Singapore woman uses maths to show why Zika is a problem the government cannot solve alone

A Singapore woman, Aresha Krishnan-Harling has taken to the social media to campaign for the mass support of the citizenry and not the government alone in combating the recent outbreak of Zika virus in the Island state.

She posted the below message on her Facebook page:

Many are taking the view that Zika is a small issue that will be unfortunate for a small minority – and that it doesn’t really concern them because they are not trying to conceive or pregnant. This couldn’t be more incorrect.

I’ve calculated some basic statistics based on the outbreak in Singapore to put it into perspective. There is a critical role that everyone needs to play by protecting themselves with repellent and protecting their homes in order to not feed the reproductive cycles of the Aedes mosquito.

A lot of people believe that public fogging and removing of stagnant water is enough – and that the Government is responsible for getting it under control. But Aedes eggs are fog-proof and can survive without water for more than a year.

One of the most important things we can do is to stop “feeding” the reproductive cycle and transmission. We all need do this with adequate repellent coverage and preventative measures at home/work.

Let me explain why:

1. The virus can only be transmitted through the female Aedes aegypti mosquito, as she needs to feed on blood in order to lay eggs. She prefers and thrives primarily on human blood to reproduce. Given our urban city – we are probably the largest source of “blood” feeding her reproductive cycle.

Seeing that the eggs are only laid if she has a full abdomen of blood, the reproductive cycle is interrupted if she has reduced sources to feed.

2. In addition, without the ability to feed on humans, she is therefore unable to spread the Zika transmission to a non-carrier human. This also prevents a non-carrier mosquito from contracting the virus and becoming a positive carrier.

I can’t stress the importance of those two points any more than to illustrate how fast a female Aedes Mosquito propagates.

To put it into context:

A single female Aedes mosquito lays 300 infected eggs per week in her 5 week lifetime. If we calculate the proportion of female blood feeding offspring at a 1:1 male to female ratio then her zika infected population legacy at the end of her lifetime would grow at the following rate:

1 -> 150 -> 22500 -> 3,375,000 -> 506,250,000

By her fifth generation, she would have laid 506 million hardy clusters of eggs waiting for water conditions in order to metamorphosise within 3 days, therefore increasing the density of infections within her said vector at a phenomenal rate.

If we look at rate of infection, it is said that a single female aedes mosquito feeds every 2 hours for up to 3 days. In volumetric needs, that is 6µl (0.006ml) of blood to achieve a full stomach conducive for a 300 egg-laying cycle. At an extremely conservative rate, that would mean approximately (6µl/0.4µl per bite) 15 bites per mosquito. Assuming it bites three times per person… that’s 5 people per mosquito infected in a single week per egg cycle.

If we work backwards and assume that there are 246 zika infections in the highest density vector cluster (Sims Avenue) in a single week (41 active symptoms + a probable 205 undetected cases), then that would mean an estimated > 50 or so infected mosquitoes in circulation.

Now if we conceal the infected mosquitoes within the vector cluster that is approximately 1.2km² and assume that there are 100 total mosquito population per m² that means the Zika virus will increase in density per vector at the following rate:

[FIRST WEEK/EGG CYCLE]

120,000 mosquitoes / 50 infected in circulation, that’s a probability of 1:2400 chance of contracting Zika.

[SECOND WEEK/EGG CYCLE]

However, after these 50 lay their 7,500 infected eggs, assuming attrition from drain oiling at 80% – 1,500 survive. And in as little as 4 days, that increases the probability up to 1:80

[THIRD WEEK/EGG CYCLE]

By the third week, (assuming attrition… fogging happens once a week and 80% adults die and a further 80% eggs/larvae gets obliterated from oiling) a further 9,000 female zika carrier eggs will be borne out of the second generation survivors. Bringing the probability up dramatically to 1:13… and so on so forth.

This is not taking into consideration the rate of growth in vector size at 400 metres per week, per egg cycle.

I can only conclude this humble Facebook post by reminding you about the extremely important role you play in stopping the spread of this horrendous disease.

I urge you to protect yourself and educate those you know to take a vested interest in this – for the sake of your fellow Singaporeans who are worried about their their unborn babies. The pregnancy journey should be a joyous and happy occasion and not one filled with panic and uncertainty.

It is not just the responsibility of those who don’t want to catch it, it’s everyone’s moral and social responsibility to help reduce and ultimately prevent the spread.

🙂 <3

DISCLAIMER

1. Conservative assumptions have been made as stated to calculate the probabilities in this post. It is by no means an official statistic or fact.
2. Repellent by means of blood supply is not the only means of defence, but a strong contributor.
3. Singapore is largely an urban city and while mammals/livestock can also contribute to the Aedes blood meal, it is a Singapore context of being a concrete, densely populated city that has led me deduce that the majority of the blood feed for mosquitoes here come from humans. I attach a study that states that the Aedes mosquito preferentially and frequently feeds on human blood (see link). In addition Zika has not yet been detected to transmit or infect animals – and they are at the very least, not an active transmitter/carrier of the virus to humans.)
4. Please feel free to correct or make suggestions, this isn’t intended to mislead or misinform, but to encourage the wider public that they have a role to play. And it’s not just the responsibility of those who are trying to conceive or pregnant.

http://www.ncbi.nlm.nih.gov/pubmed/11372967

https://www.cdc.gov/…/resour…/30Jan2012/aegyptifactsheet.pdf

Live Zika case map:
https://www.google.com/maps/d/viewer…

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