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The ultimate goal of a deworming program is to improve the health and performance of the horse. Additionally, specific diseases associated with parasites should be prevented. Today, diseases caused by parasites are unusual; by deworming heavily, we are actually "fine-tuning" management.
Specifically, the aim is to promote growth, utilization of nutrients, athletic performance, physical condition (fitness), and longevity.
The key to success is reducing the number of parasites on the pasture or in the paddock, such that horses will not automatically be re-infested with parasites when they graze. Dewormers prevent larvae (incapable of making eggs) from developing into adults, and kill adults while they are attempting to make eggs. Simply deworming horses on a regular basis (such as every eight weeks) may be excessive (depending on the drugs used) in many circumstances and fails to take into account the variable risk for parasite challenge at different times of the year.
Until proven innocent, all adult horses should be considered a threat to the pastures by passing their parasite eggs in 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 to larvae on the ground.
The higher the temperature, the more rapid the maturation of the egg. While colder temperatures lead to "hibernation" of the eggs, larvae that mature at higher environmental temperatures quickly die. Parasite eggs survive the cold weather much better than hot summer weather. Horses can be easily infected by digging through snow to graze the underlying grass.
As a rule, the important parasite of adult horses (Cyathastomes) cannot infect horses in the stable. The manure is cleared away before the eggs develop into infective larvae. Furthermore, the parasite eggs tend to be neutralized by the ammonia produced in soiled bedding.
In contrast, the risk of new infection in the spring is very high. Spring pastures contain an abundance of infective larvae that are ready to infect horses - having matured from the last grazing season (and surviving through the winter). Pasture contamination with larvae, which developed from eggs dropped in the spring, declines with the warming weather and is quite low by early June in temperate climates. 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.
Eliminating Parasites from Pastures
The infective parasite larvae must move away from the piles of manure in order to be eaten by horses. Parasites can be helped 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.
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 United States. 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.