A Pathogen’s Dilemma: The Virulence-Transmission Trade-Off Oakem Kyne explores why pathogens evolve to cause different levels of harm What causes some pathogens, disease-causing
microorganisms, to become more harmful than others? How does our behaviour impact the way they evolve? Evolutionary biology helps us consider these questions.
The Virulence-Transmission Trade-Off Model | Pathogens and their hosts have constantly been coevolving over millions of years, mainly resulting from interactions between the two species. This coevolution can be observed in a pathogen’s virulence, which is the relative ability of a microorganism to cause harm to a host organism. A common way of assessing this is by tracing mortality rates. Historically, organisms were thought to reduce their virulence over time, as the pathogen relies on the host for transmission. Thus, host mortality would reduce the amount of transmission time available to the pathogen. However, this pattern was not observed and so subsequent models of host-pathogen evolution have been proposed, with the coevolution of myxoma virus with the European rabbit being a prime example. Myxomatosis is a lethal disease in rabbits caused by the myxoma virus, resulting in blindness and swelling. Originating from South America, the myxoma virus was intentionally introduced to the Australian rabbit population in the 1950s in an attempt to control their numbers. As these rabbits had never been in contact with this virus before, they had not yet coevolved or developed any resistance. When the virus was first introduced, it led to a dramatic reduction of the population, killing more than 90% of the rabbits. However, over time the genetic resistance of the rabbits increased due to the strong selection pressure. Eight years later, the same strain had a mortality rate stabilising at around 30%. So, why was an intermediate level of virulence favoured in the population? Myxoma virus is transmitted via airborne droplets or arthropods, such as fleas or mosquitos. Researchers
14
A Pathogen’s Dilemma
found that highly virulent strains quickly produced large numbers of the virus on the skin, increasing the transmission rate to the pathogen’s vectors, an organism of a different species that transmits the disease, whereas strains with low virulence never obtained numbers as high. However, strains with intermediate virulence took longer but reached higher overall levels eventually. This meant that viruses with intermediate virulence levels were more effective at transmission as they had fairly high virus numbers on the skin, for a prolonged period compared to highly virulent ones. This is the basic concept behind the ‘virulence-transmission trade-off theory’, which argues that intermediate virulence maximises pathogenicity as a result of a trade-off between virulence and transmission. While the replication rate of a pathogen increases with virulence, the duration of transmission is negatively impacted by it, due to host mortality. As a result, there will be an optimum level of virulence, where the overall transmission of the pathogen is maximised. This will be the most evolutionarily favourable level of virulence for a pathogen. What Causes The Observed Differences In Virulence? As a result of coevolution, population structure can influence host-pathogen interactions. A good example is host population density. High host population densities favour increased transmission rates, as more potential hosts can be encountered over a set period of time. This implies that optimal transmission occurs with higher virulence. This may be particularly important to us as a species as our population and population density increase. What happens to virulence when the host and pathogens have not coevolved? Pathogens crossing between species, so-called zoonotics, can lead to increased virulence. Because coevolution has not occurred, virulence is often far higher
Michaelmas 2020
