By Dr Lal Jayasinghe-July 25, 2017, 8:42 pm
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.”
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”
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.