Over two years ago I wrote that “new viruses are constantly being discovered… Then something comes out of the woodwork like SARS (severe acute respiratory syndrome) which causes widespread panic.” The Zika virus infection bids fair to repeat the torment. On 28 January 2016 the BBC reported that the World Health Organization had set up a Zika “emergency team” as a result of the current explosive pandemic spread of the virus with up to four million cases predicted.
Why the concern? It can be summed up with these words: Global epidemic, Brazil, Rio de Janeiro Olympic Games, and microcephaly. The most dreadful consequence of Zika infection is microcephaly (small head) of babies born to women who are infected during pregnancy. But let us begin at the beginning.
In 1947 a novel virus was serendipitously discovered in blood taken from a monkey kept in a cage in the jungle of Zika Forest in Uganda as a sentinel for yellow fever infection – hence its name. The following year this same virus was isolated from Aedes africanus mosquitoes in that forest, showing that the virus was an arthropod-borne or arbovirus. Word of this discovery did not find its way into print until 1952 when the British virologist George Dick and two colleagues described the isolation of the virus and then Dick went on to report that 6% of 99 humans tested had antibodies against this virus in their serum indicating its transmission to humans. The virus itself was first isolated in Nigeria from a human in 1954, then two years later it was shown that Aedes aegypti could also be infected experimentally and transmitted the infection to a monkey.
During the next few years serological studies of populations found the presence of Zika virus antibodies over a wide area ranging from many countries in Africa through India to the Philippines. Zika virus was shown to be an RNA virus in the genus Flavivirus, a genus which also contains such viruses as yellow fever virus and dengue virus. Nevertheless, for six decades after its discovery Zika virus remained a curiosity and was consigned to the small print of textbooks; by 2007 only ten or so human cases had been described in Africa, Malaysia, and Indonesia. It seemed to be a mild disease characterised by transient fever, lassitude, rash and aches and pains.
Change was in the air in 2007 when an epidemic of fever, conjunctivitis and muscle pains occurred in dozens of patients in the Pacific island of Yap in Micronesia. The infection then disappeared once more from view until 2013 when there was an outbreak affecting several thousand patients in French Polynesia which in turn was spread by travellers to other Pacific islands. Furthermore, some of these patients appeared to develop a complicating paralytic disease called Guillain-Barré syndrome (GBS). By 2015, the infection had appeared in a number of South American countries from whence it spread north to Mexico and the Caribbean. Studies of the virus in the Pacific and Americas showed that it was a strain that had originated in Asia.
A temporary flu-like illness is one thing. A prolonged paralysis with GBS is another and is consumptive of scarce health resources. But life-long microcephaly and its consequences is orders of magnitude worse. By 28 November 2015, 646 cases of microcephaly had been reported in Pernambuco state of Brazil. On 1 December 2015 the Pan American Health Organization issued an alert after finding a 20-fold increase in the rate of microcephaly in all of Brazil that year, with the virus being found in the amniotic fluid of two pregnant women whose fetuses had serious neurological malformations. Although the causation of microcephaly with Zika virus infection has not yet been conclusively proved, the association is so strong that health authorities in many countries are urging pregnant women not to go to endemic areas while countries in which infection is endemic are suggesting that women postpone pregnancy if possible.
Why has all this happened? We do not yet know but it may be that in recent times mutation has occurred in the viral genome which has facilitated its spread and enhanced its pathogenicity.
So what can be done? There is no specific treatment and there is no vaccine. People, and especially pregnant women, need to protect themselves from mosquito bites by using all available measures. Aedes mosquitoes are adapted for indoor and daytime biting in urban areas, especially during the early morning and late afternoon. They are known to breed in aquatic environments such as small puddles, pots, old tyres, and the leaf axils of plants. Unfortunately, decades of determined efforts to control the breeding of Aedes aegypti in Central and South America has had only limited success.