For your purpose of choosing the best vaccination and deworming strategy for your horse, these are the three basic equine lifestyles:
Open herd: This is your horse's situation if he travels frequently to shows or other events where he comes in contact with unfamiliar horses whose worming and vaccination schedules you don't know. Even if he's stay-at-home, he's in an open herd if he boards in a large facility with a high turnover and even more so if manure is not removed regularly from the turnout areas.
Semi-open herd: If your horse and those around him are on regular vaccination and deworming schedules and their fecal egg counts are monitored and all newcomers to the barn are immediately vaccinated and dewormed, your horse fits here.
Closed herd: This lifestyle gives you horse the least exposure to worms and germs. He qualifies if he's in an extremely controlled environment with rare exposure to unfamiliar horses, is turned out in regularly cleaned paddocks and benefits from regular vaccination and deworming program that includes fecal egg counts.
Now that you know your horse's herd type, use the chart below to develop a custom plan that optimally immunizes your horse against disease and controls his exposure to parasites.
This chart originally appeared in the article "Protect Your Horse from Worms and Germs" in the April 2005 issue of Practical Horseman magazine.