1. Introduction
Vector-borne parasites are common, important biological enemies of
humans, animals, and plants, transmitted by one living organism to
another. Despite the recent gains in reducing the overall global burden
for parasites like malaria (Gething et al. 2010; Bhatt et
al. 2015; Ashepet et al. 2021), vector-borne diseases still
account for 17% of all infectious diseases and cause 700,000 deaths in
humans annually (W.H.O. 2020). Livestock and crop systems are also
plagued by vector-borne diseases, which place serious constraints on
agricultural production globally (Döring 2017; Garros et al.2017), and vector-borne diseases can be devastating in wildlife
populations, particularly when introduced to new areas. Collectively,
tens of billions of dollars are spent every year on control, medical
interventions, and mitigating loss of productivity (Warner 1968; Georgeet al. 2015; Stuchin et al. 2016; Aguirre 2017; Weaveret al. 2018).
The dependence of many pathogens on ectothermic arthropod vectors for
transmission means that vector-borne diseases are highly sensitive to
variation in the environment. Arthropod vectors experience a complex
suite of environmental factors, both abiotic (e.g., temperature,
rainfall, humidity, salinity) and biotic (e.g., biological enemies,
inter- and intra-specific interactions, and variation in habitat
quality). These factors vary in their relative effects on organismal
fitness, can synergize (Kleynhans & Terblanche 2011; Huxley et
al. 2021, 2022; Liu & Gaines 2022), and exert their effects at
different spatial scales (Cohen et al. 2016) with important
consequences for the abundance and distribution of arthropod vectors
(Ryan et al. 2015; Evans et al. 2019), vector population
dynamics (Murdock et al. 2017), and pathogen transmission (Samuelet al. 2011; Mordecai et al. 2013; Murdock et al.2014a, 2016; Mordecai et al. 2017; Huber et al. 2018;
Shocket et al. 2018b; Tesla et al. 2018; Wimberly et
al. 2020; Ngonghala et al. 2021).
In vector ecology, there has been a strong emphasis on studying the
effects of temperature on mosquito-borne pathogen transmission (reviewed
in Mordecai et al. 2019). In addition to temperature, water
availability is another critical abiotic variable influencing ectotherm
biology, and both play important roles determining the distribution and
abundance of ectotherms (Chown & Nicolson 2004; Deutsch et al.2008; Steiner et al. 2008; Kearney & Porter 2009; Roura-Pascualet al. 2011; Lenhart et al. 2015; Rozen-Rechels et
al. 2019; González-Tokman et al. 2020; Klink et al. 2020)
and species richness (Jamieson et al. 2012; Calatayud et
al. 2016; Beck et al. 2017; Cardoso et al. 2020; Pilottoet al. 2020; Hamann et al. 2021). Body temperature has
important effects on the rates of enzymatic processes as well as the
structural integrity of cellular membranes and proteins (Angilletta
2009), while all cellular processes rely on water as a solvent for
biochemical reactions and for trafficking nutrients into, within, and
out of cells (Chown & Nicolson 2004; Chaplin 2006). Temperature also
affects the amount of desiccation stress an organism experiences due to
the fundamental relationship between ambient temperature and the amount
of water the surrounding air can hold (Lawrence 2005; Romps 2021). Other
fields at the climate-health interface have explored the effects of wet
heat vs dry heat on the energy budgets of endotherms in the context of
human heat stress and climate change (Buzan & Huber 2020). We
anticipate that variation in relative humidity is also an important
force shaping the thermal performance of ectotherms, including
mosquitoes. Whereas metabolic theory has been well developed and widely
applied in ecology to understand temperature effects (Brown et
al. 2004; Dell et al. 2011; Corkrey et al. 2016) we
currently lack a similar framework for understanding how humidity and
temperature interact to influence mosquitoes and their pathogens.
In this Perspectives, we explore the effects of humidity on the thermal
performance of mosquito-borne pathogen transmission. We begin by
summarizing what is currently known about how temperature and humidity
affects mosquito fitness, population dynamics, and pathogen
transmission, whilst highlighting current knowledge gaps. We present a
conceptual framework for understanding the interaction between
temperature and humidity and how it shapes the range of temperatures
across which mosquitoes persist and achieve high transmission potential.
We then discuss how failing to account for these interactions across
climate variables hinders efforts to forecast transmission dynamics and
to respond to epidemics of mosquito-borne infections. We end by
outlining future research areas that will ground the effects of humidity
on thermal performance of pathogen transmission in a theoretical and
empirical basis to improve spatial and temporal prediction of
vector-borne pathogen transmission. Such a framework will inform
multiple fields (thermal, disease, and landscape ecology and
epidemiology) and a diversity of vector-borne disease systems (human,
wildlife, domestic animals, and plants).