schistosomes & snails

Question 1

schistosomes

Answer 1

• diegenean trematodes
  
  • alternate between sexual and asexual stages
  • sexual in mammals, asexual in snails
• platyhelminth flatworms
• cause schistosomiasis
• distributed in subsaharan africa as well as south east asia and south america

Question 2

schistosome lifecycle

Answer 2

• sexual reproduction in mammals producing eggs
• eggs deposited in urine/faeces
• eggs hatch into miracidia in freshwater
  
  • short lived, non-feeding
• uses glands to target and penetrate snail host
• two development generations in snails
  
  • forms primary sporocyst
  • then secondary/daughter sporocysts
• secondary sporocysts mature to form cercariae
  
  • target mammals, short lived

Question 3

mammalian schistosome infection

Answer 3

• adults in sexual stage residue in venules around bladder or large intestine
• form reproductive pairs and produce eggs
• eggs make their way out through gut or bladder wall

Question 4

urogenital schistosomiasis

Answer 4

• caused by schistosomes living around the bladder
• blood in urine and damage to the bladder wall due to eggs pentrating the wall
• can result in bladder calcification and cancer
• also kidney and genital damage

Question 5

intestinal schistosomiasis

Answer 5

• caused by schistosomes that don’t make it through the gut wall and are swept around circulation
  
  • end up in capillary beds, particularly in the liver
• causes liver and intestine damage
• enlargement of liver and spleen
• severe inflammatory response leading to tissue fluid leakage

Question 6

DALYs

Answer 6

• disability adjusted life year
• measure of disease burden
• years lived with disability added to years of life lost
• schsitosomiasis secondary only to malaria in terms fo DALYs

Question 7

discovery

Answer 7

• 1851 by Theodor Bilharz - S. haematobium
• life cycle elaborated using liver fluke lifecycle (also use snails)
• more than 20 species now
  
  • divided into clades
  • japonicum, mansoni and haematobium mainly infect humans
• really a group of diseases caused by different species transmitted by different snails

Question 8

schistosome pairs

Answer 8

• females closely assoicated with males within the gynaecophoric canal for egg production
  *

Question 9

schistosomiasis treatment

Answer 9

• MDA of praziquantel
  
  • kills adults stages to break transmission
  • no apparent resistance problems
  • cheap
• schistosomiasis control initiative using MDA in africa
  
  • compliance issues
• MDA needs to be in combination with:
  
  • sanitation
  • snail control (niclosamide, non-specific)
  • reduced entry of water by humans

Question 10

snails

Answer 10

• distribution and ecology of host snails has huge impact on transmission
• different species have different preferences for habitats and environmental conditions
• distribution affected by climate and anthropogenic change

Question 11

Bulinus snails

Answer 11

• host for S. haematobium
  
  • urogenital disease
• small ponds and rivers

Question 12

Biomphalaria snails

Answer 12

• host for S. mansoni
  
  • intestinal disease
• large lakes and rivers

Question 13

Oncomelania snails

Answer 13

• host for S. japonicum
• amphibious - in and out of water, sides of drainage ditches, rice paddy edges
• can move between bodies of water

Question 14

Radix snails

Answer 14

• host for S. incognitum
• widespread - lakes, rivers etc.

Question 15

south american schistosomiasis

Answer 15

• porbably transmitted form africa via slave trade
• compatible snails probably already present in south america
• ancestral biomphalaria may have been transferred to africa in a rare event and radiated from there

Question 16

Senegal river

Answer 16

• manmade dams lead to huge outbreak of S. mansoni as environment became more sutiable for biomphalaria snails
• changed river from estuarine to freshwater affecting prawn populations

Question 17

parasite penetration of snails

Answer 17

• miracidia use secretions from glands at apical papilla to penetrate
• locate snails by swimming upwards towards light
  
  • where snails are
• use chemotaxis once close enough
  
  • amino acids, glycoproteins, fatty acids and ammonia
  • released by snail
• miracidium sheds ciliated plates and encysts
• form primary sporocyst in head or foot of snail
• daughter sporocysts migrate to digestive gland

Question 18

parasite-snail compatibility

Answer 18

• sometimes non-compatible snails attract parasites that penetrate but won’t develop any further (killed by snail)
• compatiblity is apparently genetic
• low compatibility snails will often die when penetrated

Question 19

snail immune response

Answer 19

• innate only
• carried out by hemocytes activated by hemolymph factors
  
  • FREPs - fibrinogen related peptides
• lectin-type interaction
• hemoctye: ROS and phagocytosis
• low prevalence in snails suggests innate genetic resistance
  
  • low snail prevalence can still cause high human prevalence

Question 20

schistosome repsonse to snail immune reponse

Answer 20

• antioxidant and protease inhibitors counteract ROS and phagocytosis
• sporocyst evasion may be aided by developing tegument/plates
  
  • point of contact between host and parasite
  • molecular mimicry? antigenic variation?

Question 21

cercariae

Answer 21

• infective and phototactic
• emerge from snails around 30 days after infection
• human infecting - emerge at midday
• rodent infectin - emerge at night
• upon host penetration, tails are shed
  
  • → schistosomulum, circulates around host
• immune response to cercaria results in swimmers itch (cercarial dermatitis)