Differences and similarities in life cycle strategies of human and bird schistosomes

Petr HORAK
Department of Parasitology, Charles University, Prague, Czech Republic

Experimental work on bird schistosomes was scarce in the past. However, the importance of these parasites is growing at present. This is caused by several facts:

1. These parasites belong to important pathogens of birds, mainly in areas where an intimate contact with infected snails occurs.
2. Bird schistosomes are frequently responsible for human cercarial dermatitis.
3. Infections of laboratory mammals by bird schistosomes may include parasite migration to the lungs and central nervous system.
4. Due to some similarities in parasite adaptations, bird schistosomes (genus Trichobilharzia) may be considered as a useful model for human schistosomes.
   
   

   HUMAN SCHISTOSOMES (genus Schistosoma)	BIRD SCHISTOSOMES (genus Trichobilharzia)
   Egg laying and release
   Eggs released with the host faeces or urine in the outer environment; eggs already mature with miracidia; hatching in hypo-osmotic conditions (Schistosoma, T. ocellata, T. szidati, T. franki, etc.)
   Eggs in the nasal discharge (S. nasale)	Eggs hatch in the host tissue; iso-osmotic conditions, miracidia move freely in the nasal mucosa (T. regenti)
   Many eggs trapped in the tissue, immune response, granulomas
   Infection of intermediate snail hosts
   Usually strict specificity to snail species or even geographical populations (strains); given by specific host-recognition (miraxons), immune evasion of parasites within snails and immunological properties of snails
   Penetration of vertebrate skin
   Specific recognition of vertebrates by cercariae; usually unsaturated fatty acids as stimuli
   Specificity to relevant hosts (S. haematobium; limited spectrum, S. japonicum=broad spectrum of hosts); penetration into non-host mammals and birds - ???	Low specificity ­ cercariae penetrate the skin of birds and mammals 9the latter are even more attractive)
   Transformation of parasites to schistosomula
   Loss of resistance to hypo-osmotic water environment, switch to anaerobic metabolism, changes in surface properties (shedding of carbohydrate-rich glycocalyx, antigen exchange, building of a protective double membrane); in case of bird schistosomes ­ the same changes in birds and mammals
   Migration of parasites within vertebrates
   Skin ­ blood circulation ­ lungs ­ blood network of target organs ??? (S. nasale)	Skin ­ blood circulation ­ lungs ­ blood network of target organs intestinal wall as target tissue (T. szidati) peripheral nerves ­ CNS ­ nasal cavity (T. regenti)
   Duration of infections and fate of parasites
   Usually years, maturation in specific mammalian host	Usually weeks or few months, maturation in specific bird host, failure in mammals

For more information on bird schistosomes see: http://www.natur.cuni.cz/~horak/

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created 21/12/00