Untitled Quiz

Parasitic Nematodes

• Nematodes are also called roundworms
• They are a diverse group of free-living and parasitic worms found in soil and water
• Approximately, 500,000 nematode species exist
• Nematodes have a more developed body cavity (pseudocoelomate), alimentary canal (complete), are dioecious (separate sexes), head with buccal capsule, and can have teeth or cutting plates.
• They have a bilaterally symmetrical body, while the head is radially symmetrical
• Body wall is composed of a tough, acellular cuticle (outer layer) and longitudinal muscles (inner layer)
• Locomotion occurs through contraction of the longitudinal muscles
• Excretory system is rudimentary; methods include: through anus, excretion of nitrogenous waste as ammonia, and an H-shaped canal to regulate nutrients and waste content. In some, an excretory gland is near the esophagus.
• Most nematodes are free-living, found in soil and water
• Nematodes have a long, convoluted tube (in the male) which is differentiated into testes, vas deferens, seminal vesicle, and ejaculatory duct
• Accessory copulatory organs include a copulatory bursa with two spicules and gubernaculum
• Female reproductive system consists of two (or one) convoluted tubes differentiated to ovary, oviduct, seminal receptacle, and uterus.
• Nervous system consists of a circular nerve ring (brain), six longitudinal nerve trunks (one dorsal, one ventral, and four lateral), and some species possess sensory structures such as sensory papilla and phasmids. These are chemoreception organs that are covered by the cuticle.

Nematode Characteristics

• Morphology: Elongated, cylindrical, unsegmented worms with tapering ends
• Size: Typically 5mm (hookworm) to 1 meter (Dracunculus); Male is usually smaller than female, with a curved or coiled posterior end.
• Symmetry: Bilaterally symmetrical body with a radially symmetrical head.
• Body wall: Tough acellular cuticle (outer layer), longitudinal muscles (inner layer)
• Locomotion: Move by contraction of the longitudinal muscles
• Digestive tract: Complete alimentary canal
• Sexes: Dioecious (separate sexes)

Classification of Nematodes

• Classification is based on systematic classification of phylum Nematoda (Anderson et al., 1974)
• Classification is also based on habitat (intestinal or somatic) and the type of infection of humans.

Developmental Stages

• Adult worm
• Egg stage
• Four larval stages (L1-L4).
• Molting, the shedding of the cuticle

Modes of Infection

• Ingestion of eggs or larvae
• Penetration of skin
• Ingestion by blood-sucking insects
• Inhalation of dust containing eggs

Classification based on egg/larvae production

• Oviparous: Produce eggs that hatch into larvae in the environment
• Viviparous: Give birth directly to larvae
• Ovoviviparous: Produce eggs containing larvae that hatch immediately.

Pathogenesis (Trichinella spiralis)

• Stage of intestinal invasion (first stage)
• Stage of muscle invasion (second stage)
• Stage of encystation (final stage)

Pathogenesis (Trichuris trichiura)

• Mechanical distortion
• Allergic response (Increased numbers of macrophages)
• Common manifestations include: Abdominal pain, anorexia, bloody or mucoid diarrhea that resembles inflammatory bowel disease, Iron deficiency anemia, Recurrent rectal prolapse (due to heavy worm load and malnutrition)
• Growth retardation and impaired cognitive function.

Pathogenesis (Enterobius vermicularis)

• Asymptomatic in some
• Symptomatic patients: pruritus (often worse at night)
• Excoriation of perianal skin
• Abdominal pain and weight loss

Laboratory Diagnosis

• Direct slide technique and H and E stain
• Serology (ELISA, CIEP) – antibody detection
• Bachman intradermal test
• Animal inoculation in rats
• Stool examination – detection of barrel-shaped eggs with mucus plugs
• Cellophane tape method
• NIH swab method

Taxonomy - Examples:

• Trichinella spiralis: Causes trichinosis; Pig is the primary host
• Trichuris trichiura: Whipworm
• Enterobius vermicularis: Pinworm
• Ascaris lumbricoides: Roundworm
• Wuchereria bancrofti: Causes elephantiasis; humans are the definitive host
• Dracunculus medinensis: Guinea worm
• Loa loa: African Eye Worm

Life Cycle (Ascaris lumbricoides)

• Migratory phase: Eggs hatch, liberate L2 larvae in the duodenum, and travel through lungs, trachea, and pharynx eventually settling in intestines; L4 forms mature adults. The adults then lay eggs per day, causing a chronic infection.
• Intestinal phase: L4 larvae undergo final molt, develop into adults in the small intestine, start laying eggs. Embryonated eggs survive for a chronic time in the environment

Characteristics and Classification

• Class Adenophora/ Enoplea (includes Trichinella spiralis, Trichuris trichiura).
• Class Secernentea/ Chromaorea (includes Enterobius vermicularis, Ascaris lumbricoides, Wuchereria bancrofti, Dracunculus medinensis, Loa loa).

Pathogenesis (Wuchereria bancrofti)

• Chronic filariasis develops over 10-15 years after infection.
• Chronic host immune response to the dead worm leads to granuloma, thrombi, and formation of fibrosis in the lymph vessels, and severe lymphatic obstruction. This results in pedal edema.

Pathogenesis (Dracunculus medinensis)

• Man is the definitive host
• Cyclops is the intermediate host
• Larvae infected Cyclops ingest. L3 larvae infect man through drinking contaminated freshwater sources.
• Migrating of larvae, penetration of skin, formation of blisters and discharging of L1 larvae into water.

Pathogenesis (Loa loa)

• Vector (Chrysops species) transmits infective (L3) larvae through blood meals.
• Larvae migrate to subcutaneous tissues and develop into adult worms.
• Adult worms produce microfilariae, which are released into the blood.

Life Cycle (Loa loa) and (Dracunculus medinensis)

• Cycles are described in details based on the stages (man and intermediate host)

Symptoms of infections are described in details based on parasitic species.