Virulence-mediated infectiousness and activity trade-offs and their impact on transmission potential of influenza patients

Abstract

Communicable diseases are often virulent, i.e. they cause morbidity symptoms in those infected. While some symptoms may be transmission-enhancing, other symptoms are likely to reduce transmission potential. For human diseases, the reduction in transmission opportunities is commonly caused by reduced activity. There is limited data regarding the potential impact of virulence on transmission potential. We performed an exploratory data analysis of 324 influenza patients at a university health centre during the 2016/2017 influenza season. We classified symptoms as infectiousness-related or morbidity-related and calculated two scores. The scores were used to explore the relationship between infectiousness, morbidity (virulence), and activity level. We found a decrease in the activity level with increasing morbidity scores. There was no consistent pattern between an activity level and an infectiousness score. We also found a positive correlation between morbidity and infectiousness scores. Overall, we find that increasing virulence leads to increased infectiousness and reduced activity, suggesting a trade-off that can impact overall transmission potential. Our findings indicate that a reduction of systemic symptoms may increase host activity without reducing infectiousness. Therefore, interventions should target both systemic- and infectiousness-related symptoms to reduce overall transmission potential. Our findings can also inform simulation models that investigate the impact of different interventions on transmission.

Social hosts evade predation but have deadlier parasites

“…Thus, diverse systems likely meet the essential assumptions of our model: predators may frequently shift the antagonistic interplay of host sociality and parasite virulence, driving hosts into the arms of more virulent parasites. Conversely, these results also indicate that social distancing may select for lower virulence when parasites exhibit multiple infections and a virulence-transmission trade-off (e.g., influenza A virus 13,49 ). Host behaviour that reduces contact may thus effectively control both the spread and virulence evolution of pathogens and parasites.…”

“…Parasites exploit their hosts to replicate and transmit, but increasing this exploitation harms parasite fitness by killing hosts ('virulence'), which reduces the time window for successful transmission, or the 'infectious period' 1 . With appropriate curvature, this trade-off should result in stabilising selection for intermediate virulence [13][14][15][16] . While theory predicts various mechanisms can shift the costs and benefits of exploitation, altering virulence evolution [2][3][4]7 and host disease outcomes 12,15,17,18 , empirical evaluation of their relative importance remains scant 12,19,20 .…”

Detection, not mortality, constrains the evolution of virulence

“…For example, if contact rates between hosts declined with increasing disease severity, there could be strong selection pressure on the pathogen to reduce its severity. Such a relationship between disease severity and contact rates has been observed (McKay et al 2020), yet with few exceptions (e.g. Ewald 1983Ewald , 1994, the role of behavior on virulence evolution has been largely ignored.…”

“…Ewald (1983Ewald ( , 1994 long ago proposed a trade-off between virulence and transmission mode that implicitly included a virulence-detection trade-off. This idea was later formalized (Day 2001(Day , 2002a, and the concept has been discussed by many others (for example, Alizon and Michalakis 2015;McKay et al 2020). Likewise, Ebert and Bull (2003) and Bull and Lauring (2014) previously discussed that virulence, in the context of mortality, is likely to impose only an indirect and weak evolutionary cost.…”

Unravelling Transmission in Epidemiological Models and its Role in the Disease-Diversity Relationship

“…However, pathogen load is not always a good predictor of transmission success. For example, HIV has highest transmission potential at intermediate viral loads (McKay et al, 2020).…”