SIX ESSENTIAL ASPECTS TO A PARASITE LIFE CYCLE

 

          1.  Find a Host

 

                   A.  Active:  Host produces either chemical, thermal or light signals to which the infective stage is sensitive.

 

                   B.  Passive:  Infective stages are dispersed passively through environment, such as waves or water currents.

 

          2.  Enter a Host

 

                   A.  Active:  Infective stage may burrow into skin, as cercariae of blood flukes do.

 

                   B.  Passive:  Host will ingest or inhale infective forms.

 

          3.  Overcome Host Defenses:  Mechanisms include

 

                   A.  Antigen Shielding:  Surface of parasite adsorbs host derived antigen, so that parasite is recognized as “self”.  (Documented among adult blood flukes)

 

                   B.  Surface Antigen Shifting:  Proteins forming protein surface change so that immune reactions lag behind development of the parasites.  (Documented in Trypanosoma gambiense and T. rhodesiense (African trypanosomiasis.)

 

          4.  Derive Nutrients From Host

 

                   A.  Aerobic Metabolism

 

                   B.  Obligate Anaerobic Metabolism

 

          5.  Reproduce More Individuals

 

                   A.  Hermaphroditism (especially simultaneous)

 

B.  Polyembrony

 

C.  Very high egg output: among some tapeworms, daily output can be in millions, but parental care is negligible.

 

          6.  Disperse Young to New Hosts

 

A.     Presence of Obligate Free-living Stage:  Needs to tolerate changes in temperature, osmotic pressure, desiccation.

B.     Passive or Active Dispersal:  Cercariae of digenetic trematodes burrow out of snail host in search of 2nd intermediate host or of definitive host.

C.     Parasite-Induced Change in Host Behavior:  Intermediate stages will induce qualitative changes in the intermediate host such that the infected intermediate host is more likely to be captured by a predator which is the definitive host.

D.     Parasite-induced change in host morphology, making them less able to avoid predators, as in the case of Ribeiroia ondotrae metacercariae among green frogs.