1
THE IMPACT OF GLOBAL CHANGE ON MOSQUITO-BORNE DISEASE
Lydia H.V. Franklinos
1,2*
MSc, Kate E. Jones
1,3
PhD, David W. Redding
1
PhD and Ibrahim
Abubakar
2
PhD.
Affiliations:
1
Centre for Biodiversity and Environment Research, Division of Biosciences, University
College London, Gower Street, London, WC1E 6BT, UK.
2
Institute for Global Health, University College London, 30 Guilford Street, London, WC1N
1EH, UK.
3
Institute of Zoology, Zoological Society of London, Regent’s Park, London, NW1 4RY,
UK.
*Corresponding author: Lydia Franklinos, Lydia.Franklinos.16@ucl.ac.uk, +44 (0)203 108
7691, Centre for Biodiversity and Environment Research, Division of Biosciences,
University College London, Gower Street, London, WC1E 6BT, UK.
In preparation for submission to the Lancet Infectious Diseases.
Word count (max 4500): 4419 (4479 including titles)
Abstract word count (max 150): 149
2
SUMMARY
Over 80% of the global population is at risk of a vector-borne disease, with mosquito-borne
diseases being the largest contributor to human disease burden. Although many global
processes such as land-use and socioeconomic change are thought to influence mosquito-
borne disease dynamics, research to-date has strongly focused on the role of climate change.
We show, through a review of contemporary modelling studies, that there is no consensus on
how future changes in climatic conditions will impact mosquito-borne diseases, possibly due
to interacting effects of other global change processes which are often excluded from
analyses. We conclude that research should not focus solely on the role of climate change but
instead consider growing evidence for additional factors that modulate disease risk.
Furthermore, future research should adopt new technologies, including developments in
remote sensing and system dynamics modelling techniques, enabling a better understanding
and mitigation of mosquito-borne diseases in a changing world.
3
THE GLOBAL THREAT OF MOSQUITO-BORNE DISEASE
Diseases transmitted by arthropod vectors such as mosquitoes and ticks are major
contributors to the global burden of infectious disease,
1
with nearly half the world’s human
population being infected with a vector-borne pathogen at any moment.
2
In particular,
mosquito-borne diseases (MBDs) are a key group of concern, as they include both very high
burden and important emerging diseases such as: Human malaria (~212 million cases per
year) , Dengue (~96 million cases per year), Chikungunya (~693 000 cases per year) and Zika
virus disease (~500,000 cases per year) (Table 1).
3
Globally, many MBDs are thought to be increasing in incidence and geographic distribution;
both emerging in new areas,
4,5
and re-emerging in previously eradicated regions.
6,7
For
example, there has been a 30-fold increase in the global incidence of dengue over the past 50
years, following its expansion into many new countries,
5,8,9
while yellow fever cases are
reported to be increasing again in many endemic countries, after previous dramatic declines.
7
These diseases, with their corresponding high levels of morbidity and mortality, have the
potential to exert significant negative financial and societal effects and can dramatically
inhibit the development and structure of economies, societies and politics.
6
As a
consequence, much research has been targeted at understanding the current and future
geographic distributions of disease risk, in the context of on-going global change, to help
guide interventions and safeguard public health.
10–12
In this context, there has previously been a strong research focus on modelling the direct
effects of climate change on spatial and temporal disease risk,
13–15
paying less attention to
other factors that are already known to interact with both climate change and vector-borne
diseases, such as land-use and socioeconomics (e.g. poverty, trade and travel).
3,16,17
Indeed,
these additional global processes, and the interactions between them, may reasonably be
4
shown to have a stronger immediate impact on future MBD burden than climate change
effects.
18
This would mean a more complete understanding of the role of global change in
modulating the spatial and temporal distributions of MBDs will be essential for the successful
prediction and management of disease risk in the future.
19
In this review, we synthesise
current knowledge on the relative impact of global change processes on MBD risk and
critically examine how these have been incorporated into existing analyses. We argue that the
current focus on the effects of climate change is insufficient, considering growing evidence
for the key role of other global change processes in modulating MBD risk. We suggest an
alternative approach to modelling MBD risk and recommend future directions for research.
CLIMATE CHANGE AS A DRIVER OF MOSQUITO-BORNE DISEASE
Systematic review of current literature
We conducted a systematic literature search to better understand the scope and outcome of
climate-based MBD modelling studies (search strategy and selection criteria), structuring the
search to explore two main axes: First, we considered different mechanisms examined by
each study as climate and climate change may affect MBD epidemiology via different
pathways, such as influencing pathogen development within the mosquito, and vector
population dynamics.
20,21
Second, we examined how different modelling approaches, such as
mechanistic and correlation-based methods, have been used to predict the effect of climate
on the risk of multiple MBDs over different geographic and temporal scales.
22
Within this
search we defined climate change as an alteration (either observed or projected) in climatic
parameters over several decades, with changes in MBD risk being inferred from variations in
disease incidence or vector populations.
5
Of 234 papers identified, 46 met the inclusion criteria (Supplementary materials; Table S1).
Overall 54% of studies demonstrated a positive relationship between climate change and
MBD risk, with increased variations in meteorological values associated with increased
vector abundance or disease incidence. However, the proportion of studies showing this
positive relationship varied depending on the geographic scale of the study (Figure 1a). Of
those studies that predicted increased disease risk with climate change, less than half included
key biological information, such as vector critical climatic thresholds and 28% considered
other global processes (Figure 1b). Global change processes examined in the 46 studies
included land use in 17%, human population density in 11%, of which less than half
considered future human density projections, and socioeconomics in 7%.
Temperature, precipitation and humidity were the main parameters used to model climate
change (Supplementary materials; Table S1). Over 97% of studies included the effect of
temperature change in their analyses, whereas 78% included precipitation and 22%
considered humidity. Temperature has been a predominant research focus since mosquitoes
are ectothermic and so ambient temperature strongly influences important epidemiological
processes including vector development, biting rates and pathogen development rate within
the vector (Reiter 2001; Mordecai et al. 2017). Precipitation is regularly included as
parameters in models of MBD risk as water pools are required for mosquito development and
associated humidity levels influence mosquito survival and flight.
23,24
Changes in these meteorological variables were determined from recorded climatic data or
from projected climatic values using different scenarios of climate change (e.g. IPCC
emissions).
25
Regarding modelling approaches, over 50% of the studies used correlative
models to investigate statistical associations between MBD risk and explanatory variables
22
.
Other studies used mechanistic models which incorporated biological or environmental
mechanisms assumed to drive disease dynamics (e.g., increased rainfall providing water
