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Scientific progress

05 - 03 - 2018

Immunity against a pathogen can promote the proliferation of more virulent forms

Vaccination that only partially protects can sometimes select for more virulent forms of the pathogen. Researchers have noted that mycoplasmosis epidemics are becoming more severe in birds. They have tried to find out if this mechanism existed in nature, without the intervention of vaccines.


For this purpose, they studied the house finch, Haemorhous mexicanus, a passerine victim in the United States of an increasingly serious mycoplasmosis epidemic due to the bacterium Mycoplasma gallisepticum. This disease causes conjunctivitis, blindness and breathing disorders that can lead to death.


The researchers infected experimentally the birds with more or less virulent strains of the bacterium and observed the immune response and susceptibility to re-infection. They found that the less virulent strains caused immunity that protected low virulent strains, but not very virulent strains.



Then they used these results in a model of diffusion of the bacterium and they found that this selective advantage favoured the proliferation of very virulent strains and thus the severity of the disease and the epidemic.


In nature, the immune system that protects pathogenic germs may therefore in some cases favour the spread of more virulent forms of the same germs. This discovery is an element to add to the study of the epidemic risk for animals and humans.