Tapeworms and Bladderworms

DALE A. PORTER AND KENNETH C. KATES.

DOMESTIC ruminants in the United States may harbor three species of adult tapeworms and five species of bladderworms, or larval tapeworms.

These larval tapeworms develop to maturity in certain nonruminant hosts, such as dogs and related animals and man.

Adult tapeworms of ruminants are flat, ribbonlike, segmented parasites up to several yards long and three-fourths inch wide. They are attached to the lining of the small intestine by a small head, or scolex, which has four suckers without hooks. The segmented body, or strobila, consists of hundreds of segments, or proglottids.

Two species, Moniezia expansa and M. benedeni, commonly occur in the small intestines of domestic ruminants and some of the wild ruminants in the United States. Both parasitize cattle, sheep, and goats, but M. benedeni is the species usually found in cattle and M. expansa is the one usually found in sheep.

Of all the parasites of ruminants, these tapeworms are the best known to farmers because of their large size and because ripe segments can be seen easily in fresh manure. Their mode of transmission was unknown until 1937. Dr. H. W. Stunkard, of New York University, then discovered that small mites, known as oribatid or beetle mites, were the intermediate hosts of M. expansa. Parasitologists in the Soviet Union have since determined that M. benedeni also is transmitted by oribatid mites.

When microscopic tapeworm eggs, or the segments containing eggs, are voided on pasture with the droppings of infected ruminants, each egg already contains an oncosphere, or small tapeworm larva. Oribatid mites eat the tapeworm eggs with their food. Inside the mite the oncosphere penetrates to the body cavity and develops into the next larval stage, known as a cysticercoid, in about 2 months. Large numbers of oribatid mites live in the humus layer of soil and migrate onto forage plants, especially in the early morning, when the grass is moist with dew.

Ruminants swallow infected mites while grazing on contaminated pastures. In their digestive tracts the cysticercoids escape from the mites.

The young head of the tapeworm attaches itself to the lining of the small intestine, and from it grows the long strobila. These tapeworms become mature in about 35 days. Thereafter segments and eggs are discharged with the droppings.

The effect of these tapeworms on ruminants is still unsettled. Because of the simplicity of making a diagnosis and the large size of these worms, they have been often held responsible for outbreaks of disease and losses of stock when other parasites, or other disease-producing agents, may have been the causes. Diarrhea, emaciation, convulsions, and death have been ascribed to tapeworm infections in calves, lambs, and kids and to adult animals as well. Convincing proof that these parasites were primarily responsible usually has been lacking.

Investigations with tapeworm infections uncomplicated by other agents of disease have been carried out only with M. expansa in lambs. The results showed that this parasite produced at most only minor effects of doubtful significance to the health of the animals involved.

Control of these two tapeworms can be achieved through medication to remove them from their hosts. Spray grade of lead arsenate, administered in doses of one-half gram for lambs and kids, 1 gram for adult sheep and goats, and up to 2 grams for calves and cattle, is safe and effective treatment for removing the worms.

Infected mites may live a year or longer on pasture, and there is no practical method of destroying them over a large area.

ANOTHER TAPEWORM, Thysanosoma actinioides, commonly known as the fringed tapeworm because of a fringed rear border of its segments, occurs primarily in range sheep in the Western States. It occurs infrequently in cattle and goats and is not uncommon in various wild ruminants, such as deer, elk, moose, and antelope. It is shorter than the moniezias. It is often found in the main bile ducts of the liver as well as in the small intestine.

A high percentage of western range lambs at slaughter have fringed tapeworms in their livers. It has been estimated that more than 2 million sheep livers, worth more than half a million dollars, are condemned annually by meat inspectors because of this parasite.

Its life history is unknown. An intermediate host is probably involved. Many attempts at direct transmission have been unsuccessful. Perhaps small insects or other small invertebrate animals act as intermediate hosts.

The economic importance of the fringed tapeworm is due primarily to its effect on the bile ducts of the liver, which become enlarged and inflamed and make the liver unfit for human food. Sheep in most instances suffer only slight ill effects from the fringed tapeworm.

Efforts to control the fringed tapeworm must be largely limited to development of treatments for their removal from sheep until the life history of the parasite is known. Several chemicals have been tried, but none has proved to be entirely satisfactory.

THE BLADDERWORMS that parasitize cattle, sheep, and goats are Cysticercus bovis, which occurs in the adult stage (Taenia saginata) in man, Echinococcus granulosus, Coenurus cerebralis, Cysticercus tenuicollis, and Cysticercus ovis, which occur as adults in dogs and related canines. Ruminants serve therefore as their intermediate hosts.

The bladderworms are larval tapeworms with a small scolex, or head, inverted into a small, membranous, bladderlike sac, or cyst, which contains fluid. The scolex of the larva is similar to (but smaller than) that of the adult.

Sometimes, as with the gid bladder-worm (Coenurus) and also the hydatid worm (Echinococcus), many immature tapeworm heads, or miniature bladderworms, develop within a cyst derived from a single tapeworm egg.

The life histories are much the same. Dogs harbor the adult tapeworms after eating the cysts in dead animals left unburied on the range or in offal from cattle or sheep slaughtered on the farm or at unsupervised slaughterhouses, where there is improper disposal of viscera and diseased parts of carcasses. Infection of man occurs through eating the bladderworm in raw or incompletely cooked beef.

After the proper host swallows the bladderworm, the tapeworm head or heads, as the case may be, push out from the bladder, attach to the intestinal lining, and develop to maturity. Adult tapeworms produce large numbers of eggs, which leave the host in the feces.