A Brief Colonial History Of Ceylon(SriLanka)
Sri Lanka: One Island Two Nations
A Brief Colonial History Of Ceylon(SriLanka)
Sri Lanka: One Island Two Nations
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Thiranjala Weerasinghe sj.- One Island Two Nations
?????????????????????????????????????????????????Thursday, July 27, 2017
Fight Dengue: Break mosquito – man – mosquito cycle
By Dr Lal Jayasinghe-July 25, 2017, 8:42 pm
www.eradicatedengue.blogspot.com
laljayasinghe@hotmail.com
Do mosquitoes transfer dengue virus to its offspring? If so, how does it affect the dengue epidemic?
There are several issues to discuss. First, what has actually been
found? Does it happen in nature? If it does, to what extent? How does it
affect the progress of the epidemic and its control?
If anyone were to Google "vertical transmission of dengue virus" or
something similar, dozens of sites will turn up. This may give someone
the impression that vertical transmission or transfer of the dengue
virus by an infected female mosquito to its offspring or vertical
transmission (VT) is a well-established fact and is a common occurrence.
The reality is, VT is neither a well established fact nor a common
occurrence. In fact, it is more implied or inferred or even speculated
rather than scientifically proved.
Vertical transmission was thought to have happened because dengue
outbreaks appeared in a locality after a long period being free of
dengue cases. It is known that the eggs of the Aedes mosquito, which
transmit the dengue virus from person to person, survive in a quiescent
state for a long period in nature, even in a drought situation when all
pools of water have dried out. Following rains, these eggs hatch and
mosquitoes emerge. The suggestion was, therefore, made that the epidemic
suddenly appeared because the dormant eggs had dengue virus in them. In
other words the mother mosquito had passed the virus to the eggs, and
the virus had survived for a long period in the eggs and when the eggs
hatched the virus was already in the newly hatched mosquito, which then
infected one or two persons and the epidemic started. Although this had
not actually been proved, the explanation was more or less accepted as
being a possibility.
Now with advances in molecular biology it is possible to test if this
happens in nature. There are lots of articles from all over the world
"proving" that vertical transmission is actually taking place. Once
again it is more implication rather than proof. What has been shown is
that the virus is found in nature inside aedes mosquito larvae and in
male mosquitoes. The argument then is that the virus cannot be in larvae
unless it was in the eggs, and it cannot be in a male mosquito because a
male mosquito does not suck blood from humans who may carry the virus.
In other words, the male must have got it from its mother through the
eggs, i.e. vertical transmission. There are two problems with this
theory. First, the virus can be acquired orally, in other words it can
be eaten by the larvae from the environment, i.e. from the water in the
pool it lives in. The way the pool acquires the virus is from the mother
mosquito while laying eggs or infected mosquitoes dying in the pool and
the larvae cannibalising them. There is a way a male mosquito can
acquire the virus other than via vertical transmission. It can acquire
the virus from a female mosquito during the mating process. So there are
alternative explanations to how the virus could get into larvae and
males other than through vertical transfer.
As I said before, there are dozens of articles in the net. If you only
read the TITLES of the articles, one gets the impression that vertical
transmission is proven fact. But if you read the articles further, you
will find that in fact what they have found is that, after examining
hundreds of samples they have found DNA that belong to dengue virus in a
very few samples. It is not actually a case that they isolated the
virus from eggs and grew the virus in a tissue culture or something
similar. We need not go through all these articles (which are quite
boring), because researchers from the Institut Pasteur, Paris, with
knowledge in these matters have studied all of the articles and come to
the following conclusions:
" In general, VT contribution to arbovirus persistence is likely modest
because vertically infected mosquitoes are rarely observed in nature.
Taken together, however, our results call for caution when interpreting
VT studies because their conclusions are context and method dependent."
https://www.ncbi.nlm.nih.gov/pubmed/25077992
Similarly researchers from Universities of Exeter UK and University of
California, US, have also studied this issue and come to the following
conclusion:
"Given the evidence from mathematical models and the number of studies
that failed to find evidence of vertical transmission, vertical
transmission is unlikely to be important for the persistence of DENVs at
a local or regional level. A combination of asymptomatic DENV infection
in humans and the movement of viraemic people may well be more
important in virus recurrence"
https://www.ncbi.nlm.nih.gov/pubmed/26545718
The next question is, if VT transmission actually takes place to even a
modest degree how will it affect efforts at controlling the epidemic?
Some people in Sri Lanka believe that isolating dengue patients, which I
am advocating as the quickest means of eradicating dengue from Sri
Lanka (please visit the blog), will not work because of VT.
The reason is that the Aedes mosquito is born already infected.
Therefore, there is no need to bite a dengue patient to acquire the
virus. Therefore, it is pointless screening dengue patients. What this
means is that in order to eliminate dengue the, entire population of
aedes mosquitoes will have to be destroyed to stop dengue, because even
if a few are left behind they will restart the epidemic. In other words,
we will have to use tons of chemicals like we did in trying to control
malaria, to eradicate the entire Aedes mosquito from Sri Lanka. There is
another flaw in their argument. Mosquito is the intermediate host in
the dengue life cycle. Man is the definite host. This is similar to
malaria, where the mosquito is the intermediate host and man the real
host, where the parasite lives. For the survival of the organism, for
malaria as well as dengue, both hosts are necessary. Remember that the
dengue virus has to live for 8-10 days in the mosquito before it becomes
infective. The process in the mosquito is well understood with malaria,
but still not with dengue. However, it is necessary for the virus to go
through man to complete the cycle. Breaking the Man-Mosquito-Man cycle
should stop the epidemic.