Flatworms: Trematoda Notes (Lecture Transcript Summary)
Trematoda (Flukes) — Overview
Flatworms in the phylum Platyhelminthes; two main classes covered here: Trematoda (flukes) and Cestodes (tapeworms).
General morphology: all are flat and dorsoventrally flattened.
Trematoda vs Cestodes:
Trematodes: not segmented; often leaf-shaped; referred to as flukes.
Cestodes: flat and long but segmented (tapeworms); described as more ribbon-like.
Trematoda anatomy to know:
Anterior end contains the mouth, surrounded by muscular oral suckers.
They attach to a host with a muscular structure called the acetabulum (a sucker).
They lack an anus and regurgitate their digested material back into the tissue of the organ they infest (colloquially called “fluke puke” during necropsy).
Digestive tract: esophagus leads to a blind sac (really the cecum).
Important anatomical distinction:
There are two acetabula in some contexts, but in this lecture the focus is on the acetabulum as the attachment organ at the mouth region.
Reproduction and eggs:
Trematodes are hermaphroditic (both male and female reproductive organs within the same animal).
Eggs (ova) have a single operculated end (operculum) and are passed out of the uterus via the genital pore into the environment through the host’s feces.
Habitat and life cycle:
All trematodes live in the gastrointestinal tract (GI tract) of the definitive host, though they may migrate to other tissues depending on the species.
Life cycle is indirect, requiring at least one intermediate host (typically a snail).
Five larval stages in the life cycle (varicutum → sporocyst → rhodea → cercaria → metasurgi state → merricurnium), with the metacercaria forming the infective stage for the definitive host via ingestion of contaminated vegetation or water. Depending on species, the cercaria can penetrate the definitive host’s skin.
Defining example of interest (Fasciola hepatica):
Predilection site is the liver (hepatic bile ducts).
Acute infection: large parasite burden can cause necrotic liver disease.
Chronic infection: fibrotic liver changes due to ongoing obstruction and tissue damage.
Diagnosis commonly via ova in fecal sediment exams.
PBS/necropsy observations may include liver rot in cattle.
General diagnostic and treatment themes:
Ova/ova-like eggs are key diagnostic targets in fecal exams.
Anthelmintic therapy often involves benzimidazole derivatives (e.g., albendazole) and related azole compounds; specific choices vary by species and host.
PPE is important due to zoonotic potential in some species and during necropsy.
Drug exposure regimens often require multiple days of treatment (not a single-dose approach) to cover different life stages.
Notable cautions and practical implications:
Snail intermediate hosts limit prevalence in some regions.
Some trematodes can be zoonotic or have zoonotic potential; proper PPE and handling protocols are essential during diagnosis and necropsy.
Drug safety margins vary by breed (e.g., Collies may have sensitivity to certain benzimidazoles).
Topics for the second section (to be discussed then): Cestodes (tapeworms) and their differences from Trematoda.
Fasciola hepatica (Fassiola hepatica in transcript)
Common name and host range:
Liver fluke with a strong association to sheep and cattle as typical hosts.
Accidental hosts possible.
Intermediate host:
Snail.
Adult morphology:
Leaf-like, flat adults with a broader anterior end than posterior.
Color ranges from grayish-brown to tannish/brownish; some individuals may appear grayish-brown to brownish.
Egg morphology:
Ova are yellowish-brown and oval in shape.
Each egg has a single end with an operculum (operculum).
When observed under the microscope, the operculum is visible at the end of the egg (arrow in images).
Site of predilection and pathology:
Predilection site is the liver, specifically the bile ducts.
Acute infection: necrotic disease of the liver due to a high parasite burden.
Chronic infection: fibrotic changes in the liver from ongoing obstruction and tissue remodeling.
Diagnostics:.
Best diagnostic method is detection of ova in fecal sediment exams.
Treatment options mentioned:
Iromet Plus (drug name as stated in transcript).
Albendazole (Albenzazol as stated).
Necropsy findings:
Liver rot in cattle due to obstruction and liver tissue damage.
Zoonotic and safety considerations:
The disease is described as zoonotic in the transcript (oaknotic); proper PPE is advised during handling and necropsy.
Other notes:
In some cases, flukes may migrate to lungs, but this is not typical for Fasciola hepatica.
Paragonimus kellicotti (Paragonimus kellicotti in transcript) — Lung fluke
Host range and ecology:
Commonly referred to as the lung fluke.
Host range includes dogs and cats as typical definitive hosts.
Intermediate hosts include snails and crayfish.
Adult morphology and eggs:
Adults are barrel-shaped and reddish-brown in color.
Eggs are yellowish-brown and have a single end with an operculum (operculum).
The eggs have a shell with a thicker opposite wall from the operculum side.
Predilection site and disease:
Predilection site is the lungs.
Pathophysiology can include eosinophilic peritonitis (note: peritoneal involvement is described here; in practice this may reflect pleural or pulmonary involvement depending on case and staging).
Diagnosis:
Tracheal wash cytology can reveal parasitic spheres when stained.
Fecal sediment examination may reveal eggs/ova.
Zoonotic potential:
Not zoonotic (or not commonly zoonotic) according to transcript; precautions still required.
Treatment options mentioned:
Praziquantel.
Fenbendazole (fentbenzanol as stated).
Albendazole (albenzanol as stated).
Notes on imaging/diagnosis in practice:
A tracheal wash can be used to obtain samples from the lungs for microscopic visualization of the parasite.
General notes:
Like other trematodes, this species requires an indirect life cycle with intermediate hosts before reaching the definitive host.
Neophytus samicola (Neophytus samicola in transcript) — Salmon poisoning fluke
Primary clinical association:
Common in dogs; not very common in the particular region discussed, but noted in the Pacific Northwest of North America.
Associated with salmon poisoning disease when the dog ingests second intermediate hosts (salmon) carrying the parasite.
Hosts and life cycle:
Intermediate host is a snail.
Second intermediate host is salmon (salmon species carry metacercariae that infect the dog when ingested).
Egg morphology:
Eggs are opiculated with a small blunt point at the end (small blunt point feature).
Adult morphology:
Adults are the smallest among the trematodes discussed and are white/cream in color.
Predilection site:
Small intestine.
Pathophysiology:
Acts as a vector for ricocelia agents and produces salmon poisoning, which can cause hemorrhagic diarrhea in the infected host.
Diagnosis:
Eggs in stool (fecal examination) can be used for diagnosis.
Zoonotic potential:
Rare zoonosis; precautionary handling advised but not typically zoonotic.
Treatments and pharmacology notes mentioned:
Drug classes referenced: azoles, specifically benzimidazoles, phenyl benzazoles, and benzomyzoles (collectively described as azoles in the transcript).
Drugs are described as requiring exposure over 3–5 days (three to five days) for effectiveness; some regimens may be a single-dose dewormer in certain circumstances.
Some regimens may use a combination of benzimidazoles and phenyl/benzazole derivatives for broader efficacy across different life stages.
Common drugs listed for trematodes include praziquantel and albendazole; fenbendazole and related benzimidazoles are mentioned as effective options.
Practical cautions:
Three-to-five-day exposure regimens are emphasized for effectiveness and safety; overdosing should be avoided.
Breed sensitivities to certain drugs (e.g., Collies and praziquantel or related compounds) may require caution.
Real-world relevance:
Salmon poisoning (Neophytus samicola) has a zoonotic and veterinary significance due to the vector role and potential severity in canines; awareness of regional prevalence is important for diagnosis and treatment planning.
Summary of key takeaways
Trematodes (flukes) share a leaf-shaped, flattened body plan and an indirect life cycle with snail intermediate hosts; they lack true segmentation.
They attach to host tissue using an oral sucker and acetabulum, lack an anus, and regurgitate digested material back into host tissue.
Eggs typically have a single operculum and are shed in feces; life cycles include multiple larval stages leading to metacercariae that infect the definitive host upon ingestion.
Fasciola hepatica primarily affects the liver and bile ducts in sheep and cattle, with potential occasional lung involvement; diagnosis via ova in feces and treatment with benzimidazoles/albenzazoles; liver rot is a necropsy finding.
Paragonimus calicati is a lung fluke with canine/feline hosts; diagnosis via tracheal wash and fecal eggs; treatment with praziquantel, fenbendazole, or albendazole.
Neophytus samicola causes salmon poisoning disease in dogs; a snail and salmon life cycle; hemorrhagic diarrhea is a key symptom; diagnosis via stool eggs; drug regimens emphasize benzimidazole/azole classes and praziquantel where appropriate.
Ethical and practical implications include proper PPE during handling (zoonotic risk), region-specific prevalence, and breed sensitivities to certain anthelmintics.
Cestodes (tapeworms) will be covered in the next section to contrast their long, ribbon-like, segmented morphology with the flattened, non-segmented trematodes discussed here.
Key terms to review
Acetabulum: the muscular sucker used for attachment to host tissue.
Operculum: the lid-like opening on trematode eggs; the single end with an operculum is a diagnostic feature.
Metacercaria: the infective encysted larval form for many trematodes after a series of larval stages.
Eosinophilic peritonitis: an inflammatory response involving eosinophils; mentioned in the context of lung fluke infection.
Hemorrhagic diarrhea: a key clinical sign in Neophytus samicola (salmon poisoning) in dogs.
Zoonosis: diseases that can be transmitted from animals to humans; PPE is advised for handling suspected cases.
Quick practice prompts
Describe the life cycle stages commonly seen in trematodes and identify where the snail intermediate host fits in.
Compare and contrast Trematoda and Cestodes in terms of morphology and life cycle.
List the primary predilection sites for Fasciola hepatica and Paragonimus calicati and the typical diagnostic methods used.
Outline the treatment considerations and potential breed sensitivities when using benzimidazole-based anthelmintics.
Explain why Neophytus samicola is particularly significant in certain U.S. regions and what clinical sign is most characteristic in dogs.