S. Promprou, M. Jaroensutasinee*, K.
Jaroensutasinee
Institute of Science, Walailak University, Thailand
This study investigated climatic factors associated with Dengue
Haemorrhagic Fever (DHF) incidence in southern Thailand, and
compared the differential effects of climatic factors on the
incidence of DHF in the areas bordering on the Andaman Sea and
those on the Gulf of Thailand side of the peninsula. Climatic
factors comprised rainfall, rainy days, relative humidity, maximum,
minimum, and mean temperatures. The result indicated that the mean
temperature, rainfall, and relative humidity were associated with
DHF incidence in the areas bordering the Andaman Sea. Minimum
temperature, rainy days, and relative humidity were associated with
DHF incidence on the side of the southern peninsula Gulf of
Thailand.
Key words: Dengue Haemorrhagic Fever, temperature, rainfall,
relative humidity, Thailand
INTRODUCTION
Dengue haemorrhagic fever (DHF)
is one of the most serious public health problems in Thailand and
in many other tropical countries around the world. The disease
affects hundreds of millions of people every year [1-2]. DHF is
transmitted predominantly by the mosquito (Aedes aegypti)
adapted to living near areas of human habitation [3-4]. Dengue
transmission occurs throughout the year in endemic tropical areas,
but there exists a distinct cyclical pattern associated with the
rainy season [1]. In tropical and sub-tropical regions, temperature
and rainfall levels enable adult vectors to remain active all year
[5]. This results in a continuous transmission cycle that makes the
disease endemic.
The transmission of dengue viruses is climatic sensitive for
several reasons. First, temperature changes affect vector-borne
disease transmission and epidemic potential by altering the
vector’s reproductive rate, biting rate, the extrinsic incubation
period of the pathogen, by shifting a vector’s geographic range or
distribution and increasing or decreasing vector-pathogen-host
interaction and thereby affecting host susceptibility [6]. Second,
precipitation affects adult female mosquito density. An increase in
the amount of rainfall leads to an increase in available breeding
sites, which in turn leads to an increase in the number of
mosquitoes. An increase in the number of adult female mosquitoes
increases the odds of a mosquito obtaining a pathogen and
transmitting it to a second sensitive host [7]. Third, a distinct
seasonal pattern in DHF outbreaks is evident in most places. In
tropical regions where monsoon weather patterns predominate, DHF
hospitalisation rates increase during the rainy season and lessen
several months after the cessation of the rains [8-9]. This decline
may be related to a decrease in mosquito biting activity, a
decrease in longevity of female mosquitoes, or both. In Thailand
where the vector life cycle is highly domiciliary, temperature and
humidity condition during the rainy season favour survival of
infected mosquitoes [10].
There has been an upward trend in the incidence of DHF, an acute
and severe form of dengue virus infection, since the first DHF
epidemic outbreak in 1958 [11], with a cumulative total of
1,369,542 DHF cases in 2001. The Bureau of Epidemiology (2000)
reported that there had been several regular outbreaks in Thailand.
From 1992 to 2002, the Southern Epidemiology Department reported
42,692 cases of DHF in southern Thailand including 123 deaths. This
indicates that DHF is a major health risk in southern Thailand.
Most studies on dengue have been done in the central part of
Thailand [12-15] and on Samui Island, southern Thailand [16-17]
where the climate differs significantly from that of the other
southern regions of the country.
Southern Thailand is a narrow peninsular that separated into two
coasts that are under different monsoon seasons: southwest and
northeast monsoon. On the Andaman Sea side of the Southern
peninsula, the wettest period of the year is from August to
September from southwest monsoon. On the Gulf of Thailand side, the
wettest period of the year is from November to January from
Northeast monsoon. The impact of climatic factors on DHF in
Thailand is probably the least well understood. A good
understanding of the current causal relationship between climatic
factors and DHF is essential for a study of the impact of potential
climate change on DHF in future [18-21]. The aim of this study is
to investigate the relationship between climatic factors and the
incidence of DHF in southern Thailand, and to compare the
differential effects of climatic factors on the incidence of DHF in
the areas bordering on the Andaman Sea and those on the Gulf of
Thailand side of the peninsula.
MATERIALS AND METHODS
Southern Thailand is
located at 5° 37’-11° 42’ N, 98° 22’-102° 05’ E, and covers
70,715.2 km2. It is bordered on the eastern side by the
Gulf of Thailand and on the western side by the Andaman Sea. There
are many hills and mountains bordered by the seas. Southern
Thailand is composed of 14 provinces (Figure 1). The climate is
equatorial and humid with rainfall, high temperature of over 20 °C,
and relative humidity of 80% throughout the year [11].
Climatic data for southern Thailand over the period 1993-2002 were
provided by the Climatology Division of the Meteorological
Department. The monthly DHF incidence data over the same period
were collected by the Centre of Epidemiological Information, Bureau
of Epidemiology, Ministry of Public Health. Climatic data comprised
monthly rainfall, rainy days, maximum temperature, minimum
temperature, mean temperature and relative humidity.
All variables were tested for normality using Kolmogorov-Sminov
test and transformed when necessary. DHF was logarithmic
transformed to achieve normality. Independent t-tests were
used to test for significant differences in both climatic factors
and DHF incidence across two regions. Pearson’s correlation
coefficient test was used to detect primary association between DHF
incidence and climatic factors. Stepwise regression technique was
employed to explore and identify statistically significant climatic
risk indicators for DHF.
RESULTS
DHF incidence rates in southern
Thailand varied from 0.00-192.73 per 100,000 population. The
highest incidence of the disease was observed in July 1995 in Trang
Province on the Andaman Sea side with 192.73 cases per 100,000
populations. DHF incidence mean rate was 9.91 17.42 with the median
3.09 (Figure 2a). Rainfall varied from 0.00 -713.20 mm (Figure 2b).
Rainy days were 14.22 7.06 days per month with the range of 0-31
days (Figure 2c). Relative humidity mean were 79.89 3.39 % with the
range of 72.4-86.0% (Figure 2d). Maximum, minimum, and mean
temperatures varied from 29.40-40.30, 13.00-26.60, and 23.90-31.20
°C, respectively (Figure 2e).
DHF incidence rates and climatic factors varied in both side of southern Thailand (Table 1). The DHF incidence rate and climatic factors on the Andaman Sea side showed statistically significant differences from those for the Gulf of Thailand side in all categories (Table 2). DHF incidence per 100,000 population on the Andaman Sea side was lower than that for the Gulf of Thailand side (Table 2). Rainy days, rainfall, mean, maximum temperature, and relative humidity on the Andaman Sea side were higher than those on the Gulf of Thailand side. However, minimum temperature was lower on the Andaman Sea side than on the Gulf of Thailand side (Table 2).
Pearson’s correlation coefficient test was used to detect primary
association between DHF incidence and climatic factors (Table 3).
On the Andaman Sea side, the significant variables were mean
temperature (x11) (t597 =
7.77, P < 0.001), relative humidity
(x14) (t597 = 2.73,
P < 0.001), and rainfall (x16)
(t597= 3.55, P < 0.01). Therefore,
the selected regression model was y1 = -6.522 +
0.338x11 + 0.180x14 +
0.147x16 (R2 = 0.15,
F3,594 = 26.11, P < 0.001).
On the Gulf of Thailand side, the significant variables were
minimum temperature (x23)
(t862 = 3.16, P<0.01), rainy days
(x25) (t862 = 4.03,
P<0.001), and relative humidity
(x26) (t862 = -3.73,
P < 0.001). Therefore, the selected regression model
was y2 = 0.072x23+
0.015x25 - 0.017x26
(R2 = 0.34, F3,838 =
144.85, P < 0.001).
DISCUSSION
The results of this study indicate
that climatic factors may play a part in the transmission cycles of
DHF. However, the relative importance of these climatic factors
varied with geographical areas. DHF incidence rate on the Andaman
Sea side was lower than that on the Gulf of Thailand side. This
could be due to different monsoon seasons between these two sides.
This result contradicted the findings of the study by [22], which
concluded that the seasonal patterns of DHF incidence on the
Andaman Sea side and the Gulf of Thailand side were similar. The
difference in findings may be due to two possible reasons. First,
the data for the two studies were specific to different time-spans.
The data for the present study were collected during the period
1993-2002 while those for their study covered the 1978-1997 period.
From 1997 to 2002 several significant outbreaks of DHF were
reported in southern Thailand. Secondly, the data for the present
study were collected from all 14 provinces of southern Thailand,
but their data were derived from only four provinces. It is
reasonable to assume that the data used in the present study is
more comprehensive and representative of southern Thailand.
Changes in climate may influence the abundance and distribution of
vectors [23-24]. Precipitation is an important factor in the
transmission of DHF. All mosquitoes have aquatic larval and pupae
stages, and therefore require water for breeding [23-24]. Rainfall
events and subsequent floods can lead to outbreaks of DHF mainly by
enabling breeding of vector mosquitoes [24]. The timing of rainfall
is as important as the amount of rain. The pattern of rainfall may
also play a part. Extremely heavy rainfall may flush dormant
mosquito larvae away from breeding sites or kill them outright
[23]. More frequent, lighter rains may replenish existing breeding
sites and maintain higher levels of humidity that assist in
dispersal and survival of adult mosquitoes [23-24]. In this study,
it was found that rainfall and rainy days were two important
determinants in the DHF transmission in southern Thailand. Rainy
days were significantly associated with DHF incidence in both [22]
and our studies. According to [22], the number of rainy days was
associated with DHF incidence rate on both sides of the peninsula,
but our study showed that rainy days were associated with DHF
incidence rate only on the Gulf of Thailand side. This divergence
may be due to the differential data in terms of scope and timing of
data collection for the two studies. The number of rainy days may
influence either the life cycle of a mosquito or viral replication
rates since a certain number of rainy days are generally favourable
for mosquito development. If the number of rainy days were too low,
there would not be enough water for mosquito larvae to complete
their development.
Warmer temperatures can increase the transmission rates of DHF in
various ways. First, warmer temperature may allow vectors to
survive and reach maturity much faster than at lower temperatures
[24]. Secondly, warmer temperature may reduce the size of mosquito
larvae resulting in smaller adults that have high metabolism rates,
require more frequent blood meal, and need to lay eggs more often
[11, 19, 25]. Thirdly, environmental temperature has a marked
effect on the length and efficiency of the extrinsic incubation
periods (EIPs) of arboviruses in their vectors [23-24]. This means
that mosquitoes exposed to higher temperatures after ingestion of
virus become infectious more rapidly than mosquitoes of the same
species, which are exposed to lower temperatures [24]. Therefore,
transmission of arboviruses may increase under warmer conditions as
more vector mosquitoes become infectious within their lifespan.
Higher temperature may reduce the length of viral EIPs in
mosquitoes [15, 26-27]. At 30°C, the duration of dengue virus EIPs
is 12 days compared with only 7 days at 32-35°C [26]. Moreover, a
5-day decrease in the duration of incubation period can triple the
transmission rate of dengue [28]. It was found in this study that
mean and minimum temperatures were positively associated with the
transmission of DHF in southern Thailand. As minimum temperature
increased, the transmission rate of DHF also increased. It is
possible that most of the physiological functions of vectors in
this area are subject to optimal minimum temperature.
In this study it was found that relative humidity had a positive
association with the transmission of DHF on the Andaman Sea side,
but a slightly negative association on the Gulf of Thailand side.
The disparity may be due to the differences in some climatic
factors. The Andaman Sea side has higher temperature, humidity,
precipitation, more rainy days, and slightly lower minimum
temperature than the Gulf of Thailand side of the peninsula.
Relative humidity influences longevity, mating, dispersal, feeding
behaviour and oviposition of mosquitoes, and rapid replication of
the virus [4, 23, 29]. At high humidity, mosquitoes generally live
longer and disperse further. Therefore, they have a greater chance
of feeding on infected people and surviving to transmit the virus
to other people. Relative humidity also directly affects
evaporation rates of vector breeding sites.