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Host density and the evolution of parasite virulence
1Medizinische Hochschule, Hannover, Federal Republic of Germany.
Abstract:
Social and cultural habits of human populations affect the biological evolution of the agents of infectious diseases. Measles and similar diseases have evolved in the Old World and cannot have existed in their present form before the rise of the great river valley civilizations. It is suggested that increased virulence of measles in white and indigenous communities in America 1500-1800 may be due to a rare strain of the virus, which was selected during transfer from Europe. The release of viruses for biological pest control has provided new material for the study of the co-evolution of host-parasite systems, which has upset the dogma "evolution tends to avirulence". It is pointed out that this issue is closely related to the group selection debate among ethologists, i.e. to the problem: how can group selection overcome individual selection? A model is proposed in which differential growth of two strains of a parasite within the host and their transmission to new hosts is considered. It is supposed that transmission stages excreted by infectious hosts enter a common pool where they are mixed before infecting new hosts. Under these conditions, selection of the slower strain is possible only if the mean size of parasite inoculum is very small, i.e. if the density of transmission stages in the environment is low. The impact of this result on host pathology depends on the relation between virulence and transmission efficiency of the parasite.
Insights
Human social behaviors influence infectious disease evolution. A new model suggests parasite evolution towards increased virulence is possible under specific low-transmission conditions, challenging the "tendency to avirulence" dogma.
Area of Science:
- Evolutionary biology
- Epidemiology
- Microbial evolution
Background:
- Human social and cultural practices significantly impact the biological evolution of infectious disease agents.
- Historical and contemporary observations challenge the traditional view that pathogens inevitably evolve towards lower virulence.
Purpose of the Study:
- To investigate the evolutionary dynamics of host-pathogen systems, specifically addressing the potential for increased virulence.
- To explore the conditions under which parasite strains with higher virulence can be selected, even against the prevailing dogma of avirulence.
Main Methods:
- A mathematical model was developed to simulate the differential growth and transmission of two parasite strains within a host population.
- The model incorporates a common pool for transmission stages, allowing for mixing before infecting new hosts.
- Analysis focused on the conditions favoring selection of a slower-growing (potentially less virulent) strain based on inoculum size and transmission density.
Main Results:
- The study demonstrates that selection of a slower-growing parasite strain is possible only when the mean inoculum size is very small (low environmental transmission density).
- This finding challenges the assumption that evolution invariably leads to decreased virulence in parasites.
- The relationship between parasite virulence and transmission efficiency is crucial in determining the overall impact on host pathology.
Conclusions:
- Parasite evolution is complex and can lead to increased virulence under specific ecological conditions, particularly low transmission rates.
- The study provides a theoretical framework for understanding pathogen evolution that accounts for group selection dynamics and environmental transmission levels.
- These findings have implications for managing infectious diseases and understanding host-parasite co-evolutionary processes.