L36 protozoa and helminths

• Protozoa


  Diverse group of single celled "animal-like" eukaryotes, 2-100µm
• Protists


  Includes protozoa, slime moulds, unicellular algae
• Protozoa


  • Most are free-living, some are pathogens
  • Prevalent in (sub)tropical regions
  • Can infect tissues/organs
  • Intracellular parasites in cells
  • Extracellular parasites in blood, intestine, urogenital system
• Transmission of protozoa


  By vector (e.g. malaria, sleeping sickness) or contaminated water/soil/food (e.g. amoeba, toxoplasmosis)
• Toxoplasmosis


  Common infection that you can catch from the poo of infected cats, or infected meat. It's usually harmless but can cause serious problems in some people.
• Classification of protozoa


  • Amoeba- move by pseudopodia
  • Flagellates- have flagella
  • Ciliates- have cilia- most are not pathogenic
  • Apicomplexa (sporozoa)
  • E.g Plasmodium
• Malaria is found in >100 countries, but mainly tropical areas of Africa, Asia and South America
• 40% of world population at risk of malaria, but 90% of deaths are in Africa
• Number of malaria deaths have decreased over past 5-10 years thanks to better prevention methods and new drugs such as artemisin
• Malaria causative agents


  Most common and severe disease is caused by Plasmodium falciparum
• Life cycle of Malaria

  1. Parasites injected with saliva of blood-feeding female mosquitos

  2. Multiplies in liver ~2 weeks

  3. Released from liver then infect RBCs repeatedly

  4. Mosquitoes inject parasites with blood meal

  5. Parasites go through reproductive phase inside mosquito
• Symptoms of uncomplicated malaria

  • Hot stage- fever
  • Cold stage- shivering
  • Sweating stage
  • Other: headache, body aches, nausea, vomiting, weakness, enlarged spleen
• Symptoms of severe (complicated) malaria

  • Cerebral malaria- abnormal behaviour, seizures, coma (25-50% mortality rate)
  • Shock
  • Severe anaemia
  • Pulmonary oedema
  • Liver failure, jaundice
  • Swelling, rupturing of spleen
• Chemoprophylaxis for malaria


  • Areas without drug resistance: Chloroquine, proguanil
  • Areas with drug resistance: Doxycycline, atovaquone-proguanil
• Helminths


  • Multicellular, with differentiated organs; no circulatory tract
  • Vary greatly in length <1mm to >10mm
  • At anterior end some have suckers, hooks or plates that are used for attachment
  • Most do not have full life cycle in humans
  • Tough cuticle
  • Some camouflaged by coating with host molecules
  • Parasitic helminths feed on body fluids or intestinal contents
• Classes of helminths


  • Nematodes (roundworms)
  • Cestodes (tapeworms)
  • Trematodes (flukes)- leaf shaped
• Intestinal nematodes


  • Large roundworm- most common, heavy infection
  • Threadworm- mild, anal itching
  • Hookworm- slows growth and development, induces anaemia
  • Whipworm- usually asymptomatic
• Ascariasis


  • Adult worms- 10-30cm, may escape from anus, mouth, nose, or ears
  • Max intensity of infection in children age 5-10
  • Migration of larvae- to lungs- causes most of the damage
  • Heavy infection: abdominal pain, malnutrition
  • Severe infection: blockage of intestines
  • Adult parasites can migrate to other organs
• Life cycle of Ascariasis

  1. Ingestion of eggs

  2. Larvae hatch and migrate to lungs

  3. Mature worms in intestines
• Blood and tissue nematodes (filariae)

  • Live in subcutaneous tissues and lymphatic vessels
  • Adults discharge larvae
  • Microfilariae circulate in blood, ingested by insects
  • In insect develops into infective larvae
  • Can be asymptomatic for many years
• Diseases caused by blood and tissue nematodes

  • Lymphatic filariasis (elephantiasis): causes enlargement of parts of body
  • Onchocerciasis (river blindness): microfilariae enter internal tissues of eye; causes inflammation and bleeding- leading to blindness
  • Loiasis: adult Loa loa worm travels through subcutaneous tissues; causes inflammation in skin and eye.
• Cestodes (tapeworms)


  • Adult tapeworms that infect humans can live for decades, grow up to 10m long
  • Humans are usually primary host- ingestion of cyst from undercooked meat (usually pork or beef)- worm in lumen of gut, symptoms are minor
  • Rarely secondary host: ingestion of egg, and larval stages will invade tissue (muscle, brain)
• Trematodes (flukes)


  • Live for decades in tissue or blood vessels
  • Cause progressive damage to vital organs
  • Use snail as intermediate host- some use secondary host
  • Lung fluke- from eating infected crab and crayfish
  • Liver fluke- from freshwater fish
• Schistosomiasis (snail fever)


  • After malaria 2nd greatest economic impact of parasitic diseases (~200,000 deaths annually)
  • Caused by Schistosoma spp. (blood flukes); mainly S. mansoni, S. haematobotium, and S. japonicum
  • worms= 0.6-2.6 cm
  • Main route of contamination through infected water (eggs excreted in faeces or urine)
  • Symptoms: Within days: development of rash or itchy skin ("swimmers' itch"); After 1-2 months: fever, chills, cough, muscle aches
  • Main damage by eggs stuck in tissues and subsequent response of immune system
  • Organ damage because of inflammation and scarring e.g. liver, intestines, lungs, bladder; can lead to bladder cancer.
• Helminth diagnosis


  • Intestinal nematodes/cestodes: Eggs or larvae can be detected in faecal sample; Sometimes incidental diagnosis; e.g coughing up or vomiting ascarids
  • Schistosomes: Eggs in faecal sample or urine
  • Tissue nematodes: Adults in tissue (difficult); microfilaria in blood
• Some helminths survive for decades in humans as they are able to modulate the immune system
• Several studies have shown that nematode infection protects against allergic diseases, for instance asthma and dust-mite allergen