Roundworms of the Digestive Tract

KENNETH C. KATES. REX W. ALLEN, AND JAMES H. TURNER.

ROUNDWORMS, or nematodes, in the digestive tract may seriously affect the health of sheep and goats. It is not uncommon for sheep and goats to die of the effects of roundworms, but even greater losses come about through reduced weight gains, less meat and fiber production, lower vitality of breeding animals, and a considerable rise in cost of production.

More than twenty-four species of roundworms occur in the digestive tract of sheep and goats in the United States. The worms are tiny, cylindrical, threadlike parasites. Some are hard to see with the unaided eye.

THE COMMON and scientific names of roundworms that occur more or less frequently in sheep and goats in this country are given in the list that follows. (When more than one species is listed under one common name, the first species named is usually the most important one. Practically all species listed occur in sheep and goats, but some are commoner in sheep than in goats, and vice versa. Some species normally are parasites of cattle but have been reported also in sheep and goats. In fact, about half of these species have been found in cattle and several kinds of wild ruminants.)

IN ESOPHAGUS AND RUMEN gullet worms: Gongylonema species.

IN FOURTH STOMACH (abomasum) large stomach worm: Haemonchus contortus; medium stomach worms: Ostertagia circumcincta, O. trifurcata, O. occidentalis, Marshallagia marshalli, Pseudostertagia bullosa; stomach hairworm: Trichostrongylus axei.

IN SMALL INTESTINE intestinal hair-worms: Trichostrongylus colubriformis, T. vitrinus, T. capricola; hookworm: Bunostomum trigonocephalum; thread-necked worms: Nematodirus spathiger, N. filicollis, N. abnormalis; threadworm: Strongyloides papillosus; cooperias: Cooperia curticei, C. punctata, C. oncophora, C. pectinata; capillarids: Capillaria brevipes, C. bovis.

IN LARGE INTESTINE and blind gut, or cecum nodular worms: Oesophagostomum columbianum O. venulosum; whip-worms: Trichuris ovis, T. globulosa, T. discolor; large-mouthed bowel worm: Chabertia ovine; pinworm: Skrjabinema ovis.

Parasitic gastroenteritis, a term that means irritation or inflammation of the stomach and intestines caused by parasites, usually roundworms, often is applied to a disease caused by several kinds of roundworms. It is not always possible to separate the effects of one kind of parasite from the effects caused by others.

The term "parasitic gastritis" is used when only the stomach is involved. "Parasitic enteritis" is used when only the intestines are involved. Sometimes more specific terms derived from the common or the scientific names of the parasites are applied when only one kind of roundworm is concerned in disease. For example, "haemonchosis" refers to infections with the large stomach worm, "ostertagiasis" to medium stomach worms, "trichostrongylosis" to hairworms, "hookworm disease" or "bunostomiasis" to the hookworm, "strongyloidiasis" to the thread-worm, and so on.

The injurious effects of roundworms on sheep and goats are many and varied according to the species, numbers of parasites present, the age and nutritional status of the animals, and other factors.

Some kinds of roundworms are well tolerated by their hosts or seldom occur in numbers large enough to produce perceptible effects.

Some of the clinical symptoms of roundworm infections are diarrhea (scours), anemia (reduced number of red blood cells and quantity of their contained red pigment), edema (swelling caused by excess fluid in the tissues), emaciation (unthriftiness from failure to gain or from loss of weight), loss of appetite, and physical weakness. The acute effects of roundworms, or less severe chronic effects of long duration, may cause death of the host.

Both immature and adult roundworms may injure the tissues of the digestive tract. For example, larvae of the nodular worm stimulate the formation of pea-sized nodules in the wall of the small and large intestines. Medium stomach worms and cooperias cause the formation of very small nodules in the wall of the stomach and small intestine, respectively. The large stomach worms, the hookworms, and large-mouthed bowel worms cut or pierce the inner lining of the part of the digestive tract in which they live with special mouth structures, at the same time ingesting blood, lymph, and cells from the gut wall.

Additional blood is lost from such small cuts or abrasions by seepage into the cavity of the digestive tract, from which it is eliminated in the droppings. When the worms make many small perforations of the gut lining, the total daily loss of blood may be greater than the animal can replace, and anemia ensues.

Hairworms, thread-necked worms, and threadworms do not usually inflict discrete, visible injuries to the tissues of the gut or cause anemia but cause diffuse damage to the gut lining, often to the extent that it may be partly destroyed.

Affected animals refuse food. Fluid may be lost from the body from a diarrhea and excessive urination. Normal digestion and absorption of nutrients are interrupted.

THE LIFE HISTORIES of these roundworms are called direct, as transmission from host to host occurs without intervention of an intermediate host, such as an insect, mite, or snail. The gullet worms are exceptions; their larvae develop in dung beetles.

Adult worms of both sexes live in the digestive tract. (The threadworm is an exception; only its females are known.)

Females deposit eggs in the cavity of the gut after fertilization by the males. The eggs are very small, usually contain one to many cells or a small larva, and are eliminated from the host in the droppings. On pasture under favorable conditions, the free-living development takes place. Small, active larvae, infective to sheep and goats, develop in a few days from the eggs. When a film of moisture is present on vegetation, they move onto the plants.

Infective larvae, after being swallowed by the host, develop to maturity in the digestive tract in 1 to 5 weeks, depending on the species. The young worms penetrate into the lining of the gut of the host, or are lodged between the small, fingerlike projections (villi), and usually cause damage to the tissues. As the worms mature, they usually, but not always, move to the cavity of the gut and spend their adult lives there.

Some variations of this typical life history occur in some species of roundworms. Knowing what they are helps one to understand the damage done and the symptoms.

Infective larvae of the hookworm and threadworm, which likewise can produce infection after being swallowed, also can enter the animal's body through the skin.

Many larvae penetrate the small blood vessels of the skin and are carried by the blood to the lungs. In the lungs they migrate from the small blood vessels through the lung tissue to the small air sacs. Infective larvae of these roundworms when swallowed may also penetrate the gut tissues, much as they penetrate the skin, and may be carried to the lungs by the blood. When the larvae reach the air passages in the lungs, they migrate up the windpipe or are coughed up and swallowed. Development to maturity takes place when they again reach the small intestine.

An interesting variation from the typical life history occurs in the thread-worm, besides the fact that only females are known in the parasitic phase. Some of the small larvae, which are in the eggs when deposited in manure on pasture, may develop directly into infective larvae. Others, however, may develop into adult, free-living males and females that differ in structure from the parasitic females. These in turn give rise to a much larger number of infective larvae than would develop directly from eggs deposited on the pasture.

In the following more detailed account of these parasites, emphasis is placed on the roundworms that cause the greatest losses to farmers. No further mention will be made of the gullet worms, which seldom occur in large numbers in sheep and goats. Therefore, the following discussion concerns the roundworms of the abomasum (hereafter referred to as "stomach"), the small intestine, and large intestine, including the blind gut, or cecum.

IN THE TRUE STOMACH of sheep and goats are three important kinds of roundworms, the large stomach worm, several species of medium stomach worms, and one species of hairworm. These are the common roundworms found in the stomach of these hosts, but occasionally some of the species that normally live in the small intestine are found in the stomach, and vice versa.

When small numbers of stomach worms are recovered from the intestines after death, it is usually thought these worms are being eliminated from the host. The stomach not only performs important digestive functions; it also insures the regular passage of food materials into the small intestine in the proper form and quantity for further digestion. Therefore, when the stomach cannot function normally because of the injurious effects of roundworms in it, the digestive processes, appetites, and general health of the parasitized animals are seriously impaired.

The large stomach worm is the main roundworm of sheep and goats in most of the United States. Its life history is direct. Heavy infections and clinical disease, called haemonchosis, are more common in localities with long summers and moderate to heavy rainfall or where pastures are irrigated than in cold, and localities. The reasons are that infective larvae develop best during warm, wet weather, and the free-living stages are killed by relatively short exposure to drying and subfreezing temperatures. When arid land in warm localities is irrigated and made into productive pasture, it becomes ideal for the development of the larvae of large stomach worms and other roundworms. Heavy infections are rapidly acquired by stock under such conditions, and losses are serious. This roundworm is also common in cattle and wild ruminants throughout the country, and its injurious effects on these animals are much the same as for sheep and goats. This parasite is such a common cause of losses that sheep-men and goat raisers routinely take measures to prevent haemonchosis from developing in their flocks and herds.