How To Read An Epidemic: An Excerpt From “The Secret Life of Zika Virus”

05 August, 2017

Between November 2016 and February 2017, three people in Ahmedabad, Gujarat, tested positive for the Zika Virus. The first was a woman who developed a fever after delivering a baby, and who recovered within a week. The other two cases—a pregnant woman in her early 20s, and a 65-year-old man—were identified during screenings conducted in January and February 2017. The existence of these cases however, was not publicly known until much later—on 26 May, the World Health Organisation published a report regarding these cases on its website. The WHO said that the Indian government had notified it of the cases only in mid May—over six months after the first case was diagnosed. The report was followed by a significant outrage among the public-health community, much of which was severely critical of the government’s delay in informing WHO.

Zika is a vector-borne virus—it is transmitted by a living organism—which can lead to microcephaly, a birth defect due to which children develop unusually small heads and resultantly, neurological defects. A Zika epidemic broke out in Brazil in 2015, during which over 50 children were born with microcephaly. Countries such as Brazil and India, due to their dense population and tropical climates, are particularly prone to the spread of such a virus. In the book The Secret Life of Zika Virus, Kalpish Ratna examines the evolution of the Zika virus, and what India can learn from the spread of the disease in Brazil. In the following extract, Ratna examines two ways to read an epidemic: one is scientific, and the other, personal.

This extract is part of The Caravan’s short series on Zika. The series includes a photo essay on the lingering effects of the virus in Recife, Brazil, and a reporton why the Indian media’s response to the emergence of Zika in India fell short.

Perhaps the time is right to step back a little and review what a year of living with Zika has meant. How do we read an epidemic?

The index used to gauge the transmissibility of a disease, and therefore the progress of an epidemic, is called the Basic Reproductive Number, R0 (R Naught). This number quantifies how many other individuals one patient can infect with an illness.

If R0 is less than 1, the infection will die out in the long run.

But if R0 is greater than 1, the infection will be able to spread in a population.

Since the outbreak in September 2014, R0 has been calculated for Brazil, and also for French Polynesia and Yap.

The R0 at the early stage of an epidemic predicts how fast the disease will spread. In an epidemic with few geographic barriers, like this one, R0 can also alert us to the ways of transmission.

Let us look at figures from Rio de Janeiro. Like India, urban expansion in Brazil has been explosive. It has scythed across natural habitats, displaced populations, and created disparate microclimates—all of which compel extreme makeovers in the history of vector-borne epidemics.

The Brazilian R0 was calculated from the 25,213 suspected cases of Zika fever notified between January 2015 and April 2016. It was 3.93.

This means that one person with Zika fever had infected four other people.

In Rio, the estimated R0 for dengue, in 2012, was 1.7.

Dengue is also transmitted by Aëdes aegypti [the scientific name for the yellow fever mosquito, which can also transmit dengue, chikungunya and zika fever, among other diseases]. The other variables, population size and climatic conditions being constant, what explains this disparity between zika and dengue?

Also, the few studies that have been published have shown poor vector competence to Zika Virus in Aëdes aegypti as compared to its competence for DENV [the Dengue virus].

Does this mean there are other vectors transmitting Zika Virus?

Other mosquitoes, Culex quinquefasciatus, and recently, Anopheles have harboured Zika Virus. But do they transit the disease?

What about other methods of transmission?

Sexual transmission has been inadequately investigated. The reports, so far, have been sporadic, almost anecdotal. (The exclusion of a general survey of women in affected areas is misogynistic. Extended virus retention in the genital tract mean more opportunity for transmission. We know this to be true of the male genital tract, but this is an investigation that should have logically been centred on women.)

There is another possibility—perhaps the Zika Virus is far more infective in humans than it is in mosquitoes.

The R0 is not a constant for a disease (or a virus). It is specific to the area under scrutiny.

The R0 calculated for the Yap and French Polynesian Zika epidemics was 4.3 to 5.8 and 1.5 to 3.1 respectively, both values similar to those for dengue and chikungunya. This suggests the replication of Zika Virus within the vector and its transmissibility is similar to that for DENV [the Dengue virus] and CHIKV [the Chikungunya virus].

Island populations are ideal for studying the course of an epidemic as they are comparatively isolated from extraneous factors. The two island groups that suffered Zika outbreaks had an epidemiological profile similar to dengue and chikungunya. Based on this observation, and presuming Zika infection confers lifelong immunity, a ten-year—or longer—periodicity of outbreak is expected.

In the Pacific islands, dengue outbreaks occur every 12 to 15 years.

Island populations are small. Below a critical population size, the endemicity of a disease cannot be sustained as the number of susceptible people will be too small. Conversely, when the population density is greater, the endemicity will be sustained and outbreaks will become more frequent.

Climate and environmental change are the strongest drivers for dengue and chikungunya on our subcontinent. Zika seems no different.

But the experience of Zika is what really makes up the epidemic. Not just the physicality of illness, but the momentum of the disease.


I imagine myself in Rio.

On Day 1, a mild fever, usually discounted. Here comes the hurdle—it is October 2016. I already know my neighbourhood has Zika Fever.

So, is it Zika I have? I check for a rash. Nothing.

I am swept up in the day’s urgencies, and—Ha! That was just Monday morning megrims.

But I’m not my usual self for the rest of the week, though my fever is long gone.

Eventually, I see the doctor.

“Is this Zika?” I ask. “How do you know it isn’t dengue? Chikungunya?”

I surrender samples of blood and urine. They are whisked off to be tested for Zika Virus, DENV and CHIKV by RT-PCR (Reverse Transcription Polymerase Chain Reaction). This will take a while.

I am shooed away after being advised to report if I develop any new symptoms.

I feel guilty for having taken up the doctor’s time—the young women in the queue are wide-eyed with anxiety. Some of them are visibly pregnant. They too will be tested just like I was, and in addition, they will get sonograms. Most of them will emerge smiling from the ordeal—everything is fine.

For now.

Some will return for more invasive investigations when future sonograms reveal worrying features. More blood tests, perhaps amniotic fluid sampling.

Meanwhile, I’m fine, just fine. My RT-PCR test comes back negative.

So does that of the young woman I met on my first visit.

Does this mean we both don’t have Zika Fever?

Not really.

A positive test would have confirmed that we are indeed infected, but a negative test doesn’t rule it out.

My young friend is pregnant. The doctor advises more tests: serology to see if she has antibodies against Zika Virus. There is a snag here too. These tests are cross-reactive for DENV and CHIKV.

“I’ll do whatever it takes to make certain my baby is safe,” she whispers earnestly.

Her husband draws me aside. “They say she could have got it from me. Is that true?”

The burden of dread on this young couple is heavier than the viral load.

This is what an epidemic means.

Try reading it in from your own vantage, from your own street in your own city or town or village, wherever you are, and you will understand what I’m saying.

An epidemic changes your life, even if you are not ill.

This is an extract from The Secret Life of Zika Virus by Kalpish Ratna, published by Speaking Tiger. It is part of The Caravan’s short series on Zika. The series includes a photo essay on the lingering effects of the virus in Recife, Brazil, and a reporton why the Indian media’s response to the emergence of Zika in India fell short.