral muscles and cervical region in six horses. The optimal device was then selected and used for intramuscular vaccination with the pING/tyrosinase plasmid vector in a group of five horses. Biological activity was measured through antibody response to the protein encoded by the plasmid on days 0, 14, 28, 42, and 56 postvaccination. Optimal intramuscular dose delivery was obtained in the pectoral muscle site using the VitaJet-3. No significant pain responses were noted. Dependent edema was seen at vaccination sites 24 hours after therapy. Antibody responses to the protein encoded by the DNA plasmid vector significantly increased after vaccinations in all horses. The VitaJet-3 is easy to use and is effective for delivering intramuscular vaccinations with DNA plasmid vectors in horses. This device allows for vaccination with vectors that exhibit low immunogenicity and/or that require targeted delivery to specific tissue planes.