Histological Evaluation of Equine Cadaver Skin Cores Dependent on Needle Type and Skin Preparation Method to Investigate Intramuscular Injection Complications in Horses

• Equine skin cores may contain bacteria inhabited structures of the skin.
• Skin cores must be discussed as initiators of injection site infections.
• Using smaller needles leads to a lower incidence of skin coring.
• Smaller needles produce smaller skin cores which may contain less bacteria.

To histologically determine number and size of skin cores and hair fragments resulting from intramuscular injection techniques using equine cadaver skin explants. Clipped (clip) and nonclipped (no.clip) skin specimens, of the hamstring region (n = 10 horses), were obtained from freshly euthanized horses and punctured with 18G needles (18G), 18G needles with stylet (18GM), and 22G needles (22G). Needles were flushed with 0.2 mL of sterile saline solution. The saline was collected on slides and stained (Pappenheim). Skin cores, produced with 18G, were fixed with formalin solution (4%) and embedded in paraffin. Sections were stained with Masson's trichrome. Light microscopy was used for evaluation. Each different injection technique resulted in the identification of skin cores within a subset of slides obtained from the different 10 skin samples. Displaced skin cores were identified more frequently in clip samples than no.clip samples; 18G needle punctures resulted in the highest number and largest surface area of skin cores in both no.clip and clip samples as compared to 22G and 18GM punctures (P < .05); 18G needles with stylet punctures in no.clip samples resulted in the lowest number of skin cores. Histology of paraffin sections identified connective tissue (14/16 samples), stratum corneum (13/16), epidermis (12/16), isolated hair shafts and sweat glands (11/16), hair shafts integrated with epidermis and sebaceous glands (6/16), and hair bulbs (5/16). Skin cores can contain bacteria which may initiate infective injection complications. The coring incidence and hair fragmentation correlate with needle size; therefore, smaller needle sizes should be used.