Tapeworms

Adult sheep tape worm (Moniezia expansa)

Tapeworms are obligate parasites, which means that the adult stage cannot survive away from the host. They are zoologically classified in the Phylum Platyhelminthes (literally "flat worms") and Subphylum Cestoda. All tapeworms have indirect life cycles, which means that they utilise more than one host. The final host harbours the adult or sexually mature stage of the worm, whereas the immature stages occur in one or more intermediate hosts.

These immature stages occur in a variety of forms and sizes, but can all broadly be described as "bladder worms" or "cysts". Adult tapeworms are hermaphroditic.Tapeworms of veterinary importance are either adult, in which case they colonise the gut or bile ducts, or immature. Adult tapeworms do relatively little direct damage to the host, in comparison with the immature stages, which can cause major tissue destruction, depending on the tissue they parasitise. The immature forms parasitise skeletal muscle (called "measles" due to their appearance) or other organs such as the central nervous and respiratory systems and the liver. These immature forms are of interest not only for their pathogenic potential to the host, but also from a public health point of view where food animals are concerned, because many of them are potentially zoonotic.

Tapeworms in sheep

Tape worm in gut lumen

The small intestinal tapeworms of major importance in sheep are Moniezia expansa (although Moniezia benedeni can also be found in sheep, this is predominantly a parasite of cattle), Thysaniezia giardi, and Avitellina centripunctata.

Other cestodes, such as Stilesia globipunctata and Thysanosoma actinoides occur less frequently, or only in specific regions. Stilesia hepatica parasitises the bile ducts of sheep and other ruminants. Moniezia expansa adults can be from 1 to 6 m long. 

Sheep dropping with tape worm segments

The adult tapeworm consists of a "head" or scolex and a "body" or strobila and usually dwells in the intestinal tract (some adult tapeworms occur in the common bile duct) of the host. The head contains four suckers which assist in anchoring the parasite to the host. Depending on the species, these suckers may be armed with hooks, which are used for taxonomic classification of the worm. The head does not contain any mouth parts. Adult tapeworms "feed" by absorbing their nutrients through the cuticle from their immediate environment and excrete waste products by the same route; there is neither a gastro-intestinal tract nor a urinary system.

The body is divided into "segments" or proglottids which are visible macroscopically (to the naked eye) in larger species. Each segment is a complete reproductive unit, with male and female genital organs. After self-fertilisation, the eggs develop within the proglottid. Proglottids containing mature eggs continually break off at the distal end of the tapeworm (either singly or in groups) and are excreted intact (as an "egg packet" which later disintegrates) or disintegrate prior to excre-tion, thus liberating the eggs. The egg is eaten by the intermediate host and the bladder worm develops. The intermediate hosts of the gastro-intestinal tapeworms of sheep are free-living mites of the Family Oribatidae (hence 'oribatid mites'). The bladder worm which develops in this case is called a cysticercoid. In the case of Stilesia hepatica and Houttuynia struthionis the intermediate hosts have not yet been identified, but are probably similar to those of the sheep gastrointestinal tapeworms.
The life cycle is completed when another final host ingests the intermediate host containing the cysticercoid. The bladder wall breaks or is digested, the scolex attaches to the gut wall and the worm starts growing. In the case of Moniezia expansa the first eggs are excreted in the faeces between 4 and 6 weeks after infestation.

Pathogenicity and economic importance

This subject has been debated extensively and the debate has not been concluded. The reason is the large number of publications which fail to convincingly demonstrate that tapeworm infestations are harmful to the host. There are no doubt many reasons for their failure, but the most important one is the fact that most trials are done with natural infestations which vary greatly in age and magnitude.

On the other hand, there have been authors who have unambigiously attributed visible detrimental effects to infestation with cestodes in sheep. These include diarrhoea, reduced live mass gain, polydpsia, depression and ill-thrift.

Most authors agree that Moniezia spp. infestations are a problem of young animals up to the age of weaning. Infestations with the other species tend to occur later in life and are not as easily recognisable.

Control

The modern trend in countering parasite infestations is towards integrated control programmes. Integrated control signifies the coordinated utilisation of all possible (chemical and non-chemical) means. Under non-chemical means, one understands concepts like habitat manipulation, limiting contact between host and parasite, etc.

Although there are a few non-chemical control measures which can be used against cestode infestations, these are not practically applicable in all situations. Where sheep are kept on cultivated pasture, the intermediate host habitat can be destroyed through plowing of the field. This will only have a temporary effect however, and a new oribatid mite population will probably be established within about two years, depending on local environmental conditions. Artificial pastures are seldom replanted every two years.

Management systems where the sheep are stabled overnight and let out to graze during the day, offer the possibility of temporal separation of host and parasite. It has been shown that oribatid mites leave the relative shelter of the grass roots and migrate up the blades of grass only during the cooler hours. At the hottest time of the day, when the mites are in hiding, grazing animals are thus less exposed to infestation.
The most trustworthy method of tapeworm control remains the treatment of infested final hosts with an effective cestocide. This not only protects those animals from the harmful effects of the parasite, but also breaks the life cycle in that it prevents further contamination of the pasture with tapeworm eggs.