Other trematodes
Other trematodes
Editor: Stephen D. Vaughan
Section Contents
In the previous module, we described esquistosomiasis - perhaps the most medically relevant trematode. In this module, we will describe four major trematodes (flukes) that you should be familiar with - Fasciola hepatica (sheep liver fluke), Paragonimus westermani (lung fluke), Clonorchis sinensis (liver fluke) and Opisthorchis viverrini (liver fluke). Clonorchis and Opisthorchis produce very similar disease processes and, thus, will be grouped together. While all four flukes are fundamentally different, they too share similarities that will hopefully help you remember them: (i) they are all food-borne and zoonoses (come from animals); (ii) all of their life cycles involve a snail as the first intermediate host. Okay enough of a preamble. Let’s begin.
Fasciola hepatica (sheep liver fluke)
Background & Epidemiology
Fascioliasis is a zoonotic infection caused by Fasciola hepatica, and is acquired by eating leafy water plants - such as watercress - that carries the encysted metacercariae that grows in bodies of freshwater [if you want to know more about growing plain watercress, can check out this Youtube Video. Infection is widely distributed across sheep- and cattle-rearing areas worldwide. In humans, adults of F. hepatica live inside the biliary tree.
Life Cycle
Figure 1. Fasciola cercariae
Figure 2. Life cycle of Fasciola hepatica
Clinical Presentation & Pathogenesis
After an incubation period of 6-12 weeks, fascioliasis goes through two distinct phases:
Diagnosis
Suspect fascioliasis in an individual with a compatible clinical picture, eosinophilia and a history of ingesting water plants - such as watercress!!! Diagnostic modalities will vary with each phase:
- Acute phase: remember that larvae do not lay eggs, so the stool microscopy (“ova and parasites”) can be negative during the acute phase. Your best friend during the acute phase is the serology. Look for those ill-defined hypoattenuating tracks across the liver on CT.
- Chronic phase: after adults start laying eggs, now stool microscopy becomes your best friend. Serology can still help. Filling defects or the adult flukes can be visualized in the biliary system (like in this video, isn’t it amazing?!)
Figure 3. Adult fluke of Fasciola hepatica
Figure 4. Fasciola hepatica adult
Treatment
Unlike every other trematode, the drug of choice for fascioliasis is triclabendazole! Please remember this because it is the ONLY PARASITE in this curriculum where you will hear this drug.
Prevention
- Avoid eating potentially contaminated watercress and other leafy vegetation. NOTE: washing watercress with clean water may not prevent the infection as the metacercaria are encysted "embedded" within the leaf, not on the surface
- Sanitary disposal of feces
- Public health education & snail control
Paragonimus westermani (lung fluke)
Background & Epidemiology
Paragonimiasis is a zoonotic infection caused by Paragonimus spp., and is acquired by eating raw/undercooked crab and crustaceans that carries the infectious metacercariae. In humans, adults of P. westermani (the most common species) live in pairs in the lungs. Paragonimiasis is endemic in parts of Africa, the Americas and East and Southeast Asia. However, most of the world's burden of P. westermani is in East and Southeast Asia - the story of the "drunken crab" = Kejang from Korea might help you remember this! Three Cases of Paragonimiasis in a Family - PMC
However, for a detailed distribution of paragonimiasis, please refer to this video:
Life Cycle
Figure 5. Life cycle of Paragonimus spp.
Clinical Presentation & Pathogenesis
After an incubation period of 6-12 weeks, paragonimiasis goes through two distinct phases:
Diagnosis
Suspect paragonimiasis in an individual with a compatible clinical picture, eosinophilia and a history of ingesting freshwater raw crab/crustaceans! Diagnosis is supported by the following:
- Smear microscopy: during the chronic phase, Paragonimus eggs can be seen in stool, sputum or in the pleural fluid!!! This is one of the few instances in parasitology where you will order an “ova and parasite” from the sputum. Bronchoalveolar lavage can have eosinophilic predominance. Charcot-Leyden crystals can be seen in sputum (from the eosinophils)
- Serology: can be helpful especially if stool or sputum microscopy is negative
- Imaging (CXR, CT chest)
Figure 6. Adult fluke of Paragonimus spp.
Figure 7. Egg of Paragonimus spp.
Answer
In paragonimiasis and strongyloidiasis!!!