If the spread of the Zika virus wasn't bad enough, scientists have found that prior exposure to the dengue virus can make Zika infections lethal.

Researchers at the Flordia Gulf Coast University found that previous exposure to the dengue virus could increase the spread of Zika around the body. This is because the Dengue antibodies bind to the new virus but can't fight it, leading them to transport the disease around the body.

The virus then reproduces rapidly, greatly enhancing the effects of the virus.


An artists' impression of the Zika virus with red blood cells urfinguss/iStock

The Zika virus was declared a global public health emergency in February by the WHO. At the time, it organised a committee in a response to the "explosive" spread of the Zika virus.

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The WHO's director-general, Margaret Chan, described the virus as being "deeply concerning" since it had spread to more than 23 countries across central and South America. It has also spread to the UK, other locations in the US, and Northern Ireland.

The Zika virus, which is spread by mosquitoes, has been causing birth defects in thousands of children. It has "explosive pandemic potential" two US scientists pointed out, encouraging the World Health Organisation (WHO) to take urgent action against it.


Writing in the Journal of the American Medical Association, Daniel Lucey and Lawrence Gostin highlighted the WHO's "failure to act decisively" during the Ebola crisis, potentially costing thousands of lives.

But what is the Zika virus, how did it spread, and how can people stay safe?

What is Zika?

Aedes Aegypti mosquitos, which carry the Zika virus, as photographed in a laboratory in San Salvador Mario Tama/Getty Images

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Zika is a virus spread primarily through the bite of the Aedes mosquito – the same species that spreads dengue. First identified in a rhesus monkey in 1947, the virus was isolated for the first time in 1968.


Infection first took hold in Africa – in Nigeria, then in the Central African Republic, Egypt, Gabon and Uganda – and then Asia, affecting India, Thailand, Vietnam and more.

The virus belongs to the Flavivirus group of viruses, which also includes West Nile, dengue and yellow fever. Each of the viruses in this family contain a single strand of RNA - a nucleic acid present in all living cells - but they're notoriously difficult to manipulate and clone.

How is it spread?

Scientists at the University of Liverpool believe the combination of El Niño, the weather phenomenon, and climate change may have enabled the spread of the virus in 2015.

The combination of El Niño, which causes above-average temperatures in the Pacific Ocean as well as extreme weather around the world, and climate change created favourable conditions for the mosquitoes which carry Zika to spread the virus.

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The researchers used the findings to develop a model to help predict the risk of future Zika outbreaks as well as other diseases including Dengue fever.

What are the effects of the virus?

The effects and symptoms of Zika vary but in most people, the virus causes a mild disease lasting between two and seven days, with key symptoms including fever, rash, aches, conjunctivitis and eye pain.

In a minority of people, Zika has been found to cause Guillain-Barré syndrome, an auto-immune disease which makes the immune system attack the body's nerve cells, resulting in weakness of the muscles and even paralysis. The effects typically last between a few weeks and a few months, but are sometimes permanent.

A more recent report found that almost half of pregnant women infected with Zika suffered miscarriages or birth defects. Of those infected in the first trimester, 55 per cent experienced adverse outcomes including miscarriages and abnormal infant brain development. A little over half of those infected in the second trimester suffered adverse outcomes, with 58 (46 per cent) of the women experiencing such outcomes.

How does Zika compare to other diseases?

Around 20 per cent of those infected with the Zika virus become ill, with common symptoms including fever, rash, joint pain, conjunctivitis and headaches. Although the virus rarely causes death it has been responsible for a spike in babies being born with microcephaly, a congenital condition associated with incomplete brain development.

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There are multiple ways in which the virus can spread; through contact with mosquitoes in the genus Aedes, person-to-person via sex, through blood transfusions and mother-to-child during pregnancy. The study focused on mosquito-driven transmission.

Brain scans of a two-month-old baby with microcephaly are displayed at a hospital in Recife, Brazil Mario Tama/Getty Images

Before the virus hit Brazil the number of babies born with microcephaly was 156 a year – a number that increased to 4,000 between October 2015 and January 2016. This spike has been attributed to Zika. A similar spike has been observed in babies born with Guillain-Barre Syndrome – a rare condition of the nervous system that causes rapid-onset muscle weakness.

Ebola, on the other hand, is far more deadly – there have been 28,638 cases of Ebola and 11,316 deaths, with symptoms including nausea, diarrhoea, bleeding, severe weight loss and intense pain. Ebola has a fatality rate of 71 per cent – 51 per cent more than the mere infection rate of Zika.

How dangerous is the threat of the virus?

Zika is no longer considered a global health emergency, as it was recently downgraded by the World Health Organisation to a chronic threat level.

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Representatives from affected countries including Brazil, Thailand and the US met with WHO to present information on the links between Zika and the birth defect microcephaly, along with an update on the measures implemented to attempt to control it. The board decided that rather than treating the virus as an emergency, it can now shift to looking at long-term control measures.

The risks are still huge, however, with WHO calling it “a significant enduring public health challenge requiring intense action”. In an interview with the New York Times, executive director of the WHO’s health emergencies program reiterated this: “We are not downgrading the importance of Zika. We are sending the message that Zika is here to stay, and the WHO response is here to stay.” It has been decided a new, “robust”, longterm strategy is needed as a result of the proven link between Zika and the birth defect microcephaly. The WHO called for “sustained research” to better understand the consequences of the disease.

Where has Zika been reported?

The current epidemic began in early 2015 in Brazil, and since then has spread throughout South and Central America.

In February 2016, the World Health Organisation declared the pandemic a public health emergency and five months later, the US Centres for Disease Control and Prevention reported the first case of mosquito-borne infection in the US in residents of Florida, in a neighbourhood near Miami.

Zika is currently on the rise in New York, for example – although there have been no local cases yet. Further along the east coast, four individuals in Miami-Dade and Broward counties were recently found to have been infected with the virus by local mosquitoes.

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Florida health officials said they are the "first known cases" of the virus being transmitted by mosquitoes in the continental US and while no mosquitoes trapped have tested positive for Zika virus, the officials believes they were "likely" transmitted through infected mosquitoes in the region.

The statement from the officials continued that this transmission is confined to a small region of Miami, as small as a ZIP code, but officials are extending their investigation across the state. In particular, they are asking residents for urine samples. This latest transmission takes the total number of cases in Florida up to 386.

Then, in November, health officials in Texas announced they had recorded the first case of the Zika virus transmitted by a local mosquito. Before this announcement, the only cases of Zika found in Texas had been contracted while travelling to other regions where the virus has been found.

Public Health England released an update at the end of November, stating that a total of 256 cases of the virus had been reported in the UK, including at least one case which had been transmitted sexually.

Professor Dilys Morgan, Zika incident director at PHE said in a statement: "It is important to remember that the main risk relates to travellers to countries classified as high or moderate risk for Zika infection.

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"Zika infection is usually a mild, self-limiting illness, and PHE’s advice is based on the fact that our main concern is to avoid infection in pregnancy, in order to avoid risk to the unborn child."

Where will the virus go next?

Using data such as climate and socioeconomics, researchers from the University of Kansas recently mapped Zika risk around the world.

The team adapted a method typically used to predict the distribution of species across the globe and their map predicts the likelihood of Zika exposure in the coming years by dividing Earth's landmasses into squares of 5km x 5km each. The grey sections are regions in which contracting Zika is, and will be, unlikely in terms of local cases.


Aedes Aegypti mosquitos, which carry the Zika virus, as photographed in a laboratory in San Salvador Mario Tama/Getty Images

Residents in the purple regions are only likely to get Zika if the virus is brought into the country by other residents or tourists who have visited currently infected zones.

Zika will likely spread into the orange regions due to their climates and the blue regions are most at risk because they are popular tourist attractions, have Zika-friendly climates and are already infected or could support the spread of infected mosquitoes.