Is there a reliable test to check the titer of immunity before we revaccinate each year for flu, rhino, etc.? Why is a tetanus shot good for five or more years in humans and only one year or less in the equine?

The equine veterinary community has been doing extensive investigation into appropriate vaccine schedules and the best methods of administration. But even if we don't know the "optimum" schedule for every horse, there is essentially no evidence that "overvaccination"–that is, vaccinating more often than the horse needs–causes horses any harm. Unfortunately, we don't know what significance titers have, and in my opinion, it is less expensive and likely to be more effective to vaccinate more often than to run titers that have an unknown significance.

As to why tetanus is such an effective vaccine: The tetanus toxin provokes a very strong and long-lasting immune response due to its chemical make-up. This is unlike viruses, which don't provoke a strong immune response (one of the reasons people get flu shots every year).