The researches find that the authors believe that, once resistance spreads in host species, virulence may be driven by other means such as by manipulating host immune systems. The research examined the spread of bacteria called Mycoplasma gallisepticum among house finches a rare example of a well-studied host-bacteria evolution where humans have not intervened with antibiotics or vaccines. “They actually have a very poor understanding of how pathogens evolve in response to natural host resistance,” said Dr. Camille Bonneaud; of the Centre of Ecology and Conservation on Exeter’s Penryn Campus in Cornwall.
Manipulating host immune systems
“This is because there are very few systems in the wild that have been monitored in sufficient detail; without being subject to human intervention. “We typically assume that pathogens respond to host resistance (including to vaccines) by increasing their rate of replication; allowing them to transmit faster to other hosts before they are cleared by their current host. “However; our study shows that pathogens can evolve to become more virulent without increasing their rate of replication.
“They hypothesise that the increase in virulence that we observed in this study was driven by an improved ability of the pathogen to manipulate the host immune system in order to generate the symptoms necessary for its transmission.” An immune system is a collection of biological processes within an organism that protects against disease by identifying and killing pathogens and tumour cells. It detects a wide variety of agents; from viruses to parasitic worms; and needs to distinguish them from the organism’s own healthy cells and tissues in order to function properly.
Viruses to parasitic worms
Detection is complicated as pathogens can evolve rapidly; producing adaptations that avoid the immune system and allow the pathogens to successfully infect their hosts. To survive this challenge; multiple mechanisms evolved that recognize and neutralize pathogens. Even simple unicellular organisms such as bacteria possess enzyme systems that protect against viral infections. Other basic immune mechanisms evolved in ancient eukaryotes and remain in their modern descendants; such as plants, fish, reptiles, and insects.
The authors say this could lead to new approaches for tackling pathogens. For example; if trying to kill the pathogen inevitably leads to more virulent infections, it might be worth trying to slow down pathogen evolution by combining treatments that both eliminate the pathogen and prevent it manipulating host immune systems. Some populations of house finches have been expose to Mycoplasma gallisepticum for more than 20 years, while others have not—and have therefore not developed resistance.
In the study, carry out in Arizona and support by Arizona State University and Auburn University, 57 finches from previously unexpose populations are expose to the pathogen. The findings show virulence has increase consistently over more than 150,000 bacterial generations since outbreak (1994 to 2015). By contrast, while replication rates increase from outbreak to the initial spread of resistance (1994 to 2004), no further increases have occurred subsequently (2007 to 2015).