Schistosomes Flashcards
What are schistosomes?
Trematodes (parasitic flatworms) that cause schistosomiasis. They have a complex life cycle
What is the general life cycle of schistosomes?
Eggs (water) -> Miracidia (water-snails) -> Sporocysts (snails) -> Cercariae (snails-water-human) -> Schistosomula (humans) -> Adult worms (humans) -> Eggs (excretion-water)
How are eggs released from humans?
They are released either in faeces (S. mansoni) or urine (S. haematobium)
What are miracidia?
Eggs hatch releasing miracidia, which are free-swimming larvae.
Miracidia infect freshwater snails through penetration of soft tissue.
What is the specificity of miricidia for snails?
Each type of Schistosoma can be transmitted by only a few specific species of snail
What are 2 examples of schistisoma that infects specific snails?
S. mansoni infects Biomphalaria spp.
S. haematobium infects Bulinus spp.
What are sporocytes?
Once inside the snail, miricidia transform into sporocytes, which produce cercariae
What are cercariae?
Fork-tailed larvae that are released from snails into water, and then penetrate human skin
What does the penetration of cercariae cause in humans?
“Swimmers itch”
How do cercariae become schistosomula in humans?
They lose their tails
What is the movement of schiststosoma once the skin is penetrated?
Bloodstream -> Lungs -> Liver (where they mature into adult worms)
Where do adult worms reside and mate?
In veins; different species reside in different veins within their host
Give 2 examples of specific species, and where they reside in and mate?
S. mansoni resides in the mesenteric veins (intestines)
S. haematobium resides in bladder veins
How do mature worms produce eggs?
Male and female adult worms pair and mate, and the eggs spread into either faeces or urine (dependent on whether the worm resides in intestines/liver or bladder)
How do schistosomes evade immune system?
Through their tegument
What is schistisoma tegument?
A cytoplasmic layer covering the parasite
How does the schistisoma tegument help with immune evasion?
- Tegument intrinsically resists immune defences
- Rapidly renewing layer; repairs damage and helps prevents immune recognition
- Adult worms can coat their tegument with host antigens (molecular mimicry)
What are the early symptoms of schistomiasis?
- Rash/itchy skin
- Fever
- Diarrhoea
- Stomach pain
What are the chronic symptoms of schistosomiasis?
- Severe abdominal pain and swelling
- Anemia
- Liver hypertension and scarring (S. Manson
- Bladder/kidney issues (urogenital schistosomiasis by S. haematobium)
- Can in rare cases affect CNS; seizures and neurological problems
What is Katayama fever?
An acute symptom of schistosomiasis (occurs in early migratory/eggs laying stage). Causes a fever and eosinophilia. Often seen in travellers/newly endemic areas, not in those with chronic infections
What factors promote schistosomiasis transmission?
Freshwater contact, snail habitats, poor sanitation.
Who are the high-risk groups for schistosomiasis?
Children, farmers, fishermen (frequent freshwater exposure).
What are some severe effects from S. mansoni infection?
- Enlarged liver
- High blood pressure in liver (portal hypertension)
- Liver fibrosis
What are some severe effects from S. haematobium?
- Blood in urine
- Bladder fibrosis
- Bladder cancer
What causes pathology in chronic schistosomiasis?
Granuloma formation around eggs -> damages tissue and causes fibrosis
How is S. haematobium diagnosed?
Urine filtration microscopy to detect terminal-spined eggs
How is S. mansoni diagnosed?
Stool microscopy (Kato-Katz technique), look for lateral-spined eggs
What is a symptom that can be used to diagnose schistosomiasis?
Enlarged liver (bloated stomach) is indicative of a later stage infection of S. mansoni.
Blood in urine is indicative of later stage infection of S. haematobium
What is the drug of choice for schistosomiasis?
Praziquantel (PZQ)
How does praziquantel work?
Disrupts the tegument, causing a calcium influx, leading to paralysis of adult worms
Why is praziquantel ineffective against juvenile schistosomes?
Juvenile worms have immature tegument → PZQ cannot bind effectively
What are the major schistosomiasis control strategies?
Mass Drug Administration (MDA)
Snail control
Sanitation improvements
What are the concerns about PZQ resistance?
Reports of reduced efficacy (Senegal, Egypt) due to tegument mutations
