What is Zika?

Previously virtually unheard of, it wasn’t until a big outbreak in Latin America in 2015-16 that Zika hit the headlines.

In fact, until then, most doctors, public health experts and even virologists would have probably never heard of the virus.

Though most of you will now have heard of Zika through the news, it’s slipped out of the limelight a little since 2016. This article will cover some of the basics about the course of the outbreak, what we now know about the virus and why it remains such a concern.

What is the Zika virus?

Zika is a member of a family of viruses called the ‘flaviviruses’, which also includes Dengue, West Nile Virus, Japanese Encephalitis, Yellow Fever and Hepatitis C.

How do you catch Zika virus? Am I at risk?

The virus is transmitted by mosquitoes, mainly a species called Aedes aegypti. When a hungry mosquito bites a human with Zika it becomes infected itself; it then bites other humans and transmits the disease as it does so.

In addition to the mosquito bites, it’s been shown that Zika can also be sexually transmitted. However, it’s thought that the sexual transmission is less important than mosquito-borne transmission, which accounts for the majority of the spread.

Therefore, Zika virus only really affects people who live or have travelled to the areas where these mosquitos live (tropical and sub-tropical regions). If you live in a place which doesn’t have the Aedes aegypti mosquito (such as the UK), you would only be at risk if you had sex with a returning traveller who had caught the virus while abroad.

aedes aegypti distribution.png
Map showing probability that the Aedes aegypti mosquito can be found in this part of the world. 1 (red) means we are 100% certain that Aedes aegypti lives here, 0 means we are 100% certain there are no Aedes aegypti here.

Zika – that’s a funny name? Where does it come from?

zika forest

The virus is named after the Zika forest in Uganda where it was originally discovered in 1948.

Before the big Latin-American outbreak in 2015-16 Zika was mostly confined to Africa where it was almost exclusively a disease of monkeys. Mosquitos would transmit the virus among monkeys, with small outbreaks in humans occurring only infrequently as the virus skipped over from monkeys to humans. These small outbreaks never affected many people in total and it seemed the disease was mild and without any complications. Consequently, no-one ever paid much interest.

zika cycle.png
Zika was confined mostly to transmission among monkeys (a ‘sylvatic’ cycle), with only occasional human cases until very recently.

Then, in 2015-16, there was a huge outbreak across Latin America and the Caribbean, starting initially in Brazil.

It was only at this point that the international community really became interested in Zika and in Feb 2016, the WHO declared a “Public Health Emergency of International Concern,” to muster the support of governments across the world to help study and control the virus.

Today there are 61 countries affected (with ongoing transmission)[1]

zika countries
Source: CDC

I’ve written more about the ‘story’ of Zika and where it came from in another post, here.

What sort of disease does the Zika virus cause?

Most people with Zika will never get any symptoms at all. Initial reports suggested that as may as 80% of cases are asymptomatic, although some argue that this figure is due to under-reporting of symptoms and the true number of completely asymptomatic may may be smaller. What is for sure is that it generally only causes a very minor illness (one of the least serious of all the flaviviruses).

The main symptoms are:

  • Fever and flu-like symptoms
  • Joint and muscle pains
  • An itchy skin rash
  • Conjunctivitis (red, itchy eyes)

Generally, the symptoms last only around 5 days, after which patients get better.

There have been some very, very rare cases of patients getting more severely unwell even a handful of deaths from Zika — perhaps around 18 deaths in total [2]. Bear in mind that the common flu kills people as well, as many as 250-500,000 across the globe every year [3].

The fact that the disease is so often asymptomatic presents difficulties for researchers and public health teams because it makes it difficult to track the outbreak and work out just how widespread it is.

zika symptoms.jpg
Symptoms of Zika

If Zika causes only a very minor illness, why’s everyone been so concerned over it?

The big worry with Zika came when it was discovered that, during the same period as the 2015-16 outbreak, there had been a large increase in the number of babies born with foetal malformations.

Specifically, an increase was detected in cases of ‘microcephaly. This is essentially just the medical word for a baby being born with an under-developed brain, which results in a small overall head size.

microcephaly diagram.jpg
Microcephaly: the medical term for a baby born with a small head, as a result of having an under-developed brain.

It didn’t take long for evidence to build up and in March 2016, the scientific community reached an agreement that the Zika virus definitely was responsible for the increase in number of cases of microcephaly in Latin America.

microcephaly case nos brasil
In 2015-16, the number of cases of microcephaly in Brazil shot up to 15-30 times the number it had been in previous years. In about half these cases there was also evidence that the mother had had Zika virus during her pregnancy.
Zika’s effects on babies: more than just microcephaly…

We now realise that in addition to causing microcephaly, there are some other effects of Zika on the developing baby in the uterus, including more subtle problems with brain development, eye problems, hearing problems and problems of the joints and muscles. The term ‘congenital Zika syndrome’ which describes a range of birth defects, is therefore now preferred to talking about microcephaly alone.

I’ve written in more detail about Zika’s affects on babies here.

Zika and neurological complications

It was also discovered that the Zika outbreak had coincided with an increase in the number of cases of a neurological condition called Guillain-Barré Syndrome (GBS), a condition causing paralysis shortly after having an infectious disease. This was first noted in French Polynesia in 2014, a little before the outbreak across Latin America and the spike in GBS cases repeated itself each time a new country became affected by Zika.

Zika’s ability to cause GBS was soon confirmed, and it has since been shown to cause other neurological complications in adults, such as meningitis, encephalitis and myelitis.

It’s these neurological complications in adults that are the focus of the project I’ll be working on during my time in Brazil.

I’ve written more about what GBS is (here) and its links with Zika virus (here).

What is Zika doing now? (more)

There’s been less fuss about Zika in the news since 2016, partly because our improved understanding of the virus means there’s less of a threat from the ‘unknown’ and partly because the number of new cases seem to be dwindling away.

The disease is seasonal as the mosquitos which transmit it become more abundant during the rainy season. Brazil’s 2017 rainy season is now in full swing and we’re seeing fewer cases than were observed in 2015 and 2016. This is likely because lots of the population have now already had the virus and developed immunity to it.

Where will Zika go from here? (more)

One prediction is that the virus will continue to affect people here at low, ‘background’ levels, with larger outbreaks only occurring every few years once the level of ‘herd immunity’ in the population wears off. However, these outbreaks are likely to be smaller than the 2015-16 outbreak because there will still be some herd immunity, whereas during the 2015-16 outbreak the population of Latin America was completely ‘immunologically naïve’ to the virus.

I’ve written more about what the virus is doing now and what we can expect it to do next here.

Does that mean we can relax and forget about Zika?

Absolutely not! There are still lots of good reasons to continue to concentrate on the virus. We need to better understand the effects on children during pregnancy, we need a better understanding of the nature of the neurological complications and why they occur, we need better diagnostic tests and although cases are dwindling, the disease is still prevalent in Latin America and has potential to cause far more disability and suffering if we’re not careful – vaccine development and mosquito control programmes are therefore crucial. We ignore Zika at our peril!

~~~~~

Related

Follow this link for a list of all my other posts on the Zika virus.

Footnotes:

[1] http://www.who.int/emergencies/zika-virus/situation-report/10-march-2017/en/

[2] https://en.wikipedia.org/wiki/2015%E2%80%9316_Zika_virus_epidemic (See table)

[3] http://www.who.int/mediacentre/factsheets/fs211/en/

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