Schistosomiasis Flashcards
Number of people infected with schistosomiasis.
200 million
Percentage severe morbidity.
10%
What host is each trematode from?
S.mansoni
S.haematobium
S.japonicum
S.mansoni - biomphalaria
S.haematobium - bulinus
S.japonicum - oncomelania
Regions each species native to.
S.mansoni - Africa
S.japonicum - China
S.haematobium - America and Africa
Species of schistosomiasis of importance in humans
S. mansoni, S.japonicum, S.haematobium, S.intercalatum, S.guineensis, S.mekongi
West African schistosomiasis zoonosis detection.
Recent detection of novel hybrids between S.haematobium and S.bovis revealed unexpected zoonosis.
Schistosomiasis life cycle.
Egg hatches > miracidium > miracidium penetrate snail intermediate > multiply in successive generations of sporocysts > snail releases free-swimming cercaria which penetrates host skin > cercaria tail falls off, forms a schistosomulum in skin tissue > enter circulation, travel to portal vein to mature to adults > paired adults migrate to mesenteric/bladder veins
What are cercariae morphology adapted for?
Motility and penetrating unbroken skin with an acetabular gland which produces proteases and elastases.
Aetiology of symptoms
From egg presence, 100s eggs released per day. Most released but some remain in tissue.
Immune response from soluble egg antigen (SEA), the immune response involves various cell types (T cells, B cells, eosinophils, mast cells, [granulomatous or multinuculated giant cells], macrophages) and cytokines.
Pathophysiology of acute symptoms
Granulomatous reaction to eggs in mesentery/urogenital veins
Symptoms of chronic infection
Hepatosplenomegaly, portal hypertension and varacies. Blood in urine/faeces.
Diagnosis
Clinical presentation
Microscopic methods (Kato-Katz, Flotac, centifruged pr filtered urine/semen)
Parasite protein detection (CCA, CAA, SEA)
Haematuria (visible or cryptic)
Antibody detection - don’t indicate current infection concerns over sensitivity or specificity. Patterns of An and Ab can be confusing.
Treatment of schistosomiasis
Praziqunatel (?voltage gated Ca2+ ion channels. This may damage worm tegument and expose surface to immune dependent attack)
Limitations of praziquantel
- Active against adult worms alone, doesn’t reverse pathology.
- Tablets hard for children to swallow.
- Performance of drug enhanced with ingestion of fatty or high carb meals.
- Cure rates by praziquantel sensitive to baseline prevalence of infection (higher infection, the lower the cure rate), particularly true in pre-school children
Why is there a need for mapping of schistosomiasis?
For effective control of disease by targeting resources. This is a fundamental concept in the WHO strategic plan for schistosomiasis.
Methods of schistosomiasis control?
Vaccines/ Molluscicides/Prevention chemotherapy
Schistosomiasis and vaccines
Glutathione-S-transferase (GST) has been investigated as a potential. GST based vaccines potentially don’t produce and appropriate response. Bourke et al (2014) showed performance varies on patient age and prior schistosomiasis/praziquantel exposure.
Schistosomiasis and molluscicides
Expensive. Large ecological impact.
Schistosomiasis and preventative chemotherapy
Large efforts in up-scaling drugs in sub-Saharan Africa. Single dose of praziquantel given to millions of children from improved political advocacy, and the increase of Merck’s donation from 25 to 250 million tablets a year.
Why are school aged children prioritised for schistosomiasis treatment?
- Higher parasitaemia burden
- Greater vulnerability to disease
- Easier accessibility within educational infrastructure (attending primary school) as supported by universal primary education
WHO recommendations regarding praziquantel treatment.
Schools in high risk areas (>50% parasitological methods, >30%questionnaires for frank haematuria) - all school age children annually, improve sanitation, water and health education
Schools in moderate risk areas (>10% and 1%
Schistosomiasis treatment challenges.
- Treatment doesn’t prevent/guard against subsequent infections (pathology eggs induce diminished by reduction in their production)
- Schistosomiasis detection in infant/pre-school children that highlights other shortcomings in present control tools
Why is socio-anthropology of interest to epidemiology?
Cultural KAP may inhibit success of otherwise logical disease interventions. Information, education, communication.
Barriers to disease acceptance.
Disease aetiology unknown, not recognised as low perceived morbidity, not known as disease can be prevented, other priorities (e.g harvesting(, limited resources (time/money/materials) to take effect, solutions may undermine standing of existing community leaders/doctors
Kick out Kichoco. What is it?
2003-2007, Zanzibar. Aimed to provide praziquantel, monitor disease and develop appropriate strategies for health education. Books with locally attractive story lines printed for children to improve awareness and reduce bad behaviours.
Result: Knowledge didn’t improve, practice of urinating in water decrease (from survey data)
