Lecture 3: Helminthology Study Notes

Medical Microbiology - Parasitology & Virology Lecture 3

Helminths

  • Definition: Helminths are parasitic worms that cause a wide variety of infectious diseases.

  • General Characteristics:

    • Multicellular parasites: Cells are differentiated into organs, each having a special function.

    • Outer protective covering: The cuticle or tegument resists intestinal digestion.

    • Locomotion: Do not possess organs of locomotion (utilize muscular contraction and relaxation).

    • Reproductive systems:

    • Monoecious: Male and female sex organs in the same individual, allowing self-fertilization as well as cross-fertilization.

    • Dioecious: The two sexes are separate.

    • Lack of vascular and respiratory systems.

    • Growth: Unlike other pathogens, helminths do not proliferate within their hosts; they grow, moult, mature, and then produce offspring, which are voided from the host to infect new hosts.

    • Distribution: Worm burdens in individual hosts and the severity of infection depend on intake (the number of infective stages taken up).

    • Disease characteristics: Diseases caused by helminths are slow in onset and chronic in nature due to the slow development of these worms compared to other infectious pathogens.

Life Cycle Stages

  • Helminths typically form three main life-cycle stages:

    • Eggs

    • Larvae

    • Adults

  • Definitive hosts: Adult worms infect definitive hosts where sexual development occurs.

  • Larval stages: May be free-living or parasitize vectors, intermediate hosts, or paratenic hosts.

Classification of Helminths

  1. Phylum Nemathelminths (Round Worms)

    • Class: Nematoda (Nematodes or Roundworms)

  2. Phylum Platyhelminths (Flat Worms)

    • Class: Trematoda (Trematodes or Flukes)

    • Class: Cestoda (Cestodes or Tapeworms)

Helminth Features

Cestodes

  • Shape: Segmented flat.

  • Body Cavity: Absent.

  • Digestive Tube: None.

  • Sex:

    • Hermaphroditic, e.g., Taenia (tapeworm).

Trematodes

  • Shape: Unsegmented flat.

  • Body Cavity: Absent.

  • Digestive Tube: Ends in cecum.

  • Sex: Hermaphroditic, except for Schistosoma which are dioecious.

Nematodes

  • Shape: Cylindrical.

  • Body Cavity: Present.

  • Digestive Tube: Ends in anus.

  • Sex: Dioecious, e.g., Ascaris (roundworm).

Nematodes: General Features

  • Body: Long, cylindrical, smooth, unsegmented, and flesh-colored.

  • Body ends: Pointed posterior and rounded anterior.

  • Protective coating: Highly resistant cuticle.

  • Digestive system: Complete system including mouth, esophagus, midgut, and anus.

  • Sexual dimorphism: Usually separate sexes, females typically larger than males.

  • Size range: Adults can vary greatly in size, from about 1 mm to 1 m in length.

  • Reproduction:

    • Oviparous (producing eggs), viviparous (producing larvae), or ovoviviparous (laying eggs containing larvae that hatch out immediately).

  • Classification: Divided into two main categories based on location:

    • Intestinal nematodes

    • Tissue nematodes (filariae)

Specific Nematodes

Trichinella spiralis

  • Disease: Trichinosis, trichinillosis.

  • Length of adult worm: 3-4 mm for males, 1.4-1.6 mm for females.

  • Geographical distribution: Worldwide.

  • Mode of transmission: Ingestion of larvae in undercooked pork; pigs are the main reservoir.

  • Site of infection: Striated muscles.

Life Cycle of Trichinella spiralis

  1. Ingestion of infective encysted larvae.

  2. Excystation in the small intestine.

  3. Development into adults.

  4. Fertilized females release motile larvae.

  5. Transported in circulation to different parts of the body.

  6. Larvae deposited in muscles.

  7. Larval development in muscles.

  8. Encystation occurs, remaining as infective larvae.

  • Infective stage: Encysted larva.

  • Diagnostic stage: Larvae in muscles and tissues.

Clinical Findings

  • Asymptomatic infection common; acute fatal illness is extremely rare.

  • Classification according to life cycle stages:

    1. Enteric phase (2-30 hours after ingestion): Symptoms include nausea, diarrhea, abdominal cramps, and vomiting.

    2. Migratory phase (1-4 weeks post-infection): Symptoms include fever, edema of the face, weakness of affected muscles; potential complications.

    3. Encystment phase (1-8 months post-infection): Cysts calcify; symptoms may subside.

Laboratory Diagnosis

  • Muscle biopsy for larvae identification.

  • Eosinophilia indicated (increased eosinophils/white blood cells).

  • Serologic tests.

Treatment

  • Severe infections: Steroids plus mebendazole; alternatively, thiabendazole.

Prevention

  • Proper cooking of pork.

  • Smoking, salting, and drying may not kill infective larvae.

  • Avoid feeding pigs raw garbage.

  • Control rat populations on pig farms.

Enterobius vermicularis

  • Common name: Pinworm, oxyuris, seatworm.

  • Disease: Enterobiasis (oxyuriasis).

  • Length of adult worm: 8-13 mm for females, 2-5 mm for males.

  • Geographical distribution: Worldwide, prevalent in temperate regions and crowded places.

  • Mode of transmission:

    • Ingestion of embryonated eggs.

    • Autoinfection via scratching perianal area.

  • Site of infection: Large intestine.

Life Cycle of Enterobius vermicularis

  1. Ingestion of eggs containing infective larvae.

  2. Larvae hatch in the small intestine.

  3. Maturation into adults in the large intestine.

  4. Nighttime migration of female to the anus for egg-laying.

  5. Eggs fully formed and infectious after 4-6 hours.

  • Infective stage: Embryonated egg.

  • Diagnostic stage: Egg found in nails or perianal area.

Clinical Findings

  • Common in children. Symptoms include severe irritation and perianal pruritis (especially at night), appendicitis, and abdominal pain.

Laboratory Diagnosis

  • Characteristic eggs collected from the perianal area using transparent adhesive tape (Scotch tape technique).

  • Adult worms may be found in the perianal area.

Treatment

  • Pyrantel pamoate and mebendazole for single-dose therapy.

  • Piperazine requires daily administration for one week.

  • Repeat treatment after two weeks to ensure complete elimination.

  • Treat entire family or group of children due to high transmission risk.

Control

  • Health education promotes personal and community hygiene (cutting nails, boiling clothes).

  • Emphasis on sanitation for food, water, and sewage.

  • Group therapy recommended.

Ascaris lumbricoides

  • Common name: Roundworm.

  • Disease: Ascariasis.

  • Length of adult worm: 20-35 cm females, 15-30 cm males.

  • Geographical distribution: Worldwide; especially high in tropical and subtropical regions with poor sanitation and use of human feces as fertilizer.

  • Mode of transmission: Ingestion of embryonated eggs in food contaminated with human feces.

  • Site of infection: Small intestine and lungs.

Life Cycle of Ascaris lumbricoides

  1. Mature egg containing larva infects humans.

  2. Larva hatches in the small intestine.

  3. Larva penetrates gut wall, enters bloodstream, and migrates to the lungs.

  4. Moulting occurs in the lungs, reaches pharynx, and is swallowed to mature in the small intestine.

  5. Adult worms become sexually mature; females lay eggs which pass in feces.

  • Infective stage: Embryonated egg.

  • Diagnostic stage: Egg in feces.

Clinical Findings

  • Pathogenic effects due to allergic reactions during larval migration.

  • Symptoms in the lung: Ascaris pneumonia (low-grade fever, inflammation, and pulmonary symptoms like dry cough, asthmatic wheezing, urticaria, and eosinophilia).

  • Mechanical effects can cause occlusions due to masses of worms or individual worms infiltrating critical areas.

Laboratory Diagnosis

  • Identification of characteristic eggs in feces (oval, brown with a colorless shell containing a large fertilized unsegmented ovum).

  • Larvae can also be identified in sputum or gastric aspirates.

  • Adult worms may pass in stool.

Treatment

  • Medications include Pyrantel pamoate, piperazine, or mebendazole.

Prevention

  • Elimination of fecal contamination of soil is essential for preventing ascariasis.

  • Ascaris eggs are highly resistant; preventive measures include treating vegetables and garden crops with iodine-containing water to kill eggs and larvae.

Hookworms

  1. Ancylostoma duodenale (Old World hookworm)

  2. Necator americanus (New World hookworm)

Ancylostoma duodenale

  • Common name: Hookworm.

  • Disease: Ancylostomiasis.

  • Length of adult worm: 10-13 mm.

  • Geographical distribution: Worldwide, especially in warm, moist climates (Middle East, North Africa).

  • Mode of transmission: Filariform larvae in moist soil penetrate the skin.

  • Site of infection: Small intestine, heart, and lungs.

Life Cycle of Ancylostoma duodenale

  1. Filariform larvae penetrate human skin (typically feet).

  2. Larvae enter venules and circulate in the bloodstream.

  3. Moulting occurs, leading to the development of a buccal capsule and growth into adults.

  4. Adult worms in the small intestine become sexually mature and lay eggs.

  5. Eggs passed in feces develop in soil.

  • Infective stage: Filariform larva.

  • Diagnostic stage: Egg in feces.

Clinical Findings

  • Symptoms include local itching ("ground itch"), risk of secondary bacterial infection, iron deficiency anemia from blood loss, and respiratory symptoms during the pulmonary phase.

Laboratory Diagnosis

  • Microscopic examination of characteristic eggs in stool (oval, colorless, thin-shelled, segmented 4-8 cells).

  • After 24 hours or more, hatched larvae may also be present.

  • Adult hookworms may be seen in feces.

Treatment

  • Medications include albendazole, mebendazole, or pyrantel pamoate; iron supplementation for anemia relief.

Control

  • Prevention of soil pollution with feces is crucial.

  • Wearing footwear can prevent the entry of larvae through the skin of the foot.

  • Use of gloves can protect hands of farm workers.

  • Simultaneous treatment of all patients and carriers limits the infection source.

Larva Migrans (Creeping Eruption)

  • Definition: A zoonotic infection caused by animal hookworm species (dogs and cats) that don't use humans as definitive hosts.

  • Types:

    • Cutaneous larva migrans: Results from hookworm larvae penetrating the skin, causing dermatitis, erythema, and itching (e.g., Ancylostoma braziliense).

    • Visceral larva migrans: Affects internal organs leading to liver enlargement and fever or lung symptoms (e.g., Toxocara canis).

Trichuris trichiuria

  • Common name: Whipworm.

  • Disease: Trichuriasis, trichocephaliasis.

  • Length of adult worm: 35-50 mm for females, 30-45 mm for males.

  • Geographical distribution: Worldwide; high prevalence in tropical and subtropical areas with inadequate sanitation.

  • Mode of transmission: Ingestion of embryonated eggs from contaminated food.

  • Site of infection: Large intestine (cecum).

Life Cycle of Trichuris trichiuria

  1. The mature embryonated eggs containing infective larvae are ingested.

  2. Eggs hatch in the small intestine; larvae emerge.

  3. Mature adults develop in the large intestine.

  4. Freshly passed eggs in feces are not infective but contain unsegmented ovum.

  5. The eggs must develop in the soil.

  • Infective stage: Embryonated egg.

  • Diagnostic stage: Egg in feces.

Laboratory Diagnosis

  • Microscopic examination of stool for characteristic eggs (brown, barrel-shaped with polar plugs).

  • Adult worms may be identified via rectal mucosa examination with proctoscopy.

Treatment

  • Alternatives include albendazole or mebendazole.

Clinical Findings

  • Often asymptomatic except in heavy infections.

  • Heavy infections can lead to gastrointestinal issues like abdominal pain, diarrhea, and rectal prolapse, along with anemia and malnutrition.

Prophylaxis

  • Proper disposal of feces to eliminate transmission.

  • Avoid eating unwashed fruits and vegetables grown in polluted areas to minimize infection risk.