Medical parasitology: Helminthes

Phylum Platyhelminths
Phylum Nematoda
Helminths

Pretty planaria, not all flatworms are ugly parasites

trematodes or flukes - when they say ‘flat’ worms they mean it
All digenea are parasitic
Small dorso-ventrally flattened worms with simple anatomy and without segmentation
No coelom (secondary body cavity lined by mesoderm), but animals are filled with mesodermal parenchyma
No blood vessels, simple ladder nervous system

trematodes or flukes -
know your worm
Digenea posses two suckers (oral and ventral acetabulum) which they use to attach within the host
Oral sucker contains the mouth
Muscular pharynx permits the worm to pump food into the blind ending gut
Most trematodes are hermaphrodites (they are male and female, and cross as well as self-fertilization occurs)

trematodes or flukes -
know your worm
The gut of trematodes is blind ending but can be quite extensive and highly branched (here shown in living Fasciola (liver) flukes, the dark staining is due to bile)
Smooth muscle fibers (longitudinal and cross) run under the tegument and around all the organs (the gut is shown in this picture)

trematodes or flukes -
know your worm
The gut is not the only organ these worms use for food uptake
The tegument (“skin”) is highly active in nutrient uptake
The epidermis is essentially a single cell (a syncytium formed by fusion of multiple cells)
The tegument’s cell bodies and nuclei underlie the two muscle layers
Actin spines are found in many species and help the worms to anchor themselves

trematodes are massively fertile – but their love life is complex
To enhance the chances to complete the complex life cycle trematodes produce massive number of offspring
The adults are hermaphrodites
The reproductive systems takes up a large portion of the body of the animal
In particular the female system is complex and different physiological functions are distributed onto different organs

Trematode life cycles
Trematodes produce an enormous number of offspring by combining sexual and asexual reproduction cycles
Asexual reproduction occurs in germinal balls. These areas are home to omnipotent (stem cell-like) progentior cells that can initiate the development of embryos without fertilization
All have at least two hosts of which one is a snail
Not all stages are found in the life cycle of all species
Miracidia and cercariae are infective (invasive) stages
Intermediate host
Definitive host

Trematode life cycles --
the egg
The egg contains an embryo rather than an oocyte
Eggs are shed at different degrees of maturity by different flukes
Eggs have to leave the body of the final host to continue development
The mature miracidium within the egg uses light, osmolarity and temperature as clues to when hatching is appropriate
Hatching proceeds in most species through a preformed “door” the operculum

Trematode life cycles --
the miracidium
The miracidium is highly motile due to the cilia on its surface
Miracidia have simple eyes (they avoid light) and several chemical and mechanical receptors which they use to find the intermediate snail host
Penetration glands secrete proteases and other lytic enzymes on contact with appropriate host
Miracidia of flukes with land snails as intermediate host will hatch upon ingestion by the snail and penetrate the gut epithelium

Trematode life cycles --
the miracidium

Trematode life cycles --
the sporocyst
After penetration the miracidum undergoes metamorphosis into the sporocysts
This stage has most organ systems reduced to the bare minimum and acts as a germinal sac
The sporocyst takes up nutrients only over its tegument and the germinal mass expands and develops into daughter sporocysts, redia or cercaria

Trematode life cycles --
the redia
Sporocyst can produce cercaria or a next amplification generation the redia
Redia have features of the adult fluke like oral and ventral sucker, a gut and “birth pore” to release cercaria
Redia are mobile in the snail and can prey on sporocysts and redia of the same or other species (competition)

Trematode life cycles --
the cercaria
Cercaria are the stages that leave the intermediate host and infect the final host
There can be many consecutive waves of “shedding” from the snail
Cercaria already show many anatomical features of the adult fluke

Trematode life cycles --
the cercaria
Reflecting the ecology of their hosts cercaria have developed an array of adaptations to achieve successful infection
Direct penetration of host skin upon water contact (Schistosoma),
Encystation within the muscle of intermediate hosts (e.g. metacercaria in fish Clonorchis)
Encystation on plants (Fasciola)

Trematode life cycles --
enhance transmission
Dicrocoelium dendriticum the lancet fluke
One metacercaria becomes the ‘brain worm’ and lodges into the central ganglia of the end
The brain worm manipulates the behavior of the ant. In the evening when the temperature drops they experience spasms of their manidibles

Trematode life cycles --
enhance transmission
Leucochloridium sp. is a tiny digenic trematode living in the gut of small song birds
Worm eggs are passed with the feces and are taken up by amber snails.
Miracidia hatch, penetrate the gut epithelium and develop into sporocysts within the hepatopankreas.
Within the sporocyst cercaria develop which infect birds that eat infected snails.

Trematode life cycles --
enhance transmission
Amber snails (uninfected, upper
panel and infected, lower panel) and
Leucochloridium sp. sporocyst
dissected from a snail (lower right)

Trematode life cycles --
enhance transmission

Trematodes of medical importance
Schistosoma, blood flukes
Clonorchis & Opistorchis, liver flukes with metacercaria in fish
Paragonimus, lung flukes with metacercaria in crabs
Fasciolopsis, Fasciola, Dicrocoelium, intestinal and liver flukes with metacercaria on plants

Human liver fluke disease
Caused by Clonorchis sinensis and Opistorchis felinus and O. viverini
All locally common in East Asia and Eurasia
~20 million people infected

Human liver fluke disease
Clonorchis and Opistorchis are quite similar causing similar disease

Human liver fluke disease
Metacercaria are found in many fish especially various carp related species
Raw or undercooked fish dishes are a source of human infection
Fertilization of ponds with untreated night soil boost infection in fish
Cats, dogs and other carnivores can be additional hosts and reservoirs

Human liver fluke disease
Pathology depends on worm burden, generally infections are light and free of major symptoms
Heavy infections Flukes residing in the bilary ducts can chronically iritate the epithelium resulting in hyperplasia of the epithelium and fibrosis around the ducts (pipe stem fibrosis)
Blockage of bile ducts and impairment of liver function, liver swelling

Human liver fluke disease
Diagnosis occurs by microscopic demonstrations of fluke eggs in the feces (~30x15 μm)
Prepatency is a month
Readily treated with Praziquantel

Human lung fluke disease
Paragonimus westermanii is best known but a number of other species infect humans around the world
Several carnivores serve as reservoir
Upon eating crabs by the final host metacercariae excyst in the duodenum and penetrate the gut, penetrate the diaphragm and pleura and enter the bronchioles, mature in 12 weeks
May end up in ectopic locations like brain, skin and mesentery

Human lung fluke disease

Human lung fluke disease
Adults are encapsulated in a granuloma (often two at a time)
Cyst rupture can result in cough and increase sputum, and chest pain
Chronic high worm burden can result in chronic bronchitis and dyspnea and increasing fibrosis -- symptoms can be very similar to pulmonary tuberculosis
Cerebral paragonimiasis produces headaches, fever, nausea, visual disturbances and convulsive seizures

Fasciola & Fasciolopsis
Important parasite of livestock, ocasionally infects humans
Symptoms similar to Clonorchis but Fasciola is much bigger
Fasciolopsis buski the human intestinal fluke has similar ecology
Usually asymptomatic if not heavy burden

Schistosomiasis
Schistosomiasis (also known as bilharzia) is an infection with blood flukes and is a major infectious diseases.
More then 200 million people are infected worldwide with these flukes, which they acquire swimming or walking in water in which the intermediate snail host lives

Schistosomiasis
Schistosome eggs enter the water when infected people urinate or defecate in or near water.
Eggs hatch and the miracidium seeks out a snail. Inside the snail the parasite develops into a sporocyst and asexual reproduction takes place. Cercaria eventually are released into the water.

8.9a and b
Schistosome
life cycle

Schistosomiasis
When a schistosome cercaria swims it takes care to avoid UV light which can damage it, but is very sensitive to the scent of humans.
When it senses molecules from human skin it swims rapidly and jerks around looking for the person. When it makes contact it releases chemicals that soften the skin and it burrows and shedding its tail at the same time.

Schistosomiasis
The fluke searches until it finds a capillary and enters it.
The capillary is only barely wide enough for the fluke and it moves along using its pair of suckers. Eventually, it reaches a larger blood vessel in which it can float until it reaches the lungs and enters an artery and eventually makes its way to the liver.

Schistosomiasis
Once in the liver, the fluke feeds on blood and begins to mature and develops ovaries or testes depending on its sex.
The fluke grows dozens of times larger in the course of a few weeks and then begins to search for a mate.

Schistosomiasis
The fluke produces chemicals to attract members of the opposite sex.
Females are slender and delicate, whereas males are much bigger and have a spiny trough or groove into which the female fits and locks in.

Schistosomiasis
Once paired up, the pair mature sexually and travel from the liver to a permanent home that is species-specific.
In Schistosoma mansoni it is near the large intestine, in S. haemotobium it is the bladder, and in S. nasale, a blood fluke of cows, it is the nose.