Lec 12: Nematoda

• Cuticle

  • a nonliving external layer secreted by the epidermis
  • stiff or hardened outer body wall, restricts growth and must be molted by ecdysis.
• Regulation of molting is achieved by the hormone ecdysone.
• Ecdysozoa comprises those taxa that molt the cuticle as they grow.
• Roundworms live in the sea, in freshwater, and in soil, from polar regions to the tropics, and from mountaintops to the depths of the sea.
• Free-living nematodes feed on bacteria, yeasts, fungal hyphae, and algae.
• Free-living nematodes may be saprozoic or coprozoic (live in fecal material).
• Predatory species may eat rotifers, tardigrades, small annelids, and other nematodes
• Nematodes are unusual animals because they have a fixed number of cells in the body, a condition called eutely.
• Nematode outer body covering is a relatively thick, noncellular cuticle secreted by the underlying epidermis (hypodermis). This cuticle is shed during juvenile growth stages.
• Nematodes do not have cilia, so they could not use protonephridia for excretion. Instead, their excretory system consists of one or more large gland cells opening by an excretory pore, or a canal system without gland cells, or both cells and canals together
• Most nematodes are dioecious. Males are smaller than females, and their posterior end usually bears a pair of copulatory spicules
• Fertilization in nematodes is internal, and eggs are usually stored in the uterus until deposition
• Nearly all vertebrates and many invertebrates are parasitized by nematodes. A number of these are very important pathogens of humans and domestic animals.
• Nematodes do not have circular body-wall muscles to antagonize the longitudinal muscles; therefore the cuticle must serve that function.
• The hydrostatic skeleton of nematodes is formed by the fluid filled

  pseudocoelom gets support by transmitting force of muscle

  contraction to the enclosed noncompressible fluid.
• Alimentary canal of nematodes consists of mouth, pharynx, nonmuscular intestine, short rectum, and anus
• Amphids are a pair of sensory organs on the sides of head near the cephalic circle of papillae
• A.megalocephala, lives in the intestines of horses.
• Ascaris lumbricoides is one of the most common nematode parasites found in humans; it occurs in warm, humid regions of the earth, and about one billion people worldwide are likely to have this worm.
• Adult Ascaris worms live in the small intestine of their host.
• Ascaris females may reach 30 cm in length and can produce 200,000 eggs a day.
• Fertilized eggs of Ascaris leave the host’s body with the feces and are amazingly tolerant to dessication or lack of oxygen, but are killed by direct sunlight and high temperature.
• Shelled juveniles of Ascaris can remain viable for many months or even years in soil, so they may be transmitted to a new host by consumption of contaminated vegetables or ingestion of soil
• Hookworms are so named because the anterior end curves dorsally, suggesting a hook.
• Hookworms consume much more blood than they need for food, and heavy infections cause anemia in patients.
• Life cycle of Hookworms

  • Eggs pass in the feces
  • Juveniles hatch in the soil, where they live on bacteria
  • When human skin comes in contact with infected soil, infective juveniles burrow through the skin to the blood
  • They then reach the lungs and finally the intestine
• Trichinella spiralis is one of several species of tiny nematodes responsible for the potentially lethal disease trichinosis or trichinellosis.
• When raw or poorly cooked meat containing encysted Trichina juveniles is swallowed, the worms are liberated into the intestine, where they mature.
• Trichinella spp. can infect a wide variety of mammals in addition to humans, including hogs, rats, cats, and dogs.
• Pinworms, Enterobius vermicularis, cause relatively little disease, but they are the most common nematode parasites in the United States
• Adult pinworm parasites live in the large intestine and cecum.
• Wuchereria bancrofti and Brugia malayi infect more than 120 million people in tropical areas, known as “elephantiasis”

  • Causes inflammation and blockage of the lymphatic vessels
  • Females release live young, tiny microfilariae, into blood and lymph
  • Mosquitoes ingest microfilariae when they feed on infected animals
  • These develop to infective stage and move into another host when the mosquito feeds again
• 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