Nematodes

• Nematodes

  Also known as Roundworms, most common parasite that infects humans
• Nematodes


  • Unsegmented, elongated, and cylindrical worms, with both ends often pointed
  • Size may vary from <5mm up to 1 meter
  • Body is covered with a tough cuticle/integument that lines the buccal cavity, esophagus, vagina, excretory pore, cloaca, and rectum
  • Lack a circulatory system
  • Have a complete digestive system
  • Can be classified according to their habitat
  • Absorb oxygen through their skin via diffusion
  • Have 2 sensory organs: Amphids (found at the anterior end, all nematodes have this) and Phasmids (found at the posterior end, not all nematodes have this)
  • Aphasmid nematodes include Trichinella, Trichuris, and Capillaria
  • Possess a complete reproductive system, most are dioecious but some can be parthenogenetic
  • Posterior ends of male nematodes are usually curved and function to hold the female during copulation
• Intestinal nematodes


  • Ascaris lumbricoides
  • Trichuris trichiura
  • Hookworms
  • Strongyloides stercoralis
  • Enterobius vermicularis
  • Capillaria philippinensis
• Tissue nematodes


  • Wuchereria bancrofti
  • Brugia malayi
  • Loa Loa
  • Parastrongylus cantonensis
  • Dracunculus medinensis
  • Trichinella spiralis
• Final host


  Host to sexual reproductive stage
• Intermediate host


  Host to asexual stage
• Ascaris lumbricoides


  Most common intestinal nematode of man, also known as Giant Intestinal Roundworm
• Ascaris lumbricoides egg


  • Remains viable in moist soil for a very long period of time
• Ascaris lumbricoides: adult worm

  • Resembles the common earthworm, whitish or pinkish in color, has a mouth with 3 lips and sensory papillae
  • Males are smaller (10-31 cm) compared to females (22-35 cm)
  • Males have a ventrally curved posterior end with 2 spicules, females have paired reproductive organs in the posterior 2/3 while males have a single, long, tortuous tube
• Ascaris lumbricoides pathogenesis and clinical manifestations

  1. Larvae (heart-lung migration) can cause pneumonitis with increased IgE, peak eosinophilia, and asthma-like symptoms

  2. Adult worms can cause light infections that are asymptomatic, moderate infections with lactose intolerance and vitamin A malabsorption, or heavy infections with bowel obstruction, eosinophilia, fever, abdominal distention and tenderness, vomiting

  3. Adult worms can migrate to other organs causing organ-specific disease
• Trichuris trichiura

  Also known as Whipworm
• Trichuris trichiura egg


  • Barrel-shaped, football-shaped, Japanese lantern, with plug-like translucent polar prominences, yellow outer and transparent inner shell
• Trichuris trichiura adult worm

  • Resembles a whip, with a thinner whip-like anterior portion and a thicker, dilated posterior end
  • Males are smaller with a coiled tail and single spicule, females are slightly longer with a blunt and straight tail
• Trichuriasis


  1. Can cause abdominal pain, mucus diarrhea, anemia with moderate eosinophilia, rectal prolapse (coconut cake rectum)

  2. The anterior portion embedded in the mucosa can predispose to amebic dysentery
• Hookworms


  Include New World Hookworm (Necator americanus) and Old World Hookworm (Ancylostoma duodenale)
• Hookworm egg

  • Thin shelled, oval, colorless, contains 2-8 cells (blastomeres)
• Hookworm larva


  • Has two forms: rhabditiform larva (feeding stage) and filariform larva (infective stage that can only survive up to 2 weeks without a host)
• Hookworm adult worm


  • About 1 cm in length, female slightly larger than male, head slightly bent in relation to the rest of the body, male posterior end expanded to form a copulatory bursa
  • Buccal capsule morphology differs between species: N. americanus has 1 pair of cutting plates, A. duodenale has 2 pairs of ventral teeth, A. braziliense has 2 pairs of teeth, A. caninum has 3 pairs of teeth
• Hookworm pathogenesis and clinical manifestations

  1. Larval stage can cause eosinophilia, ground itch (human hookworms), cutaneous larva migrans (animal hookworms), pneumonitis (Wakana's syndrome), Loeffler syndrome

  2. Adult worms can cause iron deficiency anemia due to GI bleeding, N. americanus can suck 0.05 mL/day, A. duodenale can suck 0.2 mL/day (more severe)
• Strongyloides stercoralis


  Also known as Threadworm
• Strongyloides stercoralis egg


  • Deposited in intestinal mucosa, rarely passed in stool, appearance similar to hookworm egg
• Strongyloides stercoralis larva

  • Has two forms: rhabditiform and filariform
• Strongyloides stercoralis adult worm


  • Exists as free-living (male and female) and parasitic (parthenogenetic female) forms, males are relatively shorter than females
• Strongyloides stercoralis pathogenesis and clinical manifestations

  1. Means of infection include skin penetration, autoinfection (same parasite), and hyperinfection (same species but different parasites)

  2. Can cause a wide spectrum of diseases including pneumonitis, diarrhea, malabsorption syndrome, ground itch, hyperinfection syndrome (increased L1 transforming to L3 in gut), and disseminated strongyloidiasis in immunocompromised hosts
• Enterobius vermicularis


  Also known as Pinworm or Seatworm
• Enterobius vermicularis egg


  • D-shaped, with a flattened side and convex other side, translucent shell with an outer albuminous covering
• Enterobius vermicularis adult worm

  • Light yellow to white in color, has a lateral wing/cephalic alae at the anterior end and a prominent esophageal bulb
  • Females are longer with a long, thin, and pointed tail, males have a curved and flat tail with a single spicule
• Enterobiasis or Oxyuriasis


  1. Means of infection include ingestion of eggs, inhalation of eggs, and retroinfection

  2. Can cause pruritus ani (nocturnal perianal itching) due to hypersensitivity reaction to secretions and excretions of the worm
• Capillaria philippinensis

  Also known as Pudoc worm
• Capillaria philippinensis egg

  • Peanut or guitar-shaped, with a striated shell and flattened bipolar plugs
• Capillaria philippinensis adult worm

  • Has two types of female worms: typical oviparous with 8-10 eggs in uterus, and atypical larviparous
• Transmission


  Inhalation of eggs
• Retroinfection


  Reinfection of the host by the parasite
• Enterobiasis or Oxyuriasis


  Disease caused by Enterobius vermicularis (pinworm)
• Pruritus ani


  Nocturnal perianal itching due to hypersensitivity reaction to secretions and excretion produced by worm
• DIAGNOSIS: Definitive Diagnosis: Scotch Tape Swab
• TREATMENT AND CONTROL: Mebendazole: drug of choice
• TREATMENT AND CONTROL: Albendazole: alternative
• TREATMENT AND CONTROL: Treatment of entire household is recommended
• TREATMENT AND CONTROL: Test of cure: 7 negative perianal swabs done on consecutive days