introduction to parasites

Question 1

what is a parasite

Answer 1

organism that lives in or on another organism (host) and derives its nutrients at the expense of this host

Question 2

where are parasites particularly problematic

Answer 2

resource-poor countries

most parasites are present in the tropics and subtropics

Question 3

what is a vector

Answer 3

an organism which acts as an intermediate host for a parasite then transmits the parasite to the next host e.g. Anopheles mosquito for malaria

Question 4

classification of parasites

Answer 4

parasites

• protozoa - malaria, amoebae, flagellates
• helminths (worms) - roundworms, tapeworms, flukes
• ectoparasites - lice, ticks, mites

Question 5

what are the 4 key points regarding parasites

Answer 5

• Parasites live inside the body or some live on it
  
  • They are classified by cellular structure and life cycle
  • They are mainly spread by vectors (intermediate hosts)
    Predominantly a problem of resource - poor countries

Question 6

protozoa

Answer 6

• Microscopic, single celled organisms
• Can be free living or parasitic
• Able to multiply in humans - key aspect of their life cycle
• Transmission:
  ○ Protozoa that live in a human’s intestine –> faecal-oral route
  Protozoa that live in the blood or tissue of humans –> insect vector

Question 7

malaria as an example of a protozoa

Answer 7

commonest cause of death by parasitic infection globally

most deaths are in African region

it is a preventable disease,

Mosquito-borne disease caused by Plasmodium parasite, patients experience fever, chills and flu-like illnesses, can be fatal if left untreated

P. falciparum has the highest mortality

Question 8

malaria prevention

Answer 8

A = Awareness of the risk 
B = Bite prevention 
C = Chemoprophylaxis (appropriate choice of antimalarial medication and compliance with the regime 
D = Diagnosis (prompt diagnosis and treatment without delay)

Question 9

amoebic dysentery as an example of a protozoa

Answer 9

• approx. 50mln cases per annum
• Causes bloody diarrhoea (dysentry)
• Entamoeba histolytica invade gut wall
• Microscopy - cysts in formed stool
  Clinically: intestinal and extra-intestinal infections, liver abscess in late disease
  Entamoeba histolytica - causes amoebic liver abscess, reddish-brown pus

Question 10

Leishmaniasis as an example of a protozoa

Answer 10

• There are several Leishmania species
  ○ Cutaneous or mucocutaneous leishmaniasis
  ○ Visceral leishmaniasis (kala-azar): Fever, weigh loss and hepato-splenomegaly
  
  • Infection by bite of sandflies (vector)
  • Diagnosis by histology and PCR of tissues
    cause infection in a variety of body systems and can cause complications if left untreated

Question 11

Enterobiasis (nematodes)

Answer 11

(pin worm): 1cm, threadlike, found in children
○ Cause anal itch, especially at night (autoinoculation)
○ Diagnosis: press adhesive Sellotape against perianal region in the morning
Ova seen on microscopy

Question 12

ascaris lumbricoides (nematode)

Answer 12

○ Ingested eggs hatch in the intestine, ova seen in faeces by microscopy
○ Largest nematode in humans (15-35cm)
○ Often asymptomatic (may cause failure to thrive in children or developmental delay)
○ Mass of worms may obstruct small intestine or common bile duct
Females are generally larger than males
very common globally

Question 13

cestodes

Answer 13

long segmented tapeworms

Question 14

Taenia saginata (beef), Taenia solium (pork) (cestodes)

Answer 14

• Larval cysts ingested (intermediate host) - eating raw or undercooked meat
• Adult tapeworms in humans (definitive host) can live in gut for years
  Ova in stools on microscopy
  neurocysticercosis - Cysts from pork tapeworm can form in the brain following ingestion of egg and heavy infection

Question 15

echinococcus (cestode)

Answer 15

carried by dogs, wolves, foxes
- Humans ingest eggs (dog faeces)
- Eggs hatch and enter circulation
Hydatid cyst forms in liver - surgical resection must involve whole cyst

Question 16

Trematodes

Answer 16

(flatworms, flukes)

Different species cause human infection in various body sites (blood, lung, liver, pancreas, intestines)

Question 17

Schistosomiasis (trematode)

Answer 17

• 200mln affected worldwide, 20 000 deaths p/a in tropical areas
  
  • 3 major species of Schistosomes: S. haematobium (bladder), S. mansoni (intestinal), S. japonicum (intestinal)
    Cases occur near affected fresh water - snails important in life cycle - swimmer’s itch is a common sign
    can lead to complications such as liver failure and bladder cancer if left untreated

Question 18

ectoparasites

Answer 18

• Parasites that live outside of the body

Most are insects/arachnids (8 legs e.g. ticks), many are vectors of infection (e.g. ticks can spread Lyme disease)

Question 19

transmission of helminths

Answer 19

Most are transmitted by faecal-oral rout but some use an intermediary host/vector

Question 20

diagnosis of parasites

Answer 20

• Definitive diagnosis: identification of parasites in host tissue or faeces/urine
  
  • Blood microscopy for parasites: thick and thin blood films for malaria
  • Stool microscopy for ova/cysts/parasites
    Worm infection usually accompanied by Eosinophilia and elevated IgE

Question 21

nematodes

Answer 21

round worms

Question 22

blood films for malaria

Answer 22

specially stained blood film shows infected red cells (parasitaemia), appear on microscope slide as ring shaped with double chromatin dots or crescent shaped (mature form)

Question 23

why is malaria hard to eradicate

Answer 23

it has a complex life cycle

Question 24

what is the role of the anopheles mosquito in malaria

Answer 24

it acts as the vector