Concisely discusses the role of weather and climatic changes in the transmission of dengue fever

1. The Role of Weather and Climatic
Changes in the Transmission of
Dengue
Emmanuel O. Taiwo
(1st May 2014)

2. Introduction: what is dengue?
Spatial and temporal spread
Climate/weather change: how does it
drive dengue?
Future scenarios
Solutions: dengue prevention & control
Contents

3. Source: http://www.gatesnotes.com/Health/Most-Lethal-Animal-Mosquito-Week 30/04/2014)

4.  An infectious disease transmitted by mosquitoes (mainly
“Aedes aegypti ”) carrying the virus
 an RNA virus, genus: Flavivirus, family: Flavivirida
 Other vectors: Aedes albopictus, polynesiensis & scutellaris
 World’s most widely distributed mosquito-borne viral disease
 A feverish illness with symptoms appearing 3-14 days after an
infective bite
 Complicated cases could lead to severe condition called
‘dengue hemorrhagic fever’
Introduction: what is dengue?
(WHO, 2009)

5. Symptoms of dengue
(Source: http://en.wikipedia.org/wiki/File:Dengue_fever_symptoms.svg , 15/04/2014)

6.  Transmitted through “Infected-person-to-mosquito-to-
another-person” pathway
 One bite can cause the disease
 Aedes mosquitoes flourish in rainy seasons
 However, they can breed in pools, water-filled vases, plastic
bags, and cans all year-round
(Cunha and Stoppler, 2013)
 22,000 deaths yearly, mostly among children
 There is currently NO CURE OR VACCINE for its treatment
 However, early detection and access to proper medical care
lowers fatality rates below 1%
(WHO, 2014)
Introduction (2)

7.  Prevalent in tropical and sub-tropical regions
 Predominantly in urban and semi-urban areas
 Leading cause of child mortality in many Asian and Latin
American countries
 50-100 million cases worldwide each year
 22,000 deaths yearly, mostly among children
 Its global incidence has grown rapidly in recent decades
 30-fold increase over the past 50 years
 About 50% of world's population now at risk
 Most rapidly spreading mosquito-borne viral disease
(WHO, 2014)
Spatial and temporal spread

8. Source: WHO cited in Alvarado (2013)

9. 1955 – 2007: The upward trend
Average annual number of dengue fever (DF) and dengue haemorrhagic fever
(DHF) cases reported to WHO, and of countries reporting dengue, 1955–2007
(Source: WHO, 2009)

10.  2009: Key West, Florida, 28 to 55 cases (Cunha and Stoppler, 2013)
 2011: Severe outbreak in Paraguay, hospitals became
overcrowded (Cunha and Stoppler, 2013)
 2011: Bolivia, Brazil, Columbia, Costa Rica, El Salvador, Honduras,
Mexico, Peru, Puerto Rico, and Venezuela (thousands infected)
 2012: first sustained transmission in Europe since 1920s reported
in Madeira, Portugal (Sousa et al., 2012)
 2013: Outbreaks in the Caribbean: Puerto Rico, Virgin Islands and
Cuba (Cunha and Stoppler, 2013)
 2013: Thailand’s worst outbreak in 20 years, 126 deaths, 135,344
people infected as of October (Cunha and Stoppler, 2013)
 Imported cases (e.g. tourism): Tahiti, Singapore, S.E. Asia, West
Indies, India and Middle East (Cunha and Stoppler, 2013)
Dengue: Some recent outbreaks

11.  Weather and climate are variable, naturally
 Human influences accelerating the rate of climate change, thus
causing chaotic global warming
 Ecosystems and living organisms depend on the stability of
atmospheric services
 These atmospheric changes (whether short-term or long-term)
have effects on living organisms (and their welfare)
- IPCC (2013, 2014)
Weather and Climate Variability

12.  Generally increasing temperatures
 Heat waves
 Floods
 More erratic changes in wind patterns
 Changing landscapes
 Rising sea levels
 Severe storms
 Increased severity of floods, droughts and fires
 Food insecurity
 Wildlife risks and possible extinction
 Economic risks
 Increased occurrence and spread of diseases
 Other health-related issues IPCC (2013, 2014)
Ongoing climate change: Features,
impacts and threats

13.  Strong links between climate and the distribution, spread and
severity of pests, diseases and human health issues (IPCC, 2014)
 Regardless of the mode of disease spread: vector-, air-, water-,
soil-, or food-borne
 Perhaps the strongest link of climate to human health has been
drawn to vector-borne diseases (Morin et al., 2013)
 Especially mosquito-borne ones: malaria, yellow fever and
dengue (Morin et al., 2013)
Climate change and diseases/health

14.  IPCC AR5 WG II report (2014): As a vector borne disease
only dengue fever was associated with climate
variables at both the global and local levels (high
confidence)
 “The disease is linked with climate on spatial, temporal
and spatiotemporal scales”
 “The principal vectors for dengue, Aedes aegypti and
Aedes albopictus, are climate-sensitive”
 (IPCC, 2014)
Dengue & weather/climate change

15.  Climate affects the dengue virus and vector populations both
directly and indirectly (Gubler et al. 2001)
 Climate influences dengue ecology by affecting:
 vector dynamics
 agent development (e.g. viral replication within mosquito)
 mosquito/human interactions
- Morin et al. (2013)
 Temperature influences vector development rates, mortality,
and behaviour (Tun-Lin et al., 2000)
 Vector biology and viral replication are temperature- and
moisture- dependent (Thai and Anders, 2011)
Dengue & weather/climate change (2)

16.  Temperature interacts with rainfall as the chief regulator of
evaporation, thereby also affecting the availability of water
habitats.
 Indirect climatic influence by changing LU/LC, which may in turn
enhance or reduce vector populations
 For e.g. dengue incidence has been correlated with vegetation cover,
grasses etc. (as breeding spaces for mosquitoes) (Troyo et al. 2009 in Morin
et al., 2013)
 Climate also influences people’s use of land, and this can affect
mosquito populations, breeding etc. (Vanwambeke et al., 2007)
Dengue & weather/climate change (3)

17. Interactions between climate, vector & virus
Dengue : ecological relationships
(Morin et al., 2013)

18.  China: Distribution of Ae. albopictus in northwestern China highly
correlated with annual temperature and precipitation
 Temperature, humidity and rainfall are positively associated with
dengue incidence in Guangzhou, China
 Wind velocity is inversely associated with rates of the disease
 (Li et al., 2011; Lu and Lin, 2009; Wu et al., 2011)
 Taiwan: Usually high incidence of dengue after typhoons
 The extreme rainfall, high humidity and water pooling =
more/fresh breeding sites for mosquitoes (Lai, 2011)
 Bangladesh: observed records of high dengue incidence with
precipitation extremes : a) high river levels b) drought
(Padmanabha et al., 2010)
Some empirical studies:
South East Asia

19.  23 years of dengue reports from 9 climatic regions of Mexico
 Findings: statistically significant (but non-linear) effects of
weather on dengue
 Temperature: <5°C Tmin = almost no effect on dengue incidence
 >18°C Tmin = rapidly increasing effect on dengue
 Tmax >20°C = also increasing effect on dengue incidence,
 Dengue incidence peaked around 32°C, after which effect
declined
 Rainfall: increasing effect up to 550 mm, beyond which such
effect declines
- Colón-González et al. (2013)
More empirical studies: Mexico

20.  Effects of higher temperatures:
 Increased larva development rate
 Reduced time for virus replication within vector
 However, extremely high temperatures may reduce vector life span
 Effects of increased/varying precipitation:
 variable effects on vector breeding sites
 depends on where the breeding grounds are
 Humidity:
 Higher humidity supports greater vector lifespan
 Perhaps the most important climatic predictor of dengue globally
- Thai and Anders (2011)
Dengue & weather/climate change (4)

21.  Mexico: Via modelling, up to 40% increase in dengue incidence by 2080
was estimated under climate change (holding other drivers constant)
(Colón-González et al., 2013)
 Thai and Anders (2011): dengue projection for 2055:
 Citing Hales et al. (2002): In 1990: 30% world population (1.5 billion)
 if climate remains at 1990 baseline values: 34% (3.2 billion)
 But with IPCC’s (2007) CC projection: 44% (4.4 billion)
 Although several other studies speculate that up to 55% of world
population are already living in areas with dengue risk (Thai and Anders,
2011)
 Inconsistent results from models to predict projected CC effects on
dengue fever occurrence (Future CC impact on incidence uncertain)
(Morin et al., 2013)
Future scenarios: CC & Dengue

22. Observed dengue incidence in Mexico (2000) and predicted dengue incidence for
2080 using (Source: Colón-González et al., 2013)

23.  Ae. aegypti mosquito population density usually 4 – 9 times higher
during the wet season
 About 80% of dengue fever cases in Trinidad were often recorded
within the wet season
(Chadee et al., 2007, IPCC, 2014)
 In 2003, a vector control program was developed
 Target: to reduce vector (mosquito) population before the onset of
the rains
 Strategy: application of insecticides (temephos) into the water drums
that serve as primary breeding sites of vector
 Result: The one-off treatment effectively controlled mosquito
populations for several weeks (the peak of rains)
(Chadee, 2009, IPCC, 2014)
Short-circuiting the seasonal influence of
climate on dengue: A case of Trinidad

24. Rainfall, temperature, Breteau index & and dengue fever cases in Trinidad
from 2002-2004. (Source: Chadee et al., 2007)

25. Efficacy of pre-seasonal treatment with temephos on Ae. aegypti ovitrap egg counts in
Curepe (treatment) and St. Joseph (control), Trinidad (2003) (Source: Chadee, 2009)

26.  Anthropogenic CC may be mitigated
 But, the climate will always be variable, naturally,
 There will always be seasonal variations, and some extreme
events
 The key is to be able to adequately forecast these changes
 And to predict the implications on vector ecology
 Then, planned and managed vector control strategies should be
implemented to systematically limit breeding sites and reduce
vector populations
Concluding remarks

27.  Alvarado, M.R. (2013): Dengue fever spreading: The view from Brazil.
Humanospher (online): http://www.humanosphere.org/2013/08/dengue-fever-
spreading-brazil/, accessed 20/04/2014
 Bhatt, S., Gething, P.W., Brady, O.J., Messina, J.P., Farlow, A.W., Moyes, C.L.
(2013): The global distribution and burden of dengue. Nature 496(7446):504–
507.
 Chadee, D.D., Shivnauth, B., Rawlins, S.C. and Chen, A.A. (2007): Climate,
mosquito indices and the epidemiology of dengue fever in Trinidad (2002–
2004). Annals of Tropical Medicine and Parasitology, 101(1): 69-77.
 Chadee, D.D. (2009): Dengue cases and Aedes aegypti indices in Trinidad,
West Indies. Acta Tropica, 112(2): 174-180.
 Colón-González, F.J., Fezzi, C., Lake, I.R., Hunter, P.R. (2013): The Effects of
Weather and Climate Change on Dengue. PLoS Negl Trop Dis 7(11): 2503.
 Cunha, J.P. and Stoppler, M.C. (2013): Dengue Fever. Medicine Net (online):
http://www.medicinenet.com/dengue_fever/article.htm, accessed 15/04/2014
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