Week 5 - Evolution of virulence Flashcards
What are serial passage experiments?
They are laboratory experiments that mimic endless within-host growth with guaranteed transmission and no host evolution
Do parasites express an optimal level of virulence?
Not always, there are examples where parasites in new populations have are overly virulent and will adapt to be fitter. There is a trade-off between virulence and transmission which must be balanced. The environment also influences virulence.
What is the role of multiple infections?
Competition between multiple infections within the same host is thought to favour mutants with a higher competitive ability. This can be mediated by enhanced growth rates which may correlate with higher virulence. The fastest replicating genotypes cause the most negative effect for the host and potentially outcompete other genotypes and infections. Thus, faster replicating and more virulent parasites have an advantage under conditions with frequent multiple infections. This results in the evolution of levels of virulence which are higher than predicted by the trade-off model. Multiple infections have been suggested as one of the single most important factors for the evolution of high levels of virulence
Can we use the trade-off model to predict the evolution of virulence?
There are no good examples of predicted changes in virulence using the model. Few experiments have been conducted and all within extreme conditions with weak responses. Comparitative evidence explains only a small portion of the variance in virulence
What is the coevolution hypothesis allowing for variation in the host population?
The thoery is that hosts evolve to reduce virulence but that parasite adaption is specific to host genotypes meaning that virulence is driven by host-parasite co-evolution. Parasite variation and specialisation is due to host evolution and variation to resist infections drive this process.
What are some of the short-sighted aspects of the evolution of virulence?
In some instances parasites may evolve within host traits that are beneficial against local conditions but may be detrimental for long term transmission. These mutants have a short term advantage at the cost of a long term disadvantage. An example of this is bacterial meningitis, when it enters the cerebral spinal fluid it is capable of entering and proliferating in the brain which has a local advantage as there are no other bacteria and only modest host defenses. However this is often fatal, killing the host and ending the parasite without transmission. Polio virus is similar acting on the central nervous systems
What is the problem of imperfect vaccines for evolution of virulence?
Evolution is driven by replication and growth of the parasite, which can be stopped by effective treatments and vaccines which eliminate the population. Impfect treatments don’t kill all parasites and allow resistant mutants to further evolve and proliferate. This is a concern especially for malaria vaccine develops who worry that antigenic escape of plasmodium could increase virulence in the face of vaccinations. Experiments were conducted on P.chabaudi through serial passage and saw increases in viruelence.
