February 4, 2016 | Posted in News

The Zika outbreak:
Interview with Professor George K. Christophides, Professor of Infectious Diseases and Immunity, Imperial College London


Is the Zika virus new?

The Zika virus was first documented in 1947, when it was isolated from rhesus monkeys and then from mosquitoes in the Zika forest of Uganda, including the species of mosquito that is responsible for the current outbreak in the Americas, Aedes aegypti. The disease has been known to circulate among human populations throughout Africa since that time. A study in the 1970s showed that a large fraction of people tested in Nigeria had been infected with Zika at some point in their life. The most recent outbreaks were in 2007 in the island of Yap in the Federated States of Micronesia and in 2013-2014 in the French Polynesia and other island nations of the Pacific. It is thought to have subsequently spread to Brazil. Therefore, Zika is not new, it can be very prevalent among human populations, is known to have been transmitted by Aedes mosquitoes in the past and is known to have caused outbreaks.


Why is the outbreak in the Americas so explosive?

The spread of the disease in the Americas indeed appears to be explosive, spreading much faster than Dengue, an illness caused by a virus that is related to Zika and transmitted by the same mosquitoes. The reason for this explosive outbreak is unknown, and in many ways resembles the 2005-2006 outbreak of Chikungunya in the Indian Ocean, India and South East Asia, which infected as many as 2 million people in record time. Notably, Chikungunya, like Zika, is commonly transmitted by Aedes aegypti mosquitoes, but a mutation in the viral genome is thought to have caused it to spread faster and also via Aedes albopictus, the so-called tiger mosquito, that was prevalent in the Indian Ocean and South East Asia and is now also very common in the Americas. It remains to be seen whether there is a similar story for Zika, but the pattern is suspicious: the disease first appeared in Brazil in 2014, in a population that had no immunity against the virus, causing several cases during the southern hemisphere summer months when Aedes mosquitoes thrive. It remained rather silent during the winter and boomed at the beginning of spring of 2015. It is therefore possible that the virus after its first arrival acquired mutations that enabled its adaptation to the local mosquito populations and rapid spread throughout Brazil and neighbouring countries where these mosquitoes are also present. A factor that might have contributed to this explosive spread is the very strong El Niño that caused exceptional climatic conditions in northeastern South America, where Zika is currently most prevalent, in the winter and spring of 2015. The very high temperatures in periods that mosquitoes are commonly less active might have contributed to an explosion of mosquito populations and rapid dispersal of the virus.


Is Europe and the UK in danger?

The answer is that we don’t know yet. The mosquito Aedes aegypti that is thought to be the main vector of Zika was eliminated from Europe many decades ago. However, we currently don’t know whether the closely related tiger mosquito Aedes albopictus is also partly responsible for the transmission of Zika in the Americas. If it is, then Europe may be in some danger. The tiger mosquito has invaded south European countries in the past 15 years and since then it has expanded its territory dramatically. It is now prevalent in Italy, Greece, Albania, Croatia, Serbia, France and Spain, and has been detected as north as in the Netherlands. Therefore, the importation of Zika to Europe from travellers to the Americas raises some concerns. The fact that the Rio Olympic Games coincide with the European summer period when the tiger mosquito thrives is of particular concern. Fortunately, the tiger mosquito is currently not present in the UK, but a suitability analysis shows that it could potentially expand as far north as Scotland. The environmental thresholds previously proposed to limit the expansion of Aedes albopictus appear to have been conservative, as this mosquito seems to be capable of withstanding harsher winter conditions than previously thought.


A tiger mosquito habitat suitability analysis for Europe for the period of 2000-2009 from Proestos et al. (Philos Trans R Soc Lond B Biol Sci. 2015). The habitability is given as a measure between 0 and 100. Green circles show where the tiger mosquito has been already detected.

How can we protect against Zika?

To shield against Zika, a protective vaccine against the virus or control of its spread through mosquitoes is needed. In the absence of a vaccine against Zika, and until this is produced and tested, which may take months or years, most efforts must be placed on mosquito control. These efforts must now be intensified, perhaps by employing radical measures such as re-introduction of DTT that has been key in the eradication of Aedes from many countries in the Americas in the past. Additional methods, such as the use of genetically modified mosquitoes to reduce wild mosquito populations are promising and shall be applied provided that a full risk analysis has been conducted and the risk is indeed negligible. Yet, what is of utmost importance is the introduction and implementation of rigorous public policies and public engagement. The Aedes aegypti and related mosquitoes such as the tiger mosquito live in close association with humans and mostly breed in man-made containers, which are particularly prevalent in urban and suburban environments and are easily identifiable. Therefore, targeted education programs and engagement of the public into eliminating possible mosquito breeding sites may produce very good results, which together with intensified vector control measures can contain the spread of the disease. In the meantime, standard methods such as avoiding exposure to mosquitoes and use of repellents must be considered at all times when travelling to affected areas. The fact that Zika appears to be additionally sexually transmitted must be also considered. Finally, given the probable association of Zika with microcephaly, a devastating defect where a baby’s head is smaller than expected, which is often linked to underdevelopment of the brain, pregnant women should avoid visiting areas where Zika is now present. The association between Zika infection and microcephaly or Guillain–Barré syndrome, a related condition resulting from damage of the peripheral nervous system, is not shown to be a causal one yet, but this is indeed highly probable.


Is there any link between the GM mosquito releases and the Zika outbreak and microcephaly in Brazil?

The theories appearing in the various media linking the release of GM mosquitoes to supress local mosquito populations with the Zika outbreak and microcephaly are at the moment entirely unsubstantiated. The fact that Zika is most prevalent in areas that are relatively close to those where GM mosquitoes were firstly released appears to be coincidental. The populations of mosquitoes that can carry Zika are very dense in these areas that are therefore affected the most by the disease. The releases concern male mosquitoes that never come in contact with the virus since they do not bite people. It is only female mosquitoes that after they bite a Zika infected person could transmit the virus to another person during a subsequent bite. It is true that during GM male mosquito releases, a very small number of female mosquitoes may also be released, however, their numbers are so small that they are highly unlikely to contribute to disease transmission. Furthermore, the released males are sterile and therefore cannot pass their GM characteristics to the wild mosquito populations. The GM mosquito releases in Brazil are shown to dramatically reduce the densities of local mosquito populations, even more so than insecticides, and therefore entertain an enthusiastic public acceptance. Although all hypotheses must be thoroughly examined by scientists and the relevant authorities, the propagation of unsubstantiated theories may harm the efforts to control Zika and other devastating diseases that mosquitoes can carry.