zoolec 12: nematoda

• Nematodes


  The most important pseudocoelomate animals, both in abundance and impact on mankind
• Nematodes are found worldwide in soil, oceans, freshwater habitats, plants, and all kinds of animals
• Most people only know nematodes as parasites in both humans and their domesticated animals
• Ecdysozoans


  • Many protostomes possess a cuticle, a nonliving outer layer secreted by the epidermis
  • Cuticle is a stiff, hardened outer body wall that restricts growth and must be molted via ecdysis
  • Ecdysozoans, such as roundworms and arthropods, molt cuticle as they grow
  • Regulation of molting achieved by the hormone ecdysone
  • Scientists assume that all ecdysozoans have similar biochemical steps in molting
• Phylum Nematoda: Roundworms


  • About 25,000 species are described
  • Many prefer the name Nemata for this phylum
  • May be as many as 500,000 species
  • Found in virtually all habitats in all biomes such that some good topsoil may contain billions of nematodes per acre
  • Nematode parasites are found in nearly all animal and plant species, which makes nematodes infestation studies important in agriculture and biomedical sciences
• Types of free-living nematodes

  • Feed on bacteria, yeasts, fungal hyphae, and algae
  • Saprozoic or coprozoic
  • Predatory nematodes eat rotifers, tardigrades, small annelids, and other nematodes
• Caenorhabditis elegans is an important model for studies of genomics and cell development and differentiation
• Nematode distinguishing characteristics

  • Express eutely, a set number of cells
  • Cylindrical shape
  • Nonliving cuticle that is shed during juvenile growth stages
  • Lack motile cilia or flagella, except for one species
  • Longitudinal muscles in body wall
  • Lack protonephridia
  • Pharynx is muscular with a triradiate lumen similar to gastrotrichs and kinorhynchs
• Nematode form


  • Usually less than 5 cm long; parasitic forms less than 1 mm
  • Outer, thick, noncellular cuticle secreted by underlying epidermis called hypodermis
  • Hypodermis is syncytial with nuclei located in four hypodermal cords projecting inward
  • Dorsal and ventral hypodermal cords with longitudinal dorsal and ventral nerves
  • Lateral cords with excretory canals
  • Cuticle has three layers of crisscrossing collagen, providing elasticity but restricting lateral expansion to allow for hydrostatic pressure when fluid is exerted from pseudocoelom
• Nematode body wall muscles

  • Lie underneath the hypodermis and contract only longitudinally
  • Muscles are in four bands, marked off by hypodermal cords
  • Each muscle cell has a contractile fibrillar spindle portion and a noncontractile sarcoplasmic cell body portion
  • Spindles are striated while sarcoplasmic cell bodies extend into the pseudocoel and stores glycogen
  • Unusual cell body has a process or muscle arm extending to the ventral or dorsal nerves
• Nematode hydrostatic skeleton


  • The hydrostatic skeleton formed by the fluid filled pseudocoelom gets support by transmitting force of muscle contraction to the enclosed noncompressible fluid
  • Muscles normally work antagonistically with contraction and relaxation of opposing muscles
  • Nematodes do not have circular muscles to work with longitudinal muscles so the cuticle plays that role
  • Compression and stretching of the cuticle returns the body to resting position when muscles relax as seen in nematode thrashing movements
• Nematode digestive system


  • Alimentary canal consists of mouth, pharynx, nonmuscular intestine, short rectum, and anus
  • Pharynx sucks food in via rapid contraction of anterior body and opening of lumen
  • Relaxation of muscles closes lumen and enables food to move into the intestines
  • Food moves back as new food enters and worm moves around
  • Defecation by opening the anus and allowing high pseudocoelomic pressure to expel waste
  • Some are obligate aerobes; others have anaerobic metabolism
• Nematode nervous system


  • Ring of nerve tissue and ganglia around the pharynx gives rise to small nerves at anterior end and two nerve cords (dorsal and ventral)
  • Sensory papillae are concentrated around the head and tail
  • Amphids are a pair of sensory organs on the sides of head near the cephalic circle of papillae
  • Amphidial opening leads into a deep cuticular pit with modified sensory cilia
  • Parasitic nematodes have reduced amphids, but have a bilateral pair of phasmids near posterior end that resemble amphids
• Figure 18.3 shows a diagram of an amphid in Caenorhabditis elegans

  
• Nematode reproduction


  • Most are dioecious with males smaller than females
  • Fertilization is internal where eggs are stored in uterus until deposited
  • Development is usually direct for most free-living forms
  • Cuticle is shed between each of four juvenile stages
• Figure 18.4 shows a cross section of a male nematode and the posterior end of a male nematode

  
• Ascaris nematode parasites


  • Ascaris is a well-studied genus
  • Ascaris lumbricoides is now uncommon in the United States, but about 1 billion people are affected worldwide
  • A. megalocephala is found in intestines of horses
  • A. suum is found in pig intestines
  • Female Ascaris may lay 200,000 eggs a day, which pass out in host's feces
  • Embryos develop into infective juveniles in 2 weeks but can be killed by direct sun and heat
• Common parasitic nematodes of humans in North America

  • Hookworm (Ancylostoma duodenale and Necator americanus)
  • Pinworm (Enterobius vermicularis)
  • Intestinal roundworm (Ascaris lumbricoides)
  • Trichina worm (Trichinella spp.)
  • Whipworm (Trichuris trichiura)
• Figure 18.5 shows the intestinal roundworm Ascaris lumbricoides, with the male being smaller and having a sharp kink in the tail, and the intestine of a pig blocked by Ascaris suum

  
• Ascaris environmental tolerance


  • Ascaris eggs have amazing tolerances to adverse conditions
  • Can tolerate desiccation or lack of oxygen
  • Shelled juveniles survive for long periods of time in soil, sometimes even years
  • Viable eggs remain after signs of fecal matter have disappeared
  • Infection can occur when eggs are ingested with uncooked vegetables or when children put soiled fingers into their mouths
  • Unsanitary conditions contribute to higher infection rates as soil and drinking water may be "seeded" from infected individuals
• Figure 18.6 shows the mouth of a hookworm displaying cutting plates and a section through the anterior end of a hookworm attached to a dog intestine
• Hookworm life cycle in a host

  1. Eggs pass out of host in feces

  2. Juveniles hatch in soil and live on bacteria

  3. If human skin comes in contact with infected soil, the infective juveniles burrow through skin to get to the blood vessels

  4. Infective juveniles then travel in the blood to the lungs, are coughed up and swallowed

  5. Juveniles then move from stomach to intestines to mature, much like Ascaris sp.
• Figure 18.8 shows a muscle infected with the trichina worm Trichinella spiralis
• Trichina infection


  • When poorly cooked meat containing encysted juveniles is eaten, worms are liberated and mature in the host intestine
  • Trichinella sp. can infect humans, hogs, rats, cats, and dogs
  • Hogs can become infected when eating uncooked scraps of infected meat or by eating infected rats or their droppings
  • T. spiralis has four other sibling species with variable distribution, infectivity to various hosts, and freezing resistance
  • Heavy infections cause death but lighter infections are more common worldwide
• Filarial worm nematode parasites

  • Eight species of filarial nematodes infect humans and some cause serious diseases
  • Wuchereria bancrofti and Brugia malayi infect more than 120 million people in tropical areas, known as "elephantiasis"
  • Cause inflammation and blockage of the lymphatics vessels
  • Females release live young, tiny microfilariae, into blood and lymph
  • Mosquitoes ingest microfilariae when they feed on infected animals and these develop to infective stage and move into another host when the mosquito feeds again
• Filarial worm diseases


  • Elephantiasis is caused by repeated reinfection with swelling and growth of connective tissue causing enormous enlargement of many body parts
  • River blindness, onchocerciasis, is carried by black flies and infects 37 million people in Africa, Arabia, and the Americas
  • Dog heartworm, Dirofilaria immitis, is carried by mosquitoes and is the most common U.S. filarial worm but is also found in cats, ferrets, and humans