Introduction
Chikungunya virus (CHIKV), is transmitted to humans throughStegomyia mosquitoes. Kerala was the worst affected State in
Indian during 2007 with a total of 55.8% of the reported Chikungunya
fever cases in the country from the State (NVBDCP, India). Almost all
the districts of Kerala were affected with the infection during
2006-2007. The factors attributed to this outbreak were attributed to
crucial mutation “A226” acquired by the virus in 2007 and the
abundance of the vector species Stegomyia albopicta in the region
(Kumar,et al ., 2008). Stegomyia aegypti andStegomyia albopicta , the vectors for CHIKV were widely
distributed and abundant during the pre and post monsoon season. The
prevailing climate, terrain and agricultural practices in these
districts were highly conducive for the breeding of St. albopicta
and St.aegypti , the vector of Chikungunya and Dengue in Kerala. The
worst affected region was the mid-highland regions which harbour the
vast rubber plantation sectors in the State. The hilly and semi-forested
districts of Kottayam and Pathanamthitta were the most affected
districts during 2007 epidemic, which constituted the major portion in
the rubber plantation sectors of Kerala. About 63.0% of people living
in the rubber plantation areas (Kerala contributes 80.0% of the rubber
production in India), was afflicted with this disease (Kumar et
al ., 2011), St. albopicta acted as main vector species (Kumaret al ., 2008).
Incidence of Dengue fever, another mosquito borne arbo-viral disease is
also on an increasing trend in Kerala (Kumar et al ., 2013).St. albopicta (Skuse) 1894 (Diptera: Culicidae), the predominantStegomyia species prevalent in Kerala was recorded as the vector
species of both these arbo-viral diseases (Kumar et al ., 2008;
Thenmozhi et al ., 2007). Innumerable Discarded Latex Collection
containers (DLCCs), unused Fixed Latex Collection Containers (FLCCs),
and tree holes, leaf axils of pine-apple plants and fallen leaves of
areca-nut tree etc. were recorded the key breeding habitats of this
species in rubber plantation sectors in Kerala (Kumar et al .,
2011; Sumodan, 2003) .
Dengue a vector – borne and emerging infectious disease is estimated to
affect 50-100 million individuals each year in tropical and subtropical
areas (Gubler, 2006; Halstead, 2007). Demographic and societal changes,
such as population growth, unplanned urbanization, microevolution of the
virus, climatic change and modern transportation, have greatly
contributed to the increased incidence and geographical spread of Dengue
virus infection in recent decades and distributed among about 120
countries globally. Dengue is caused by four serologically distinct
types of Dengue virus. DENV-1, DENV-2, DENV-3 and DENV-4, belonging to
the family Flaviviridae , genus Flavivirus . The viruses are
transmitted to human beings through the bite of Stegomyiamosquitoes such as St. aegypti and St. albopicta which are
considered as primary and secondary vectors of Dengue respectively (WHO,
2015) . The incidence of Dengue fever has increased significantly around
the world in recent decades. About half of the global population lives
under the risk of infection of this arbo-viral disease(WHO, 2009)
.St. albopicta is becoming an increasingly important vector of
Dengue because of its rapidly changing global distribution (Rezza, 2012)
and it is generally believed to be a less efficient vector of
arboviruses than St.aegypti . St. albopicta is highly
adaptive and therefore can survive even in cooler temperate regions. Its
spread to new areas is due to its tolerance to temperatures below
freezing, hibernation and ability to shelter in microhabitats (WHO,
2015).
St. albopicta is (the Asian tiger mosquito), is an aggressive
day-biting species known in different parts of the world including South
East Asia (Russell et al., 1969) , and Southern China (Qui
et al., 1981) . It belongs to the East and Southeast Asia, where
it was originally lived at the edges of forests breeding in tree hole,
containers and other natural reservoir (Rezza, 2012) .Worldwide trade in
second hand tires transported to various places, which often contain
water could be an ideal place for eggs and larvae of these mosquitoes,
has been a key factor in the large-scale conquest of St.
albopicta in new areas which easily adapts to new environments, even in
a temperate climate. This expansion is creating new opportunities for
viruses to circulate in new areas, becoming a common cause of epidemics
in St. aegypti -free countries from Hawaii to Mauritius. HoweverSt. albopicta is considered as an inefficient vector of Dengue
because not well adapted to urban domestic environments and is less
anthropophilic than St.aegypti .
The entire population of India lives under the risk of Dengue. In India
the first case of Dengue has been reported from Kolkata (previously
Calcutta) during 1963. The number of Dengue cases had been an increasing
trend since 2001 and the maximum number of suspected cases (28055) has
been recorded during 2010. In Kerala Dengue fever was first recorded in
Kottayam District in 1997 (Kalra and Prasittisuk, 2004). This district
continues to contribute the maximum number of cases of Dengue fever in
the State, next to Trivandrum District in the south Kerala every year.
Topographically, Kottayam District is semi-forested region with hills
and hillocks located at the foothills of the Western Ghats. This
district is the abode of rubber plantations in the country and grows
rubber in about 60% of its net sown area under agriculture. St.
albopicta the vector of Dengue fever, is abundant throughout the
plantation belt of Kerala, including Kottayam District (Kumar et
al. , 2011). All the four Dengue serotype are prevalent in the State,
however, Dengue 2 and 3 are the major serotypes prevalent in Kerala
State (Kumar et al ., 2013).
Rubber plantations in the region could be classified as immature ones
and mature ones based on the age and productivity of the trees, broadly.
Latex collection is carried out by placing hemispherical plastic
containers of either 450ml/600ml/900ml capacity, attached to the bark of
the mature trees about 7 years old. A small portion of bark of the tree
is removed in a horizontal oblique manner by a sharp knife in an
interval of 2-5 days and the latex oozing out is collected into the
containers fitted to the tree. Also rain guards are fitted to the latex
yielding section of the bark to prevent rainwater interfering with the
tapping process and collection of latex. The latex oozing out of the
scar is removed about 3 hours after making the cuts in the bark. The
container is kept in the tree itself after latex collection in upright
position to collect the residual latex.
In absence of effective vaccines or prophylaxis against most of the
arboviruses, control of these vector borne diseases heavily rely on
vector control. Community oriented Integrated Vector Management (IVM)
strategies involving inter-sectoral collaboration is the best approach
proposed for the same. However, during outbreak situations, usage of
adulticidal measures using insecticides are being resorted to, towards
rapid management of the outbreaks (WHO, 2008). Recently, studies have
indicated that Stegomyia species when infected by the
intracellular proteobacteria called Wolbachia limits pathogen
replication, in particular the enveloped, positive single-stranded RNA
viruses such as dengue (DENV), yellow fever (YFV) and chikungunya
(CHIKV) (Moreira et al ., 2009; Bian et al ., 2010; Walkeret al ., 2011). Based on this observation, a global campaign to
eliminate dengue (http://www.eliminatedengue.com) was initiated to
control dengue in countries such as Australia, Vietnam, Brazil,
Colombia, Indonesia etc. by introduction of Wolbachia into the
populations of these vector species. However, the mechanism of pathogen
inhibition by Wolbachia is still being investigated, but this
blocking phenomenon has been linked to priming of the host innate immune
system and competition for limited resources between pathogens andWolbachia (Caragata et al., 2013). Wolbachia is
found in many arthropods such as insects, mites and isopods and also in
filarial nematodes. This natural insect symbiont is reported to be most
concentrated in reproductive organs, but often have a wide tissue
distribution within the host insect, including midgut and salivary
glands. Wolbachia infection in St . albopicta was
discovered by Wright and Barr in 1980 and is reported to be super
infected with wAlbA and wAlbB strains. These strains are
known to play a crucial role in dynamics, evolution and reproductive
system of their host. Usually the arboviruses acquired through the blood
meal must invade midgut cells and ultimately disseminate to the salivary
glands to be transmitted, so that Wolbachia and the virus can
potentially be present within the same cells. Wolbachia itself is
not an infectious agent, but is maternally transmitted in the egg
cytoplasm and is able to invade host populations by manipulating their
reproduction. Hence, considerable scientific attention was directed at
better understanding the impact of Wolbachia infections on the
evolution and ecology of host populations (Charlat et al , 2003).
Moreover, the self-spreading ability of Wolbachia makes it an
attractive biocontrol agent to interrupt or reduce arbovirus
transmission (Sinkins, 2013). However, the infection prevalence details
of Wolbachia in natural populations of St. albopictus in endemic
regions of the arbo-viral Diseases as Dengue and Chikungunya remains not
well documented. Hence, we undertook a study to elucidate the natural
infection status of this parasite in this vector species in a highly
endemic state of Dengue Kerala.