The trematodes (flukes) . Schistosomes are flukes that inhabit the portal
vascular system. All species have
similar life cycles.
Schistosoma mekongi (rare)
- Schistosoma intercalatum (rare)
Humans are the principal hosts for the major species.
- Persistence of schistosomiasis
depends on the presence of an appropriate snail as an intermediate host.
- Eggs excreted
in stool (S mansoni and S japonicum) or urine (S haematobium) into fresh water
hatch into motile miracidia, which infect snails.
- After development in the snails, cercariae
emerge and penetrate the skin of humans encountered in the water.
- The cercariae pass through lungs to liver where they
mature, mate and pass down into mesenteric or vescical venules to begin egg
- Communicability lasts as
long as live eggs are excreted in the urine and feces.
- Adult worms of the S mansoni
species have been documented to live as long as 26 years in the human host.
schistosomiasis can be diagnosed in patients many years after they have left the
Swimmer's itch occurs in all regions of the world after exposure to fresh, brackish, or
salt water containing larvae that do not complete their life cycle in humans.
The incubation period is variable but is approximately 8 weeks for S haematobium
and 4 weeks for S mansoni and S japonicum.
and clinical manifestations
are 3 clinicopathologic stages of schistosomiasis.
stage - Initial penetration of skin by cercariae results in
hypersensitivity and a transient pruritic., papular skin rash
- After penetration, the
organism enters the bloodstream and migrates through the lungs.
- Each of the 3 major
human schistosome parasites lives in some part of the venous plexus that drains the
intestines or the bladder, depending on the Schistosoma species.
- Four to 8 weeks after
exposure, an acute illness can develop, manifested by fever, malaise, cough, rash,
abdominal pain, diarrhea, nausea, lymphadenopathy, and eosinophilia (Katayama fever).
In acute infections with heavy infection due to Schistosoma mansoni or Schistosoma
japonicum, a mucoid bloody diarrhea accompanied by tender hepatomegaly occurs.
schistosomiasis - retained eggs induce the formation of eosinophilic
granulomas, fibrosis and scarring, probably mediated by cytokines and
involves T helper lymphocytes.The severity of symptoms associated with chronic disease is related to the worm
burden. Persons with low to moderate worm burdens can be asymptomatic. Obstruction
to blood flow is common.
Swimmer's itch (cercarial dermatitis) is caused by the larvae of other avian and
mammalian schistosome species that penetrate human skin but do not complete the life
cycle and do not cause chronic fibrotic disease.
- Manifestations include mild to moderate
pruritus at the penetration site a few hours after exposure, followed in 5 to 14 days by
an intermittent pruritic, sometimes papular, eruption.
- In previously sensitized persons,
more intense papular eruptions may occur for 7 to 10 days after exposure.
Schistosoma mansoni occurs throughout tropical Africa, in several Caribbean islands
including Puerto Rico, and in Venezuela, Brazil, Suriname, and the Arabian peninsula.
Schistosoma japonicum is found in China, the Philippines, and Indonesia.
Schistosoma haematobium occurs in Africa and the eastern Mediterranean region.
- Schistosoma mekongi is limited to a small area of the Mekong delta in Southeast Asia
(Kampuchea and Laos).
- Schistosoma intercalatum is found in Central Africa.
Children frequently are involved in transmission because of habits of uncontrolled
defecation, urination, and frequent wading in infected waters.
Infection with S mansoni and other species (except S haematobium) is determined by
microscopic examination of concentrated stool specimens to detect characteristic eggs.
In light infections, several specimens may have to be examined before eggs are
- A biopsy of the rectal mucosa may be necessary.
- The fresh tissue obtained should
be compressed between 2 glass slides and examined under low power (unstained) for
- Schistosoma haematobium is diagnosed by examining filtered urine for eggs.
excretion often peaks between noon and 3 PM
Biopsy of the bladder mucosa may be necessary.
- Serologic tests may be
particularly helpful for detecting light infections or before eggs appear in the stool or
Swimmer's itch can be difficult to differentiate from other causes of dermatitis. A skin
biopsy may demonstrate larvae, but their absence does not exclude the diagnosis.
is no specific therapy for dermatitis or Katayama syndrome.
is active against all 3 species.
may be used for S. hematobium and oxamniquine for S. mansoni as
- No satisfactory alternative drug for S
japonicum is available.
The key elements of current control measures.
Isolation of the Hospitalized Patient:
Standard precautions are recommended.
- Treatment of infected populations
- Sanitary disposal of human waste
about the source of infection
to endemic areas should be advised to avoid contact with freshwater streams and lakes.
Elimination of the intermediate snail host is difficult to achieve in most areas.