Introduction to Nemathelminthes

Introduction to Nemathelminths

• Nematodes, also known as roundworms, are a group of helminths (parasitic worms)
• Key examples include Ascaris lumbricoides, Ancylostoma duodenale, Oxyuris vermicularis, Wuchereria bancrofti, and Trichinella spiralis

Learning Objectives

• The general features of nematodes
• Their life cycle stages
• Modes of infection
• How to employ relevant control strategies
• The classification of nematodes by habitat
• Comparison of different helminth classes

Case Scenario

• A 7-year-old presented to the emergency unit with abdominal pain and vomiting for 3 days
• Physical examination revealed abdomen tenderness, and X-rays showed signs of intestinal obstruction
• Ultrasound identified masses in the intestinal lumen
• A laparotomy was performed, and unsegmented cylindrical worms, around 20-30 cm were extracted

Parasites of Medical Importance

• Helminths are one category of parasites of medical importance
• Other categories of parasites include protozoa and arthropods
• Platyhelminths(Trematodes and Cestodes) and Nemathelminths (Nematodes) are types of helminths

General Characteristics of Nematodes

• Nematodes are elongated, cylindrical, and unsegmented worms
• They have separate sexes and are parasitic to humans and animals

Nematodes Affecting Humans

• These worms are cylindrical and thread-like, with both male and female forms
• Adults vary greatly in size, ranging from millimeters to meters in length
• The body is covered by a tough outer cuticle, beneath which lies a muscle layer
• Viscera are present in the body cavity

Nematode Digestive System

• The digestive system is complete, consisting of a mouth (anterior)
• This continues to an esophagus which varies in shape and structure
• Ending with an intestine followed by a rectum, which opens through the anus

Nematode Morphology

• Nematodes can have a buccal capsule and esophagus
• A buccal cavity, copulatory spicules (male), and a copulatory bursa (male)

Nematode Reproduction

• Males contain a simple tubule differentiated into a testis, vas deferens, seminal vesicle, and ejaculatory duct that opens into the cloaca
• Males also possess copulatory structures such as a bursa and spicules
• Females have an ovary, oviduct, seminal receptacle, uterus, and vagina, which opens in the genital pore
• Female nematodes may either lay eggs (oviparous) or larvae (viviparous)

Morphological Differences Between Male and Female Nematodes

• Male nematodes are generally shorter and thinner than females
• Female nematodes are longer and thicker than male nematodes
• The posterior end of males is coiled or curved and has penial spicules, and sometimes a bursa for copulation
• The posterior end of females is straight

Life Cycle of Nematodes

• Humans are the optimal host for all nematodes
• Most nematodes complete their life cycle in one host
• Some need a second host, either an animal or arthropod
• Eggs develop into rhabditiform larvae, which then develop into filariform larvae
• Most nematodes live in the intestinal tract and lay eggs which exit via the stool (diagnostic stage), except for Trichinella, which deposits larvae
• Trichinella adults live in the small intestine but deposit larvae that encyst and encapsulate in striated muscles
• Some nematodes reside in tissues and deposit larvae

Intestinal Nematodes

• Most intestinal nematodes inhabit the intestinal lumen
• They are oviparous, laying either mature or immature eggs
• Eggs are passed in feces and develop in the environment
• They typically do not require an intermediate host
• Infection occurs through ingestion or skin penetration by infective stages (mature eggs, or filariform larva)

Tissue Nematodes

• Some nematodes inhabit extraintestinal tissue (muscles, eye)
• They are larviparous, directly laying larvae
• Larvae reach their intermediate hosts for further development
• Larvae mature inside the intermediate host (usually an arthropod) and become infective filariform larvae
• Infection is mediated through the transmission of filariform larvae from their intermediate hosts (vector)
• Trichinella spiralis is both an intestinal and tissue nematode

Modes of Infection

• Types of infection include Ingestion (Ascaris, Enterobius, Trichinella, Dracunculus medinensis), Inhalation (Ascaris, Enterobius), Skin Penetration (Ancylostoma, Strongyloides), and Insect Bites (Filaria)

Prevention and Control

• Health education can also reduce infection and reinfection
• Improved sanitation mitigates soil contamination with infective stages
• Proper sanitary treatment of human waste is also helpful
• Regular treatment of infected individuals and reservoirs is also useful
• Regular handwashing with soap and clean water, especially after handling raw meat, pets, or pigs, helps
• Avoiding eating raw pork is useful
• Vector control is also a good preventative control

Nematode Classification By Habitat

• Intestinal nematodes include Ascaris lumbricoides, Ancylostoma duodenale, Strongyloides stercoralis, Trichinella spiralis (small intestine), and Enterobius vermicularis (large intestine)
• Somatic nematodes include Wuchereria bancrofti (lymphatics), Trichinella spiralis (muscles), Loa loa (eye), and Medina worm/Onchocerca volvulus (skin/subcutaneous tissue)

Summary

• Nematodes are cylindrical worms that affect humans
• Nematode life cycles occur in one or two hosts
• They have male and female worms (sexually differentiated)
• Nematodes can live in the intestine or other parts of the body
• Transmission occurs through ingestion or inhalation of the infective stage, skin penetration, or vector bites

Public Health Importance

• More than 1.5 billion people (24% of the world's population) are infected with soil-transmitted helminths, primarily in tropical and subtropical regions
• About 10,000 cases of trichinosis occur globally each year
• Pinworm is common worldwide, especially in temperate zones and developed countries
• Roughly 68 million people, in tropical and subtropical areas, are affected by Lymphatic filariasis.

General Characteristics of Helminths

• Trematodes: Leaf-shaped (except schistosomes), flattened dorsoventrally, bilaterally symmetrical, unsegmented, has no body cavity fixes via oral/ventral suckers and hermaphroditicreproductive systems.
• Cestodes: Flattened and segmented, has no body cavity fixes via hooks/rostellum/spines and hermaphroditicreproductive systems. -Nematodes: Cylindrical, unsegmented, has a body cavity, has lips/teeth/cutting plates for fixation and a separate sex reproductive system

Digestive system

• Trematodes have a mouth, oesophagus, and intestine
• Cestodes have no digestive system
• Nematodes have a mouth, oesophagus, intestine, and anus

Life Cycle Comparison

• Trematodes: Adults live in small intestine, biliary passages, blood vessels or lungs, eggs are operculated, mature/immature laid in fresh/brackish water.
• Cestodes: Adults live small intestine, eggs are non operculatedand mature, survive months in soil
• Nematodes: Adults live in small/large intestine/tissues, eggs are mature/immature laid in soil.

Life Cycle & Host Comparison

• Trematode Larval stages: Miracidium, Sporocyst, Redia (except schistosomes), Cercaria, Encysted metacercaria (except schistosomes) require a snail Intermediate Host

• Cestode Larval stages: Cysticercus larva, Cysticercoid larva, Hydatid cyst, Coenurus Most do not require snail Intermediate Host.

• Nematodes have rhabditiform/filariform larval stages and do not require snails.

• In terms of hosts: -Trematodes use two hosts. -Cestodes use two hosts, except Hymenolepis nana -Nematodes may use an arthropod vector or animal host, while others develop with no I.H.