Community Medicine / Fourth Stage / Dr. Ali / Lecture 10
Is a parasitic disease caused by several species of fluke (flatworms) of the genus Schistosoma.
Schistosoma Mansoni (and S. intercalatum ) → cause intestinal schistosomiasis
S. Haematobium → causes urinary sc histosomiasis
S. Japonicum (and S. mekongi) → cause Asian intestinal schistosomiasis
Intermediate Snail Hosts
S. mansoni: Biomphalaria
S. haematobium: Bulinus
S. japonicum: Oncomelania
Human → S. Haem. & S. Mansoni & S. Intercalatum.
Man, dog, cat, pig, cattle &wild rodent → S. Japonicum.
Incubation period: 2 -6 weeks
Mode of transmission : Infection is
acquired from water containing free -
swimming larval form (cercariae)
that have developed in snail .
1- Person distributionAge : Any age but children most
affected, 5 -15 years (peak incidence)
Sex : Males are more exposed than
2- Time distribution:
Infection with schistosomiasis is more common in summer due to: -
Swimming and contamination of canal water by urine or faeces is more common in
The out -put of cercariae from infected snails is more in summer, as the light is strong and
heat is maximal, since the cercariae prefers heat and light.
Minimal infection occurs in winter as the out -pu t of cercariae from i nfected snails are
sluggish, as well as the cycle inside the snails is reduced during winter.
3- Place distribution
S. haematobium: is found in large parts of Africa, parts of t he Arabia, the Middle East ,
Khuzestan Province in Iran, Madagascar and Mauritius.
S. mansoni: is found in many countries in Africa, South America (Brazil, Surinam and
Venezuela), the Caribbean and in parts of the Middle East.
S. japonicum: is found in the Far East, particularly China and the Philippines, but not any
longer in Japan where successful contro l programs have been implemented.
Schistosomiasis Diag nosis :
Eggs in the urine or stool
Three Major FactorsThree major factors are responsible for the occurrence of schistosomiasis
The method of disposal of human excreta
The presence of the snail intermediate host
The contact with cercaria -infected water
Determinants or Ecology of Schistosom iasis:
The disease is affected by the following factors:
1) Human health habit
Defecation and urination in canal water.
Swimming and bathing in canal water.
Irrigation is considered as one way for infection.
2) Factors related to the distribution of snails
Nature of water : It must be fresh with some degree of salinity , Depth of water : Snail
live usually in marginal water , that is why they are present in tertiary canals in high
Water Current : snails usually prefer sluggish current of water,
Dryness: Although these snails are aquatic , they still resist dryness but prolonged dryness
kill the snails.
Sun light: is needed for reproduction and survival of the snails.
Temperature: The suitable temp. needed ranges from 22ºC –28ºC , the temp. is needed
for ovi -position and hatching of eggs
Oxygen: Snails need slightly polluted water , but heavy pollution of water leads to death
It runs in a period of generalized reaction during penetration of the cercaria, but this is
Urinary Bilharziasis is characterized by dysurea and terminal haematuria.
Intestinal bilharziasis is characterized by dysenteric s ymptoms .
Serious complications may occur as carcinoma of the bladder and liver fibrosis.
Period of c ommunicability : So long as the case passes living ova in stool or in urine.
Hot weather and south breeze (or no breeze) sometimes brings a parasite which causes a
condition called "swimmers itch" or "green itch ".
This parasite causes small mosquito -bite -sized spots on the skin which can itch like crazy. Most
susceptible to this are kids who play in the shallow water near shore on still, hot days.
To avoid this condition, please follow this advice:
When you leave the water, do not allow the lake water to dry on your skin.
Rinse off with a hose and towel dry to remove the parasite before it reacts with your skin.
Prevention and control:
The program is composed of four approaches that must run together:
I- Chemotherapy : Praziquantel , Oxamniquine, Metrifonate
II- Snail control
III- Health education
IV - Environmental sanitation
There are 3 ways for providing the drug
1) Mass comprehensive treatment :
It is mass treatmen t to the entire population without prior investigation. It is indicated when
the disease is highly endemic , the drug given must be cheap effective, with minimal side effects.
2) Selec tive population treatment :
The drug is given after urine and stool examination for positive cases only.
3) Selected group treatment :
Provision of the drug for selected group like school children
This aims at reducing the number of snails.
It can be done by:
1-Mechanical Methods as:
- Cleaning of vegetations
- Proper banking of canals
- Establishment of straight canals.
- Application of nets to prevent the passage of snails,
2-Chemical methods: ( Molluscicides)
- Copper sulphate (20 -30 ppm.)
- Sodium pentachlorophenate (5 -10 ppm)
- Bayluscide (0.5 ppm)
3- Biological control (some species of fish).
III -Hea lth education
The aim of health education is to change the habits of the people in order to acquire
IV -En vironmental sanitation:
Provision of pure water supply
Provision of sanitary W.C.
Mechanical methods of irrigation.
Establishment of recreation places for children.
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