6.08 Cestodes Flashcards
Class Cestoda common name
tapeworms
Class Cestoda: Genuses/species of importance to us
- Taenia spp.*
- Echinococcus spp.*
- Dipylidium caninum*
Cestodes; Definitive vs Intermediate hosts; host specificity, replication, pathogenicity
Definitive Hosts
* usually highly host specific
* do not replicate in DH
* usually NOT overtly pathogenic as adults
Intermediate Hosts
* usually host specific (but exceptions)
* usually a prey species
* may be pathogenic to IH’s
Cestodes; Morphology - Adults
- some small (a few mm in length)
- most large (measured in meters)
- 3 body regions
- scolex (holdfast)
- neck (germinal region)
- strobila (chain of developing segments)
- no mouth or digestive tract - absorb nutrients
- hermaphroditic
Cestodes; Morphology - Eggs
- hexacanth embryo fully developed within embryophore
- eggs shed in feces or, more commonly, shed into
environment within degenerating segments - morphology varies according to type of tapeworm (e.g. taenids, hymenolepids, etc.)
Cestodes; do all have the same immature stages?
great variety of forms that vary by tapeworm group and intermediate host infected
Cestodes; how many intermediate hosts?
- most species only have a single intermediate host (the prey item)
- some (usually tapeworms using aquatic hosts) incorporate a second IH
Cestodes; General Life Cycle
- usually indirect (except Hymenolepsis nana)
- eggs (free or in gravid segments) in feces
- hexacanth embryo infects IH
- embryo migrates to final site of development and transforms into infective form
- transmission occurs when the IH is eaten
- scolex of juvenile tapeworm attaches to gut and matures (begins to produce eggs)
Taenia pisiformis
Rabbit Tapeworm
Taenia pisiformis - Rabbit Tapeworm; General Features; what is the adult host? larval host? transfer of stages?
- a typical taeniid tapeworm (example for all Taenia species)
- adult tapeworms infect small intestine of dogs
- the larva (a cysticercus) infects rabbits
- predator-prey transfer of stages
Taenia pisiformis - Rabbit Tapeworm; Morphology - Adults
- shiny white strobila, up to 2 meters long
- scolex with 4 suckers and 2 rows of hooks on the rostellum
- single genital pore on each rectangular segment
Taenia pisiformis - Rabbit Tapeworm; Morphology - Egg
- typical “taeniid-type” egg, ~ 40 μm
- has a thick radially striated embryophore
(shell) - found as single eggs (not in groups like
Dipylidium)
Taenia pisiformis - Rabbit Tapeworm; Morphology - Larva
- a cysticercus (“Cysticercus pisiformis”)
- fluid filled bladder containing invaginated scolex
- “bladder worm” usually surrounded by host cyst
Taenia pisiformis - Rabbit Tapeworm; life cycle
- a typical predator-prey life cycle
- eggs released from segments are ingested by the IH
- hexacanth larva hatches and migrates to the peritoneal cavity or liver to mature to a cysticercus
- when a dog eats the rabbit, the scolex in the cysticercus evaginates, attaches to the gut and begins to form a strobila
- PPP - 6 to 8 weeks
Taenia pisiformis - Rabbit Tapeworm; Pathogenesis/Lesions/Clinical Signs
- non-pathogenic
- presence of motile gravid segments in feces or perianal regions (rarely, eggs in feces)
Taenia pisiformis - Rabbit Tapeworm; Treatment/Prevention
- specific cestodicides - praziquantel (Droncit®), nitroscanate (Lopatol®), epsiprantel (Cestex®)
- prevent hunting to block transmission
Taenia taeniaeformis - Rat Tapeworm; General Features; hosts, transfer of stages
- felids as definitive hosts
- infects the small intestine
- rodents (especially rats and mice) as IH
- found predominantly in liver
- predator-prey transfer of stages
Taenia taeniaeformis - Rat Tapeworm; Morphology - Adults
- scolex with 4 suckers and 2 rows of hooks on the rostellum; strobila up to 60 cm long
Taenia taeniaeformis - Rat Tapeworm; Morphology - Larva
- a strobilocercus (“Cysticercus fasciolaris”)
Taenia taeniaeformis - Rat Tapeworm; Morphology - Eggs
- typical taeniid eggs
Taenia taeniaeformis - Rat Tapeworm; life cycle
- similar in most respects to Taenia pisiformis
Taenia taeniaeformis - Rat Tapeworm; Pathogenesis/Lesions/Clinical Signs
- non-pathogenic
Taenia taeniaeformis - Rat Tapeworm; Treatment/Prevention
- specific cestodicides - praziquantel (Droncit®), nitroscanate (Lopatol®), epsiprantel (Cestex®)
- prevent hunting to block transmission
Taenia crassiceps; General Features; hosts, how it amplifies infections, threat to humans
- predator-prey cycle (foxes and groundhogs)
- budding cysticercus amplifies infections
- zoonotic threat if dog becomes infected (hunts)
Taenia crassiceps; Treatment
- typical cestodicides such as praziquantel (Droncit®), nitroscanate (Lopatol®), epsiprantel (Cestex®)
- prevent hunting to block transmission
Taenia saginata; General Features; hosts, larva, basic adult morph, disease imortance?
- predator-prey cycle (humans and cattle)
- simple cysticercus in flesh of cattle
- normal human parasite – unarmed rostellum
- bovine cysticercosis is a reportable disease
Human Taenia spp. – General Life Cycles
- Eggs or gravid proglottids in feces and passed into the environment
- cattle (T. saginata) and pigs (T. solium) become infected by ingesting vegetation contaminated by eggs or gravid protoglottis
- oncospheres hatch, penetrate intestinal wall, and circulate to musculature
>oncospheres develop into cysticerci in muscle - humans infected by ingesting raw or undercooked infected meat
- scolex attaches to intestine
- adults in small intestine
Echinococcus granulosus; General Features; cycle, distriution/hosts, site of infection, threat to humans
- typical predator-prey cycle
- cosmopolitan distribution
- in Canada, exists in a sylvatic moose- wolf cycle
- adults infect the small intestine of wolf
- immature stages found in the liver and other organs of the moose
- NOTE: Zoonotic Threat
Echinococcus granulosus; Morphology - Adults
- smallest tapeworms of dogs, less than 1 cm long
- scolex has 4 suckers and 2 rows of prominent hooks
- strobila only 3 or 4 segments long
Echinococcus granulosus; Morphology - Immatures
- hydatid cyst - massive asexual replication
- some cysts up to 50 cm in diameter
Echinococcus granulosus; Morphology - Eggs
- typical taeniid egg (radially striated) ~40 μm in diameter
Echinococcus granulosus; Pathogenesis/Lesions/Clinical Signs
- non-pathogenic to canids
- ruminant IH’s usually unaffected
- hydatid cysts in humans cause a space occupying lesion with pressure necrosis as they grow - rupture of the cyst can cause anaphylactic shock and rapid death
Echinococcus granulosus; Diagnosis/Treatment - Canids
- eggs in feces
(cannot distinguish from Taenia eggs) - praziquantel
Echinococcus granulosus; Diagnosis/Treatment
- Humans and other IH’s
- cysts usually identified radiographically and commonly misdiagnosed as tumours
- surgical intervention - cysts can usually be removed successfully – supplemented with anhelmintics
Echinococcus multilocularis; cycle, distribution/hosts, site of infection, threat to humans
- typical predator-prey cycle
- cosmopolitan distribution, will infect dogs and cats
- in Canada, exists in a sylvatic fox-rodent cycle
- adults infect the small intestine of fox
- immature stages found in the liver and other organs of the rodent
- NOTE: Dangerous Zoonotic Threat
Echinococcus multilocularis; Morphology - Adults
- smallest tapeworms of dogs, less than 1 cm long
- scolex has 4 suckers and 2 rows of prominent hooks
- strobila only 3 or 4 segments long
Echinococcus multilocularis; Morphology - Immatures
- alveolar hydatid cyst - massive asexual replication
- exogenous budding of cyst acts like a neoplasm
Echinococcus multilocularis; Morphology - Eggs
- typical taeniid egg, ~40 μm in diameter
Echinococcus multilocularis; Pathogenesis – intermediate hosts including humans
- Massive replication – exogenous budding and metastasis makes control difficult
Echinococcus multilocularis; Diagnosis/Treatment - Canids
- eggs in feces (cannot distinguish from Taenia eggs)
- praziquantel Droncit
Echinococcus multilocularis; Diagnosis/Treatment - Humans and other IH’s
- cysts usually identified radiographically and commonly misdiagnosed as tumours
- surgical removal of alveolar hydatid cyst is difficult and can be unsuccessful – can be a fatal disease
Dipylidium caninum
“Double-pored tapeworm of dogs”
Dipylidium caninum
“Double-pored tapeworm”
-hosts, site of infection
- infect dogs, cats and, rarely, humans
- uses fleas or, less commonly, biting lice as IH
(sometimes called the “flea tapeworm”) - infects small intestine of DH
Dipylidium caninum; Morphology - Adults
- scolex with 4 suckers and a rostellum with several rows of fine hooks
- strobila up to 1/2 meter long
- segments are somewhat barrel-shaped with pores on each side
Dipylidium caninum; Morphology - Larva
- cysticercoid (about 1 mm) encysted in hemoceol
(body cavity) of the insect IH
Dipylidium caninum; Morphology - Eggs
- eggs in “packets” (5 - 30 oncopheres in each)
- eggs each ~40μm with a thick unstriated embryophore (membranous shell)
Dipylidium caninum; Life Cycle
- eggs usually passed within segments (motile!)
- the liberated eggs are ingested by flea larvae
- hexacanth embryo hatches from embryophore and penetrates into body cavity of insect
- cysticercoid matures there (survives metamorphosis of its insect host)
- when adult flea is ingested, scolex attaches to gut and strobila matures – PPP - 2 to 3 weeks
Dipylidium caninum; Pathogenesis/Clinical Signs
- non-pathogenic to vertebrates
- migrating gravid segment disconcerting to animals and
humans alike
Dipylidium caninum; Diagnosis/Treatment/Prevention
- segments in feces (rarely eggs with fecal flotation)
- squashes of segments (+/- rehydration) for egg masses
- cestodicides – e.g. praziquantel (e.g. Droncit®), epsiprantel (Cestex®)
- flea control to reduce transmission
Diagnosis of tapeworms in small animals
- segments in feces (rarely eggs with fecal flotation)
- Squashes of segments (+/- rehydration) for egg masses
Anoplocephala perfoliata; hosts, site of infection, special morphological feature
- Definitive Host - horse, ponies and other equids
- Intermediate Host - oribatid mites (free-living mites on pasture)
- Located at ileocecal junction
- Scolex has “lappets”
Anoplocephala perfoliata; Pathogenesis
- suspected role in stomach upsets in horses (e.g. colic, diarrhea)
Anoplocephala perfoliata; egg morphology, Diagnosis/Treatment
- irregular eggs with pyriform apparatus
- (pyrantel pamoate) Strongid-P or Strongid-T
Moniezia species; hosts, site of infection, pethogenicity, treatment
- Definitive Host - cattle, sheep, goats
- Intermediate Host - oribatid mites (free-living mites on pasture)
- located in the small intestine
- pathogenicity of tapeworms is controversial with clinical signs only in heavily infected animals and often young animals
- Treatment - albendazole
Diphyllobothrium latum; what type of cestode, hosts, geographical origins
- a pseudophyllidean cestode that incorporates two intermediate hosts in its life cycle
- first IH is a copepod (procercoid)
- second IH’s are a wide variety of fish (pleurocercoid)
- definitive hosts are many fish-eating mammals, including humans
- an introduced parasite (originally from Europe)
Diphyllobothrium latum; Morphology - Adults
- scolex has a pair of bothria (grooves)
- large strobila (some many meters long)
Diphyllobothrium latum; Morphology - Eggs
- operculate, light brown egg (superficially looks like a fluke egg) with ciliated hexacanth larva
- ciliated hexacanth embryo called a coracidium
Diphyllobothrium latum; Pathogenesis/Lesions/Clinical Signs
- usually non-pathogenic, gravid segments disturbing
- may cause pernicious anemia (B12 deficiency) in genetically predisposed humans
Cestodes - Tapeworms; Immature Stages - Mammalian IH
- Taenia spp.
- cysticercus (single, invaginated)
- strobilocercus (single, invaginated, with developing strobila)
- coenurus (multiple, invaginated)
- Echinococcus spp.
- hydatid cyst
- alveolar hydatid cyst
Cestodes - Tapeworms; Immature Stages - Arthropodan IH
- Dipylidium, Monezia, Anoplocephala
- cysticercoid
(enclosed scolex, not inverted)
