introduction to platyhelminthes (flukes)

PHYLUM PLATYHELMINTHES

• Major characteristics

• Dorsoventrally flattened

• Acoelomates

• Digestive tract greatly reduced or missing

• Excretory system consists of flame cells or protonephridia

CLASSIFICATION

• Great deal of work being done in Platyhelminthes systematics.

• One of the most current classification schemes is given on page 189-190 of text.

• The older, more traditional classification scheme is given on page 192. We will use the older, much simpler classification.

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.

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.

Monogenea

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

• Have a single host.

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)

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

Digenetic Trematode Structure

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.

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.

Fasciola hepatica: Trematode Life Cycle Representative

• Ova or egg

• Miracidium

• Sporocyst

• (Daughter sporocyst, or redia)

• Cercaria

• Metacercaria

• Adult

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.


• 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.


• 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.


• 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.


• 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


• 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.


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

Often very host specific.


• .

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

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.

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.

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.

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.

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.

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.

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.

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

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

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.

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.•

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

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

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

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

Swimmer’s Itch

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

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

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

Fasciola hepatica adult

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.

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.

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.

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

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

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.

Order Plagiorchiata

• Adults show much variation but there are many similarities in larval stages

• Some common examples– Dicrocoelium dendriticum - lancet fluke– Paragonimus westermani

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.


• Modification of host behavior

• Causes ant to climb to tips of grasses early morning and late evenings.

• Increases odds of ant being eaten.

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).


• 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

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.


• Prevention includes– Cooking of crabs, crayfish

– Care when eating salads, no crab juice.– Proper disposal of feces and sputum.

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.

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

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.

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.

Clonorchis sinensis

• .

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.

•

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.

•

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

Epidemiology

•Why So Many?

Epidemiology•How To Break the Life Cycle

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.

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.

–

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.

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.

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.