PHYLUM PLATYHELMINTHES

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PHYLUM PLATYHELMINTHES

• Major characteristics
• Dorsoventrally flattened
• Acoelomates
• Digestive tract greatly reduced or missing
• Excretory system consists of flame cells or
  protonephridia

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CLASSIFICATION

• Great deal of work being done in Platyhelminthes
  systematics.
• One of the most current classification schemes is
  given on page 192-193 of text.
• We will use an older, much simpler classification.

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CLASSIFICATION

• Class Turbellaria - mostly free-living flatworms
  (commensals)
• Class Monogenea - parasitic on skin and gills of
  fish and amphibians. (ectoparasites)
• Class Trematoda - all parasitic, called flukes
• Class Cestoidea (formerly Cestoda) all parasitic,
  the tapeworms. Adult tapeworms found in
  intestine of definitive host.

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Class Monogenea (chapter 19)

• Most are hermaphroditic ectoparasites of fish,
  amphibians, turtles
• Some are found in urinary bladder, lower
  intestine of frogs and turtles.
• Have a direct life cycle
• In nature they cause little economic damage but
  can be very destructive in fish farming.

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Monogenea

• Most distinguishing feature is the presence of a
  large attachment organ called an opistohaptor
  (see fig 19.12).
• Have a single host.

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Some Interesting Examples

• Dactylogyrus sp - cause large kill-off in fish in
  commercial fish ponds.
• Gyrodactylus sp. can cause large fish-kills in
  fishponds. These organisms are viviparous and 4
  generations may be seen within an adult worm.
  (see fig. 19-12).
• Diplozoon paradoxum - two larval forms fuse
  together (fig. 19-13)

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Digenetic Trematodes

• Require two or more hosts
• Some very important parasites of man
• Most are hermaphroditic, Schistosomes are
  exception being dioecious.
• Some can reproduce parthenogenetically

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Digenetic Trematode Structure
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Trematode Reproduction

• Male reproductive system
• Usually contains two testes (number may vary)
• Vas efferent leaves each testis and forms the
  vas deferens.
• The vas deferens gives rise to a seminal vesicle
  which stores sperm
• the muscular cirrus pouch stores the male
  copulatory organ called the cirrus.

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Trematode Reproduction

• Usually a single rounded ovary but in some
  species may be lobated or branched.
• A short oviduct leads from the ovary and has a
  sphincter called the ovicapt which regulates the
  passage of ova.
• Vitellaria or vitelline glands are important
  because they contribute yolk to the eggs.
• Where the oviduct and the vitelline duct join
  there is a expansion which forms the ootype.
• Mehlis glands surround the ootype and in
  combination with the ootype and vitelline glands
  forms the oogenotop or egg forming apparatus.
• Beyond this, the duct expands to form the uterus
  and on to the genital pore.

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Fasciola hepatica Trematode Life Cycle
Representative

• Ova or egg
• Miracidium
• Sporocyst
• (Daughter sporocyst, or redia)
• Cercaria
• Metacercaria
• Adult

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Stages of Trematode Life Cycle

• Ova or egg - maybe a better term is shelled
  embryo.
• Contains miracidium inside shell
• Under appropriate conditions, the operculum (cap
  on shell) opens to allow miracidium to escape.
• Many of flukes have very distinctive eggs.

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Stages of Trematode Life Cycle

• Miracidium
• Ciliated organism that can be mistaken for a
  ciliated protozoan.
• In species that hatch in water, it contains
  penetration glands that release histolytic or
  proteolytic enzymes to help penetrate snail
• Some species do not hatch until eaten by snail
  host. In these the miracidium is not as
  prominent.

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Stages of Trematode Life Cycle

• Sporocyst
• The miracidium develops into sporocyst often in
  the digestive gland of the snail.
• The sporocyst is an embryonic bag or germinal sac
  that has asexual reproduction occurring.
• The sporocyst will produce many daughter stages
  called rediae or in some cases daughter
  sporocysts. The term daughter has nothing to do
  with gender.

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Stages of Trematode Life Cycle

• Rediae or Daughter Sporocyst
• In function they are very similar to sporocysts.
• Contain digestive tract and are more active
• Asexually reproduce to yield many cercariae.
• Some species they can live for many years.

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Stages of Trematode Life Cycle

• Cercariae
• Usually escape snail and often swim by some means
  of tail structure. (see fig 15.22)
• Responsible for transmission from snail to the
  next host.
• Differences in this mechanism will be discussed
  later

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Stages of Trematode Life Cycle

• Metacercaria
• Resistant stage that is formed in many species
• Cercaria that have this stage contain cystogenic
  glands that helps the organism encyst on
  vegetation.
• Cercaria that form metacercaria in second
  intermediate hosts, often have penetration glands
  that enable them to penetrate the second
  intermediate host.

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Stages of Trematode Life Cycle

• Adult
• Always found in the definitive host
• Responsible for sexual reproduction
• Often restricted to specific region of host.
  Often very host specific.

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Stages of Trematode Life Cycle

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Digenetic Trematodes Order Strigeiformes

• Superfamily Strigeoidea
• Strange looking (see fig 16-1, page 234) Anterior
  end large spoon-shaped oral sucker with
  pseudosuckers on the sides
• Alaria americana -
• may require 4 hosts snail, tadpole or frog,,
  water snake, and frog or snake eating vertebrate.
  
• Man can become infected eating infected frogs

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Digenetic Trematodes Order Strigeiformes

• Superfamily Schistosomatoidae - the blood flukes
• Differ from other trematodes
• Dioecious
• Do not have second intermediate host.
• Penetrate integument of definitive host
• Not really flat but rounded.

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Schistosoma sp.

• Adult worms are found in blood vessels of
  digestive tract or urinary bladder thus called
  blood flukes
• male worm has a split body called the
  gynecophoral canal. The female is usually found
  within this canal safe in the arms of her
  lover. She leaves only during the egg laying
  period.

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Schistosomiasis

• Migratory phase - 4-10 weeks after infection. Is
  characterized by fever and toxic or allergic
  reactions resulting from migration of immature
  organisms. Often results in bronchitis,
  hepatomegaly, splenomegaly, and diarrhea.

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Schistosomiasis

• Acute phase - 10 weeks to years. Eggs can become
  trapped and produce granulomas and scar tissue.
• Form fibrous nodules called pseudotubules.
• Eggs may lodge in gastrointestinal, renal,
  neural, and other systems.
• A person infected with 50 mating pairs would be
  exposed to about 15,000 eggs per day for several
  years. ½ of eggs might remain trapped in
  tissues.

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Schistosomiasis

• Chronic phase - persons living in endemic regions
  are often asymptotic. May have mild, chronic
  bloody stools or urine. Often have formation of
  granulomas. Hepatomegaly, Spleenomegaly, Ascites
  (accumulation of fluid in abdominal cavity fig
  16.17, page 245.

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Epidemiology

• Human waste into water.
• Moslem religious practice of ablution
• Agricultural projects
• Irrigation extends snail habitats
• Use of night soil (human feces)
• Planting and harvesting of water crops
• Dependence of people on rivers.

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Epidemiology

• Before the Aswan Dam was built, the region
  between Cairo and Aswan was subject to annual
  floods. The prevalence of Schistosomiasis was
  only about 5. Four
• years after completion of the dam the prevalence
  ranged from 19-75 (average 35) or a 7 fold
  increase.

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Schistosoma sp.

• Important parasites of man and some domesticated
  animals
• Three species infect man
• Schistosoma mansoni
• Schistosoma japonicum
• Schistosoma haematobium
• similar see table 16.1 pg 237 for differences

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Schistosoma japonicum.

• Common in parts of Japan, China, Taiwan,
  Philippines, Thailand, and other parts of
  Southeast Asia.
• Most pathogenic and most difficult to control
• Located in blood vessels of small intestine.
• Eggs may lodge in brain causing CNS damage, coma,
  and paralysis.
• Low host specificity

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Schistosoma mansoni

• Common in Egypt, the Middle East, parts of
  Africa, and parts of South and Central America.
• Found in portal veins draining large intestine
• The sharp lateral spine is distinctive
• Primary pathological effects come from the damage
  done by eggs.

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Schistosoma mansoni

• In heavy infections eggs become trapped in the
  mucous and submucosa of the gut and cause
  granuloma formation
• If extensive, they can cause colon blockage and
  significant blood loss.
• In liver can cause hepatomegaly.
• Destruction of lungs and heart tissue.
• Reservoir hosts are of limited or no importance.

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Schistosoma haematobium

• often referred to as Bilharzia after Theodore
  Bilharz who discovered it.
• found in parts of Africa, and parts of the Middle
  East, southern Europe and some parts of Asia.
• Found primarily in the veins of the urinary
  bladder. Eggs released in urine.
• They are least pathogenic

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Diagnosis, Treatment, and Control

• Finding eggs in feces or urine
• Biopsy - in chronic cases if eggs not passed
• Treatment - very difficult - page 245-246.
• Control is very difficult
• Customs and traditions
• Agricultural practices
• Socioeconomics

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Breakthroughs in Control

• new molluscicides
• Biological controls
• Compeditory and Predatory snails in Puerto Rico
  compete with intermediate snail host
• Crayfish from North America introduced into
  drainages of Kenya and have significantly reduced
  prevalence of snail hosts.
• fresh water shrimp that eat snail hosts. Natives
  eat shrimp

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Schistosome cercarial dermatitis or swimmers itch

• Schistosomes of animals other than man (usually
  rodents and birds) try to penetrate the skin of
  man, they can not establish themselves in the
  blood vascular system of man.
• Often cause a dermatitis which can be severe and
  in some cases life threatening.
• Allergic reaction

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Swimmers Itch
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Order Echinostomata

• Most parasites of wild animals, but a few cause
  diseases in man or domestic animals
• have spines or scales near anterior end
• Some common examples
• Echinostoma revolutum
• Fasciola hepatica
• Fasciolopsis buski
• Fasciola gigantica

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Echinostoma revolutum

• Cosmopolitan
• Shows little host specificity and is common in
  any bird or mammal that eats molluscs, planaria,
  fish, or tadpoles found infected with the
  metacercarial stage.
• Man usually becomes infected by ingesting
  metacercaria found in raw snails

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Fasciola hepatica

• Commonly known as the sheep liver fluke
• Important parasite of sheep and cattle (other
  grazers) can be found in humans.
• Morphology
• Large size, frequently over 30 mm long
• Characteristic cone-shaped projection at anterior
  end followed by wide shoulders

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Fasciola hepatica adult
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Fasciola hepatica

• Adult in bile duct of definitive host passes eggs
  in feces.
• If eggs land in water, they hatch into miracidium
  that actively swims until it finds an appropriate
  snail.
• Penetrates snail, develops into germinal sac
  (sporocyst), asexual stages of rediae and
  cercariae formed.

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Fasciola hepatica

• Cercariae leave snail, encyst on vegetation, and
  form metacercaria.
• Herbivore infected when it ingests vegetation
  with metacercaria.
• Metacercaria develop into adult penetrates gut
  wall, moves to the liver.
• Humans infected by eating watercress that has
  metacercaria on it.

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Fasciola hepatica Pathology

• Necrosis of the liver by wandering fluke
• Anemia common, inflammation and damage of the
  bile ducts, abscesses formed
• May damage other organs (eye, brain, lung)
• halzoun - Ingestion of raw liver - adult worms
  migrate to the lungs and cause a respiratory
  blockage.

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Fasciola hepatica Epidemiology

• liver blockage and watercress consumption
• Prevention - Eschewing (shunning or avoiding)
  watercress.
• Rabbits are probably important in spreading
• In some parts of southeastern United States, it
  is important parasite of domestic animals

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Fasciolopsis buski - Intestinal fluke of man

• large fluke infects man when he ingests
  metacercaria found on vegetation including water
  chestnuts, bamboo, and water caltrop.
• eat these raw or peel or crack with teeth.
• elimination of feces (human and animal) into
  water and use of night soil for farming

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Fasciolopsis buski -

• Pathology
• Blockage of food passage or interference with
  normal digestive processes
• Destruction of intestinal tissue resulting in
  ulcers, hemorrhages, and abscesses formation
• Chronic diarrhea
• Verminous intoxication - result of absorption of
  parasite metabolites (waste products), can lead
  to death.

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Order Plagiorchiata

• Adults show much variation but there are many
  similarities in larval stages
• Some common examples
• Dicrocoelium dendriticum - lancet fluke
• Paragonimus westermani

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Dicrocoelium dendriticum

• Common parasite of herbivores, rare in man
• Not dependent upon aquatic environment
• Eggs eaten by land snail (asexual reproduction
  occurs producing cercariae.
• Cercariae surrounded forming slime balls.
• Ant eats slime ball and metacercaria develop.
• Herbivore infected by ingesting ant.

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Dicrocoelium dendriticum
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Dicrocoelium dendriticum

• Modification of host behavior
• Causes ant to climb to tips of grasses early
  morning and late evenings.
• Increases odds of ant being eaten.

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Paragonimus westermani

• the human lung fluke, it is found in the Orient
  including India and Philippines
• Definitive host becomes infected by eating
  improperly cooked crustacean. Adult infections
  become established in lungs but larval forms may
  wander into brain, pleura, mesentery,
  etc.(ectopic infection).

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Paragonimus westermani
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Paragonimus westermani

• Reservoir hosts include - dogs, cats, pigs,
  rodents, and other animals
• Man becomes infected by eating improperly cooked
  crabs, ingestion of metacercaria from cutting
  boards where salads are fixed, medicinal use of
  crab juices)
• Smoked or pickled crab do not kill

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Paragonimus westermani Pathology

• Early invasive stages usually asymptomatic.
• In the lung or ectopic site, connective tissue
  forms pseudotubertules. In the CNS, they can
  cause paralysis and in rare cases can be fatal.
  In the heart they can cause severe damage and can
  be fatal.
• Lung infections cause chronic cough, bloody
  sputum, pneumonia -like conditions.

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Paragonimus westermani

• Prevention includes
• Cooking of crabs, crayfish
• Care when eating salads, no crab juice.
• Proper disposal of feces and sputum.

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Nanophyetus salmincola

• Small fluke commonly found in salmon
• Transmits rickettsial organism, Neonckeitsia
  helminthoeca (known as Salmon poisoning) to dogs,
  other canines, raccoons, and other fish eating
  mammals
• This rickettsial organism is highly toxic to dogs
  with up to 90 mortality if not treated.
• People become infected with the trematode but do
  not get salmon poisoning.

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Order Opisthorchiata

• Small to medium sized flukes
• Testes located at posterior end
• Lack cirrus pouch and cirrus
• Eggs hatch only after ingestion by snail
• Live in intestine of bile ducts of fishes,
  reptiles, birds, and mammals.
• Second intermediate host are fishes

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Clonorchis sinensis ( Opistochonorchis sinensis)

• Commonly called the Chinese liver fluke, Asian
  liver fluke, Oriental liver fluke, and Human
  liver fluke
• Common in Orient including Japan, China, Korea,
  Taiwan, Viet Nam.
• Many immigrants from Orient living in the United
  States are infected.

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Clonorchis sinensis

• Man becomes infected by eating improperly cooked
  fish containing metacercaria.
• Metacercaria may contaminate other foods such as
  salads, etc.
• Smoking, drying, pickling fish often fails to
  kill the metacercaria.

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Clonorchis sinensis

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Biology of Clonorchis sinensis

• Adults in bile ducts of man, eggs (embryonated
  embryos) passed out in feces.
• Do not hatch until eaten by proper snail host
• Miracidium develops into sporocyst.
• Asexual reproduction occurs with sporocyst
  producing many rediae and each rediae producing
  many cercariae.

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Biology of Clonorchis sinensis

• Cercariae leave snail, find and enter fish and
  forms metacercaria
• Definitive host (man, dogs pigs cats rats,
  camels, and other mammals) become infected when
  they ingest this fish.
• Dogs and cats serve as important reservoir hosts.

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Epidemiology

• Disease of the wealthy who can afford fancy cuts
  of raw fish (yeu-shan chuk is delicacy)
• Disease of the poor whose only source of protein
  is fish.
• 14 of Hong Kong, 80 of some rural areas, and
  100 of some villages are infected

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Epidemiology

• Why So Many?

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Epidemiology

• How To Break the Life Cycle

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Epidemiology

• Fish farming often use night soil to fertilize
  ponds.
• Frequently dry, salt, pickle, smoke, and freeze
  fish. This does not kill the organism.
• Be careful about eating dried or smoked fish that
  you can buy in China Towns and other Oriental
  food stores.

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Pathology

• Erosion of lining of the bile ducts.
• Severity of disease is dependent upon the
  intensity of the infection.
• Most infected people have 20-200 worms but as
  many as 21,000 worms have been recovered from a
  person at autopsy.

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Pathology

• Chronic destruction of the epithelial lining of
  the bile ducts leads to thickening of the walls
  and a condition known as "pipestem fibrosis
• Ascites (accumulation of large amounts of fluid
  in the abdominal area) almost always occurs in
  fatal cases.

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Pathology

• Eggs and sometimes worms have gallstones form
  around them.
• Liver cancer is higher than normal in Japan and
  Clonorchis infections are suspected but the
  relationship has not been proven.

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Prevention

• Proper cooking of fish
• Do not contaminate other foods with juices from
  infected fish. Careful with salads!
• Care must be taken not to eat fish that are
  dried, smoked, frozen, pickled etc. without
  proper cooking.
• Proper and controlled disposal of human feces.