Common Internal Parasites of Swine

Robert M. Corwin
Department of Veterinary Microbiology
College of Veterinary Medicine

Roderick C. Tubbs
Swine veterinarian
Commercial Agriculture Program

Swine performance is influenced by internal parasites, most dramatically in the young, growing pig. Parasites reside in the stomach and intestinal tract of the pig, causing irritation, impaction, indigestion and lack of appetite. Parasites cause loss of nutrients from feed consumed by competing with the host, and by causing poor digestion, gut ulceration and even blood loss. Even small numbers of the large roundworm Ascaris suum can depress feed intake and daily gain and cause a depression in gain:feed ratio. Some worm parasites may be found in the lungs, where they interfere with respiration. This in turn may result in pneumonia.

Even though internal parasites are commonly associated with pasture and dry lot husbandry, some parasites such as Isospora (neonatal infection) may be present in total confinement. Ascarid infections frequently are found in finishing barns with concrete floors. The reason for continuing parasitism in these units is that transmission is by infective eggs and oocysts, which are difficult to keep out of any environment. Therefore, appropriate deworming schedules and sanitation are mandatory for parasite prevention programs.

There are only a few target parasites of concern, while several dewormers can be used effectively. The dewormers are usually broad spectrum in that most if not all of the worm parasites are removed, and they are safe and easily administered.

Parasites: Large roundworms

Most hogs have Ascaris infections during their lifetimes. These roundworms are usually found in greatest numbers in pigs up to 2 to 3 months of age with a few in older pigs. Sows usually are not clinically affected, but serve as carriers. Roundworms are long (6 to 12 inches), stout, pinkish worms, sometimes with curved tails. The adults live in the small intestine, grazing on the gut lining and ingesting particulate and liquid materials from digesting food.

The adult females deposit round, microscopic eggs. Each female lays thousands per day beginning about two months after the pig becomes infected. Eggs may survive for 10 or more years and are quite resistant to cold and disinfectants. They can be destroyed by high-pressure steam heat and sunlight. Because they are sticky, eggs are easily transported by cockroaches, beetles, flies, birds and workers’ boots and clothing.

Eggs become infective after being outside the pig for one month. When another pig swallows them, they hatch in the stomach or small intestine. The tiny larva that emerges penetrates the gut wall and is carried to the liver through the bloodstream. In the liver, larvae migrate for one-half to one week and then are swept through the bloodstream to the lungs. From there, the larvae are coughed up, swallowed and returned to the small intestine, where they grow and mature within two months. Thus pigs may be 1-1/2 to 2 months of age before eggs can be detected in fecal samples, but immature adult worms may be passed earlier. The significance of this is that clinical signs may occur before eggs are detectable in feces.

Several clinical events may occur in the infected pig. They include:
•Inflammation of the liver due to an allergic reaction to ascarid larva migration.
•”Milk spots” on the liver that usually disappear with time.
•Another allergic reaction occurs in the lungs as larvae move through the air spaces.
•The lung tissue becomes thick and wet, leading to inefficient respiration and “thumps.” This process is made worse by dust, ammonia and bacteria.
•Colic or gut pain may result from worms in the small intestine grazing or nipping forcefully on the gut lining and stretching the gut wall as the worms grow.
•An impaction and even tearing of the gut may occur.
•Often most obvious to the producer is competition of the pig and its roundworm burden for nutrients, so that wormy pigs are set back and appear unthrifty.
•Otherwise healthy pigs with a low worm burden may appear normal, but performance as judged by feed conversion may be depressed.


Refer to Common Internal Parasites of Swine for More information