A good model system for studies on invertebrate immunity is the investigation of immunity to parasites.
Examination of eels on day 112 revealed no indication for acquired immunity resulting from the primary infection.
The presence of dead parasites may not confer an acquired immunity to re-infection.
For example, as the prevalence in dogs becomes lower the probability of immunity on exposure is decreased.
The next step is to extend these analyses to a more realistic model having both natural immunity and vaccine effects.
Above 20 % immunity the number of cases required increases dramatically.
The pathogenesis of necrotic proliferative colitis in swine is linked to whipworm induced suppression of mucosal immunity to resident bacteria.
We do not know exactly what the levels of effective immunity are in the population.
