UPDATE: 13 April 2016
US federal health officials confirmed that the Zika virus causes a rare birth defect, microcephaly, which is characterised at birth by an abnormally small head, and other severe foetal abnormalities.
Scientists at the Centers for Disease Control and Prevention conducted a careful review of existing research and agreed that the evidence was conclusive, Director Thomas Frieden said.
The CDC is launching more studies to determine whether children with that rare condition,, represent the “tip of the iceberg of what we could see in damaging effects on the brain and other developmental problems.”
The Zika virus burst into the news late last year, the first time many people had ever heard of the disease.
Its rapid spread in Brazil – a region where it was previously unknown – combined with a suggestion that it could be responsible for the rare but devastating condition of microcephaly in babies, alarmed global health authorities.
Still smarting from accusations that it was caught flat-footed in its response to Ebola, The World Health Organization (WHO) lost no time announcing its “deep concern”, warning that total infections in Latin America could reach four million before the outbreak is brought under control.
So where did this virus come from, how dangerous is it and how can we combat it, or at least contain its spread?
What is Zika and where did it come from?
Zika is a flavivirus – a genus that includes dengue, yellow fever and West Nile fever, all spread by the Aedes mosquito, and tick-borne encephalitis. But it is far from new.
It was first recognised in rhesus monkeys in Uganda in the late 1940s.
It causes a sporadic mild illness in humans with symptoms including headache, mild fever, joint and muscle aches and a rash.
While in most cases people recover naturally from the virus, researchers in 1952 showed that it preferred to lodge in the brain tissue of mice causing neuronal degeneration and brain softening.
Why have we only started to hear so much about it?
For decades Zika remained an extremely rare infection until 2007, when a large outbreak occurred in the Yap islands in Micronesia in the Western Pacific.
By 2013 other Pacific island countries were reporting cases. An outbreak in French Polynesia raised the possibilities of a link to Guillain-Barré Syndrome – an autoimmune disease that attacks the peripheral nervous system.
So how did it come to infest Brazil?
Zika virus made its first appearance in the Americas in 2014 but it was not until late in 2015 that this explosive epidemic took off.
Although accurate numbers are hard to come by for Brazil, clearly the infection rate is vast.
Associate Professor Simon Reid of the University of Queensland’s School of Public Health explains how the disease spread so far, so fast.
“This came about because the disease was new to this densely populated region of non-immune people,” he says.
“They were also living in conditions that allowed for a burgeoning mosquito population to feed freely.”
Reid believes the mild nature of the disease may well have helped Zika gain a foothold.
Around 75% of those infected will experience no symptoms at all. Consequently, the virus is likely to have become very widespread before anyone realised the scale of the problem.
Is Zika really causing microcephaly in babies?
Much of the intense alarm surrounding the Zika outbreak is due to the possible links to microcephaly, a congenital condition associated with incomplete brain development.
There has been spike in the number of cases reported in Brazilian infants born during the Zika epidemic, with around 20 times the normal incidence.
Virologists believe that, if Zika is responsible for this, it is caused when the virus crosses through the placenta and directly infects foetal brain tissue.
But this theory – indeed the link between the two – is far from fully established.
Microcephaly, so called because sufferers of the condition have a significantly reduced head and brain size, is responsible for a wide range of potentially devastating neurological problems.
It has many causes including the in utero exposure to the infections rubella, or German measles, and cytomegalovirus, a type of herpes virus. It can also be caused by exposure to toxins.
Currently there is just an “association” between Zika and microcephaly – a term used by epidemiologists when two things happen at the same time, but well before definite causation has been established.
Even “association” may be too strong a statement.
“While there is strong circumstantial evidence, on-going surveillance and a large case-control study are required to help prove a link,” Professor Allen Cheng, from Alfred Health and Monash University, told Cosmos.
“Even then establishing causation may be difficult, as defining someone as a case of Zika is hampered by limitations of diagnostic tests.”
Studies carried out on living cells are needed to determine if the virus actually causes the neurological damage, Cheng says.
As more thorough surveillance of the epidemic takes place, some have suggested the link to microcephaly may have been over-diagnosed in many of the reported cases.
Where do we go from here? Can this thing be stopped?
It looks like it is here to stay. Zika can spread anywhere there are Aedes mosquitos – just think of how widespread dengue is and you get the idea.
That means there are many vulnerable populations where outbreaks may yet occur.
As there is no specific anti-viral treatment, and no vaccine, that means the key to control the virus is through reducing its ability to spread. That will involve the same techniques as used to control dengue – fogging with insecticides to kill the mosquitos and killing larvae in water storage systems, as well as public health education campaigns to minimise exposure to mosquito bites.
Meanwhile, genetics is playing a role in insect control, too, with a pioneering genetically engineered Aedes mosquito being released in Brazil.
It is unclear whether Zika will retreat to clinical obscurity or remain endemic, with epidemic outbreaks from time to time, in the manner of dengue fever.
The disease may largely disappear, if human immunity proves to be long-lasting and new strains don’t emerge to confound antibodies – but it is too early to say.
Adam Jenney is an infectious disease physician and clinical microbiologist.
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