Nematodes & Geohelminths

• Nematodes are cylindrical or round worms and are abundant animals, with around 5 billion potentially living in each acre of fertile garden soil.
• Nematodes consume diverse organic matter, including rotten substances and living tissues of invertebrates, vertebrates, and plants.
• Nematodes vary in size from microscopic to meters long and can be free-living in marine, freshwater, or soil habitats, or parasitic on plants or animals, playing a key part in nutrient cycling.
• Most clinically important intestinal nematodes are geohelminths which are spread through soil via fecal contamination.
• Individuals become infected with geohelminths when they swallow infective eggs or larvae penetrate the skin.
• Examples of geohelminths that cause infection through egg consumption are Ascaris lumbricoides, Trichuris trichiura, and Enterobius vermicularis.
• Examples of geohelminths that cause infection through skin penetration are hookworms and Strongyloides stercoralis.
• Before maturing in the human host, hookworm, A lumbricoides, and S. stercoralis larvae migrate through the heart and lungs for about 10 days, developing as they migrate.
• Filarial nematode larvae are transmitted via insect vectors, such as Anopheles in Africa, Culex quinquefasciatus in the Americas, and Aedes and Mansonia in the Pacific and Asia.
• Trichinella spiralis is transmitted through the ingestion of larvae in infected tissue.
• Dracunculus medinensis is transmitted through the ingestion of infected intermediate hosts (Cyclops).
• Humans serve as the primary or only hosts for most nematodes of medical significance.
• Nematodes have a round body in cross-section, possess a body cavity, and have a layered collagenous cuticle (skin) that can be smooth, ridged or spined.
• Most nematodes are dioecious (separate sexes), in which the males are smaller than the females.
• Nematodes move through the contraction of longitudinal muscles, without cilia or flagella.
• Nematode body walls only have longitudinal muscles.
• Nematodes are triploblastic, bilaterally symmetrical, vermiform, unsegmented, and pseudocoelomate.
• Parasitic nematodes exhibit several evolutionary adaptations, including high reproductive potential, life cycles increasing transmission likelihood, enzyme-resistant cuticles, resistant eggs, and encysted larvae.
• Some nematodes are free-living, playing critical ecological roles as decomposers and predators.
• Parasitic nematodes directly or indirectly affect humans through domestic animals, such as Ascaris lumbricoides causing ascariasis.
• Filarial worm infections may result in diseases such as lymphatic filariasis (Wuchereria bancrofti), onchocerciasis/river blindness (Onchocerca volvulus), and dracunculiasis/Guinea worm ulcer disease (Dracunculus medinensis).
• Nematodes are classified into two classes based on morphological studies which look for the presence or absence of caudal sense organs called phasmids.
• Two nematode classes: Secernentea (Phasmidea) and Adenophorea (Aphasmida).

Class Phasmidea Characteristics

• Presence of phasmids in the tail region
• Well-developed excretory system
• Weakly developed amphids with simple pores towards the anterior end of the body
• Several parasitic forms exist in this class; free-living species primarily inhabit the soil.
• Example species in this class: Ascaris sp., Enterobius sp., Necator sp., Ancylostoma sp., Wuchereria sp., Strongyloides sp., Toxocara sp., Dracunculus sp., Loa sp., Onchocerca sp., Brugia spp., etc.

Class Aphasmidea

• Phasmids are absent
• Amphids (anterior sense organs) are present but are unlikely to be pore-like.
• Excretory system is absent or poorly developed
• Example species in this class: Trichinella spiralis, Trichuris trichiura

Important Nematode Parasites of Humans

• Grouped into Intestinal nematodes and filarial/tissue nematodes

Intestinal Nematodes

• Ascaris lumbricoides (large roundworm)
• Enterobius vermicularis (threadworm)
• Trichuris trichiura (whipworm)
• Strongyloides stercoralis
• Ancylostoma duodenale (hookworm)
• Necator americanus (New World hookworm)
• Trichinella spiralis (pork worm)

Filarial and Other Tissue Nematodes

• Wuchereria bancrofti
• Brugia spp.
• Loa loa
• Onchocerca volvulus
• Dracunculus medinensis (Guinea worm)
• Adult T. spiralis lives deeply in the mucosa of the small intestine (duodenum or jejunum) of pigs, bears, rats, or humans, while encysted larvae are in the striated muscles of these hosts.

Large Roundworm (A. lumbricoides)

• Largest nematode parasite in the human intestine
• Adult males (15-30cm) are smaller than females (20-40cm) in length
• Approximately 800 million people worldwide are infected with Ascaris lumbricoides, which live in the small intestine (jejunum)
• Widespread and heavy Ascaris infections are common in children (3-8 years), due to fingers becoming contaminated when playing outdoors.
• Factors for Ascaris infections include high egg production by fertilized females and egg viability in soil/dust for several years.
• More common in the tropics and subtropics, where environmental sanitation is inadequate, and untreated human feces are used as fertilizer.

Parasitic Adaptations of Ascaris lumbricoides

• Resistant shell protects the zygotes from unfavorable environmental factors, and increases its lifespan for years.
• Eggs' minute size and resistant nature make them withstand prolonged dryness and cold, while size aids in dispersal of the parasite.
• Tough and thick cuticle protects them from the host's digestive enzymes and antitoxins, while also secreting anti-enzymes to avoid digestion.
• Lacking digestive glands, food is ingested from a muscular pharynx
• A well-developed reproductive tract makes room for numerous eggs to be produced, so to make up for a lower chance of a successful infection in correct host.

Transmission and Lifecycle of A. lumbricoides

• Transmitted by ingesting infective eggs found in contaminated food, drinking water, or from contaminated hands which hatch in the intestine.
• The first-stage larva (L1), develops rapidly into a second-stage larva (L2), which is the infective stage.
• Larvae penetrate the intestinal wall, move throughout the circulation, migrate to the lungs, migrate up the trachea and is swallowed, the worms sexually mature in the intestine, mate and begin producing eggs.
• Eggs become infective in the soil after 30-40 days.
• A. lumbricoides causes ascariasis and malnutrition in young children with heavy infections.
• Spread by fecal contamination.
• Direct lifecycle involving a single host

Hookworms

• Hookworm infection in humans is caused by Ancylostoma duodenale (Old World hookworm) and Necator americanus (American/New World hookworm)
• Hookworm disease prevails in the tropics and subtropics, while N. americanus was found in Central and South America, Central and Southern Africa, Southern India, and the Pacific Islands.
• A. duodenale was prevalent along the Mediterranean coasts of Europe and Africa, in Northern India, China, and Japan
• Can also be found together following movement of infected people

Adaptations of A. duodenale to parasitic life

• Adults reside in the small intestine (mostly jejunum) with the male being smaller (8-11mm) than the female (10-13mm)
• Presence of thin shell on eggs for protection as they travel

Hookworm Larvae

• Possess amphids (large paired sensilla on each side of their mouths) that help humans to locate, attach to, and penetrate the host
• Juveniles and adults have two pairs of sharp teeth, allowing for attachment to the intestine
• Can consume intestinal mucous and blood of the host
• Produce a natural anticoagulant to keep blood from clotting, allowing for prolonged consumption
• Posses a hydrostatic skeleton that allows movement through a host's circulatory and digestive system

Transmission and Lifecycle of Hookworms:

• Ancylostoma duodenale and Necator americanus infection spreads through fecal contamination of soil.
• Hookworms are monogenetic, and humans are definitive hosts
• Infective filariform larvae (L3 - 3rd stage) penetrate skin, often via bare feet in infected soil, commonly between the toes, dorsum, and medial sole
• Eggs hatch on warm, moist soil, releasing L1-stage larva.
• Female A. duodenale lays more eggs than N. americanus
• A. duodenale is also transmitted by ingesting infective larvae on contaminated vegetables or fruits.
• Both hookworms are responsible for hookworm disease and infection.

The Filarial Worm

• Responsible for lymphatic filariasis/elephantiasis in humans; Brugia malayi and Brugia timori.
• Live as parasitic worms within lymphatic vessels and lymph nodes.

Microfilariae

• First stage larvae of filarial worms are found in blood and display periodicity
• This refers to higher presence during certain hours depending on the insect host vectors.

Periodicity

• Nocturnal: Microfilariae are abundant in peripheral blood during nighttime, such as periodic W. bancrofti, B. malayi, and B. timori.
• Diurnal: Microfilariae are abundant in peripheral blood during daylight, such as Loa loa.
• Nocturnal or diurnal subperiodicity: Microfilariae are consistently present in peripheral blood, but with a slight increase during day or nighttime, like subperiodic W. bancrofti and subperiodic B. malayi.
• W. bancrofti is widely distributed in the tropics and subtropics of sub-Saharan Africa, South-East Asia, India, and the Pacific Islands and South America.

Risk Factors for Elephantiasis

• Can occur at any age and in both sexes
• Residing in tropical or subtropical regions
• Frequent exposure to mosquito vectors
• Unsanitary conditions.
• W. bancrofti is transmitted by various female mosquitoes, dependent on geographic region: Anopheles gambiae and funestus (Africa), Culex quinquefasciatus (Americas), Aedes aegypti (Pacific), and Mansonia uniformis (Asia).

Transmission and Lifecycle of W. bancrofti

• Occurs when infective larvae are deposited on human skin from a mosquito, then penetrate the skin through the bite wound

In Humans

• Larvae grow into adults in the lymphatic system.
• Females release sheathed microfilariae, which then migrate into the lymph and bloodstream.

In Mosquitoes

• Microfilariae are ingested
• Penetrate into the thoracic muscles
• Develop into infective larvae that migrate to the mosquito's mouthparts and can then infect another human as the vector blood feeds
• Responsible for infection is the L3-stage
• W. bancrofti reproduces in two hosts: definite host (man) and intermediate host (female mosquitoes of different species based on area).