Comprehensive Notes on Helminthology

HELMINTHOLOGY

  • Helminths are classified under:
    • Phylum Plathelminthes
      • Class Trematoda
      • Class Cestoda
    • Phylum Nemathelminthes
      • Class Nematoda
  • Epidemiological classification of helminths:
    • Biohelminths: Larva localized in the intermediate host.
    • Geohelminths: Larva localized in the environment.
    • Contact (contagious): Transmission from sick to healthy individuals.

Phylum Plathelminthes

  • Morphological features:
    • Flattened three-layered body.
    • Attachment organs: suction cups and hooks.
    • Absence or incomplete digestive system.
    • Absence of circulatory and respiratory systems.
    • Hermaphrodites.
    • Development with metamorphosis.
    • Biohelminths: Necessarily have intermediate hosts (one or more).

TREMATODA

  • Parasites of the internal organs of humans and animals.
  • Complex development cycle with the change of one or more intermediate hosts.
  • All organ systems are developed.
  • They have suction cups.

FASCIOLA HEPATICA (Liver Fluke)

  • Fascioliasis, also known as sheep liver fluke infection, is emerging as a significant human parasitic disease.
  • Estimated that up to 17 million people are infected and another 180 million are at risk.
  • The pathogen is widespread, causing infections in Europe, Central and South America, Mexico, the Middle East, and Asia.
  • Hosts:
    • Final: humans, cattle, and small animals.
    • Intermediate host: small pond snails Lymnea truncatula and Lymnea tonantosa.
  • Morphological features:
    • Fascioliasis is caused by a leaf-shaped trematode, Fasciola hepatica.
    • The adult is brown and flat with an average size of 2.5 \times 1 cm.
    • The eggs are yellow-brown, oval, and measure 140 \times 75 μm.
  • Life cycle:
    • The adult worm lives in the bile ducts of the liver of cattle.
    • It forms many eggs that escape into the environment.

Egg of Fasciola hepatica

  • Fluke eggs must enter the water for further development.
  • In the water, a miracidium larva covered with cilia comes out of eggs equipped with a special lid.

Miracidium

  • The miracidium burrows into the body of the pond beetle and is localized in the liver or sex gland of the mollusk.
  • Here, the miracidium turns into a sporocyst, an immobile, baggy stage in which redia develop from germ balls (the next stage of fluke development).

Redii

  • The Redii are mobile, have an elongated body shape, mouth, pharynx, and a short rectum.
  • Inside the redia, either redia are formed again (the second generation), or a new generation of larvae, cercariae.

Cercaria

  • Cercariae have an ovoid body shape, two suckers — oral and abdominal, and at the rear end there is a muscular tail — a larval organ with which cercariae swim.

Adolescariae

  • The life cycle from egg to cercaria lasts 70-100 days.
  • The cercariae come out of the pond beetle's body into the water, become rounded, lose their tail, and become encrusted.
  • Such encysted cercariae are called adolescariae. They float on the surface of the water or attach themselves to aquatic plants.
  • Adolescariae are an invasive stage in the life cycle of Fasciola hepatica.
  • For further development, the adolescariums must be eaten by the final host.
  • Mechanism and path of infection: Alimentary, water-based (through raw water or vegetables soaked in water).
  • Disease Signs and Symptoms
    • Fascioliasis occurs in two stages, which differ in symptoms based on the migration of F. hepatica through various organs.
    • The first stage, or prepatent or larval period, is marked clinically by abdominal pain, fever, weight loss, and urticaria. Eosinophilia and elevations in liver transaminase enzymes may occur. The first stage can last for several months. Egg production during this time is minimal.
    • The second stage, referred to as the patent or biliary period, represents the maturation of larvae into adult flukes that pass into the biliary ducts. Symptoms during this phase are often subtle, vague, and even asymptomatic. Patients may develop intermittent right upper quadrant pain, which can mimic cholecystitis. Eosinophilia may still be present. During this phase, ova are released and may be found on careful, repeated stool examination.
    • Complications from chronic disease include anemia, cholangitis, and biliary obstruction.
    • Diagnosis: Detection of eggs in feces, which should not be confused with "transit" eggs. Diagnosis is possible only 3-4 months after infection.

THE CAT FLUKER – OPISTHORCHIS FELINEUS

  • Disease – opistorchoz
  • Leaf-shaped, 5-15 mm long, 2-3 mm wide, 2 suckers, 2 testicles in the back of the body.
  • The eggs are pale yellow, oval in shape with a lid.
  • The invasive stage is the metacercarium
  • Human infection occurs when eating raw or frozen fish.
  • Localization in humans - gallbladder, hepatic ducts, duodenum
  • Diagnostics - detection of eggs in human feces, detection of eggs and the parasites themselves during microscopy of the contents of the duodenum during probing
  • It has two intermediate hosts.
    • The eggs enter the water, where the miracidia come out of them only if the eggs are swallowed by a mollusk Bithynia leachi — the first intermediate host. In miracidia, sporocysts are formed, followed by redia, and in the latter, cercariae.
  • Cercariae enter the water and actively penetrate the skin of fish, which are the second intermediate hosts. Cercariae penetrate into the musculature of fish, where they turn into metacercariae.
  • The definitive hosts of the fluke — cat, dog, and other animals — get this parasite by eating raw fish.

Geographical Distribution of Opisthorchis felineus

  • O. felineus is endemic to Southeast Asia and Central and Eastern Europe, especially in Siberia.
  • O. viverrini occurs primarily in Thailand, Laos, and Kampuchea.
  • Estimates of infection are 10 million people worldwide, with 24 to 90 percent prevalence in Thailand and 40 to 80 percent in Laos.
  • Disease Signs and Symptoms
    • Symptoms include fever, malaise, arthralgia/myalgia, and anorexia, as well as abdominal pain and urticaria.
    • Clinical signs are usually limited to tender hepatomegaly and lymphadenopathy.
    • At this stage, peripheral eosinophilia is common, and as symptoms resolve, eggs become detectable in the stool.

Dicrocoelium lanceatum

  • The causative agent of dicrocyosis.
  • The life cycle is not related to the aquatic environment.
  • It looks similar to a feline fluke, somewhat smaller in width. The testes are located in the front of the body. The eggs are dark brown (45 microns).
  • It is widespread everywhere.
  • Intermediate host: mollusk, ant.
  • The final host: herbivores, bear, human.
  • Invasive stage: metacercariae.
  • D.lanceatum development cycle: egg – miracidium (in the external environment) – sporocyst – cercarium (in the body of a mollusk) – combined cyst (in the external environment)- metacercarium (in the body of the ant) – sexually mature form –marita (in the liver of the final host)
  • Human infection by accidental ingestion of ants with metacercariae
  • Diagnosis – detection of eggs in faeces; microscopy of duodenal contents, while it is possible to detect mature forms.