Schistomiasis

Schistosomiasis

Transmission and Life Cycle Schistosomiasis
•     Schistosomiasis: A chronic disease caused by nematodes of the genus Schistoma and is characterized by passing of bloody urine or abdominal pains and diarrhoea.
•     In more chronic forms it can cause hepatosplenomegaly and lead to portal hypertension and liver fibrosis.

Transmission and Life Cycle
•   Schistosomiasis is caused by tissue reaction against the eggs of the Schistoma worm
•   Depending of the species, the eggs are excreted in urine or faces.
•    The main schistomes which infect human in Africa are Schistoma mansoni and Schistoma haematobium.

•    Mode of transmission is by penetration to the skin by cercariae.
•    When the eggs reach water; miracidia hatch out.
•   Miracidia are free- swimming larvae, which have to reach a snail host within 24 hours or die. In the snail, the miracidia develop and multiply into many cercariae.
•    These are infective agents of schistosomiasis.
•    There are shed from the snail in 4- 7 weeks.
•   They can only live up to 48 hours unless they infect human.
•   Humans are infected when entering cercariae-infected water during bathing, playing, laundering, cultivating or fishing.
•    Once the cercariae penetrate the skin, enter the blood stream and are carried to the liver or the bladder where they develop into adult worm.
•    Each type of schistosome has different distribution depending on the type of snail vector and snail house behaviour.
•    S. haematobium live in venous plexus of urinary bladder.
•    Vector to this belong to genus bulinus which live in temporary water bodies such as ponds and dams.
•    Schisosoma mansoni lives in the mesenteric plexus of the large intestine.
•    The snail vector belongs to the genus Biomphalaria, which prefer permanent water in streams, irrigations schemes on lakes.

Clinical Features Schistosomiasis
•    Schistosomiasis is a disease with several clinical manifestations according to the stages namely:
o Invasion
o Maturation
o Established infection

o Late stage
Invasion Stage
•   Dermatitis with itching papules and local oedema
•   It is more severe when caused by bird and animal schistosome (swimmer’s itch)

Maturation Stage
•    Presents with fever, eosinophilia, abdominal pains and transient generalized urticaria
•    It is also known as Katayama syndrome

Established Infection Stage
•   Colitis with bloody diarrhoea and cramps in S. mansoni infection
•    Terminal haematuria and dysuria in S. haematobium infection

Late Stage
•    There is fibrosis and calcification which occur where there are many eggs in the tissues.
•   In the bladder this may result in:
o Obstruction to and dilatation of Ureters (hydro-ureter) and kidney (hydronephrosis) leading to kidney failure
o Pyelonephritis
o Cancer of the bladder
o Calcification of the bladder shown in an x-ray or ultrasound investigation
•    In the liver, the fibrosis is periportal, resulting in portal hypertension leading to
splenomegaly and anaemia, oesophageal varices and massive upper gastrointestinal bleeding
•     In the lungs the fibrosis results in pulmonary hypertension and leads to congestive cardiac failure (CCF)

Management of Schistosomiasis
Investigations
•     Stool/urine for Schistosoma ova
o Identification of eggs in stool or urine is a confirmatory test
o If stool or urine is repeatedly negative a biopsy (rectal snip or bladder biopsy) can be done

Treatment
•     Praziquantel is the drug of choice for all schistosoma species that occur in humans
•    It is effective for other snail-borne trematode infection
•    Dose: Praziquantel (bithicide) 40 mg/kg body weight single dose (may be divided into two doses)

Other drugs used are;
o Metrifonate for Schistoma haematobium
o Oxamniquine 15mg/kg single dose is used for treating S. mansoni when praziquantel is not available in some areas

Prevention and Control of Schistosomiasis
•    Prompt treatment of identified cases
•    Control of snail infection
o Proper use of latrines
•    Control of snails
o Molluscides – Kills both snail and fish and are harmful to humans.
o Also expensive (handle with care)
o Copper sulphate is safe it kills both snails and their eggs and is not toxic to fish as other compounds.
o Niclosamide (Bayluscide) is safe to handle and use; it kills both snails and their eggs.
      It is the cheapest per volume of water treated but clogs equipment.
•     Environmental sanitation
o Prevention of snail breeding may be more important than killing snails.
o This requires radical alteration of snail habitats by draining or filling water bodies and clearing vegetation in water bodies to deprive snails of food and resting places.
o In irrigation schemes, intermittent irrigation will result in sudden changes of water level and wave action.
o Flooding is harmful for all snails.
•   Control through environmental sanitation or chemical (molluscides) needs maintenance
over a long period – at least 10 years.
•    The most effective way to protect humans from schistomiasis is to educate them about disease and give them ample supply of safe water.
•   Water can be made safe from schistomiasis by keeping it for 48 hours using the three-pot system; within this period the cercariae will die.

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