Roundworms (Parascaris Equorm)
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There appears to be several reasons for this higher rate of infection in young horses, less than 2 years of age. First foals and yearlings don’t seem to be as particular where or what they eat therefore they have a tendency to eat in areas where the number of roundworm eggs may be greater. Also it appears that as the horse gets older they may develop immunity to the eggs or larva as they migrate through the tissues that keep them from becoming adult worms in the intestines.
The eggs will remain infective for many years in the soil and can withstand great variations in temperature, can be well below freezing for long periods of time, or very warm without killing the larva inside the eggs.
The adults look very much like some of their close relatives the earth worms, they may be from 5 inches to nearly 20 inches in length and are generally white to light grayish blue in color. They generally are described as looking like spaghetti and can cause the young horse to appear mal-nourished (rough hair coat and pot bellied). The larvas are often responsible for fairly severe respiratory problems in weanlings and yearlings as they migrate through the lungs. Large numbers of these worms can also cause impaction colic in young horses.