Nematoda

• Phylum Nematoda


  The Roundworms
• Evolution of Body Cavities

  General Description
• Roundworms (Nematoda)


  • Round worms (unsegmented)
  • Many nematodes possess a cuticle - non-living outer layer secreted by epidermis, it restricts growth and must be molted via ecdysis (shedding)
  • Regulation of molting achieved by the hormone ecdysone
  • Nematoda (Roundworms) are Pseudocoelomates - Complete Guts - mouth and anus

  
• Nematoda Characteristics


  • They are vermiform (worm-like), usually cylindrical in shape
  • Triploblastic and non-segmented, with bilateral symmetry
  • Size varies from microscopic to pencil-sized
  • Covered with cuticle, having no external cilia
  • Body cavity is a pseudocoelom, which functions as a hydrostatic skeleton
  • Muscle layers include longitudinal fibers only
  • Complete digestive system
  • Circulatory and respiratory organs lacking
  • Excretory system consists of one or more large gland cells opening to an excretory pore or canal system
  • Circular nerve ring with dorso-ventral nerve cords; sense organs include ciliated pits
  • Sexes usually separate (dioecious), male is smaller than the female
  • Fertilization internal, some with direct development, others (mostly parasites) with a complicated life history

  
• Ascaris lumbricoides


  Roundworm, occurs in up to 25% of people in some areas of the tropics and subtropics, more than 1.27 billion affected worldwide, some species found in intestine of livestock animals (horse and pigs), a female Ascaris may lay 200,000 eggs a day which pass out in host's feces, viable eggs remain after signs of faecal matter have disappeared, survive for long periods in soil, host swallows embryonated eggs, juveniles hatch, and burrow through intestinal wall, they feed on intestinal contents and may block or perforate the intestines, infection rates tend to be higher in children and males

  
• Hookworms


  • Anterior end has a hook-like curve, Necator americanus is the most common hookworm, sexes are separate, large plates in mouth cut into intestinal wall and suck host's blood, pump through more blood than they digest, heavy infections cause anaemia, eggs pass out in feces and juveniles hatch in soil, if human skin comes in contact with soil, infective juveniles burrow through skin to blood, travel in blood to the lungs, are coughed up to be swallowed, and mature in the intestine

  
• Hookworm


  • Ancylostoma (Dog Hookworm)

  
• Life Cycle of Hookworms

  Eggs pass out in feces and juveniles hatch in soil, if human skin comes in contact with soil, infective juveniles burrow through skin to blood, travel in blood to the lungs, are coughed up to be swallowed, and mature in the intestine

  
• Trichinella spiralis


  Causes Trichinellosis, encysted larvae in muscle
• Pinworms (Enterobius vermicularis)


  Most common worm parasite in the tropics and subtropics but causes little disease, adults live in large intestine, females migrate to anal region at night and lay eggs, causing itching, scratching the anal region contaminates hands and bedclothes, eggs develop rapidly and become infective within six hours at body temperature, when swallowed, hatch in duodenum and mature in large intestine

  
• Filarial Worms: Common examples

  • Wuchereria bancrofti- Elephantiasis/Lymphatic filariasis
  • Brugia malayi
  • Onchocerca volvulus- Causes River Blindness/onchocerciasis/Leopard skin
  • Dracunculus medinensis (Guinea worm)
  • Loa loa (Eye Worm)- Calabar swelling/Loasis
  • Mansonella sp.

  
• Filarial Worms


  Eight species of filarial nematodes infect humans, some cause serious diseases, Wuchereria bancrofti live in lymphatic system and cause inflammation and blockage of the lymphatics vessels, females release live young, tiny microfilariae, into blood and lymph, mosquitoes ingest microfilariae when they feed and pass to new host when they feed again, elephantiasis is caused by repeated exposure, swelling and growth of connective tissue causes enormous swelling of body parts

  
• Eight Species of Filariae Parasitizing Humans

  • W. bancrofti - Lymphatic tissues, Mosquito, Lymphatic damage, Worldwide
  • Brugia malayi - Lymphatic tissues, Mosquito, Lymphatic damage, Asia
  • B. timori - Lymphatic tissues, Mosquito, Lymphatic damage, Timor
  • Onchocerca volvulus - Subcutaneous; eye, Black fly, River blindness, Africa
  • Loa loa - Subcutaneous, Deer fly, Skin swellings, Africa
  • Dipetalonema streptocerca - Subcutaneous, Midge, Indefinite, Africa
  • D. perstans - Thoracic and abdominal cavities, Midge, Indefinite, South America
  • Mansonella ozzardi - Peritoneal cavity, Midge, Indefinite, South America
• Parasitic nematodes are associated with many plant diseases resulting in loss of agricultural productivity, some transmit other disease causing agents (e.g. viruses) to plants as they feed, some nematodes are plant feeders which live in the soil and feed on plant roots, thereby reducing the plant's uptake of water and nutrients, and reducing tolerance to other stresses such as drought
• Nematodes play vital role in public health as agents of many wasting human diseases such as Onchocerciasis or River Blindness as well as Elephantiasis amongst others
• Most species of nematodes have no effect, or have a beneficial effect, on humans and their endeavours, many species of nematodes are 'free-living', living in soil, sea and freshwater, these feed on bacteria, fungi, protozoans and even other nematodes, and play a very important role in nutrient cycling and release of nutrients for plant growth
• Some nematodes attack insects, and help to control insect pests in farms
• The nematode Caenorhabditis elegans was the first multi-cellular organism to have its DNA fully sequenced, this has led to many exciting breakthroughs in the biological and medical sciences in the last few years
• Nematodes are found in various habitats, including soil, freshwater, marine environments, plants, animals (including humans), and extreme environments.
• The phylum Nematoda is divided into three subphyla based on their morphology and life cycle.