Parasitology

• Flatworms does not only include the parasitic flukes (trematodes) and tapeworms (cestodes) but also other free-living flatworms
• Flatworms


  • Bilaterally symmetrical
  • Compressed dorso-ventrally
  • Has a definite anteroposterior axis
• All classes of flatworms possess a bilaterally similar excretory system, collecting tubules ad capillaries which terminate in flame cells
• Flatworms lack a circulatory system
• Sexual organs of flatworms are highly elaborated and complicated
• In most species of flatworms, the sexes are combined in a single organism (hermaphrodite or monoecious), while in some group, sexes are found in separate organisms (diecious)
• The life cycle of flatworms may involve a single obligatory host (some tapeworms like Hymenolepsis nana) or may require two or more consecutive hosts (some trematodes like Paragonimus westermani)
• Two classes of utmost importance under Phylum Platyhelminthes

  • Class Cestoidea
  • Class Trematoda
• Cestodes or Tapeworms


  • Inhabit the intestinal tract of vertebrates while the larva parasitizes the tissues of vertebrates and invertebrates
  • Adult cestodes are usually ribbon or tapelike segmented parasites varying in size from a few millimeter to several meters
  • Body consists of 3 distinct regions: Head or colex as holdfast organs, Neck which is the region of growth, Strobili or body, consist of series of segments or proglottids
  • Each segment or proglottid is a complete reproductive unit with male and female sex organs
• Life cycle of cestodes

  1. Egg with a hexacanth embryo or oncosphere

  2. Larval stage (cysticercus, cysticercoid larva or coracidium, procercoid and plerocercoid larva)

  3. Adult stage
• All cestodes require an intermediate host although in some species, the definitive host can serve as the intermediate host
• Trematodes or Digenetic Flukes

  • Complicated life cycle involving alternation of generations and hosts
  • Usually primary hosts are snails
  • Most flukes which parasitize man are also animal parasites and their non-human vertebrate hosts serve as reservoirs for human infections
  • Affect various areas of the body: Circulatory System, Intestines, Liver, Lungs
  • Vary in size from 1 mm to several centimeter in length
  • Possess suckers as organs of attachment (oral or ventral suckers)
  • Alimentary canal is present but incomplete, anus is absent
  • Esophagus bifurcates in front of the ventral sucker into a pair of blind intestinal caeca which maybe simple or branched or may reunite to form a single caecum
  • Reproductive system is highly developed and complete in each individual
  • They are oviparous and the eggs are operculated except for schistosomes and can develop in water
• Life cycle of trematodes

  1. Egg stage

  2. Larval stage (miricardium, sporocyst, redia, cercaria, metacercaria)

  3. Adult stage
• The definitive host is the one in which a parasite reaches the adult or sexually mature stage
• The intermediate host is the host in which the parasite passes one or more of its asexual stages, usually designated first and second if more than one
• Trematodes (Flukes) that inhabit different parts of vertebrates


  • Portal Blood Stream: Schistosoma japonicum, Schistosoma mansoni, Schistosoma haematobium
  • Liver: Fasciola hepatica, Clonorchis sinensis, Opistorchis felineus
  • Small Intestine: Fasciolopsis buski, Echinostoma ilocanum, Heterphyid group
  • Lungs: Paragonimus westermani
• Schistosoma japonicum


  • Common Name: Oriental Blood Fluke
  • Pathogenicity: Schistosomiasis japonica or Oriental schistosomiasis
  • Digenetic trematodes inhabiting the veins of vertebrate host
  • Mode of transmission: Skin penetration
  • Infective stage to Intermediate host: miracidium
  • Infective stage to Definitive host: cercaria
  • Diagnostic stage: eggs in feces
  • First intermediate host: Snail (Oncomelania quadrasi)
  • Reservoir host: infected domestic animals
• There are 10 species reported to parasitize man but only 3 are of major medical importance: Schistosoma japonicum and mansoni- parasitizes the branches of portal vein causing primary hepatointestinal schistosomiasis, Schistosoma haematobium- inhabits veins of urinary bladder causing schistosomiasis
• The three Schistosoma species have different geographic distribution depending on their intermediate host, the snail
• Schistosoma japonicum egg


  • 70 – 90 u by 50 – 65 u
  • Depending on the stage of development at the time they passed out with the feces, one may find them from multicellular stage to full embryonation with a developed miracidium within the shell
  • Distinguishing characteristic: small knoblike projection or spine on one side
• Schistosoma japonicum male adult

  • Shorter and more sturdy individual with length from 1.2 cm –1.75 cm by 0.05 mm in breadth
  • Has both oral and ventral suckers
  • Bigger but shorter than female
  • Distinguishing characteristic: Behind the ventral sucker and extending to the posterior end is a groove called gynecophoral canal, this is where the female is held by the male during most of their life
• Schistosoma japonicum female adult

  • Longer and more slender than the male
  • 2 – 3 cm by 0.3 mm, oral and ventral suckers are also seen
• Schistosoma japonicum miracidium


  • Ciliated, pyriform in shape with a primitive gut
  • Two pairs of flame cells and germ balls as its posterior end
  • Distinguishing characteristic: Pair of penetration glands (for movement), Cilia- for motility, Sac-like organism with germ balls
• Schistosoma japonicum cercaria


  • Body and tail of approximately equal length from 100 – 150 u
  • Breadth of tail is 1/3 of that body
  • Oral and ventral suckers are developed
  • Fork of the tail is situated at the posterior third of the tail
  • Distinguishing characteristic: When discharged from snail, tail is typically forked → bifid tail (like mermaid's tail)
• The pathology of schistosomiasis is primarily dependent on host reaction to the deposition of eggs in the tissues although lesions and clinical manifestations due to skin penetration of the cercariae and due to migrating schistosomules has been described
• Demonstration of ova from stools by DFS (directly smeared) or by concentration technique (fecal debris is removed) or from rectal or liver biopsies is the main diagnostic method
• Immunodiagnostic test to demonstrate antibodies, more commonly the circumoval-precipitin test, is the most specific diagnostic method
• Treatment


  Praziquantel in dose of 40 – 50 mg/kilo as a single dose or 25 mg/kilo in two doses or 3 doses of 20 mg/kilo given every 4 hours
• Regression of manifestations of hepatosplenic disease follows after treatment
• Epidemiology of Schistosoma japonicum in the Philippines

  • Distribution follows that of the snail intermediate host, Oncomelania quadrasi
  • Found in Sorsogon, Oriental Mindoro, Samar, Leyte, Bohol (Talibon), and all the provinces of Mindanao except Oriental Misamis
  • These areas have rainfall throughout the year which favors breeding of the snail host
  • Prevalence ranges from 10% and as high as 60%
  • Transmission requires contact of man and other mammalian host, with waste water and agricultural water at breeding sites from the snail host
  • In endemic areas, the water which support the snail population are also utilized for occupational, domestic and recreational needs therefore the amount of water contact dictated by activities of a particular population group now determines prevalence
• Prevention of schistosomiasis includes avoiding contact of direct skin to snail infected water, proper disposal of human feces including control of domestic animals since they may be infected animals and serve as reservoirs of infection, and snail control by use of molluscicides and ecological methods to radically change the environment
• Paragonimus westermani


  • Common Name: Oriental Lung Fluke
  • Pathogenicity: Paragonimiasis, Pulmonary Distosomiasis, and Endemic Hemoptysis (coughing of blood)
  • Mode of transmission to 2nd intermediate host: Ingestion of infected snail
  • Mode of transmission to definitive host (man): eating of raw or insufficiently cooked infected crab
  • Infective stage to 2nd intermediate host: cercaria
  • Infective stage to man: metacercaria
  • 1st Intermediate host: Snail (Antemelania asperata, Antemelania dactylus)
  • 2nd Intermediate host: mountain crab (Sundathelpusa philippina, formerly Parathelpusa grapsoides)
  • Reservoir host: dogs, cats, field rats, and other rodents
• Paragonimus westermani adult


  • Reddish brown in color
  • 7 – 12 mm in length, 4 – 6 mm in width, 3.5 – 5 mm in thickness
  • Rounded anteriorly and somewhat tapering posteriorly
  • Tegument is covered with single spaced spines
  • Testes are deeply lobed and situated opposite each other almost midway between the ventral sucker and the posterior border of the body
  • Ovary is located posterior to the ventral sucker has six (6) long unbranched lobes
  • Vitellaria are extensively branched
• Paragonimus westermani egg


  • Golden brown in color
  • Oval in shape
  • 80 – 188 by 48 – 60 u
  • Immature when released from the adult worm
  • Distinguishing characteristic: Flattened prominent operculum, Opposite the operculum is a thickened abopercular portion
• Paragonimus westermani cercaria


  • Round
  • Measures average of 381 – 457 u in diameter
  • Distinguishing characteristic: Ellipsoidal, small spherical tail covered with spines
• Paragonimus westermani is found in countries of East and Southeast Asia and due to its spotty distribution and low prevalence rate in these endemic foci, it is not considered a major public health problem in the Philippines
• The known endemic foci of Paragonimus westermani in the Philippines are Camarines, Sorsogon, Mindoro, Samar, Leyte, and some provinces in Mindanao, with a prevalence rate of less than 5%
• Transmission of Paragonimus westermani is by consumption of infected crabs and also by eating meat of paratenic hosts that harbor immature parasites
• Treatment of Paragonimus westermani

  Emetine hydrochloride, bithionol and praziquantel are the drugs of choice
• Paragonimus westermani can be prevented by eating sufficiently cooked crabs and meat, and taking care not to contaminate the kitchen utensils when preparing the crabs as there might be metacercariae that can attach to it