Interventions can shift the thermal optimum for parasitic disease transmission

Abstract

Temperature constrains the transmission of many pathogens. Interventions that target temperature-sensitive life stages, such as vector control measures that kill intermediate hosts, could shift the thermal optimum of transmission, thereby altering seasonal disease dynamics and rendering interventions less effective at certain times of the year and with global climate change. To test these hypotheses, we integrated an epidemiological model of schistosomiasis with empirically determined temperature-dependent traits of the human parasite Schistosoma mansoni and its intermediate snail host (Biomphalaria spp.). We show that transmission risk peaks at 21.7 °C (Topt), and simulated interventions targeting snails and free-living parasite larvae increased Topt by up to 1.3 °C because intervention-related mortality overrode thermal constraints on transmission. This Topt shift suggests that snail control is more effective at lower temperatures, and global climate change will increase schistosomiasis risk in regions that move closer to Topt. Considering regional transmission phenologies and timing of interventions when local conditions approach Topt will maximize human health outcomes.

Thermal limits in the face of infectious disease: How important are pathogens?

“…We have so far considered how pathogens themselves are able to shape the thermal performance of the host and discussed how an understanding of the thermal ecology of individual host and pathogens, and even populations distributed across geographic regions, may be required to accurately predict, and control, the spread of infectious disease (see also Cator et al, 2020; Sternberg & Thomas, 2014). A recent study by Nguyen et al (2021), however, provides the first evidence that measures implemented to control the spread of pathogens can themselves shape the thermal optima and thermal limits of host and pathogen populations. By simulating the consequences of controlling the vector, a freshwater snail, of the human pathogen underlying schistosomiasis, they show how removal of snails from a population could potentially shift the thermal optimum for disease transmission upwards by nearly 2°C (Nguyen et al, 2021).…”

“…A recent study by Nguyen et al (2021), however, provides the first evidence that measures implemented to control the spread of pathogens can themselves shape the thermal optima and thermal limits of host and pathogen populations. By simulating the consequences of controlling the vector, a freshwater snail, of the human pathogen underlying schistosomiasis, they show how removal of snails from a population could potentially shift the thermal optimum for disease transmission upwards by nearly 2°C (Nguyen et al, 2021). This occurs because the additional vector mortality induced by artificial snail removal reduces the relative contribution of natural temperature‐driven mortality, and so allows for an increase in the temperature at which transmission risk will likely peak.…”

“…Interventions that increase the thermal optima for disease transmission could thus facilitate disease outbreaks during warmer periods of the year, and when exposed hosts are more likely to experience thermal stress and temperatures close to their upper thermal limits (discussed in Nguyen et al, 2021). Where infection also reduces a host's thermal limits (as is likely, see Figure 1), this may substantially reduce the thermal safety margins of an individual or population, placing a host at greater risk of mortality and local extinction.…”

“…immune activity, nutritional intake; Andersen et al, 2010; Hector et al, 2020; Hoffmann et al, 2012), and as a result, potentially influenced by many drug, supplement or sanitation‐based control measures. Understanding the conditions in which the attempted control of infectious disease might lead to host or pathogen thermal optima or thermal limits to increase (as in Nguyen et al, 2021), or even decrease, will shed new light on the management of infectious disease under global change.…”

Scaling effects of temperature on parasitism from individuals to host–parasite systems

“…Indeed, in research on the effects of anti-parasite treatments, models and field experiments have shown that treatment effects on individual hosts do not necessarily lead to equivalent effects in host populations (Fenton, 2013;Nguyen et al, 2021;Pedersen and Antonovics, 2013). The process of parasite transmission among hosts can potentially decouple population dynamics of hosts and parasites from dynamics within host individuals, particularly if the traits that shape the transmission process are temperature dependent (e.g., temperature-dependent activity rate; Casey, 1976).…”