Biology of medically important parasites

Chapter 8

Biology of medically important parasites

26 topics

Overview and terminology

  1. 78Overview of parasitismA parasite lives on or in another organism, its host, and feeds at its expense. The words for parasites and their hosts (definitive, intermediate, paratenic, reservoir, accidental, vector) are the grammar of every life cycle in this chapter, and knowing which host a person is tells you how the infection is caught, what it does and how it is stopped.

Protozoa

  1. 79Protozoa. Phylum Sarcomastigophora. Subphylum Mastigophora (Flagellata). Trypanosoma brucei gambiense, Trypanosoma brucei rhodesiense. Trypanosoma cruziWhat protozoa are and how the classic scheme sorts them, the flagellates and their four body forms, and the two trypanosomes that cause sleeping sickness in Africa and Chagas disease in Latin America.
  2. 80Leishmania donovani. Leishmania tropica. Leishmania braziliensisLeishmania, a flagellate that lives inside our macrophages and is carried by sand flies, its two body forms and its life cycle, and the three diseases its species cause in the skin, the mucosa of the nose and mouth, and the organs.
  3. 81Trichomonas vaginalis. Trichomonas tenax. Giardia lamblia (Lamblia intestinalis)Three flagellates that live on our surfaces and never invade them, Trichomonas vaginalis in the urogenital tract, Trichomonas tenax in the mouth and Giardia lamblia in the small intestine, how each is built and passed on, and the diseases two of them cause.
  4. 82Subphylum Sarcodina. Entamoeba histolytica. Entamoeba dispar. Entamoeba gingivalisThe amoebas of the gut and the mouth, Entamoeba histolytica, its harmless double Entamoeba dispar and Entamoeba gingivalis, how they are built and passed on, and amoebiasis, in the colon and in the liver.
  5. 83Phylum Apicomplexa. Toxoplasma gondiiToxoplasma gondii, the apicomplexan that lives inside our cells, its three infective forms and the apical complex it enters cells with, its life cycle between cats and every other warm-blooded animal, and toxoplasmosis, in the healthy, in pregnancy and when immunity fails.
  6. 84Plasmodium vivax. Plasmodium malariae. Plasmodium falciparum. Morphological differences in the erythrocyte schizogony of the genus PlasmodiumPlasmodium, the apicomplexan that causes malaria, its life cycle between the Anopheles mosquito and a person, through the liver and again and again through red cells, how P. vivax, P. malariae and P. falciparum differ in the red cell, and malaria itself, from the fever to the severe disease, its diagnosis, treatment and prevention.
  7. 85Phylum Ciliophora. Balantidium coliBalantidium coli, the only ciliate that infects people and the largest protozoan parasite of humans, how its trophozoite and cyst are built, how it passes from pigs to people, and balantidiasis, from silent carriage to dysentery.

Metazoa

  1. 86Metazoa: important evolutionary changes associated with the transition to multicellularity. Phylum Cnidaria (Coelenterata). Genus Hydra. Genus ObeliaAnimals are built of many cells that share out the work, held together by a collagen matrix, and every one grows from a hollow ball of cells. The cnidarians, Hydra and the jellyfish among them, are with the comb jellies the simplest animals with true tissues, two layers of cells with a jelly between them, a gut with one opening, a nerve net and stinging cells, which make some of them a hazard to swimmers.
  2. 87Phylum Platyhelminthes: important evolutionary changes. Class Trematoda. Fasciola hepaticaThe flatworms and what changed in them on the way from the cnidarians, the flukes and their life cycles, and Fasciola hepatica, the sheep liver fluke, from its organs and larvae to fascioliasis in people who eat raw watercress.
  3. 88Dicrocoelium dendriticum (D. lanceatum)Dicrocoelium dendriticum, the lancet fluke of sheep and cattle, a fluke of dry land whose cycle runs through a land snail and an ant it drives to the tips of the grass, and the rare human infection, told apart from eggs that only pass through.
  4. 89Opisthorchis felineus. Genus Schistosoma and its speciesTwo flukes that reach people through fresh water. Opisthorchis felineus, the cat liver fluke, is eaten in raw fish and lives in the bile ducts; the schistosomes, the blood flukes, bore through the skin and live as pairs in the veins, where their eggs cause the disease.
  5. 90Class Cestoda. Diphyllobothrium latumThe tapeworms, flatworms without a gut that live as ribbons of segments in the intestine of a vertebrate, their body, their larvae and their hosts; then Diphyllobothrium latum, the broad fish tapeworm, from its grooved scolex to the copepod and the perch it passes through, and the vitamin B12 it takes from people who eat raw fish.
  6. 91Taenia saginata. Taenia soliumThe beef and the pork tapeworms, how to tell their scolices and their proglottids apart, how cattle and pigs carry their cysticerci, and why only the pork tapeworm is dangerous, its eggs growing into cysticerci in people too, in the brain the commonest preventable cause of epilepsy in the world.
  7. 92Echinococcus granulosus. Hymenolepis nanaTwo small tapeworms with outsized effects. Echinococcus granulosus, a few millimetres long in a dog's gut, whose larva grows in a person's liver or lungs into a hydatid cyst that can hold a litre of fluid and millions of protoscolices; and Hymenolepis nana, the dwarf tapeworm, the commonest tapeworm of people, which can complete its whole life cycle inside one person.
  8. 93Phylum Nematoda. Ascaris lumbricoidesThe roundworms, the round, unsegmented worms with a complete gut and a false body cavity, how their body is built and how they grow; then Ascaris lumbricoides, the large roundworm, the commonest worm of people, whose larvae travel through the blood and the lungs before they settle in the gut.
  9. 94Enterobius vermicularis. Trichuris trichiura (Trichocephalus trichiurus)Two small roundworms of the large intestine that need no other host. The pinworm, the commonest worm infection in western Europe, whose eggs infect within hours and pass from the anus to the mouth on the fingers; and the whipworm, which threads its thin front into the lining of the caecum, and whose eggs must ripen in the soil.
  10. 95Trichinella spiralisA tiny roundworm caught from undercooked pork or game. Its adults live a few weeks in the small intestine; their young settle in skeletal muscle, where each larva remakes a muscle cell into a nurse cell and lives on for years. Fever, swollen eyelids and aching muscles are its mark.
  11. 96Ancylostoma duodenale. Strongyloides stercoralisTwo small roundworms of the small intestine whose larvae go in through the skin. The hookworm holds on to the gut wall with its mouth and feeds on blood, a major cause of iron-deficiency anaemia in the tropics; Strongyloides, a roundworm that can multiply inside its host, can stay for decades and, in a patient given corticosteroids, overwhelm them.
  12. 97Dracunculus medinensis. Filarial worms: Wuchereria bancrofti, Loa loa, Onchocerca volvulusRoundworms of the tissues, not the gut. The Guinea worm female, about a metre long, comes out through a blister on the leg, and people catch it from drinking water; it is nearly eradicated. The filariae, Wuchereria, Loa and Onchocerca, are passed on by biting insects and cause elephantiasis, the eye worm and river blindness.
  13. 98Phylum Annelida: important evolutionary changes. Lumbricus terrestris. Hirudo medicinalisThe annelids, the segmented worms, are the first animals in this course with a true coelom, a body built of repeated segments and a closed blood system. The earthworm shows the plan most clearly; the medicinal leech has turned it into a bloodsucker whose saliva gave medicine the anticoagulant hirudin and whose bite still saves replanted fingers and flaps.
  14. 99Phylum Arthropoda: important evolutionary changes. Class Crustacea: general characterisation. Class Arachnida. Orders Scorpiones and AraneaeThe arthropods, animals with jointed legs and an outer skeleton of chitin, are over four fifths of all known animal species. Among them the crustaceans live mostly in water, and a tiny one, Cyclops, carries the guinea worm and the fish tapeworm; the arachnids took to land with eight legs and no antennae, and two of their orders, the scorpions and the spiders, hold the venomous arachnids a doctor in southern Europe will meet.
  15. 100Order Acari. Medical importance of ticks and mitesTicks and mites are arachnids, not insects. Ticks are large; hard ticks feed on blood for days and pass on many infections, from Lyme disease, the commonest tick-borne disease of the northern hemisphere, to tick-borne encephalitis; mites are tiny, and the itch mite burrows in the skin and causes scabies.
  16. 101Class Insecta. The role of insects as vectors and mechanical carriersInsects are the largest class of animals, and a few of them shape human health more than any other group. Mosquitoes, sand flies, tsetse flies, bugs, lice and fleas carry malaria, dengue, leishmaniasis, sleeping sickness, typhus and plague inside them as biological vectors; the housefly carries germs on its body and in its vomit as a mechanical carrier.
  17. 102Pediculus humanus. Phthirus pubis. Cimex lectularius. Pulex irritansFour wingless insects that live on us or beside our beds and feed on our blood. The head louse is common in schoolchildren; the body louse, a parasite of poverty and war, carries epidemic typhus, trench fever and relapsing fever; the crab louse is passed on by sex; the bed bug and the human flea bite and itch but carry little.
  18. 103Genus Culex and genus Anopheles: morphological differences. Phlebotomus papatasiThe two-winged flies that carry the most disease. Anopheles carries malaria, and filariae in the countryside; Culex carries filariae in towns and viruses such as West Nile. They are told apart at every stage, from the egg to the resting female. The sand fly Phlebotomus papatasi, a tiny hairy fly that rests with its wings raised, carries Leishmania and the sand fly fever viruses.