Roundworms, Equine - contd.
Parascaris equorum usually infects young horses or foals living on pastures with other infected horses. Parascaris equorum eggs are shed by infected horses, and may survive in the soil for many years, thus contaminating entire pastures for long periods of time.
Once ingested, roundworm eggs will hatch into larvae once within a horse's intestines. These larvae migrate out of the intestines and into organs such as the lungs and liver. Some larvae move from the lungs up the trachea, or windpipe, and into the oral cavity. Here they are swallowed again, reaching adulthood in the small intestines.
The migrating larvae damage the lungs, liver, and small intestines. The lungs and liver may suffer significant damage resulting in respiratory signs such as coughing, nasal discharge, and, in severely affected horses, pneumonia. Other symptoms include depression, loss of appetite, and debilitation.
Adult worms are large, white, and as long as 20 to 30 centimeters. Large numbers of adult worms may clog the digestive tract and block the passage of digesta, decrease normal gut movements, and may cause the intestines to rupture their contents into the abdomen. Consequently, roundworm infection in horses is a serious condition that may cause sudden death in the case of intestinal rupture or obstruction.
Because their immune systems are not developed enough to kill the worms, foals and young horses are most susceptible to infection with Parascaris equorum. As horses mature, their immune systems develop a resistance to the parasites. However, adult horses usually retain enough worms to continually shed eggs into pastures, thus infecting new foals each year. After about three months of age, foals also begin to pass eggs into the environment, further contaminating the soil.
