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Three species infect humans: Schistosoma mansoni (Africa and
South America); S. japonicum
(Far East); and S. haematobium
(Africa). Eggs are excreted in the faeces and urine of infected
humans. In areas with poor sanitation, the eggs hatch, releasing a
miracidium that invades a snail. After development in the snail,
the schistosome cercariae emerge into the environment. They
actively penetrate intact skin and develop into male and female
adult worms that migrate to the superior, inferior mesenteric or
vesical plexus, depending on species, where they lay eggs.
Pathogenesis and Clinical features
- Initial infection: fever, hepatosplenomegaly, rash and
- Egg expulsion: bloody diarrhoea or haematuria.
- Later: symptoms and signs are caused by the fibrotic reaction
to the eggs in the liver (hepatic fibrosis and portal hypertension),
the lungs (pulmonary fibrosis) and bladder. Space-occupying
lesions in the brain and spinal cord may lead to seizures.
Microscopy for eggs in stool, urine, rectal snips or other tissue
biopsy. An enzyme immunoassay (EIA) that detects antischistosomal
antibody is useful, especially in travellers, and antigen detection
methods are available as research tests.
Prevention and control
- Avoidance of contaminated water and the wearing of appropriate
clothing when working in the fields.
- Control programmes that target snails.
- Mass treatment can control the disease if sufficient resources are
Lymphatic filariasis is caused by Brugia malayi and Wuchereria
bancrofti and transmitted by the mosquito Aedes aegypti throughout
the tropics. Onchocerciasis is caused by Onchocerca volvulus
and transmitted by the blackfly, Simulium damnosum, in West
Africa and South and Central America. Loa (loiasis) is caused by
Loa loa and transmitted by Chrysops flies in West Africa.
Lymphatic filariasis is characterized by acute attacks of fever and
lymphoedema, which may be complicated by secondary bacterial
infection. After repeated attacks lymphatic vessels are permanently
damaged, leading to lymphoedema in the leg, arm or
scrotum. Inflammation arises from the response to endobacteria
, related to Rickettsia
) that infect the filariae.
Onchocerca adults are located in nodules and microfilariae
migrate in the skin, resulting in pruritus and dry, thickened skin.
Inflammation in the eye causes blindness.
Loiasis is less damaging and diagnosis is based on fleeting subcutaneous
swellings, known as Calabar swellings. Infection may
be associated with fever and abnormalities of renal function.
- Lymphatic filariasis: identification of microfilariae in a peripheral
blood sample taken at midnight.
- Onchocerciasis: microscopic examination of 'pinch biopsies'
taken from any affected area, plus the shoulder-blade, buttocks
and thighs. If the biopsy is negative, a 50-mg dose of diethylcarbamazine
will induce increased itch (the Mazzotti reaction).
- Loiasis: Loa loa is detected in daytime blood films.
- EIA is also used for diagnosis.
Treatment of filarial infection may stimulate an acute inflammatory
Prevention and control
- Lymphatic filariasis: diethylcarbamazine or ivermectin together
- Onchocerciasis: ivermectin (the addition of tetracycline enhances
the effect on microfilarial load by sterilizing the adult worms).
An international onchocerciasis control programme is under way,
using mass treatment of whole populations with ivermectin and
doxycycline. Lymphatic filariasis is prevented by mosquito-control
Clonorchis sinensis (Opisthorchis sinensis)
Infection is acquired, mainly in the Far East, by eating undercooked
fish that contains metacercariae. The adults live in the bile
ducts, and eggs are passed in the faeces. Light infections are usually
asymptomatic, but heavier infections result in cholangitis and pancreatitis;
biliary obstruction and cirrhosis may develop. Cholangiocarcinoma
is a late complication. The diagnosis is made by
identifying the characteristic eggs in faeces. Patients may be treated
with praziquantel. Infection is prevented by adequate cooking of
potentially infected fish.
Humans are accidental hosts of this sheep and cattle parasite. The
infective stage is found on freshwater plants such as watercress
which, if eaten without cooking, can result in infection. The larvae
hatch in the intestine; after maturation and migration, the adults
are located in the liver. Patients present with fever and right upper
quadrant pain. Low-grade biliary symptoms and liver fibrosis may
denote continuing infection. Treatment is with praziquantel.
. infect different organs: P. westermani
and P. mexicanus
, brain. This rare infection follows the ingestion
of undercooked crustaceans. Acute, non-specific symptoms, such
as fever, abdominal pain and urticaria, are followed by specific
symptoms and signs, such as chest pain, dyspnoea and haemoptysis
or central nervous system signs. Diagnosis is made by identification
of the characteristic eggs in sputum, imaging, serology or
tissue biopsy. Lung fluke is treated with praziquantel; cerebral
disease with a combined surgical and medical approach.