6.7 Introduction to Helminths and Trematodes Flashcards
Helminth:
WORM; esp. an intestinal worm ‐ used esp. by parasitologists
Helminths; phyla
- Phylum Nematoda: the roundworms
- Phylum Platyhelminthes:
Class Monogenea
Class Cestoda ‐ tapeworms
Class Trematoda ‐ flukes - Phylum Annelida: Leeches, earthworms, etc.
- Phylum Acanthocephala: Thorny‐headed worms
Flatworms are what phylum?
Phylum Platyhelminthes
Phylum Platyhelminthes; Classes
Flatworms
- Class Monogenea
- Class Trematoda (trematodes)
‐ Aspidogastrea
‐ Digenea - Class Cestoda (tapeworms)
Flukes are what? Phylum, classes, common hosts:
Phylum Platyhelminthes
Class Monogenea (“Anchor worms”):
* generally ectoparasites (often gills of fish) of fish, amphibians or reptiles
Class Trematoda:
* Order Aspidogastrea
‐ common parasites of molluscs, reptiles and amphibians (aquatic hosts)
* Order Digenea
‐ definitive hosts often mammals (most important from veterinary perspective)
orders of class trematoda and features hosts:
Class Trematoda:
* Order Aspidogastrea
‐ common parasites of molluscs, reptiles and amphibians (aquatic hosts)
* Order Digenea
‐ definitive hosts often mammals (most important from veterinary perspective)
Flukes ‐ General Morphology; Adults ‐ Digeans
- often dorso‐ventrally flattened
- spiny cuticle often observed
- two suckers (ventral and oral)
- esophagus and blind ceca for food
- hermaphroditic in many, but not all, species
- female ‐ ovary, prominent uterus, vitellaria
- male ‐ paired testes
- sex organs share a common genital pore
Flukes; Immature Stages ‐ Digeans
What are the life stages? Where are they found?
- miracidium > sporocyst > redia > cercaria (optionally > meta/mesocercaria)
- most stages are found within the intermediate host(s) but some may also be free in the environment or attached to vegetation
Flukes ‐ General Morphology; Eggs ‐ Digeans
- oval or egg‐shaped
- operculate ‐ has a “hatch” from
which the miracidium emerges - usually yellow to golden brown smooth shell
- contents of egg usually completely fill the egg
features typical of all fluke/trematode eggs
- operculate ‐ has a “hatch” from which the miracidium emerges
- usually yellow to golden brown smooth shell
- contents of egg usually completely fill the egg
Digeans ‐ General Life Cycle, hosts
- digeans have an indirect life cycle
- snails/slugs are always the first intermediate host
- digenetic flukes are normally highly host‐specific for their intermediate mollusc host
- somewhat less specific in their choice of definitive vertebrate host
- geographic distribution of parasite
usually follows that of its intermediate snail host - miracidium leaves egg through operculum
- miracidium penetrates snail tissues
(host‐specific event) - asexual replication (sporocyst and redia) give rise to a large number of cercariae
- cercariae then do one of the following:
> penetrate the definitive host (form adults)
> encyst on vegetation or in an intermediate host (forming metacercaria)
> penetrate intermediate host (forming mesocercaria)
Alaria spp.; Hosts affected
- cats or dogs act as definitive hosts
- snails and tadpoles act as intermediate hosts
- numerous possible paratenic hosts (including humans ‐ zoonotic threat)
Alaria spp.; Morphology ‐ Adults
- small fluke about 1 cm long
- body has two regions (anterior flattened part and cylindrical posterior region)
- anterior region has the oral and ventral suckers
Alaria spp.; Morphology ‐ Eggs
- 130 μm or less, yellow brown, operculate
- segmented embryo in fresh feces
Alaria spp.; Life Cycle
- indirect life cycle (like all digenetic trematodes)
- immature eggs passed in feces (uninfective)
- miracidium matures and eggs hatch in water
- miracidium penetrate snails
- cercariae penetrate tadpoles and become mesocercariae (unique to Alaria ‐‐ this form is a potential zoonotic threat to humans, usually through ingesting undercooked paratenic hosts)
- fluke completes development when the intermediate or paratenic host ingested by DH
- mesocercariae migrate to lungs and become metacercariae
- metacercariae mature, migrate up respiratory tract and are swallowed
- adult flukes mature in small intestine
- PPP‐5weeks
Alaria spp.; Pathogenesis
- adult flukes found attached to small intestine, usually in tightly packed clusters
- not normally pathogenic except in massive infections
- parasites do not replicate in definitive host or paratenic hosts (only in snails)
Alaria spp.; Diagnosis
- typical eggs on fecal flotation
Nanophyetus salmincola
“Salmon Poisoning Fluke”
Nanophyetus salmincola; General; range, significance
- restricted to the Pacific Northwest and Siberia * not of clinical significance on its own
- acts as a vector for Neorickettsia helminthoeca
Nanophyetus salmincola; hosts
- DH ‐ dogs, cats and mink
- IH ‐ snail and then fish
Nanophyetus salmincola; Morphology ‐ Adults
- small (2 mm or less), creamy white
Nanophyetus salmincola; Morphology ‐ Eggs
- small (up to 80 μm)
- indistinct operculum
- yellowish brown shell
Nanophyetus salmincola; Life Cycle
- typical digenetic life cycle
- eggs passed in feces hatch in water
- miracidia penetrate snails
- cercariae produced in snails penetrate fish and encyst (metacercariae)
- DH ingests IH to release fluke which then moves to small intestine
- PPP about 1 week
Nanophyetus salmincola; Pathogenesis
- normally none from the flukes
- Neorickettsia helminthoeca causes a sudden febrile illness with fever, vomiting, diarrhea, weight loss, lymphadenopathy
- rickettsial disease causes high mortality
Nanophyetus salmincola; Treatment
- rickettsia can be treated with tetracyclines
Paragonimus sp.
“Lung Flukes”
Paragonimus sp.; Morphology ‐ Adults
- ovoid, relatively large flukes (up to 16 mm)
- found within cysts in pairs ‐ reddish‐ brown from the eggs within uteri.
- spiny cuticle
Paragonimus sp.; Morphology ‐ Eggs
- small (up to 80 μm) with distinct operculum (look like shoulders around the “hatch”
- yellowish brown shell
Paragonimus sp.; Life Cycle, PPP
- typical digenetic life cycle
- eggs laid in cysts are shed into bronchioles where they are swept up the respiratory tract to be swallowed
‐ eggs found in feces - miracidia penetrate snails
- cercariae penetrate crayfish and encyst (metacercariae)
- DH ingests IH to release fluke which then excysts in the small intestine
- flukes migrate to lung and associate in pairs in lung parenchyma near bronchioles
- fibrotic cyst forms around the paired flukes
- PPP:
> dogs,4‐5weeks
> cats, up to 10 weeks
Paragonimus sp.; Pathogenesis/Clinical signs
- fibrotic cyst forms around paired worms with associated bronchiolitis
- golf ball‐sized cysts found in right caudal lobe
- atelectasis and adhesions may surround cysts
- often asymptomatic, may cause coughing
- may induce dyspnea and lethargy in more pronounced cases
Paragonimus sp.; Diagnosis
- clinical signs and history (access to water)
- typical eggs can be found on fecal flotation
Paragonimus sp.; Treatment
(all off‐label usage)
* Albendazole (Valbazen®)
* Fenbendazole (Panacur®)
Fasciola hepatica
“Liver Flukes”
Fasciola hepatica; General; Where found in Canada, hosts, zoonosis, eggs, diagnosis
- infrequently observed parasites of ruminants in Canada, usually imported animals
- severely restricted geographically
- large leaf‐like adults in bile ducts of sheep, zoonotic threat
– “cephalic cone” - eggs with indistinct operculum
- often only diagnosed post‐mortem
Liver Flukes – Life Cycles
Unembryonated eggs passed in feces
Eggs in feces hatch in water
Miracidium penetrates snail
>in snail tissue: sporocysts > rediae > cercariae
Cercaria liberated from the snail and encysts on vegetation as a metacercaria
When a sheep ingests the metacercaria with herbage, the juvenile fluke is liberated and migrates through the intestine, the peritoneal cavity and liver capsule. In the liver, the fluke migrates for a period through the parenchyma before attaining the bile duct to mature as an adult
PPP = 10-12 weeks
Fasciola hepatica; Pathogenesis
- located within bile ducts where an extensive fibrosis of the duct wall occurs
- so‐called “pipe stem” fibrosis of the liver
- may progress to carcinoma of the liver
Fascioloides magna
“American Liver Fluke”
Fascioloides magna; Hosts, where found, site of infection, diagnosis
- frequently seen in wild cervids in Canada
- widely distributed geographically
- large leaf‐like adults in cysts within liver with ducts leading to bile ducts
- often only diagnosed post‐mortem
Fasciola hepatica vs Fascioloides magna differentiating physical feature
Fasciola hepatica - Adult - cephalic cone
Fascioloides magna - Adult – no cephalic cone
Fascioloides magna; Pathogenesis – Wild and Domestic Cervids
- NO clinical signs
- patent infections – typical fluke eggs on fecal float
Fascioloides magna; Pathogenesis ‐ Cattle
- normally NO clinical signs
- DO NOT become patent
Fascioloides magna; Pathogenesis ‐ Sheep
- mortality is evidence of infection
- flukes migrate constantly and destroy liver through these migrations
- patent infections usually DO NOT develop – mortality first
