-Endoparasitic flatworms

           

            -Without a gut (no mouth, no digestive tract)

           

            -Acellular - syncytial tegument with microtriches

            

            -Suckers and/or other modifications at anterior end

 

-Sexually mature adult tapeworms live in intestine or its diverticula in all classes of vertebrates.

a)     All adults (except for one species) are parasites in the gut and 

Lacking a digestive tract, derive all nutrient molecules from the host across the tegument.

 

Intestines have a very low oxygen tension creating a necessity for anaerobic metabolism.

 

Glucose very important nutrient.  Stored in worm as Glycogen.

ATP produced during glycolysis

Order Pseudophyllidea

 

"broad fish tapeworm"

 

Parasitizes several larger mammals, including humans 

(definitive hosts = fish eating carnivores)

 

Scandinavia, USSR, South America, North America--infects up to 9 million people world-wide

 

Adult worms may attain a length of 10 meters, may be 10-22 mm wide, and may consist of more than 3,000 proglottids. 

 

LARGEST TAPEWORM OF HUMANS

 

Worms may shed up to a million eggs per day

 

Largest tapeworm, ever, isolated from a bear on autopsy= 12 M

 

Length is largely due to anapolysis-retention of terminal proglottids

 ~80% of proglottids are either mature or nearly mature

Adult worm

[attached to mucosal lining of the small intestine by both bothria

 

Ovoid, operculated eggs are released from uterine pore of proglottids

 

Eggs need 8-12 days for embryo to complete development

 

Coracidium hatches

 

Coracidium ingested by copepod

 

Hexacanth larva bores through intestinal wall into hemocoel

 

Metamorphosis into an elongated, globular procercoid

 

Infected copepod is eaten by a plankton-feeding freshwater fish

 

Procercoid penetrates intestinal wall and migrates to the body muscles

 

Transformation into plerocercoid larva

[pseudo-segmented with an adult scolex]

Infection in the definitive host results from ingestion of plerocercoids in undercooked or raw fish

 

Plerocercoid attaches to mucosa and grows at a rate of 30 proglottids per day.

-rarely more than a single worm in human host

-usually no symptoms

-sometimes: abdominal pain, weight loss, weakness

-also potential problem with pernicious anemia since worm robs host of vitamin B12

 

Diagnosis based on ID eggs in feces

 

Treatment with Niclosamide (interference with ATP synthesis) or Praziquantel

 

2) When plerocercoids are ingested in the 2nd intermediate hosts: amphibians, reptiles, birds. In China, in some regions it is common to eat raw snake-medicinal. No further develoment to the adult stage. 

 

most cases are in the Orient but Sparganosis has been reported from almost every country in the world. 

Increased awareness of cancer, lumps reveal writhing plerocercoids. 

 

-eggs shed in feces adn accidentally ingested by intermediate host. egg hatches in gut and migrates though gut wall

 

-cysticercoid develops in the hemocoel. scolex in a cavity, invaginated but not inverted. intermediate host usually an invertabrate (insect, annelid, or mollusk)

 

cysticercus- posses an inverted invaginated scolex. bladder is filled with fluid. found in species using a vertebrate intermediate host. 

 

 

Coenurus-budding of a few to many scolices (protoscolicies) from geminative layer--each protoscolex on a single stalk. 

- scolex with acetabula or various morphology

 

-rostellum may or may not be present

 

-if rostellum is present it may or may not be retractable

 

-almost all monoecious 

 

-vitellaria are compact and posterior to the ovary

-largest of the cyclophillideans

-marked segmentation

-holdfast with pronounced suckers

-extruded rostellum

-gravid uterus with median stem and lateral branches

-eggs appear striated

"pork tapeworm"

most highly potentially dangerous tapeworm of humans (ingestion of eggs instead of cysticerci leads to cysticercosis)

 

common to humans in areas where raw or improperly cooked pork is a regular element of the diet. (measley pork)

 

very small scolex (1mm in diameter) armed with two circles of rostellar hooks [non-retractable]

 

stobila can be very long-normally 6-10ft. 

 

apolytic

-humans are the definitive host

-pigs are teh intermediate host

 

group of 5 or 6 gravid proglottids each containing 1000s of eggs exit the host daily. eggs are indistinguishable from T. bovis

 

radial stiations on egg shell

 

Eggs are ingested by pigs  (pigs eat human feces – coprophagous)

 

Hexacanth oncosphere embryo penetrates the intestinal wall and gains access to the circulatory system.  Embryos are carried by blood or lymph to muscles, viscera , and other organs, where they develop into cysticerci.  White, ovoid, fluid-filled cysticerci containing a single, invaginated scolex.  When “measly pork” is consumed by a human, the scolex evaginates and attaches to the jejunal wall.  Parasite develops to maturity.  Humans are the only known natural­ definitive host.

 

Identification of proglottids in the feces. 

 

 

**********

Cysticercosis can occur if the human accidentally ingests the eggs.  The cysticercus then has the ability of developing in almost any tissue of the body.  Severe pathology can develop.  If in the brain, the condition is known as Neurocysticerosis.  The cysticercus is oval, about 0.5cm or wider and usually enclosed in a capsule.  Prevalence of human cysticercosis predictably parallels the incidence of the adult worm.  Quite common in Latin America [particularly in Mexico], Africa, Indonesia, India and China. 

 

Most common means of getting cysticercosis is by direct ingestion of eggs.  Hand to mouth self-infection, Eating foods contaminated by eggs because of unsantitary food preparation, or from consuming food or water contaminated with feces containing eggs.  Only about 25% of patients with cysticercosis harbor the adult worm.  Eggs hatch in small intestine and the escaping oncospheres penetrate the intestinal wall and enter the circulatory system by which they are dispersed throughout the body.

 

Symptoms

Skeletal muscles and brain infected most commonly.

Cysts are well-tolerated in muscles and subcutaneous tissues.  Heavy infections can produce muscle spasms, weakness, and general malaise.

The most serious symptoms occur 5-10 years after infection as a result of dead and dying cysticerci.  The degenerating parasite tissues and associated fluids also elicit a host inflammatory response which can be very severe and even fatal.   Cysts developing in the CNS, sense organs, or heart can exert mechanical pressure and cause severe neurological symptoms.  Violent headaches, convulsions, local paralysis, vomiting, and optic disturbances.

 

Diagnosis

Clinical diagnosis can be made by linking symptoms to a history of residency in an endemic region.  X-ray, CAT scan

 

Treatment

Surgery is recommended for cysticerci in the fluid spaces of body.

Praziquantel + steroids are effective in reducing edema and alleviating some of the symptoms of cerebral cysticercosis. 

"beef tapeworm"

Most common of the large tapeworms of humans

Usually 35-60cm long, specimens reported up to 225 cm

Strobila comprises approximately 2000 proglottids.

Scolex is unarmed, having neither hooks or rostellum

 

Adult resides in the jejunum of the small intestine. 

 

Apolytic -   Gravid proglottids detach single and pass out with feces.

 

Eggs, are ingested by cattle or other ungulate.

 

Liberated oncosphere penetrates the intestinal wall and is carried by the lymphatic or blood circulatory system to intramuscular connective tissue, where it develops into cysticercus.

 

Humans are infected by consuming cysticerci in beef, particularly the muscles of the head and heart.  Adult worms develop to maturity in the intestinal tract.

 

 

Distributed throughout the world

Humans acquire infection by eating undercooked beef

Cattle acquire infection by grazing in fields upon which human feces has been deposited.  Pastures flooded by rivers and creeks contaminated with human excrement are another potential source of infection.

Cattle are known to follow people in the woods to eat their feces.

Abdominal discomfort, diminished appetite, and weight loss.  Symptoms usually only in people debilitated by malnutrition or other illness.

Diarrhea and intestinal obstruction can occur.

T. saginata victims rarely ( if ever) develop cysticercosis.

 

Niclosamide and Praziquantel are drugs of choice.

Definitive hosts – canines

Adult worms in intestine

 

Characterized by developing a coenurus type of bladderworm.  Similar to cysticercus but has many rather than a single protoscolex.  Such coenuri occasionally occur in humans, particularly in the brain, eye, muscles, or subcutaneous connective tissues, where they can grow to be longer than 40mm.  The resulting pathology is similar to cysticercosis.  The adults are parasites  in carnivores (canines), the intermediate host is the jackrabbit for multiceps and rodents for crassiceps. Hunting dogs can frequently be infected.  Accidental infection of humans occurs when the eggs are ingested.

Sheep can become infected – coenuri in brain – known as staggers or gid

 

 

 

Sylvatic echinococcosis  -  in wild animals the cycle may involve wolf/moose wolf/reindeer, dingo/wallaby

 

Humans are seldom involved as accidental intermediate hosts in these cases. 

 

However, ample opportunities for human infection in situations in which domestic herbivores are raised in association with dogs.  Hydatid disease is a very serious problem in sheep-raising areas of Australia, New Zealand, N and S America, Europe, Asia, Africa

 

Dogs are infected when they are fed the offal of butchered animals

 

Herbivores are infected when they eat herbage contaminated with dog dung

 

Humans are infected with hydatids when they accidentally ingest Echinococcus granulosis eggs, usually as a result of petting dogs. 

 

Primitive tribes in Kenya - highest infection rate in world

Eat dog intestine roasted on a stick.  Contents squeezed out and external scorching is form of cooking.

 

Infants are infected in cultures where parents encourage dogs to clean up vomit or feces by licking the face or anal area. 

 

In Lebanon - dog feces are used as an ingredient in tanning solution.  Scats picked off the street are added to vats, any eggs present may infect handler by contamination.

 

E. granulosus is distributed throughout most of US.  Highest concentrations are in the deep south and far Western US. 

Cosmopolitan parasite

            Adult worm found in small intestine of domestic dogs and cats

            Human infection occurs most often in children

Each proglottid contains 2 sets of male and female reproductive structures.

            Genital pore is present on each side of the proglottid

            Possess a scolex with a retractable rostellum - rather pointed

            Armed scolex with several circles of hooks

"dwarf tapeworm" usually less than 1mm in width 10 cm in length

 

Adult worm has scolex with retractable rostellum (armed with hooks)

Proglottids are wider than they are long.

 

Cosmopolitan parasite of humans – and one of the most common tapeworm infections of humans worldwide – especially common in children.

 

Only one host is required in the life cycle – the intermediate host (flour beetle, Tenebrio) is optional.

 

Natural definitive hosts are humans – but also infects domestic mice, rats, and other rodents.

 

Adult worm in small intestine

 

Gravid proglottids released into intestine and released with feces

 

If eggs eaten directly by definitive host, the oncosphere embryo penetetrates into the villi of the intestine and becomes a cysticercoid larva. 

 

The cysticercoid then erupts from the villus and enters lumen of the intestine where it attaches to mucosa, and becomes adult worm

Alternatively –

The flour beetle ingests eggs and cysticercoid stages develop in the beetle.

The beetle is then ingested by the definitive host.

Cysticercoid gives rise to adult worms once in the intestine.

Because of the potential for a direct life cycle – autoinfection can lead to a large worm burden (fecalàoral).  Eggs can be passed to other individuals as well through contaminated hands, etc.

 

Symptoms:  loss of appetite, diarrhea, abdominal discomfort  - but normally asymptomatic unless the worm burden is heavy.

 

Praziquantel very effective

 

 

 

" common rat worm"

 

Only occasionally infects humans

 

Adult worm is much larger than H. nana – and has a scolex that lacks an armed rostellum.

 

H. diminuta eggs are larger than H. nana eggs and lack polar filaments

 

Intermediate host (flour beetle, grain beetle  - Tenebrio and Tribolium)

 

Adult worm in rat intestine

 

Proglottids released in feces

 

Insect ingests eggs in rodent feces

 

Oncosphere embryo penetrates intestinal wall and enters hemocoel of insect

 

Cysticercoid develops

 

Rat eats insect

Cysticercoid becomes adult worm

 

Epidemiology

Humans can become infected by eating cereals, dried fruits, grains – containing the infected beetles.

Children most commonly infected.  Similar symptoms as observed for H. nana infection.

 

This cestode is very commonly studies and used as a model cestode in the parasitology laboratory because of the ease of maintaining the life cycle.