Vector Volume 11 Issue 2 - 2017

Table 1: Dengue Serotypes and Epidemiology
DENV Serotype Notable related epidemiology and outbreaks
DENV-1
The most prominent serotype in 2012-2013, causing the largest-ever documented
outbreak affecting New Caledonia.[21]
DENV-2 Caused recent outbreaks in Tuvalu and a current outbreak in Samoa.[10, 22]
DENV-3
After 18 years of absence, has recently become the dominant serotype in the
Pacific islands causing five ongoing outbreaks [23].
DENV-4 Caused one outbreak since 2012, is rare in the Pacific Islands [10].
illness reported in the Solomon Islands, Vanuatu, Fiji and
Palau.[10] With this growth, some reports indicate that the
vast majority of the Pacific Island population will be infected
at some point in their lives.[1] In Samoa, one study showed
96% of the population tested positive for IgG antibodies,
indicating prior infection.[11] With 89% of 18-25 year olds
testing positive, this demonstrated that most Samoans first
contracted dengue during childhood, when dengue illness is
more likely to be fatal.[7, 11]
Outbreaks
Dengue typically follows an epidemic pattern with 1 of
the 4 serotypes causing outbreaks across the Pacific every
three to five years. However, the number of outbreaks of
concurrent serotypes has been growing.[2] After an outbreak
of a single serotype, this strain of the virus tends to circulate
throughout the region until the next outbreak of a different
strain occurs.[12] A single outbreak can affect a large portion
of the population, with the 2009 outbreaks affecting 14
Pacific nations.[13] During such outbreaks, complications
increase, placing a burden on hospital resources, with 4%
of the Federated States of Micronesia’s population requiring
hospitalisation during the Kosrae state outbreak.[14] The
frequency of outbreaks appears to be increasing,[4] though
this may be due to improved surveillance.
The virus
Dengue virus (DENV) is a single-stranded, positivesense
RNA virus of the Flavivirus genus.[15] There are
four serotypes DENV-1 to DENV-4. Though they only share
65% of their genomes, their clinical syndromes are nearly
identical, and they all occupy the same ecological niche.
[16, 17] Dengue epidemics usually result from introduction of
a single serotype from hyper-endemic countries, which will
remain dominant in the region for several years.[12,18,19]
However, in 2012, outbreaks of all four DENV serotypes were
noted in a single year [20]. Each DENV serotype has caused
outbreaks or been prevalent in the Pacific Islands at various
times (Table 1).
Repeated infection of DENV of the same serotype is
associated with increase risk of progressing to severe
dengue, which is associated with higher morbidity and
mortality if left untreated.[24] Those living in endemic areas
such as the Pacific Islands are at an increased risk of being
reinfected and thus complications are more common.
The Vector
Dengue, zika, chikungunya and other arboviruses are
transmitted to humans through the bites of infected Aedes
mosquitoes.[25] Aedes aegypti is the primary vector in the
Pacific Islands and is widespread across the region except
for Futuna and other isolated islands.[26,27] Aedes aegypti is
associated with human migration and urbanisation, enabling
it to be dominant in the region, however, Aedes albopictus,
Aedes polynesiensis and nine other potential vectors have
also been identified in the Pacific Islands.[27, 28]
Aedes mosquitoes begin their transmission cycle upon
acquiring the dengue virus from the blood of a viraemic
person; the virus then replicates in mosquito midgut
epithelium before shedding its progeny into the haemocoel,
which then disseminates into secondary target tissues such
as salivary glands.[29] During the next feeding event, the
mosquito transmits the virus to the host through saliva.[29,30]
Aedes aegypti is capable of repeatedly transmitting the virus
through this process irrespective of its number of hosts.[30]
The introduction of Aedes aegypti into different islands has
been spurred by human migration; there have been intense
population migrations in the Pacific Islands since European
colonization.[31] Though the first dengue epidemic in the
Pacific Islands was reported in the 1880s, descriptions of
Aedes aegypti didn’t emerge until the 1960s in Fiji and Tonga.
[20, 32, 33] Aedes aegypti then spread during World War II,
when travel between the Pacific Islands and Asia, Europe,
and America became more frequent.[34] Recent studies
have now identified genetic variability in nine locations
across Fiji, New Caledonia, Tonga and French Polynesia,
suggesting a link between human migration and Aedes
aegypti populations, possibly related to island isolation and
environmental conditions.[25]
Several factors influence the transmission of DENV
from mosquitoes to humans, including climate.[30] Higher
temperatures enable the virus to replicate in higher
concentrations, enhancing the vectors’ risk for pathogen
transmission and contributing to the high prevalence of
dengue infection in the tropical Pacific Islands[30] Globally,
climate-induced variations in modelled Aedes aegypti
populations were strongly correlated to historical dengue
cases between 1958 to 1995.[35] Recent research from New
Caledonia, where dengue spread by Aedes aegypti is a major
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