FOR IMMEDIATE RELEASE — MAR. 4, 2013 — One high-shedder horse can drop 6+ billion eggs that have the potential to become infective larvae in a pasture over a year’s time. One low-shedder horse can drop 1.5+ billion eggs that have the potential to become infective larvae in a pasture over a year’s time. All other shedders fall somewhere in between. Managing parasites in one horse starts with managing the parasite load of the herd, and managing the parasite load in the herd means managing the pastures.

How the Horse Gets Infected

Understanding how horses become infected is critical to understanding why and how pastures must be managed.

• Most horses do not graze pastures evenly. Rather, they have areas where they eat the grass very short (lawns) and areas where they pass most of their feces (roughs). Consequently, the eggs are going to be concentrated in the roughs.
• Most horses seem to avoid grazing in the roughs, allowing the grass to grow very lush and green in these areas because of the added nitrogen fertilizer.
• Horses that are tempted to take just a few bites of this lush green grass while on their way to and from the bathroom area are likely to pick up large numbers of infective larvae, while other horses that resist the temptation do not become as heavily infected. This is one explanation as to why some horses on a pasture have very high egg count while others on the same pasture have very low egg count.
• An infective larva generally travels no more 1.5 feet from the pile where it is passed as an egg by the horse, so there is a high concentration of eggs (potentially infective larvae) in the “rough” areas.
• Infective larvae actually climb up the blades of grass (but generally no more than 3 inches high) when it is warm and moist in order to make themselves much more available to an unsuspecting horse passing by. Grass in a pasture that is not taller than 3 inches results in a very high concentration of infective larvae at the tips of the blades of grass.
• Horses grazing on a short pasture are more likely to be exposed to more infective larvae because the larvae crawl up the blade of grass only about 3 inches.
• Other Factors Affecting Egg Distribution in Pastures
• Two other factors that affect the distribution of worm eggs in a pasture are nature and man.
• When it rains, eggs and larvae can be washed by the water over the pasture and be much more evenly distributed, thereby making it more difficult for horses to avoid them. We cannot control the rain but we may be able to control the drainage, thereby controlling the distribution.
• The best practice for dragging or mowing of pastures is to first remove the piles of manure from the roughs once a week. Then drag or mow the pasture when the weather is hot and dry, Ideally, horses should be kept off freshly-mowed or dragged pastures for five days. Larvae need moisture to survive, and exposing them to the heat causes them to use up more of their stored energy and die. Infective-stage larvae cannot eat because of a special coating and must rely on energy stored from eating bacteria during the previous stage while in the pile of manure.

Recommended Reading

Understanding parasites, their life cycles, how they affect your horse and your pasture, and how to manage everything parasite-related can be challenging—there is a lot to learn. An excellent resource for anyone who owns or manages horses is the recently published Handbook of Equine Parasite Control by Craig R. Reinemeyer and Martin K. Nielsen. The book provides basic biology, general principles of parasite control, assessment tools, case studies and advice. It is available from several sources including Amazon and Barnes & Noble, in both print and ebook versions.