A digitally-colourised transmission electron micrograph of the Zika virus (coloured blue, each 40 nm wide). Image: Cynthia Goldsmith/CDC
- Dengue is caused by four closely related viruses, called serotypes, but immunity against one type doesn’t protect against infection by another.
- As a result, a population that has developed immunity to one serotype can still have a dengue outbreak the next year, caused by a different serotype.
- This issue was exacerbated by the fact that a lot of the disease-control machinery had been diverted to manage COVID-19 in 2020 and 2021.
Different parts of India are currently reporting major dengue outbreaks. The total number of cases, according to the Centre’s National Vector Borne Disease Control Programme (NVBDCP), updated until September, is already past 60,000.
In the national capital, the NVBDCP’s case tally until September was 455 – but according to a civic report released on November 15, the city has more than 5,000 cases already.
At the same time, another vector-borne disease, Zika, has erupted in Uttar Pradesh and three months ago in Kerala. While there is no clear information on the total number of Zika cases, a source in the Union health ministry told The Wire Science that 129 cases of the disease have thus far been logged in Uttar Pradesh and 60 in Kerala.
To understand more about what is going on, including the causes of these outbreaks, The Wire Science spoke to Ashwani Kumar, the head of the Vector Control Research Centre (VCRC), Indian Council of Medical Research. The VCRC is located in Puducherry and conducts research on vectors (like mosquitoes in the case of Zika and dengue) prevailing in India.
The interview is presented in full, with light edits for style and clarity. The questions are in bold. Editor’s clarifications are enclosed in square brackets.
Both Zika and dengue are caused by the bite of the same mosquitoes – Aedes aegypti and Aedes albopictus. Is there any connection between the two diseases as a result?
There is no connection as such, other than the fact you state. Both these mosquitoes are quite well prevalent in the country.
When the Zika outbreak happened in Kerala this year, we did a study and isolated the virus from the mosquitoes. We found that other than these two species of Aedes, a third one, Aedes vittatus, was also circulating. All the three were quite likely involved in transmission [of the disease].
On the other hand, if a mosquito gets an opportunity to bite a Zika patient and a dengue patient simultaneously, both the viruses can multiply in the mosquito, and this [mosquito] in turn may bite a healthy person.
We don’t know, as of now, what kind of impact it will have on the mosquito but hypothetically, both viruses can multiply and get transmitted to a patient. So a patient can be infected with both dengue and Zika viruses at the same time. But we don’t know what their interactions will lead to, whether the virulence of one virus will go up or the other will come down.
Why is there a specific geography for Zika, like Uttar Pradesh and Kerala this year and Madhya Pradesh and Rajasthan in 2018?
The Zika virus is either introduced through some people from outside or it is already in circulation, and lying low. It surfaces when the right environmental conditions appear. So the circulation could be from overseas to the country or within the country, transmitting from one state to another. Perhaps it travelled to UP [in similar fashion].
It all depends on four factors: vector availability, suitable conditions for the vector to multiply, the virus’s availability and the host.
But why only specific states?
Zika, as we have seen in the past, is a self-limiting disease. It does not flare up like dengue. So far, there has been no recorded case of microcephaly (kids born with small heads to mothers who had been infected by the Zika virus), like it happened in Brazil in 2016. So it may get passed off to many people without them knowing, as they do not get tested.[Thus far,] there has been no systematic and periodic surveillance programme for Zika in the country, like we have for other vector-borne diseases like dengue, malaria, kala azar, filaria, chikungunya and Japanese encephalitis. [That is, Zika is tested for only in the event of an outbreak, whereas officials periodically check for the other six diseases irrespective of whether there is an outbreak.]
If there is systematic surveillance in different parts of the country where one would suspect Zika to be, or routine surveillance, then that might give us a different picture.
But people should not panic knowing this, because it is a very self-limiting disease.
Do you think the surveillance should start now?
Well, after COVID-19, we now have a network of medical colleges that have RT-PCR machines, and these machines are used to detect Zika. So now the government can think of launching Zika surveillance in different parts of the country. They look for these viruses.
Are the machines the same?
RT-PCR machines are the same. The procedures are the same and the reagents [are the same] too, by and large. But the primers and probes, which indicate the presence of the virus, are different. The hardware is essentially the same. [Different primers and probes are chemical compounds used to detect the presence of genetic material from different sources in a sample.]
Coming back to dengue – what were the factors responsible for the bigger outbreaks, scientifically speaking?
Dengue has four serotypes – 1,2,3 and 4.
What happens is these stereotypes normally take rounds.
If one type is in circulation for a few years, it will infect and immunise the bulk of the population with many not even knowing about it.
Suppose if in any given year, [serotype] 1 is, next year it might be 2, for a few years [it might be] 1 and 2. Then type 4 might come, and in between type 3 [as well]. Immunity built against one type, in the case of dengue, does not provide immunity against another serotype.
So whenever a serotype changes after several years, we see large outbreaks. There are studies suggesting that more than one serotype can prevail in a year, in combination. One may be dominant and the other may be less prevalent.
This year, serotype 3 has been seen in India after a long time. Otherwise, scientifically speaking, type 2 has the highest severity.
Is there any modelling available to project which serotype will affect more?
Not as of now.
A large part of India’s healthcare machinery was engaged in COVID management and then the vaccination drive. So mitigation measures like fogging, etc. that vector-borne diseases require were affected. Your understanding of this?
Undoubtedly, COVID-19 has affected other health programmes. Routine surveillance of vector borne diseases [has been] hit. The municipal corporations do fogging, spread anti-larval sprays or close the breeding sites of mosquitoes. These services had become deficient because most of this machinery [had been diverted to] handling pandemic. Therefore, it is not misplaced to assume that this year we are seeing outbreaks due to a shortfall in these actions.
There are articles indicating that these routine surveillance activities for vector-borne diseases have been affected not just in India and elsewhere also.
Is erratic weather in India also playing a role? The summer monsoon lasted longer into 2021.
Weather conditions definitely influence mosquito breeding and population development. So if such [rainy] conditions prevail for a long time, we will have more vector-borne diseases.