Sources

Here is a list of the material used to write this blog

1. Roberts, L.S. 2005. Foundations of Parasitology pp 311-363
2. Schojorring, S. et al. 2007. "Incestuous Mate Preference By A Simultaneous Hermaphrodite With Stong Inbreeding Depression" Evolution. 423-429
3. Akira, I. et al. 2003. "Human taeniasis and cycticercosis in Asia". The Lancet. 362: 1918-1920
4. Eckert. J. et al. 2000."Echinococcosis: a emerging or re-emerging zoonsis?" Internations Journal for Parasitology" 30: 1283-1294
5. Lightowlers. M.W. 2000. "Vaccination against Cycticercosis and Hydatid Disease" Parasitology Today. 15(5): 191-195.

Article 3.

Incestuous Mate Preference By a Simultaneous Hermaphrodite With Strong Inbreeding Depression

Schjorring, S. 2007

• Inbreeding depression is considered one of the major selective forces for evolution of mating systems, generally favoring mating among unrelated individuals
• the cost of inbreeding avoidance must be weighed against the potenetial costs of inbreeding avoidance and the potential nongenetic and genetic benefits derived from breeding with related individuals
• when unrelated mating partner are scarce relative to related partners, the cost of inbreeding depression may be lower than the cost of forgoing mation
• the aritcles tests whether the tapeworm Schistocephalus solidus discriminates between related and unrelated mating partners and the fitness of the offspring
• two experiments were done to test this:

1. worms were given choices between related and unrelated parnters and their choices were documented
2. th e eggs and offspring produced from the various matings were exanimed

• the worms maing choices were: a sibling, a unrelated worm
• and selfing (self fertilization)
• it was found that despite the evidence for strong inbreeding depression mating preference was shown for close kin (the sibling)
• evidence for inbreeding depression: eggs produced by unrelated pairs hatched at a rate 3.5 times high then those produced by siblings, eggs produced by siblings hatched at a rate of 2.3 times higher then those resulting from selfing
• 3 possible resons for the benefit of breeding kin

1. indirect fitness benefits of in breeding: increase in indirect fitness is large enough to outweigh the decrease in direct fitness due to inbreeding depression
2. direct fitness benefits of inbreeding: direct benefits associated with incestuous breeding outweigh the cost of inbreeding
3. lower extinction risk of inbreeding populations: populations in which individuals regularly inbreed are less likely to go extinct that populations in which inbreeding is avoided

Article 2.

Vaccination against Cysticercosis and Hyatid Diseases

Lightowlers, M.W. 2000.

• a study of the tapeworms Taenia solium, Taenia ovis, and Taenia saginata
• all are capable of infecting humans
• T. solium infects pigs
• dogs are the definitive host of T. ovis and humans are for T. saginata.
• article examined the need for a vaccines
• vaccines against cysticercosis already in existance
• these vaccines are for the tapeworms T. sagunata in cattle and T. ovis in sheep
• it is possible these vaccines would be just as effective for vaccination pigs against infections of T. solium due to homologues protein coding
• vaccination against hydatid dissease is done in sheep
• vaccines were created using recombinant DNA technology
• genes encoding different antigens were cloned
• the results were used to produce the vaccines
• it was found that the combination of tow recombinant protein into a single vaccine gave a highly effective result
• the benefits of livestock vaccination to human morbidity and mortality are indirect
• a more direct approach would be to use protective vaccines against cysticercosis and hydatid disease in humans
• discussions have started to begin human trials
  

Cysticercosis and Crowding Effect

Cysticercosis

In some tapeworm speices, the oncosphere penetrates the gut of the intermediate host, and upon reaching the parenteral site (in any part of the body) metamorphoses into a cysticercoid or a cysticercus type of metacestode. A cysticercoid is a solid-bodied organism with a fully developed scolex.

Virtually any organ in the body can be infected with cysticerci. The most common location is the eye followed by the brain and then muscles, heart, liver, lungs and coelem. To the right it an x-ray of a infected thigh muscle.

In whatever tissue they invade, a fibrous capsule of host origin surrounds the metacestode (the exceptiong is when the cysticercoid develops in the eye). The effect of the cysticercoid depends on its location. Ocular cysticercosis can cause damage to the retina, iris or choroid. It also runs the risk of being mistaken for a malignant tumor, leading to the unnecessary surgical removal of the eye. Cysticerci in the brain can cause severe central nervous system malfunction, blindness, paralysis, disequilibrium, disorientation or epilepsy. Cysticerci in the skeletal muscle, skin or liver usually go unnoticed, except in the case of massive infection. Taenia solium is the most common tapeworm that forms cysticercoids in its intermediate hosts.

Crowsing Effect

An important condition which can affect tapeworm growth in the intestines is "crowding effect". Crowding effect proposes that within limits, the weight of the individual worms in the intestines of a individual is inversely proportional to the number of worms present. So the total worm biomass and the number of eggs produced are the same and are maximal for the host no matter the actually number of worms present.

It has been found that tapeworms in the presents of others with secrete "crowding factors" that influence the development of the other tapeworms in the population

Development

Tapeworm development can be generalized into four basic steps:

1. embryogenesis with the egg to result in a larva called the oncoshpere - the oncospheres of some species can be free-swimming in water
2. hatching of the oncosphere after or before being eaten by the next host, where it penetrates to a parenteral site
3. metamorphosis of the larve in the parenteral site into a juvenile called a metacestode - usually has a scolex
4. development of an adult from the metacestode in the intestine (called the enteral site) or either the same host or a new host

Article 1.

Echinococcosis: in emerging or re-emerging zoonosis

Eckert, J. et al. 2000

• a study of Echinococcus multilocularis the causative agent of human AE
• human AS is caused by the metacestode stage, it is a severe, progressive disease that can lead to death in as little as 10-15 years
• treatments are surgery and chemotherapy which can prolong the patients life, but a cure is only achieved if the metacestodes are completely eliminated
• has a wide distribution in central Europe
• sources of infection involve the red fox
• humans are infected with the parasite primarily through infected fox feces
• 3 reasons are given in the article for the apparent rise in the number of human infections

1. increasing fox populations and parasite prevalences - possibly due to reduced fox mortality after anti-rabies vaccination
2. invasion of villages and cities by foxes
3. the role of domestic dogs and cats in disease transmission to humans

- dogs and cats can be definitive hosts of the parasite by ingesting rodents harbouring the metacestode stages
- however there is little information on the actual role this might play in transmission to humans

- dogs are especially highly susceptible, whereas the susceptiblity of cats appears to be reduced

• also examined the factors associated with the spread of E. multilocularis, transmission dynamics, distribution, the incidence of human AE, awareness of the disease and whether it is ermerging or re-emerging
• Is E. multilocularis spreading? - unclear
• parasites may be spreading or it might just be a case of detection in regions it was always present in, but never discovered (could be due to greater awareness of the parasite)

PITT

Tapeworms are one of the many parasitic organisms capable of inducing the phenomenon called PITT (parasitic induced torphic transmission). This is a change in the behaviour, physiology or morphology of the host due to the infection by the parasite. The result is to facilitate transmission of the parastic to the next host, usually in order to complete its lifecycle.

The tapeworm is capable of manipulation the host for it's own purposes without the host's knowledge - definitely not a passive, dependent, helpless worm!

Examples of this phenomenon can be seen in copepods infected with Triaenophorus crassus. Infection with the tapeworm causes the copepod to swim near the surface of the water, where it is much more likely to be eaten by fish. This is benefical to the tapeworm as fish are its second intermediate host.

One of the most popular parasitics that cause PITT is Toxoplasma gondii:
http://en.wikipedia.org/wiki/Gondii

Tapeworm Life Cycles

From top to bottom:

The life cycle of a beef tapeworm (Taenia saginata)

The life cycle of a pork tapeworm (Taenia solium)
The life cycle of a broad fish tapeworm (Diphyllobothrium latum)
The life cycle of a cucumber tapewrom (Dipylidium caninum)

Nervous System

Nervous System

• the nerve center of tapeworms is located in the scolex
• the complexity is depended on the complexity of the scolex - bothriate cestodes are the simplest, with only a lateral pair of cerebral ganglia united by a single ring and a transverse commissure
• worms with bothridia or acetabula are much more complex and hence have a more complex system of commissures and connectives in the scolex, with five pairs of longitudinal nerves running posteriorly from the cerebral ganglia through the stroblia

Morphology con't and Nutrition

The Tegument and Nutrition

• tapeworms absorb all their requited nutrients from the host, through the tegument
• don't have any digestive system at all - completely lost
• free-living ancestors did have a digestive system - but it is not needed in parasitic life-style
• the tegument is a living tissue with high metabolic activity
• the outer lining of the tapeworm has numerous finger-shaped projections called microtriches - these function to increase the absorptive surface area of the worm, much the same as microvilli do in human intestines
• tapeworms require a lot of carbohydrates - but can only absorb glucose and galactose
• also absorbed: amino acids, by active transport across the tegument, purines and pyrimidines by facilitated diffusion, lipids (method unknown) and vitamins (method unknown)

The tapeworm Diphyllobothrium latum can absorb so much vitamin B12 that it can cause pernicious anemia in its host.

This was featured in an episode of the television show House: http://en.wikipedia.org/wiki/Insensitive_%28House_episode%29

Morphology con't: The Scolex

The Scolex

• located at the anterior most end, the scolex is equipped with a varity of holdfast organs the purpose of which is to ensure the worm maintains its position in the gut of the host
• it can be equipped with suckers, grooves, spines, glands, tentacles or any combination of these
• suckers occur in three (very) general types

1. Acetabula: cup shaped with a heavy muscular wall, there are normally four present on one tapeworm. Hooks can be arranged in circles anterior to the suckers. These are located on a protrusible, dome-shaped area on the apex of the scolex called a rostellum.
2. Bothridia: also usually occur in groups of four. The have mobile, leaf-like margins, project sharply from the scolex and are quite muscular.
3. Bothridia: two to six can occur in a tapeworm. They are shallow pits or grooves arranged in lateral or dorsoventral pairs
   

Morphology

There are three main features to a tapeworms external morphology: the strobila (proglottids) the scolex and the tegument. This post will deal with the strobila.

Strobila/Proglottids

• unique among the metazoa


• a linear series of sets of reproductive organs of both sexs - each is referred to as a genitalium


• the area around the genitalium is a proglottid


• tapeworms with mulitple proglottids are describes as being polyzoic
  
• new proglottids are continuously differentiated near the anterior end in a process called strobilation - as each segment moves from the anterior end to the posterior end a new one takes it place
• the new proglottids are produced in an undifferentiated zone between the scolex and the strobila, called the neck - this area contains stem cells which give rise to the new proglottids


• while the segments travel down the length of the worm they sexually mature, by the time they reach the end of the worm the genitalia have already copulated and produced eggs
  
• a proglottid can mate with itself, with others in the same worm or with another worm entirely (it depends on the species of tapeworm)


• a segment containing eggs is called a gravid


• when the gravid reaches the end of the worm, it detaches and passes out of the host in the host feces
  
• from here the eggs can be ingested by a new host, leading to tapeworm infection

Tapeworm Classification!

Tapeworm is the common name given to the Class Cestoidea. This class can be placed in the phylum Platyhelminthes (the flatworms). Below the class level there are numerous orders, families and species. Here is a more in-depth look at tapeworm classification:

Supra-phylum: Platyzoa

Phylum: Platyhelminthes

Class: Cestoidea

Order: Caryophyllidea

Families: Caryophyllaeidae, Balanotaeniidae, Lytocestidae, Capingentidae

Order: Spathebothriidea

Families: Cyathocephalidae, Spathebothriidae, Bothrimonidae

Order: Pseudophyllidea

Families: Amphicotylidae, Bothriocephalidae, Cephalochlamydidae, Diphyllobothriidae, Echinophallidae, Haplobothriidae, Parabothriocephlidae, Ptychobothriidae, Triaenophoridae

Order: Nippotaeniidea

Family: Nippotaeniidae

Order: Lecanicephalidea

Families: Adelobothriidae, Disculicepitidae, Lecanicephalidae

Order: Tryanorhyancha

Families: Dasyrhynchidae, Eutetrarhynchidae, Gilquiniidae, Gymnorhynchidae, Hepatoxylidae, Hornelliellidae, Lacistorhynchidae, Mustelicolidae, Otobothriidae, Paranybeliniidae, Pterobothriidae, Sphriocephalidae, Tentaculariidae, Mixodigmatidae, Rhinoptericolidae

Order Aporidea

Family: Nematoparataeniidae

Order: Tetraphyllidea

Families: Onchobothriidae, Phyllobothriidae, Triloculariidae, Dioecotaeniidae

Order: Diphyllidea

Families: Ditrachybothridiidae, Echinobothriidae

Order: Litobothridea

Family: Litobothridae

Order: Proteocephalata

Families: Proteocephalidae, Monticellidae

Order: Cyclophyllidea

Families: Amabiliidae, Anoplocephalidae, Catenotaeniidae, Davaineidae, Dilepididae, Dioecocestidae, Diploposthidae, Hymenolepididae, Mesocestoididae, Nematotaeniidae, Progynotaeniidae, Taeniidae, Tetrabothriidae, Triplotaeniidae

You can find out lots more about tapeworms at: http://en.wikipedia.org/wiki/Tapeworms