Helminths: Parasitic Worms, Life Cycles and Diagnosis

Helminths: Parasitic Worms

Helminths are group-living and parasitic species. They include flatworms (Platyhelminthes) and roundworms (Nematelminthes), collectively known as worms.

• Acanthocephala: thorny-headed worms
• Schistosomiasis: caused by Schistosoma mansoni

Morphology: male, female and egg.

Cycle:

Habitat:

Tapeworms and Cysticercosis

Cysticercosis is caused by T. solium and T. saginata.

• The presence of changes in the skin may be caused by the adult form of T. solium and T. saginata in the bowel.
• Cysticercosis more commonly affects any tissue, and can cause brain involvement, which is more common with T. solium.
• The oncosphere is the larva contained inside the egg.

Morphology: T. saginata and T. solium have a flattened, ribbon-shaped body divided into a scolex and a strobilus.

• Scolex: functions as an organ of fixation to the bowel mucosa.
• Neck: the region where proglottids are formed.
• Strobilus: the long body of the parasite.
• Proglottids: eggs are released in mature proglottids, which are eliminated in feces. Cattle or pigs ingest the eggs; a person who eats raw or undercooked meat becomes infected. The parasite goes to the small intestine, where it develops into an adult worm and multiplies.

Cysticercosis results from ingesting eggs in food, where an oncosphere develops inside.

Diagnosis: eggs detected in feces.

Ascariasis

• Ascaris lumbricoides (humans)
• Ascaris suum (pigs)
• Toxocara canis (dogs)

Morphology: male, female and egg.

Habitat: adult worms live in the small intestine (jejunum and ileum).

Egg cycle: the larva (L1, rhabditoid) develops inside the egg, moves on to L2 and then quickly becomes L3 (filariform). The L3 larvae are ingested with contaminated food or water, attach to the intestinal wall and develop into adult worms.

Transmission: contaminated water and food containing eggs with L3 larvae.

Diagnosis: stool examination for larvae.

Enterobiasis (Pinworm)

Enterobius vermicularis (pinworm)

Morphology: male, female and egg.

Habitat: males are found in the appendix and caecum; females are found in the caecum.

Transmission: heteroinfection (person to person).

Cycle: eggs are ingested through contaminated hands or objects, pass through the stomach and reach the bowel. After maturation, adults multiply; females migrate to the anus and lay eggs around the perianal region, and the cycle repeats. Infection is most commonly found in children, usually with perianal itching (perianal pruritus).