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Climate change: A driver of increasing vector-borne disease transmission in non-endemic areas

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Abstract

In this Perspective, Shlomit Paz discusses the link between climate change and transmission of vector-borne diseases in non-endemic areas.
PERSPECTIVE
Climate change: A driver of increasing vector-
borne disease transmission in non-endemic
areas
Shlomit PazID*
School of Environmental Sciences, University of Haifa, Haifa, Israel
*shlomit@geo.haifa.ac.il
Climate change can affect human health in complex ways, both directly (e.g., exposure to extreme
temperatures) and indirectly (e.g., changes in infectious disease ecology), compounded by a multi-
plicity of biological, ecological, and socioeconomic factors.
The transmission of vector-borne diseases (VBDs) is highly complex and multifactorial,
and is impacted by a multiplicity of biological, ecological, socioeconomic, demographic, and
human-caused factors, including climate, migration, global trade, and travel among many oth-
ers. Although climate is one of several drivers, it is recognised as a major environmental factor
influencing the distribution of VBDs. Climate change exacerbates the risk and burden of both
vectors and pathogens and allows their introduction and dispersion into new regions [1]. Dis-
ease vectors (predominantly mosquitoes and ticks) capable of transmitting VBDs rely on exter-
nal sources of heat to maintain their temperature within functional limits. As such, climatic
conditions are major determinants in the physiology, ecology, development, and behaviour of
vectors and also influence biological processes in the life cycles of pathogens [1,2]. When tem-
peratures rise, these biological processes may be accelerated. For example, during heat waves,
high temperatures increase the biting rate of female mosquitoes. Since disease transmission to
humans occurs during blood feeding, higher biting rates lead to higher disease incidence [2].
Although the interaction between climatic variables and VBD transmission is complex, often
nonlinear, and variable among different vector/pathogen combinations, there is clear evidence
supporting an association between climate change and VBD transmission [3].
Malaria and dengue remain of significant concern, with 249 million malaria cases in 2022
[4] and 740.4 dengue cases per 100,000 population globally in 2019 [5]. However, substantial
declines have occurred in recent years which can be attributed to economic development and
the success of public health interventions. Between 2000 and 2019, malaria case incidence
declined globally from 81 to 57 per 1,000 population at risk and malaria deaths decreased by
one-third. Increased use of dual-ingredient insecticide-treated bed nets, improved diagnostic
testing, and expanded access to artemisinin-based combination therapies contributed to this
decline [4]. These achievements demonstrate the ability to reduce infectious disease transmis-
sion and highlight the difficulty in conclusively attributing and quantifying the impact of cli-
mate change as one of many complex factors influencing VBD transmission.
Climate change is a contributory factor in the expansion of geographical distribution of
VBDs, since warmer conditions facilitate the establishment of vectors in new regions. Cur-
rently, tropical species are spreading towards the poles, and species are established at higher
elevations due to the rising temperatures. As a result, we now observe the spreading of disease
vectors to new, including non-endemic, areas due to improved (warmer) habitat suitability
[6]. Pathogens may be dispersed into non-endemic regions through travel, trade or migration,
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OPEN ACCESS
Citation: Paz S (2024) Climate change: A driver of
increasing vector-borne disease transmission in
non-endemic areas. PLoS Med 21(4): e1004382.
https://doi.org/10.1371/journal.pmed.1004382
Published: April 4, 2024
Copyright: ©2024 Shlomit Paz. This is an open
access article distributed under the terms of the
Creative Commons Attribution License, which
permits unrestricted use, distribution, and
reproduction in any medium, provided the original
author and source are credited.
Funding: The author(s) received no specific
funding for this work.
Competing interests: The authors have declared
that no competing interests exist.
Provenance: Commissioned; not externally peer
reviewed
whereas autochthonous transmission (i.e., cases with no travel history 2 weeks before the dis-
ease) can occur in areas where the vector is established and climatic conditions are favourable
for transmission [1]. For example, global warming increases climatic suitability for Aedes albo-
pictus, one of the vector mosquitos of dengue, which is adaptable to urban, rural, and agricul-
tural habitats, and can breed in natural and human-made containers. Since A.albopictus can
survive at below-freezing temperatures, it has the capacity to colonise a broader latitudinal gra-
dient [2]. In parts of Europe, rising temperatures make conditions more suitable for virus
transmission by A.albopictus since, among other processes, the link between higher tempera-
tures and lower extrinsic incubation periods (the time between infection and the onset of
symptoms) increase the vectorial competence (vector’s ability to transmit a pathogen) [1]. Fol-
lowing heat waves in 2022, the number of autochthonous cases of dengue in France reached
their highest ever recorded [7].
Other examples of VBD transmission to non-endemic areas are the spreading of West Nile
virus in Europe following heat waves [1] and the geographic expansion in Canada of I.scapu-
laris, the main vector of Borrelia burgdorferi (a cause of Lyme disease), following elevated tem-
peratures [8]. With warming temperatures, an expansion of malaria has been observed in both
elevational and latitudinal directions within Africa. From 1898 to 2016, malaria mosquito vec-
tors (Anopheles spp.) ranges gained an average of 6.5 m of elevation per year, and the southern
limits of their ranges moved poleward 4.7 km per year. These findings are consistent with
expectations for climate-linked range shifts [6].
The examples above emphasise the pressing need for prevention, preparedness, and deci-
sive action in response to climate-sensitive VBDs by health authorities. This should include
various actions such as data collection and surveillance, epidemiological investigations of
cases, vector/pest control policies and action plans, and increased monitoring in high-risk
regions/periods [9]. Since vaccines and curative treatments are currently lacking for most
VBDs, such as West Nile fever, Zika, and dengue (for which the current vaccine only protects
individuals with past infection), vector control and prevention to minimise human contact
with the vector are crucial for protecting individuals and populations [10]. Substantial progress
has been made towards the prevention of malaria with 2 vaccines (R21/Matrix-M and RTS,S/
AS01) now recommended by the World Health Organisation (WHO) for the prevention of
malaria in children living in regions with P.falciparum malaria transmission, with the poten-
tial to save hundreds of thousands of lives [11].
Continued efforts to tackle the increased transmission of VBDs will require inter-sectoral
collaboration between governments, nongovernmental organisations, and experts from
diverse clinical and scientific disciplines, including immunology, entomology, clinical micro-
biology, and climate and health scientists [1]. Since VBDs can spread rapidly across national
boundaries, collaboration between countries to control cross-border VBDs and share insights
into challenges relating to disease transmission should be a priority for health agencies, even
where diplomatic relations are strained or absent.
Public health campaigns must go further to improve awareness of infection risk, disease
symptoms, and prevention strategies, such as the use of mosquito nets and the importance of
eliminating small breeding sites for mosquito larvae and pupae in water. To help communities
adapt to the increased risks of VBDs, it is important to build strong community-level systems
based on collaboration between officials and stakeholders, involving community leaders and
health care workers with local knowledge [12]. Special attention should be directed to vulnera-
ble populations such those as in refugee camps, which are at very high risk of outbreaks due to
high population density, poor sanitation, and inadequate access to health and social services
[2].
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According to climate models, future climate change is expected to render additional areas
suitable for the survival of vector species, due to worsening warming trends [1,2] in parallel
with degradation of other interconnected support systems such as water, soils, and ecosystems
that also influence VBD ecology. Predictive modelling of the expected impacts of climate
change on VBD transmission involving different climate and socioeconomic scenarios is criti-
cal for the development of improved early warning systems for outbreaks and to help deci-
sion-makers evaluate where or when infections will emerge or spread [1]. Without effective
surveillance, intervention and cross-border collaboration, the changing climate will continue
to contribute to the risk of morbidity and mortality from VBDs, even in non-endemic regions.
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Background There has been a growing interest in camel anaplasmosis due to its recent emergence in this reservoir species and concerns for its zoonotic potential. The epidemiology of anaplasmosis in camels therefore remains poorly understood mostly because camels belong to marginalised poor and often transhumant populations whose interests are largely neglected. Most studies of anaplasmosis in camels have relied on microscopy and serology for diagnosis and only three studies, undertaken in Tunisia, Saudia Arabia and China, have used molecular diagnostics. The present work characterises Anaplasmataceae strains circulating in the Camelus dromedarius reservoir in Morocco using PCR. Methods Camels (n = 106) were randomly sampled from 6 regions representing different agro-ecological areas in southern Morocco. Whole blood was collected and screened using PCR methods targeting the gene groEL. Anaplasmataceae strains were characterised by sequence analysis of the gene groEL. Results A total of 39.62% (42/106) camels screened were positive for Anaplasmataceae spp. GenBank BLAST analysis of five positive sequenced samples revealed that all strains were 100% identical to “Candidatus Anaplasma camelii”. Phylogenetic investigation and genetic characterisation of the aligned segment (650 bp) of the gene groEL confirmed high similarity with A. platys. Conclusion This study demonstrates the circulation of a previously unidentified species of the genus Anaplasma in Morocco which is genetically close to the agent causing canine anaplasmosis but whose main reservoir is thought to be Camelus dromedarius. Trial registration number This study is not a clinical trial and therefore a trial registration number does not apply. Electronic supplementary material The online version of this article (doi:10.1186/s40249-016-0216-8) contains supplementary material, which is available to authorized users.