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TM: Trematodes (ii) - Schistosomes (Intro (reservoirs (human, mansoni in…
TM: Trematodes (ii) - Schistosomes
Intro
aka blood flukes
16 species known to cause human diseases but 5 responsible for majority
intestinal schisto
Schistosoma mansoni
in bowel venous plexus
S japonicium
S mekongi
S intercalatum
inhabit intestinal mucosa
urogenital schisto
S haematobium
in blood vessels around bladder (sacral + pelvic vessels)
Africa, Middle East, S America, SE Asia
endemic in 76 countries
2nd only to malaria in importance in many tropical countries
commonest water-borne disease (freshwater snails)
reservoirs
human
mansoni in baboons
dogs
japonicium in horses
300 million infected
85% African
65% aged 5-14 (important reservoir - high egg output)
prevalence drops after 30 y/o
pre-patent period = time between infection - passage of eggs (4-12 wks)
Transmission factors
snail host
distribution
biology
pop dynamics
water contamination with human waste
human activities (water contact)
host-parasite relationship in both snail + human
Lifecycle
Asexual phase in intermediate snail host
Cercariae (free-swimming larval stage)
fluke passes from snail to final definitive vertebrate host
cercurial penetration of skin/mucosa, enter lymphatics, then circulation via the lungs
after about 30 says snails release them in response to sunlight
Sexual generation of adults in human vasculature
Adult male + females pair (female lies in ventral gynaecophoric canal of male) + produce numerous eggs
eggs laid @ peripheral branches of capill venules
Cytolytic secretions help egg migration through capill wall
Most eggs deposited in capills + tissues of the target organ
50% retained in tissues, 50% excreted via urine/faeces
their shape + where they were excreted from helps to dx species
the retained ones die after about 3 wks + cause foreign body reaction - granulomata - space-occupying lesion effects
some carried downstream in portal circulation
Excreted eggs hatch when in contact with freshwater + free living motile miracidium (ciliate larvae) are released + infect snails
Adult schistosomes
males 1cm, females 1.5-2.5cm
ingest blood
lifespan = 3-8 yrs but may be much longer
may embolise
Pathogenesis
granuloma formation around dead eggs
delayed T cell hypersensitivity reaction
adults do not cause disease in other ways
disease proportional to egg burden
There may be a HLA-subtype associated with severe disease
Long-lasting + persistent antibodies to various stages of development - may persist long after tx
Prevention
public health aims to reduce
no. of eggs reaching water via sanitation + education
no. of snails contacting eggs via biological control of snails
no. of cerceriae via environmental control
human contact with infected water
highly effective
remove cattle from snail-infested grasslands
provide farmers with mechanised farm equipment
improve sanitation
tap water
lavatories
pit latrines
faecal-matter containers
health education programme
Tx
Praziquantel
isoquinolone
drug of choice
only effective against mature worms
delay tx for 3 months after exposure in travellers