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The Zika Virus: an increasingly worrying problem?

By Rhoda Frost

Photo credit: Pixabay

Introduction

If you’ve been paying attention to international news lately, I’m sure you’ll have heard of the Zika virus. If not, suffice it to say that this is a virus with potentially serious consequences, that is currently spreading across Brazil, with the possibility of spreading even further afield. Should we be worried? Here are the facts of the matter.

What is it?

The Zika virus is, like all other viruses, a non-living entity essentially made up of proteins, lipids (fat molecules) and a genome (a structure similar to our DNA, containing the code for viral proteins). It is what we call a flavivirus, and is related to other viruses such as yellow fever and dengue, which are also mosquito-borne viruses found in the tropical and sub-tropical regions [1]. As viruses are non-living, they rely on cells of living things (called hosts) to provide what they need to keep replicating themselves and infecting other cells. Structurally, the Zika virus consists of an envelope around the outside, made up of proteins and lipids taken from the last host cell it infected [2]. It also has a nucleocapsid, which is the virus’ protein shell, formed around the genome [2]. The genome is read by the host’s cellular machinery, and the subsequent protein is produced by structures inside the host cell. The diagram below is a representation of what the virus might look like.

Photo credit: Rhoda Frost

Once the virus has infected a human, it generally causes mild symptoms including a rash and fever, joint and muscle pain, and often conjunctivitis. Only one in five persons infected will show any symptoms, but occasionally people can also develop more severe complications of the brain and nervous system [3] [4]. As of yet, there is no vaccine against Zika, and no cure.

How does it spread?

The virus is carried by a particular family of mosquitoes, the Aedes mosquitos, which are very tough, and difficult for public health agencies to control [5]. This is because they can reproduce in tiny amounts of water, and the eggs they lay can survive for a long time in challenging conditions [5]. The mosquito bites a human infected with the virus, taking up some of the virus in the process. It then goes on to bite another, uninfected human, delivering the virus into their blood stream and causing disease. Thus, the virus is spread. Previously, this would have been worrying, but generally contained to the local area and therefore would have been relatively manageable. In the modern age however, international trade and travel mean that a virus can travel thousands of miles to different continents with relative ease within a human host [3]. Climate change also means that the habitats of the carrier mosquitos are becoming larger, stretching further north even into the southern states of the US [3].

Where does the Zika virus come from?

The virus was first observed in the Zika forest of Uganda in 1947 where it was isolated from a rhesus monkey, hence the name ‘Zika virus’ [4]. Throughout the 1960s, a few cases were seen in parts of Asia and Africa, but the first big Zika outbreak was in 2007, on Yap Island in the Federated States of Micronesia. This was shortly followed in 2013 by an outbreak in French Polynesia, which affected around 11% of the population [4]. The first confirmed case in Brazil was in March 2015, and worryingly the problem here has only escalated [3]. There is a real potential that the virus could now spread across South America and the Caribbean, even into the southern states of the US, in which the Aedes mosquitos that carry the virus can be found. The map below shows where local transmission of the Zika virus has been recorded.

Photo credit: Pixabay and Rhoda Frost

The Brazilian problem

Through use of genetic techniques, it has been determined that the Zika virus entered Brazil around a year before the first reported case. It was likely that it was brought there from Polynesia, and could have entered the country when tourists entered Brazil for the 2014 FIFA World Cup [3]. In 2015, between 500,000-1.5 million people had been infected with the virus, which continues to spread [3]. At the same time, the country has seen a dramatic increase in the number of babies born with microcephaly, causing them to be born with abnormally small heads. Between 2010 and 2014, an average of 163 cases of microcephaly were reported each year in Brazil, but in 2015, when the Zika virus proliferated, there were 3,530 suspected cases. A concerning increase [5]. There have also been reports that the Zika virus can cause Guillain-Barré syndrome in some of its victims, a serious condition described below [2]. Considering all these factors, and the real lack of a standardised, fast, and reliable test for the Zika virus, it is all the more worrying that the 2016 Summer Olympic Games in Brazil are fast approaching, and with it, an influx of susceptible people from around the world [6].

Photo credit: Pixabay

Microcephaly

As previously mentioned, a rise in the number of people infected with the Zika virus has been accompanied by an increase in the number of children born with microcephaly. Microcephaly literally means ‘small head’ – a condition where there is a reduction in a baby’s head circumference below a certain threshold, found either at birth or shortly after [7]. This implies a significant lack of brain development, causing various levels of disability [7]. Many factors can cause the condition, including maternal exposure to alcohol, tobacco and radiation, maternal diabetes and maternal infection during pregnancy, causing certain diseases including syphilis, HIV and herpes [8]. It is therefore possible that Zika could be another causative virus.

In February 2016, the World Health Organisation stated that the link between Zika and microcephaly was strongly suspected, but not proven [5]. Despite the lack of experimental evidence proving that the Zika virus causes microcephaly, a study published in the New England Journal of Medicine on 13th April 2016 stated that, summing up all the evidence that has been collected, it could be inferred that the Zika virus causes microcephaly [9]. This was concluded for various reasons, including studies that found the virus in the brain tissue of affected babies, and the timing of the pregnant mother’s infection in relation to the stage of the baby’s development in the womb. This statement means that preventative measures and public health warnings can be more clearly made and enforced in order to reduce the risk to pregnant mothers, and to keep the public suitably informed about the threats the virus may pose.

Guillain-Barré syndrome

But the devastating effects of the Zika virus don’t stop at microcephaly; studies have also linked the virus to Guillain-Barré syndrome, a disease that causes your immune system to attack your nervous system, causing paralysis [10]. In this disease, your own antibodies and immune cells attack myelin (a protein and lipid layer that normally wraps around your nerves to insulate them, allowing them to transmit nerve signals more quickly) [11]. This destruction of myelin causes a range of symptoms such as weakness of muscles, missing reflexes, and paraesthesia (skin tingling or numbness) [11]. In severe cases, the breathing muscles are affected, and the sufferer will need to be ventilated in order to maintain normal breathing [11]. Whilst Guillain-Barré syndrome can be treated, and mortality (the proportion of sufferers who die from the disease) is only 2%, around 25% of patients will be left with some kind of disability as a result of the illness [11]. The link with the Zika virus is worrying because it means that cases of Guillain-Barré syndrome could become more common if the Zika virus continues to spread, and this may stretch the current resources available to treat it.

Conclusion

The overall picture is a worrying one. A quickly-spreading, incurable disease causing virus, with possible severe complications is currently at epidemic level in Brazil, a country which will soon welcome millions of tourists into the country for the Olympic Games held this year. Here in the UK, we appear to be relatively safe from the spread of the Aedes mosquito, at least for now. But it remains to be seen how much of the Americas will be affected by this virus before it can be brought under control, if that’s possible. The government and public health organisations advise prevention through insect repellent, avoiding mosquito bites through the day and night, and community surveillance, as the world hopes and prays that this doesn’t turn into an even bigger nightmare [6].


References

  1. [1] AS Oliveira Melo et al., Ultrasound Obst Gyn, 2016, 47(1),6-7
  2. [2] VL Pinto Junior et al., Acta Med Port, 2015, 28(6), 760-765
  3. [3] O Dyer, BMJ, 2015, 351:h6983
  4. [4] D Musso et al., Emerg Infect Dis, 2015, 21(2)
  5. [5] ES Paixão et al., Am J Public Health, 2016, 106(4), 606-612
  6. [6] II Bogoch et al., Lancet, 2016, 387(10016),335-336
  7. [7] CG Woods, Curr Opin Neurobiol, 2004, 14(1), 112-117
  8. [8] MJ Krauss et al., Am J Obstet Gynecol, 2003, 188(6), 1484-1490
  9. [9] Zika Virus and Birth Defects - Reviewing the Evidence for Causality, available from http://www.nejm.org/doi/full/10.1056/NEJMsr1604338, [accessed 07/4/16]
  10. [10] Guillain-Barré Syndrome outbreak associated with Zika virus infection in French Polynesia: a case-control study, available from http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(16)00562-6/fulltext, [accessed 07/4/160]
  11. [11] J Naish, D Syndercombe Court, “Medical Sciences”, 2nd ed, 2014,358b, Saunders Ltd

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