Study Guide on Helminths: Parasitology by Miriam Tagoe

Parasitology: Helminths

By Miriam Tagoe
Department of Microbiology
Central University

Overview of Helminths

  • Helminths are parasitic worms broadly classified into three main groups:
    • Platyhelminths (Flatworms)
    • Nematodes (Roundworms)
    • Cestodes (Tape worms)

Categories of Platyhelminths

  • Cestodes (Tapeworms)
  • Trematodes (Flukes)
  • Various helminths can mature in humans and can affect different organs:
    • Intestinal Cestodes: T. saginata, Echinococcus
    • Liver Trematodes: Schistosoma, Fasciola
    • Tissue Nematodes: Wuchereria, Onchocerca, Loa
    • Visceral Nematodes: Toxocara

Generalised Life Cycle of Trematodes (Flukes)

  • Development Stages:
    • ADULT WORM: Lives in the definitive host, usually humans or other mammals.
    • METACERCARIA: Second intermediate host, various species.
    • CERCARIA: Free-swimming stage that emerges from the snail.
    • SPOROCYST, REDIA: First intermediate host, commonly snails.
    • EGG: The reproductive output of the adult fluke.
    • MIRACIDIUM: The hatching larval form.

Blood Vessel Dwelling Trematodes

  • Schistosomiasis (Bilharziasis):
    • Different species of Schistosoma cause different types of schistosomiasis:
    • S. mansoni: Causes intestinal schistosomiasis.
    • S. haematobium: Causes urinary schistosomiasis.
    • S. japonicum: Causes intestinal schistosomiasis.
    • S. intercalatum: Causes intestinal schistosomiasis.
    • S. mekongi: Causes intestinal schistosomiasis.
    • S. guineensis: Causes intestinal schistosomiasis.

Life Cycle of Schistosomes

  • Stages Explained:
    • ADULT WORMS reside in pairs, with the female in the gynecophoral canal of the male.
    • Fertilization leads to egg production, which is released into venules.
    • MIRACIDIUM develops within the egg, which using lytic enzymes ruptures the venule and enters tissues.
    • Eggs can be evacuated via feces (S. mansoni) or urine (S. haematobium).
    • Hatching occurs upon contact with fresh water, leading to larval forms that penetrate snails, the first intermediate hosts.

Infection Process via Schistosomes

  • After hatching, miracidia penetrate appropriate snails to develop into infective cercariae.
  • Humans can contract the infection when exposed to infested water.
  • Cercariae penetrate skin, enter systemic circulation, and reach portal vessels where they mature.
  • Adult schistosomes can inhabit the host for several years.

Symptoms and Complications

  • S. haematobium infection leads to:
    • Terminal hematuria (blood in urine).
    • Painful micturition (urination).
    • Inflammation of urinary bladder (cystitis).
    • Splenomegaly and hepatomegaly.
    • Risk of squamous cell carcinoma in the bladder.
  • S. mansoni leads to:
    • Cercarial dermatitis (swimmer's itch).
    • Dysentery.

Laboratory Diagnosis

  • Detection via microscopic examination of:
    • Urine: for S. haematobium eggs.
    • Stool: for all other Schistosoma species' eggs.

Treatment of Schistosomiasis

  • Praziquantel: Effective against all Schistosoma species.
  • Oxamniquine: Effective for S. mansoni infections.
  • Metrifonate: Effective for urinary schistosomiasis (S. haematobium).

Prevention and Control Measures

  1. Health Education:
    • Promoting clean latrinization and safe water supply.
    • Advising against urination and defecation in water bodies.
  2. Snail Control:
    • Physical Methods: Removal of vegetation from canals and manual snail destruction.
    • Biological Methods: Introducing natural snail predators like Marisa.
    • Chemical Methods: Application of molluscides to kill snails in infected waters.

Cestodes (Tapeworms)

  • Tapeworms are hermaphroditic and require intermediate hosts for reproduction.
  • Adult forms found in humans are characterized by a:
    • Flat body with scolex and strobila (main body made of proglottids).
    • Lacking a digestive system; they absorb nutrients from the host's intestine.

Structure of Pork Tapeworm

  • Features:
    • Scolex: Head with chitinous hooks and suckers.
    • Proglottids: Divided segments representing reproductive units.

Taenia saginata (Beef Tapeworm)

  • Life Cycle:
    • Completed in humans (definitive host) and cattle (intermediate host).
    • Inhabits upper jejunum, surviving for up to 25 years, causing Taeniasis.
    • Gravid proglottids passed out with feces.
  • Infection Mechanism:
    • Cattle ingest infective eggs while grazing.

Life Cycle of Taenia saginata

  1. Oncospheres hatch and penetrate the intestinal wall.
  2. Migrate to musculature and develop into cysticerci.
  3. Humans get infected via undercooked meat containing cysticerci.
  4. Cysticercus releases larvae in the small intestine.
  5. Adult tapeworms develop in about 2 months.
  6. Adults can reach up to 5 meters in length.

Pathogenicity of Taenia saginata

  • Infected individuals may experience:
    • Epigastric pain
    • Abdominal discomfort
    • Diarrhea and weight loss.

Diagnosis and Treatment of Taeniasis

  • Diagnosis:
    • Finding gravid segments or eggs in stool.
  • Treatment:
    • Niclosamide: Dosage of 2 mg PO stat.
    • Albendazole: 400 mg daily for 3 days.
  • Prevention:
    • Treating infected individuals.
    • Proper cooking of meat.
    • Safe disposal of human wastes.

Infection with Taenia solium (Pork Tapeworm)

  • Similar to T. saginata infection but with specific precautions outlined.
  • Other relevant species include Hymenolepis nana (Dwarf Tapeworm).

Hymenolepis nana Infection Mechanism

  • Infections occur through:
    • Ingesting eggs via contaminated food.
    • Direct transmission from a patient (fecal-oral).
    • Auto-infection from hands contaminated with eggs.

Pathogenicity of Hymenolepis nana

  • Symptoms:
    • Light infections: asymptomatic.
    • Moderate infections: lack of appetite, abdominal pain, diarrhea.
  • Treatment: Niclosamide - 4 tablets chewed in one dose.

Nematodes

  • Characteristics:
    • Un-segmented, elongated, cylindrical worms with a complete digestive tract.
    • Can be free-living or parasitic.
  • Notable parasitic intestinal nematodes include:
    • Ascaris lumbricoides
    • Hookworms
    • Strongyloides stercoralis

Ascaris lumbricoides

  • Life stages: Egg, Larvae, Adult worm.
  • Infection occurs when contaminated eggs are ingested, leading to ascariasis.

Life Cycle of Ascaris lumbricoides

  1. Eggs ingested with contaminated food hatch in the duodenum.
  2. Larvae invade intestinal mucosa, migrate to portal and systemic circulation leading to the lungs.
  3. In lungs, larvae penetrate alveolar walls, ascend to throat, are swallowed, and re-enter the small intestine.

Egg Production of Ascaris lumbricoides

  • A female can produce up to 200,000 eggs daily.
  • Eggs require 14 days to several weeks to become infective under optimal conditions (moist, warm, shaded soil).

Pathogenicity and Clinical Features of Ascaris lumbricoides

  • Symptoms might include:
    • Localized reactions due to larvae migration.
    • Loeffler's syndrome and pneumonia-like symptoms from pulmonary migration.
    • Abdominal pain, nausea, vomiting, intestinal obstruction in adults.

Diagnosis and Treatment of Ascaris lumbricoides

  • Diagnosis:
    • Stool examination for eggs via direct saline smear.
  • Treatment:
    • Mebendazole, Albendazole, and Piperazine.
  • Prevention:
    • Proper hygiene and vegetable washing.

Hookworms

  • Species include:
    • Ancylostoma duodenale and Necator americanus.
  • Generally lead to mixed infections and reside in the small intestines.

Life Cycle of Hookworms

  1. Adult worms lay eggs passed in feces.
  2. Eggs hatch under proper conditions in soil, develop into larvae.
  3. L3 larvae penetrate skin, enter bloodstream, and reach the lungs.
  4. Migration from lungs to the small intestine for maturation.

Pathogenicity of Hookworms

  • Symptoms:
    • Leads to iron deficiency anemia due to blood feeding.
    • Inflammation may occur from lung larvae migration.
  • Diagnosis: Examination of stool for eggs.
  • Treatment: Mebendazole and Albendazole.

Cutaneous Larva Migrans

  • Caused by: Larvae from animal hookworms, e.g., Ancylostoma brasiliense and Ancylostoma caninum.
  • Symptoms: Pruritus and dermatitis from migrating larvae through skin.
  • Treatment: Thiabendazole applied topically.

Filarial Worms

  • Thread-like nematodes causing tropical diseases called Filariasis.
  • One extreme manifestation is elephantiasis.

Life Cycle of Filarial Worms

  • Female filarial worms produce microfilariae, which transition via insect vector.
  • Humans become infected when bitten by infected insects.
  • Notable species include:
    • Wuchereria bancrofti (lymphatic filariasis)
    • Onchocerca volvulus (subcutaneous filariasis).

Impact of Filarial Worm Infections

  • Lymphatic Filariasis:
    • Adult worms inhabit lymphatic vessels, causing blockages and inflammation.
    • Results in painful enlargements, elephantiasis.
  • Other Complications:
    • Ocular complications (e.g., blindness).

Diagnosis of Filarial Worm Infections

  • Detection of microfilariae in blood or through skin snips.