Life Cycles and Transmission and Control  of Schistosomes

  Once the eggs are laid by the adult female worms, the majority of them first pass through the veins of the blood vessel in which the worm is living, and then into the lumen of the intestine and are passed in the faeces (S. mansoni and S. japonicum) or into the lumen of the bladder, and are then passed in the urine (S.haematobium).

  Those eggs that reach fresh water hatch, releasing a miracidium which, to develop further must infect a specific snail species within 24 hours. The eggs of each species are markedly different but each produce virtually identical miracidium. A single miracidium can multiply in the snail to produce nearly 100,000 cercariae.

  Asexual multiplication takes place in the snail, and results in the release of cercariae (minute in size with forked tails, 200mm long) into the water about 3 – 6 weeks later. Cercariae actively swim around and when they have located, or come into contact with a definitive host, they actively penetrate the skin. They can stay  active looking for a host for 24-48 hours after which if they don’t find a host they will die.

   The head of the cercariae migrates to the liver and develops into either adult male or female worms (flukes), where they pair up and then migrate to their region of the venous blood system (species specific sites). The females leave the males and moves to smaller venules closer to the lumen of the intestine or bladder to lay her eggs (about 6 weeks after infection). The majority of adult worms live from 2-4 years, but some can live considerably longer.

Acute manifestations

a. Cercarial dermatitis, also known as swimmer's itch, is an allergic reaction caused by the penetration of cercariae in persons who have been exposed to cercariae in salt water or fresh water. Cercarial dermatitis manifests as petechial haemorrhages with oedema and pruritus, followed by maculopapular rash, which may become vesicular. The process is usually related to avian
schistosomal species of the genera Trichobilharzia, Gigantobilharzia, and Orientobilharzia, which do not develop further in humans.

b. Katayama syndrome corresponds to maturation of the fluke and the beginning of oviposition.
 This syndrome is caused by high worm load and egg antigen stimuli that result from immune complex formation and leads to a serum sickness –like illness. This is the most severe form and is most common in persons with S mansoni and S japonicum infections. Symptoms include high fever, chills, headache, hepatosplenomegaly, lymphadenopathy, eosinophilia, and dysentery. A history of travel in an endemic area provides a clue to the diagnosis.

• Chronic manifestations

a. Symptoms depend on the Schistosoma species that causes the infection, the duration and severity of the infection, and the immune response of the host to the egg antigens.

b. Terminal haematuria, dysuria, and frequent urination are the main clinical symptoms of urinary schistosomiasis.

c. The earliest bladder sign is pseudotubercle, but, in long-standing infection, radiography reveals nests of calcified ova (sandy patches) surrounded by fibrous tissue in the submucosa.

d. Dysentery, diarrhoea, weakness, and abdominal pain are the major symptoms of intestinal schistosomiasis.

e. A reaction to schistosomal eggs in the liver causes a periportal fibrotic reaction termed Symmers clay pipestem fibrosis.

f. Haemoptysis, palpitation, and dyspnea upon exertion are the symptoms
of schistosomal cor pulmonale that develops as a complication of hepatic schistosomiasis.

g. Headache, seizures (both generalized and focal), myeloradiculopathy with lower limb and back pain, paresthesia, and urinary bladder dysfunction are the noted symptoms of CNS schistosomiasis due to S.
japonicum infection.

Intestinal schistosomes

• Laboratory confirmation of S. mansoni and S. japonicum infection can be
made by finding the eggs in the faeces. When eggs cannot be found in the faeces, a rectal biopsy can be examined.

• Serological tests are of value in the diagnosis of schistosomiasis when eggs cannot be found. An enzyme linked immunosorbent assay (ELISA) using soluble egg antigen, is employed at HTD.

Urinary schistosome

• The definitive diagnosis of urinary schistosomiasis is made by finding the characteristic ova of S. haematobium in urine. Terminal urine should be collected as the terminal drops contain a large proportion of the eggs. The urine can then be centrifuged and the deposit examined microscopically for ova. Eggs can sometimes be found in seminal fluid in males.

• A bladder biopsy is seldom necessary to make the diagnosis. A rectal snip may show the presence of ova as they sometimes pass into the rectal mucosa.

  Serological tests can be of value when eggs cannot be found in clinical samples. An enzyme linked immunosorbent assay using soluble egg antigen todetect antischistosome antibody is most sensitive.


Note: There is a marked periodicity associated with the time when most eggs are passed out. Higher numbers of eggs are encountered in urine specimens passed between 10 am and 2pm, presumably as a result of changes in the host’s metabolic and physical activities.

Epidemiology of Schistosomiasis

The following factors are of epidemiological importance in the transmission of schistosmiasis:

- The presence of water bodies such as rivers, streams, lakes, dams suitable for the breeding of the snail intermediate hosts

- Presence of appropriate snail hosts necessary for the developments of the asexual stages and transmission of the infective stage to the human definitive host

- Contamination of natural water bodies with infected human urine and faeces

- Human water contact activities including swimming, laundry and fetching

- Factors that promote intramolluscan development of parasite and subsequent transmission to man

- Socio-economic status of the people such as good sanitary system and water supply

Control

- Reduction of human-water contact

- Improved sanitation by proper waste disposal

- Attacking the adult forms of parasite through chemotherapy to reduce theworm burden or egg production

- Eradication or reduction of snail population through the use of molluscicides

- Development of vaccine to induce immunity

- Modification of the ecology of the snail habitat

- Biological control through the introduction of competitors snails into the snail habitat