A rabies vaccine reaction is an area of hair loss that forms 1-3 months after a rabies vaccine has been administered. The reaction occurs at the area where the vaccine was injected and the hair loss may be accompanied by a dark pigmentation. The vaccine causes a panniculitis, which is an inflammatory reaction of the tissue where the vaccine was deposited. Other injections, such as steroids, can cause a similar reaction.
Often, a presumptive diagnosis can be made based on appearance, time frame (1-3 months after receiving the vaccine), and location (site where the vaccine was administered). All reactions should be biopsied to confirm a diagnosis and reported to the vaccine manufacturer.
Most vaccine reactions will remain small and cause no other clinical problems. In some affected animals hair will grow back. Medications to improve the underlying inflammatory response, such as pentoxifylline, tetracyclines, and essential fatty acids, should be discussed with your veterinarian or dermatology specialist. Most states and municipalities have laws in place requiring a rabies vaccination for your pet. Overall, the rabies vaccine is very safe and these reactions are uncommon.