What does Zika look like?

What does Zika virus look like?

 Viruses are small

Very small. Smaller than a speck of dust, smaller than a germ like E coli. Between 20 and 300nm [there are one million nanometers in a millimeter]. So we can't just take a photo of them. 

Here are some 3D renderings of viruses such as this one: Hepatitis C

It looks like a studio portrait, with two lights from above and a backlight helping it standing out from the monotone background. 

But it is nothing of the sort. It's made-up.

Viruses can only survive deep in body tissues, usually inside cells. If we could film viruses in the blood stream they would be like flotsam washing along the arteries and veins, dwarfed by thousands of giant red cells and a few even larger white cells. But we can't. Light microscopes can only reach a resolution of 200nm so even a giant virus is a minute dot.

But the electron microscope has a resolution of 50pm [there are a thousand picometers in one nanometer]. The problem is that the smallest electron microscope is about the size of a PC.  Here it is:

So it won't fit inside any living body. As a result for transmission electron microscopy you need to:

• Take a sample
• bring it to the lab
• fix the structure with formaldehyde
• freeze it quickly in liquid ethane and keep it frozen
• take fine slices 
• place the sample in a vacuum 
• direct electron beams at the sample

You'll be familiar with the resulting gray scale em photos like this:

Here is our first sight of Zika virus in frozen section.

It looks spherical and transparent. There is no colour. The only way we can see coloured images is by adding stains to the sample. And the only way we can see the 3D structure of the virus is by using a scanning electron microscope, which can work with thicker sections but you may have to:

• bathe the sample in electron dense solution
• dry it
• freeze fracture it to examine the broken off edge
• embed it with epoxy resin
• blow metal vapour such as platinum across it to create shadows
• take an imprint of the structures with carbon vapour
• stain it with heavy metals: lead, tungsten or uranium

As you can imagine, any of these processes may interfere with the sample to cause artefacts. So it's necessary to compare the results of several different methods to get an accurate picture.

Since viruses are essentially colourless, artificial colour is added in the staining process or in illustrating the virus structure in order to make it visible. It's guesswork:

From these sudies we know that Zika virus has a smooth surface but develops projections when it is attacked by acid inside the cell. The projections are exaggerated in some illustrations to create a more beautiful/sinister image:

 This image of Zika virus homing in on a human embryo [much reduced in size is particularly unsettling:

For me, the image which best captures the structure of Zika is this diagram: 

Why is Zika vaccine taking so long to develop? 8 Reasons

Zika Virus

Zika belongs to the flavivirus group that is spread by arthropods: Yellow Fever virus [YFV], Japanese Encephalitis Virus [JEV], Tick-borne encaphalitis virus [TBEV] Dengue [DEN] and West Nile Virus [WNV]. 

Each of these viruses has a wide and growing distribution, due to international travel and climate change favouring spread of infected people and mosquitoes:

Why is Zika virus vaccine taking so long?

1. Because Zika virus seemed to be harmless

Zika was isolated from Rhesus macaque monkeys in Zika Forest, Uganda in 1947. When human infections with Zika were detected in the 1950's, the symptoms were mild: flu-like transient fever, headache and muscle pains. 
However since then two changes have occurred. Transmission of the virus by mosquitoes and the development of Pacific strain associated with microcephaly and Guillain Barre Syndrome [GBS: a neuropathy, or nerve disease]. 
Spread of the virus has been aided by international travel and climate change favouring the spread of Aedes aegypti, the main mosquito vector. Other mosquito species such as Aedes albopictus are capable of harbouring Zika which would be a serious development.

2. Because most Zika infections go unnoticed

Zika infections usually have no symptoms. In any case flu-like symptoms are so common they are rarely investigated. However older people are at higher risk of GBS after Zika infection and the dangers to unborn children are extremely severe, especially in the first three months of pregnancy.
Japanese encephalitis virus is similar to Zika. Most JEV infections go unnoticed but one case in a thousand progresses to brain infection. A mouse brain derived vaccine is available with 80-95% efficacy and vaccination in affected countries has led to a fall in incidence since 1960. A cell-grown strain appears to be safer and more effective. A two dose regime is suggested to match the conversion rate of other vaccines.

3. Because Zika virus is constantly changing

Viruses are the fastest evolving organisms on earth. They exist in multiple forms so that drug resistant strains are already present before the drugs are even developed. Viruses exchange genetic material and switch genetic code frequently leading to multiple mutant strains, some of which survive and come to dominate the population. This is illustrated by the current change in Zika to become much more invasive and adapted to life in a mosquito.

The RNA of flaviviruses can be read backwards or start from a different base pair and still produce viable organisms. They are the masters of improvisation.

Like most viruses, JEV has shown changes in the dominant strain from genotype 3 to genotype 1 however immunisation with one strain has been shown to protect humans from all strains. 

4. Because Zika virus rapidly enters human cells

Zika's ability to penetrate human cells is the key to its success. Once inside the cell the virus is protected from the body's defences. Zika deconstructs itself and the RNA component hijacks the cell into making more viruses, ultimately leading to cell death and release of mature virus particles.

Survival inside the cell means that viral infections such as Herpes can persist for long periods. A vaccine needs to target the immediate entry of viruses in the blood stream before they enter human cells. 

5. Because Zika virus survives in other animals

YFV cannot be eradicated because a reservoir exists in non-human primates, and tropical birds and mammals 'the sylvatic cycle', in towns YFV is spread from person to person by Aedes aegypti the 'urban cycle'. In addition some mosquitoes target humans and non-human primates 'the intermediate cycle'.

Despite the wide availablity of vaccine, outbreaks still occur. So far YFV has not become endemic in Asia but a current outbreak in Angola [April 2016] increases the risk via migrant workers returning to Asia.

6. Because of the danger of brain complications from the vaccine

The first YFV vaccine was made in the 1930's and substrains of the YFV-17D vaccine, are used nowadays to protect against infection. >98% of people have an immune response to a single dose and because antibodies are detectable 30 years later, a follow-up at ten years is no longer recommended. 

However the early vaccine was complicated by brain and spine complications and the current vaccine has an adverse rate is 38/100,000. Some people are allergic to gelatin or chicken egg protein,however 1/100,000 reactions are dangerous: encephalitis or multiple organ failure.

These complications can't be detected until the vaccine is tested in a large human population. Scientists are understandably cautious about introducing the vaccine until they are sure that the vaccine is effective and safe.

7. Because detecting Zika is difficult

Host response to the Zika virus, antibodies, are only detectable in the blood of infected people briefly. Antibodies are detected in the urine for longer.  However immunity to YFV may interact with the result, complicating the issue. 

The most sensitive test  is to isolate the Zika virus itself by RT-PCR but this requires deep refridgeration at -80C to preserve the virus, which isn't widely available.

8. Despite years of study, there is no vaccine to Dengue virus

Like Zika, closely related Dengue is spread by mosquitoes and as the map shows, it's the most widespread and the most dangerous flavivirus, causing 50m infections and 20,000 deaths a year. Antibody-dependent enhancement [ADE] explains Dengue's resistance to vaccines. The body's response to infection creates an army of monocytes and macrophages that ultimately spread the infection.This may explain why second infections in an individual can provoke dangerous Dengue Haemorrhagic Fever [DHF]

Dengue has four subtypes, so a vaccine must protect against all four otherwise it might be associated with DHF. Conversion rates of 40% against all 4 serotypes are reported and 70% after two immunisations, however the risk of DHF has halted some testing.

More technical information about flavivirus infections

Disney films that feature mosquitoes: lessons for Zika

Mosquitoes feature in two Walt Disney films from the 1930's and 40's
'Camping out' and 'The Winged Scourge' 

Both films illustrate the difficulties of a light-hearted approach to a serious health risk.
'Camping out 'portrays a battle between Mickey Mouse and friends against an army of determined mosquitoes. The weapons in use include a pop gun loaded with peas and sweetcorn, a hammer and a fly swatter. 
Some scenes of multiple bites are difficult to watch as in real life the experience would be extremely painful. 

The mosquitoes are shown as resourceful, capable of considerable ingenuity in responding to each of the attacks. The mosquitoes are victims of several attacks by the campers. They are ultimately tricked into flying into an item of underwear which is kicked out of the campsite, to the campers' delight.

Mosquitoes are clearly at fault in the second film: 'The Winged Scourge' which shows the impact of malaria on the heath and work of a farmer. 
The film is best when it shows the mode of transmission: a newly-emerged female becomes infected by the malarial parasite and transmits it via her salivary secretions.
         The film weighs melodramatically on the fate of a farmer, who, we learn, has failed to protect himself with a 'few easy precautions': 

• cut emergent plants
• spray oil on water
• dust with Paris Green
• drain pools and hollow stumps
• fill ponds with earth
• screen water butts 
• spray insecticide indoors
• cover cracks in the walls and floor
• bury tin cans
• make door and window screens
• sleep under mosquito nets

Paris Green is a highly toxic preparation of arsenic no longer in use. Oil is likely to affect many aquatic animals apart from mosquitoes. While the precautions are adviseable, none of  them are 100% effective.The film sounds a warning for those people who fail to follow the advice.

"Remember: there is only one cause of malaria: the mosquito. Destroy the mosquito and you will wipe out the disease."

This underplays the role of the malaria parasite which also plays a part. Overall the film shows a wide range of practical steps twhich help reduce the chances of being bitten. The only omission is the use of insect repellent cream.

Why is Zika risk under or over-estimated? Recent papers

An article form the Infectious Diseases Study Group in Queensland, Australia on April 19 2016 [Gyawali, Narayan; Bradbury, Richard S; Taylor-Robinson, Andrew W; NLM. Infectious diseases of poverty5.1: 37.]  seeks to balance media reports of '2.1 billion people at risk' with the recent experience of Dengue, with no vaccine, theoretically also capable of spread to the United States and Australia. Their discussion reads: 

"Despite understandable concern among the general public that has been fuelled by the media, in regions where Zika is not present, such as North America, Europe and Australia, at this time any outbreak (initiated by an infected traveler returning from an endemic area) would very probably be contained locally."

   The main reasons they cite are the limited distribution of mosquitoes and less dense centres of population. Well developed infrastructure, covered drains and improved nutritional status may also contribute. 
   It is possible to find apparently scientific papers making dire predictions of numbers of people at risk: 

Mapping global environmental suitability for Zika virus

Messina, Jane P; Kraemer, Moritz Ug; Brady, Oliver J; Pigott, David M; Shearer, Freya M; et al. eLife 5 (Apr 19, 2016) 

So the media hype is understandable. But this creates difficulties for reporters who fail to distinguish peer-reviewed papers from internet publications which lack thorough critique.

Top 5 Zika websites

Top Five sites for up-to-date information about Zika virus

1. The CDC shows the number of US cases of Zika to date and how they were acquired. 

There's also good advice on how to avoid mosquito bites, how to combine insect repellant and sun cream and how to treat infants.In addition, CDC Travel Health Notices give warnings about countries with active outbreaks of Zika: 

recent warnings include Fiji, Puerto Rica and Cuba.

2. The PAHO, Pan American Health Organisation has an interactive map which shows the number of countries with Zika cases and the number of cases in each country. 

Also avaialable as a table showing the number of suspected cases. There's advice for pregnant mothers and information about microcephaly.

3. The NHS Choices site has recommendations for reducing your risk of Zika:

4. The BBC site gives sound advice and recommends using insect repellants containing picaridin as well as DEET.

However, as you can see this map is not fully up-to-date

5. A WHO map shows the number of confirmed microcephaly cases in Brazil.

Zika virus films: a review

There are several short animations about Zika virus, five of which I will review here.

1. How Zika spreads (and who's to blame) a one minute infomatic by Sohail Aljamea.
Malaria as the most dangerous disease spread by Aedes aegypti but the film focuses on the danger of yellow fever rather than dengue, a much more widespread condition which unlike yellow fever, has no available vaccine. A map of the United States showing the distribution of Aedes mosquitoes as far north as Iowa, Ohio and New York does not correspond to the likely areas of spread of Zika; the virus thrives in areas of warm temperature, dense population, open drains which are largely absent from the areas shown in red on the map.
The information is essentially accurate but the style is dramatic and might cause non-professionals to overestimate the risk of Zika infection in many of the 30 American states implicated.
Who's to blame? Presumably the Aedes aegypti mosquito, rather than any person or government.

2. "O odioso do Egito".Watch with English subtitles. roughly translated as 'Hateful from Egypt'. A mainly black and white 'woodcut style' animation from Brazil's ministry of Health, accompanied by a song in Portuguese. The film begins with a wanted poster for the mosquito. Three closely related viruses are shown: Zika, dengue and Chikungunya. Mosquito larvae are shown in an open bottle. Rain highlights the danger of any open water, Insecticide is spread on water and earth is added to the base of pot plants, a lid is put on  a water tank.
While the control measures are vital, Aedes aegypti has some defences against each of these measures: by living outside houses, laying eggs above the water level, having eggs that hatch after variable intervals, being resistant to many insecticides and entering through fine gaps to breed in covered tanks.

3. A scientifically accurate model of the Zika virus. Visually stunning, tactile, silent film showing the RNA in the centre of the the virus, protein capsid, lipid membrane and the triangular arrangements of enveloipe proteins. Probably of most use to people already familiar with an understanding of viral structure. A fascinating insight but the lack of comparative scale might make people think the virus is larger than 10nm. Doesn't explain any biological properties of the virus.

4. From Aedes to Zika: what we know and what we don't A scientifically sound film in which Professor Jason Rasgon puts the disease in context with a list of symptoms and a map showing the spread of Zika in the last twenty years. 4/5 people have no symptoms but may be carriers of the disease. Infants with microcephaly are shown. Although the link is unproven, RNA virus particles from Zika have now been isolated from a child with microcephaly, supporting a causal link. Insecticides are shown being sprayed in towns in Brazil.
.

5. Alerta Dengue, Chikungunya e Zika A health information film in Portuguese encouraging four preventive steps: 

Use insecticide, cover water, remove water from old tyres and fill the bases of plant pots.