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3 types of schistosomes
- Mansoni
- Japonicum (both gut)
- Haemotobium (bladder)
- Martha Jasmine Helminth!
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Describe schistosome life cycle (inc 4 fancy words!)
- most schistosomes crave sausages
- Eggs in water, hatch to form Miricidium
- Infect snails, reproduce as Sporocysts
- In water as Cercaria
- Penetrate skin of host (attracted by skin secretions like arginine)
- Develop in host as schistosomulum
- Adult schistosomes (male and female) in portal veins
- Eggs layed into bladder (haemotobium) or intestine (mansoni, japonicum)
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life span of adult schistosomes, miracidium and cercaria
- schistosomes: 20-30 years
- miracidium: 5-6 hours
- cercaria: 2-3 days
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general pathogenesis of schistosomiasis
- skin penetration: dermatitis - 'swimmers itch'
- acute: inflammatory response to egg deposition: 4-8 weeks after infection: fever, headaches - sudden high level of antigen exposure
- chronic: granulomas formed around eggs in wall of intestine/portal vessels / wall of bladder/uterus/genital organs
- eggs secrete antigens and cause T cell mediated recruitment of macrophages, eosinophils, fibrocytes � granuloma formation ad fibrosis
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what immune response do schistosome eggs produce?
Th2 (IL3, 4, 5): eosinophils, macrophages, fibroblasts
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Describe chronic phase of S. mansoni and japonicum
- Eggs swept back into presinusoidal capillary bed of liver
- Fibrotic granulomas in portal vessels
- Portal hypertension, splenomegaly, oesophageal varices, haematemesis
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Describe chronic phase of S. haematobium
- Fibrotic granulomas in bladder wall, ureters, genital organs
- Initially painless haematuria, may be intense inflammatory response
- Later, ulceration that may lead to cancer
- Femal genital schistosomiasis may be a risk factor in HIV transmission
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describe 4 main ways schistosomes evade immunity
- Production of blocking antibodies (specific IgE and IgG4, non-specific IgE)
- Adsorbing host proteins onto tegument
- Protease-mediated destruction of host IgE
- Production of immunosuppressive molecules eg ACTH
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describe schistosome 'blocking antibodies'
- Specific IgE and IgG4 bind to the helminth and block the binding of Fc-epsilon-R1
- Non-specific IgE binds to eosinophil Fc-epsilon-R1 and prevents the eosinophil binding to IgE actually bound to helminths
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describe function of schistosome tegument in avoiding immunity
- The outside layer of a double lipid bilayer
- Adsorbs host molecules so is seen as 'self': MHC, blood group, host antibodies via Fc, host lipids
- Rapidly replaced if damaged by host immune response
- Few schistosome antigens
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5 targets for schistosome control
- Schistosomula: vaccines
- Adult worms: chemotherapy
- Eggs in water: sanitation, education
- Worms in snails: vector control - environmental, chemical, biological
- Cercaria in water: sanitation, education
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how does schistosome immunity change with age?
- More likely to be reinfected if infected when young: change in immunity at puberty
- Infection more intense when young
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What drug is used against schistosomiasis? What does it do?
- Praziquantel: kills adult worms
- Affects Ca channels and causes paralysis
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3 types of vector control against schistosomiasis
- Environment: improve drainage, remove aquatic plants
- Biological: competitive schistosome-resistant snails
- Molluscisides: expensive, environmentally damaging
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what is concomitant immunity? Describe it in relation to schistosomiasis
- incoming parasites fail to establish an infection in the presence of an already present infection
- due to parasite crowding, host genetics, parasite genetics, host sex differences, age of host
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which gender is more susceptible to parasite infections? Why?
Men: testosterone is immunosuppressive
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What is basic reproductive ratio in relation to micro- and macro-parasites
- Micro: the average number of secondary infections triggered by a primary infection
- Macro: the average number of female offspring produced by a parasite that survive to reproduce
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