Phylum Nematoda: Roundworms

Phylum Nematoda

• Nematodes, or roundworms, are abundant animals found worldwide, with around 5 billion potentially in each acre of fertile soil.
• Nematodes eat a variety of organic substances, from decaying stuff to the living tissues of invertebrates, vertebrates and plants.
• Nematodes' sizes vary from microscopical to several meters.
• Nematodes can be parasitic to plants and other animals, or free-living in marine, freshwater, or soil environments.
• Nematodes play a role in nutrient recycling in soils and bottom sediments.
• Soil-transmitted geohelminths are the most clinically significant intestinal nematodes.
• Geohelminths spread by faecal contamination of the soil.
• A person gets infected either by; swallowing infective eggs, or by infective larvae penetrating the skin.
• Before becoming adults in humans, hookworm, A. Lumbricoides and S. Stercoralis larvae migrate through the heart and lungs for ten days.
• Filarial nematode infective larvae are transferred by insect vector bites (Anopheles in Africa, Culex quinquefasciatus in the Americas, Aedes and Mansonia in the Pacific and Asia).
• Trichinella spiralis is transmitted by eating larvae in infected tissue.
• Dracunculus medinensis is transmitted by ingesting an infected intermediate host (Cyclops).
• Humans are the primary or most important hosts for the majority of clinically important nematodes.

General nematode characteristics

• Nematodes have a round body in cross section and are covered by a collagenous cuticle (skin) that may be smooth, spined, or ridged.
• Most nematodes are dioecious, with separate sexes; males are smaller than females.
• Nematodes move by contracting longitudinal muscles, lacking cilia or flagella.
• Nematode's body wall is mad up of longitudinal muscles
• Nematodes have triploblastic body organization, bilateral symmetry, a worm-like form (long and slender), are unsegmented, and pseudocoelomate.

Parasitic nematode adaptations

• Parasitic nematodes have developed a variety of evolutionary adaptations that aid their survival like;High reproductive potential, life cycles, enzyme-resistant cuticle, resistant eggs, and encysted larvae.

Economic Importance of Nematodes

• Numerous nematodes are free-living, functioning as decomposers and predators of microorganisms.
• Parasitic nematodes affect humans directly or indirectly via domestic animals; for example, Ascaris lumbricoides causes ascariasis, whereas filarial worms cause illnesses such as elephantiasis, onchocerciasis/river blindness, and dracunculiasis/guinea worm ulcer disease.

Classification of Nematodes

• The classification of nematodes is divided into two classes based on morphological characteristics in the presence or absence of caudal sense organs called phasmids: Secernentea (Phasmidea) and Adenophorea (Aphasmida).

Class Phasmidea

• Phasmids are present in the tail region
• Excretory system is present and well developed
• Amphids are present but poorly developed in the anterior end with simple pores
• Many parasitic forms are members of this class.

Class Aphasmidea

• Phasmids are absent
• Amphids (anterior sense organs) are present but rarely pore-like
• The excretory system is absent, or if present, poorly developed.
• Trichinella spiralis, Trichuris trichiura (whipworm) are examples of nematodes in the Aphasmidea class

Nematodes of medical importance

• Nematodes of medical importance are grouped into intestinal nematodes and filarial/tissue nematodes.
• Intestinal nematodes include Ascaris lumbricoides (large roundworm), Enterobius vermicularis (threadworm), Trichuris trichiura (whipworm), Strongyloides stercoralis, Ancylostoma duodenale (hookworm), Necator americanus (New World hookworm), and Trichinella spiralis (pork worm).
• Filarial and other tissue nematodes include Wuchereria bancrofti, Brugia spp., Loa Loa, Onchocerca volvulus, and 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, whereas encysted larvae are found in the striated muscles of these hosts.

Large Roundworm (A. lumbricoides)

• It is the largest nematode parasite of human intestines.
• The adult male worm measures 15-30 cm, smaller than the female measures 20-40 cm in length.
• 800 million worldwide are infected with Ascaris lumbricoides.
• Adult Ascaris lives in the small intestine of humans (jejunum precisely).
• Ascaris infections are commonly occurring in children ages 3-8, whose fingers are contaminated through playing on open ground.
• Factors responsible for Ascaris infections are, production of many eggs per day by the fertilised worms, and viability of the eggs in soil and dust for several years.
• A. lumbricoides is particularly common in the tropics and subtropics, where environmental sanitation is substandard and human excrement is used as fertilizer.

Parasitic Adaptations of Ascaris lumbricoides

• Presence of resistant, chitinous shell on its zygotes, protects them from unfavourable environmental factors and increases the viability of the zygote for years.
• The minute size and resistant nature of the eggs makes them withstand prolonged dryness and cold.
• Presence of a tough, thick, and resistant cuticle which offers protection against digestive enzymes and antitoxins.
• Lack of digestive glands, its muscular pharynx facilitates food ingestion by its sucking action
• Well developed reproductive system, this makes room for many eggs to be produced, to make up for poor chances of the right host being reached.

Transmission and Life cycle of A. lumbricoides

• A person becomes infected by swallowing infective eggs from contaminated food, drinking water, or faecally contaminated hands with eggs.
• In the intestine, a first-stage larva (L1=rhabditiform) develops rapidly, it molts, and matures and develops into a second-stage larva (L2-filariform), the infective stage.
• The larvae then enter the intestinal wall being transported to the lungs, moult twice, migrate up the trachea and are swallowed again.
• The worms mature in the intestine and begin producing eggs.
• A. Lumbricoides speads by faecal pollution of the environment and has a direct life cycle involving only one host (man).
• The four life cycle stages of A. Lumbricoides are ingestion, migration, maturation, and reproduction.
• A. Lumbricoides causes ascariasis and malnutrition in young children.

HOOKWORMS

• Hookworm infection in man is caused by two species of hookworms: Ancylostoma duodenale (Old World hookworm) and Necator americanus (American or New World hookworm).
• Hookworm disease remains common throughout the tropics and subtropics.
• A. duodenale was prevalent along the Mediterranean coast of Europe and Africa, in northern India, China, and Japan, while N. americanus was commonly occurring in Central and South America, Central and Southern Africa, Southern India, and the Pacific Islands.
• Adult worms live in the small intestine, mostly in the jejunum.
• The male worm is smaller (8-11 mm in length) than the female worm (10-13 mm in length).

Adaptations of A. duodenale to Parasitic Life

• They have a thin shell for protection as they travel through and out of the digestive tract.
• Hookworm larvae have amphids (large paired sensilla on each side of their mouth) that allow them to locate and penetrate the host.
• The juvenile and adult forms are adorned with two pairs of sharp teeth allowing them connect to the host’s intestine, ingest intestinal mucus and penetrate blood vessels allowing them to feed on blood.
• Hookworms releases a natural anticoagulant secretion to keep the blood from clotting.
• Hookworms have a hydrostatic skeleton to make it easier for them to move throughout the substrate, circulatory system and digestive system.

Transmission and Life Cycle of Hookworms: Ancylostoma duodenale and Necator americanus

• Hookworm infection is caused by fecal pollution of the soil and is Monogenetic, with no intermediate host required.
• Infection results when infective filariform (L3) larvae penetrate the skin, especially when walking barefoot on soil.
• The skin between the toes, the dorsum of the foot, and the medial aspect of the sole are common sites of entry.
• A. duodenale can transmit through vegetables/ raw food through ingestion of infective filariform larvae.
• Female A. duodenale lay more eggs than N. americanus with eggs hatching in warm, moist soil to release L1-stage larva.
• In humans the larvae migrate, pass through lungs/trachea and are swallowed to mature attached to the intestinal wall.
• Both hookworms cause hookworm disease or infection.

The Filarial Worm

• W. bancrofti is responsible for Lymphatic Filariasis / elephantiasis, alongside two filarial worms: Brugia malayi and Brugia timori.
• The parasitic worms live in the lymphatic vessels and lymph nodes.
• Microfilariae of filarial worms in W. bancrofti are present in the peripheral blood in greater numbers during certain hours, this corresponds to the timing in which their insect vectors (mosquitoes of the genus Culex, Anopheles, Aedes and Mansonia) bite.
• During nocturnal periodicity, microfilariae are present in the peripheral blood during night hours being periodic to W. bancrofti, B. malayi and B. timori.
• During diurnal periodicity microfilariae are present in the day such as Loa Loa
• In a diurnal subperiodicity microfilariae are found throughout the 24 hours where numbers only slighty increase such as W. bancrofti, B. malayi.
• Geographic distribution of W. bancrofti is in tropics/ sub tropics of sub-Saharan Africa, South-East Asia, India, and lands of the Pacific and South America.

Risk Factors for Elephantiasis

• Elephantiasis can affect people at any age, the common risk factors include long term living in tropical/subtropical areas, mosquito vector exposure, and unsanitary living conditions.
• W. bancrofti transmission is by female mosquitoes dependent on geographic area, species include: Anopheles funestus and A. gambiae (Africa), Culen quinquefasciatus (Americas), Aedes aegypti (Pacific) and Mansonia uniformis (Asia).

Transmission and Life cycle of W. bancrofti

• Infection occurs after vector deposits infective larva on open wound on human skin.
• L3 is called filariform larvae.
• W. bancrofti passes its life cycle in two hosts.
• Definitive host - Which is man.
• Intermediate host is dependent on mosquito species, acting as vectors across different geographic areas.