Schistosomiasis

Question 1

What is Schistosomiasis caused by

Answer 1

Caused by parasitic blood flukes belonging to the genus Schistosoma.

Question 2

Schistosomiasis transmission and intermediate host

Answer 2

Infection occurs when humans come into contact with freshwater contaminated with the larval stage of the parasite known as cercariae.

These cercariae are released from infected freshwater snails, which serve as the intermediate host in the parasite’s lifecycle.

Question 3

Human infection of Schistosomiasis

Answer 3

Humans become infected when they are in contact with water containing the free-swimming cercariae.

These penetrate intact skin and enter the bloodstream.

Once inside, they mature into adult worms and migrate to blood vessels surrounding the intestines or bladder, depending on the species.

Infected individuals excrete parasite eggs in their faeces or urine, continuing the life cycle if the eggs reach water and infect snails.

Question 4

Clinical Diagnosis of Schistosomiasis

Answer 4

1. cercarial dermatitis, an itchy, red rash at the site of entry, often referred to as “swimmer’s itch”.

Katayama syndrome: This is a systemic hypersensitivity reaction occurring weeks after infection, usually when the worms start producing eggs. Symptoms can include:

High fever

Dry cough

Muscle pain

Eosinophilia

Hepatosplenomegaly (enlargement of liver and spleen)

urinary schistosomiasis, - haematuria

Question 5

Schistosomiasis Parasitological Diagnosis: microscopy and imaging

Answer 5

Kato-Katz thick smear: A standard method for detecting Schistosoma eggs in stool samples. It has high specificity (correctly identifies positive cases) but low sensitivity, especially in low-intensity infections (i.e., fewer eggs per gram of faeces).

Formalin-ether concentration or salt flotation techniques can increase egg concentration, improving chances of detection in stool or urine.

Ultrasound can reveal organ damage caused by chronic schistosomiasis

Question 6

Schistosomiasis Parasitological Diagnosis: Antibody based tests

Answer 6

Indirect Fluorescent Antibody Test (IFAT)

Enzyme-Linked Immunosorbent Assay (ELISA)

Both tests can detect antibodies produced by the host in response to infection, particularly useful in travellers or early-stage infection where eggs may not yet be detectable.

Question 7

Schistosomiasis Parasitological Diagnosis: Antigen-Based Tests

Answer 7

Detect Schistosoma antigens in body fluids rather than antibodies. The two primary circulating antigens measured are:

Circulating Cathodic Antigen (CCA)

Circulating Anodic Antigen (CAA)

These tests are particularly useful for:

Monitoring treatment efficacy, as antigen levels decline following successful therapy

Question 8

Schistosomiasis Parasitological Diagnosis: Molecular Diagnostic Methods

Answer 8

PCR and real-time quantitative PCR (qPCR):

These methods amplify Schistosoma-specific DNA from clinical samples, offering high sensitivity and specificity

qPCR can distinguish between Schistosoma species

Major drawback: These techniques require advanced laboratory infrastructure, limiting their use to research or reference labs, especially in endemic regions with limited resources

Question 9

Ascariasis: Cause, habitat, transmission

Answer 9

Causative agent: Ascaris lumbricoides, a large intestinal roundworm (nematode)

Habitat: Adult worms live in the lumen of the small intestine

Transmission:

Ingestion of embryonated eggs via the faecal-oral route, often through contaminated food, water, or hands

Question 10

Clinical Diagnosis of Ascariasis

Answer 10

Early stage:
After ingestion, larvae hatch, penetrate the intestinal wall, enter the bloodstream, and migrate through the liver and lungs

In the lungs, they cause Löffler’s syndrome: eosinophilic pneumonia with dry cough, wheezing, and transient infiltrates on chest X-rays

Late stage:
Larvae are coughed up and swallowed, returning to the intestine where they mature

In the intestines, large numbers of worms can cause:

Abdominal pain

Intestinal obstruction

Occasionally biliary or pancreatic duct blockage

Question 11

Ascariasis Parasitological Diagnosis

Answer 11

Stool examination:

Microscopy can reveal Ascaris eggs, which are large and have a distinctive appearance that makes them easy to identify compared to other helminths

Molecular methods:

PCR/qPCR techniques are now available that can:

Detect low levels of infection

Be multiplexed to detect multiple types of helminths simultaneously

But again, they require laboratory infrastructure and are costly