Leishmaniasis, a tropical neglected disease, is a vector-borne disease caused by protozoan parasites of genus Leishmania. The parasites are carried only by female sandflies as the female flies need blood for the development of eggs. They become infected with the Leishmania parasites while they suck blood from an infected person or animal and then transmit it to another host. Of 900 species of the sand fly, over 70 species are found to be associated with the transmission of leishmaniasis. One of the major vectors for leishmaniasis is the Phlebotomine sandfly. There are around 500 known Phlebotomine species, however, nearly 30 species are capable to transmit leishmaniasis ( https://www.who.int/leishmaniasis/en/).
In order to be incriminating natural vectors, they should have the
following criteria. The wild females not having recent blood meal (<36 hrs)
should contain promastigotes of Leishmania parasites. The anterior
midgut of infected females sand flies must have infective forms of Leishmania.
The flies should be attracted to and bite humans and other reservoir hosts.
They have to be strongly associated with humans and any reservoir host and
finally, the experimental transmission is achieved after infection from natural host
species or equivalent laboratory model. To date, no published data have been documented
to verify the vectors found in Thailand to prove as incriminating natural
vectors according to the criteria mentioned above. Few studies have reported the
presence of Leishmania DNA in female sandflies collected from different
provinces of Thailand by using molecular tools. Similarly, few investigators
have studied the natural habitats of the sandflies and their related host. On
the basis of these characteristics, they have suspected these vectors as the
potential natural vectors of leishmaniasis.
Chamnarn and team had collected 2401 Phlebotomine sand
flies from 16 limestone caves (temperature range 26-28°C) in Kanchanaburi province, Thailand, and identified
them following standard protocol. The study had updated to a total number of 26 species of sandflies belonged to the four genera Sergentomyia, Phlebotomus, Nemopalpus, and Chinius in Thailand.
These are C. barbazani, N.
vietnamensis, P. asperulus, P. barguesae, P. betisi, P. hoepplii, P. major
major, P. mascomai, P. philippinensis gouldi, P. pholetor, P. stantoni, P.
teshi, S. anodontis, S. bailyi, S. barraudi, S. brevicaulis, S. dentata, S.
gemmea, S. hodgsoni hodgsoni, S. indica, S. iyengari, S. perturbans, S.
phasukae, S. punjabensis, S. quatei and
S. sylvatica. The most frequent cave species found in this study were P.
major major and S. anodontis. The human biting species, P. major was also the first time reported from this study. They have observed ecological habitats and behaviors (host feeding, biting activity, resting areas
at day time, sheltering places at night) of sandflies to identify. However,
they are not concerned about whether there was a presence of Leishmania
infective forms in their midguts or not. They only proposed the sand flies as
potential natural vectors for leishmaniasis (Apiwathnasorn et al., 2011). Late
one more species, S. mahadevani had been identified, and altogether 27
species have been reported date in Thailand to date.
Among these potential vectors, different studies have
confirmed different species that transmit the Leishmania parasites. Kanjanopas
and team collected sandflies from individual households in Hat Samran District,
Trang Province, southern Thailand where coinfection of visceral leishmaniasis
and Human Immunodeficiency Syndrome (HIV) had been reported. They identified
the female sandflies with the help of Entomologists and finally sent them in
Molecular laboratory in Taiwan. They have evaluated for natural infections with
L.
siamensis confirmed by amplifying heat shock protein 70 (hsp70)
of Leishmania parasite by PCR method. Although other criteria had not
been studied, S. (Neophlebotomus)
gemmea is considered as a potential
natural vector for L. siamensis (Kanjanopas et al., 2013).
Chusri et al. carried
out active human case surveys processing blood, saliva and urine samples from
99 villagers living in an affected area in Na Thawi District. The team had also
studied details of animal reservoirs including blood samples from dogs, cats,
black rats, and Indochinese ground squirrels. Sandflies were collected from
villagers’ houses and plantation which were identified at office of Disease
Prevention and Control. The presence of Leishmania parasite in those
sandflies were confirmed by amplifying parasite specific 18s rRNA followed by
nucleotide sequencing. The study finally reported female S. (Neophlebotomus)
gemmea and female S. (Parrotomyia) barraudi were potential natural
vectors for L. siamensis (Chusri et al., 2014).
Another study by Sukra
and the team had reported six sand fly vectors of Sergentomyia; S. gemmea, S. iyengari, S. barraudi, S. indica, S. silvatica and S. perturbans as potential
natural vectors of leishmaniasis. The team had collected sandflies from three
provinces of Thailand; Phang-nga, Suratthani, and Nakonsitammarat. They had trapped the
flies at 200m around the patients’ houses by CDC light traps. The traps had
also been placed in other possible habitats such as cattle corrals, pig sites,
stacks of leaves etc. These flies were then identified, however, their role
in the transmission of Leishmania parasites was not confirmed. One of the
important vectors of leishmaniasis of genus Phlebotomus, Phlebotomus
argentipes,
was also detected. They suspected them as potential vectors because they were
found in the infected areas (Sukra et al. 2013).
Leishmaniasis
cases in Thailand constituted only imported cases before 1999. The recent
studies emphasizing indigenous leishmaniasis identify two new species, L.
siamnesis and L. martiniquensis as autochthonous species among Thai
patients. As reported by Chusri
et al., 2014 and Kanjanopas et al., 2013, S. (Neophlebotomus) gemmea and S. (Parrotomyia) barraudi as could
serve as potential vectors for L. martiniquensis (Leelayoova et al.,
2017).
Srisuton
et al. collected sand flies from endemic areas (Songkhla and
Phatthalung Provinces) and non-endemic area (Chumphon Province) of
leishmaniasis in Thailand. Head and genitalia dissection of pre-identified female
sandflies were done for morphology identification, and the remaining parts were
used to detect Leishmania and Trypanosoma DNA. One new vector identified
as S. khawi was found to carry Leishmania
and Trypanosoma parasites. The vector species was
confirmed as a potential natural vector capable of transmitting L.
martiniquensis in the human population (Srisuton et al., 2019).
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