Nemathelminthes: General Features & Reproduction

• Introduction to Nemathelminthes

Nematodes General Features

• Nematodes are elongated, cylindrical, unsegmented worms.
• They exhibit separate sexes and parasitize humans and animals.
• Nematodes are cylindrical and thread-like and have male and female worms.
• Adult sizes vary from millimeters to meters in length.
• The body is covered with a tough outer cuticle, underneath is the muscle layer.
• Viscera is present within the body cavity.
• The digestive system is complete, inclusive of a mouth, intestine, and anus.
• The esophagus varies in shape and structure.

Nematodes Reproductive System

• Nematodes possess separate male and female worms (sexually differentiated).
• Male reproductive structures include simple tubules differentiated into testis, vas deferens, seminal vesicle, and ejaculatory duct, opening into the cloaca.
• Copulatory structures in males include the bursa and spicules.
• Female structures include the ovary, oviduct, seminal receptacle, uterus, and vagina, which opens into the genital pore.
• Females may either lay eggs (oviparous) or larvae (viviparous).
• Morphological differences exist between male and female nematodes.
• Males tend to be shorter and thinner, with a coiled or curved posterior end featuring penial spicules.
• Females tend to be longer and thicker, with a straight posterior end.

Nematodes Life Cycle

• Humans are the optimum host for all nematodes.
• Most nematodes complete their life cycle within a single host; some require a second host, either an animal or an arthropod.
• Eggs typically develop into a rhabditiform larva, which then transforms into a filariform larva.
• Most nematodes reside in the intestinal tract, laying eggs, which are the diagnostic stage, and pass with the host's stool.
• An exception is Trichinella, whose adults reside in the small intestine but deposit larvae that encyst mainly in striated muscles.
• Some nematodes live in tissues and deposit larvae.

Intestinal Nematodes

• Most inhabit the intestinal lumen, whether small or large.
• Infection is mediated through ingestion or penetration of the skin by the infective stages such as mature eggs or filariform larva.
• They are oviparous, laying either mature or immature eggs.
• Eggs are deposited in stool, known as the diagnostic stage, for further development in the environment.
• They usually do not need an intermediate host in their life cycle.

Tissue Nematodes

• Some nematodes inhabit extraintestinal tissue such as muscles and eyes.
• They are larviparous, directly laying larvae.
• Larvae reach separate intermediate hosts for further development.
• Larvae mature inside the intermediate host such as an arthropod to become infective filariform larvae.
• Infection is then mediated through transmission of filariform larva from the intermediate hosts (vector).
• Trichinella spiralis is unique as an intestinal and tissue nematode.

Modes of Infection

• Ingestion of the infective stage: Ascaris and Enterobius.
• Inhalation of dust containing the infective stage: Ascaris and Enterobius.
• Skin penetration by the infective stage: Ancylostoma and Strongyloides.
• Insect bite transmission: Filaria

Prevention and Control

• Health education helps to avoid infections and re-infections.
• Improved sanitation is necessary to reduce soil contamination with infective stages.
• There should be sanitary treatment of human waste.
• Consistent treatment of infected people and reservoirs.
• Washing hands regularly with clean water and soap before eating, after handling raw meat, pets, or pigs.
• Avoid eating raw pork meat.
• Vector control

Nematodes Classification According to Habitat

• Intestinal nematodes like Ascaris lumbricoides, Ancylostoma duodenale, Strongyloides stercoralis, Trichinella spiralis, and Enterobius vermicularis.
• Somatic nematodes
  • Lymphatics: Wuchereria bancrofti
  • Muscles: Trichinella spiralis
  • Eye: Loa loa
  • Skin/subcutaneous tissue: Medina worm and Onchocerca volvulus

Summary of Nematodes

• Nematodes are cylindrical worms affecting humans.
• Their life cycle runs in one or two hosts.
• They have male and female worms (sexually differentiated).
• They live in the intestine or other parts of the body.
• They can be transmitted through ingestion or inhalation, skin penetration, or vector bite.

Public Health Importance

• More than 1.5 billion people (24% of the world's population) are infected with soil-transmitted helminths, mainly in tropical and subtropical areas.
• Approximately 10,000 cases of trichinosis occur worldwide each year.
• Pinworm occurs worldwide, mainly in temperate zones and developed countries.
• Approximately 68 million people in the tropics and subtropics are affected by Lymphatic filariasis.

Differences Between Helminths

• Nematodes shape: Cylindrical
• Trematodes shape: Leaf-like
• Cestodes shape: Flat
• Nematodes have a digestive tract, but cestodes (tapeworms) have no digestive tract
• Nematodes: separate sexes
• Cestodes and Trematodes: hermaphroditic
• Nematodes hosts in life cycle: Some need an arthropod vector or animal host, while other nematodes are only in one host
• Cestodes hosts in life cycle: Two hosts (except Hymenolepis nana)
• Trematodes hosts in life cycle: Two hosts