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Equine Parasite Control

By: Dr. Philip Johnson

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How Do Pastures Become Infested with Parasites for Horses?

Until proven innocent, all adult horses should be considered a threat to the pastures, by passing parasite eggs in their manure. Following a deworming treatment, the adult parasites in the intestine are killed and the passage of eggs in the manure stops for a while. In the meantime, new adult worms develop from larval stages in the horse's intestinal wall and, after a delayed period, start to produce eggs again. These eggs are passed onto the grass. The eggs are NOT immediately infective for other horses; they must undergo a phase of maturation on the ground. The egg must develop into an infective 3rd-stage larva to be infective. The maturation of eggs into 3rd-stage larvae is affected by the climatic condition.

How do Climatic Conditions Affect the Development of the Parasites?

The higher the temperature, the more rapid the maturation of the egg. While colder temperatures lead to "hibernation" of the eggs, larvae which mature at higher environmental temperatures quickly die (they deplete their energy reserves). Although horse owners have traditionally believed that parasites are killed by cold winter weather, the exact opposite actually happens. The parasite eggs survive the cold weather much better than hot summer weather. Horses can be readily infected by digging through snow to graze the underlying grass. The clever horse owner can thus use the weather patterns in a given area to help eliminate parasites from the horse pastures.

Can Stabled Horses be Infected with Parasites?

As a rule, the important parasites of adult horses (cyathastomes) cannot infect horses in the stable. The manure should be cleared away before the eggs develop into infective 3rd-stage larvae so that there is no risk of additional exposure to these parasites. Furthermore, the parasite eggs tend to be neutralized by the ammonia produced in soiled bedding. Therefore, when horses are stabled in the winter (as occurs in the northern USA), there is little risk of new parasite infection. However, following turnout in the spring, the risk of new infection is very high. Spring pastures contain an abundance of infective 3rd-stage larvae that are "ready to go" – having matured from the last grazing season (and surviving through the winter).

Pasture contamination larval levels decline in the warming weather and are quite low by early June. The number of larvae will remain low into summer if re-contamination of the pasture is not allowed. To that end, horses should not be allowed onto pastures if they are passing parasite eggs in their feces.

How Can Horses and Pastures be Managed to Minimize or Eliminate Parasites from the Pastures?

The infective parasite larvae must move away from the piles of manure in order to be eaten by horses because most horses will not eat grass that has been soiled by manure ("fecal avoidance behavior"). Parasites are aided and assisted in their ability to move away from piles of manure by the action of horses' hooves, farm machinery and rainfall. The areas of the pasture that are contaminated with manure are known as "roughs" and the uncontaminated areas are known as "lawns". If space is limited, horses that are low in the "peck" order (low dominance) are often forced to graze in the roughs and are therefore exposed to higher levels of parasites. If practical, the regular removal of manure from the pasture will help to reduce the number of parasites to which grazing horses are exposed. Whether or not regular manure removal is practicable for a given farm typically depends on the size of the pasture and the number of horses on the pasture. However, this technique can be used to great advantage in some farm settings.

The horse owner should take advantage of the fact that parasites in the pasture are killed by periods of hot dry weather. It should also be remembered that, although parasite larvae cannot mature in very cold weather, they are fully capable of persisting and being ready to infect new horses as the weather warms up. Local conditions obviously differ in different parts of the USA. In the southern states, the summer weather can be relied upon to effectively eliminate the majority of pasture parasites. However, in the more northern states, the "summer kill" is not so effective.

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