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Zika, Brazil and the continuing threat of mosquito borne disease

By Peter Curson and Alexandra Bhatti - posted Monday, 25 July 2016


Another year and another emerging tropical infectious disease threatens. Zika virus which continues to spread throughout Brazil is now beginning to threaten parts of North America and further afield. Zika has now joined a long list of infectious diseases that have shot to global notoriety over the last 15 years including SARS, Bird Flu, Swine Flu, Ebola, MERS, Dengue and Chikungunya.

Given our experience of all this you might think that we would have come up with well-prepared national and international surveillance and response systems designed to quickly identify and respond to emerging infectious disease threats, but not so. In the case of Zika we paid scant attention to major outbreaks in Micronesia and Polynesia between 2013 and 2014 and one could be excused for believing that it is only if a major outbreak of infectious disease directly threats the USA or parts of the developed world that we are inspired to take direct action.

There is still an all pervading belief that if an infectious disease breaks out in Africa or Asia we can watch from afar and convince ourselves that it will eventually burn itself out or retreat back into its natural animal reservoir. But in our highly mobile world where literally hundreds of thousands of people move across international borders by air every day, the threat of infectious disease spreading around the world has never been so great.  Zika and Ebola really demonstrate how little we have learnt from a decade of emerging infectious disease threats and how poorly developed are our surveillance and response mechanisms.

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But it is really much more than this. We still arrogantly cling to the belief that we are the dominant species on earth and have the ability to easily adapt to such threats. Nothing could be further from the truth. The bacterial and viral world easily outranks us and has the ability to change and mutate in relation to changes in their environment or their host as well as quickly build an ability to adapt to the drugs we might come up with. In addition we continue to ignore the important role that animals play in all of this, particularly the fact that the majority of infectious disease threats over the last century have been maintained and nurtured among wild animals.

And so to the current threat of Zika. There is little doubt that Zika is not as dangerous as Ebola but it still constitutes a major threat. Zika is however, the first mosquito-borne virus to have a confirmed link to birth defects, and it appears to cause the neurological muscle paralysis condition Gullain-Barre.  It is mainly spread via the Aedes type of mosquito. The first human cases were identified in Africa in the 1950s, with the identification that Zika causes human infection occurring in the 1960s.

It can be passed from a pregnant woman to her foetus, with infection during pregnancy causing severe birth defects such as microcephaly – where a baby is born with an abnormally small head.  Zika is also known to be transmitted sexually. The virus is generally not fatal and causes only mild symptoms. A real issue is that only 20% of those infected display these mild symptoms and a large proportion who are infected remain unaware and pose a challenging threat to controlling the spread of the disease.

The first outbreaks of Zika involving more than a handful of people occurred in 2007 in Gabon and in the small state of Yap in Micronesia, where approximately two-thirds of the population were reported to have been infected, although with no hospitalisations, neurological issues or deaths. Six years later in 2013 a major outbreak occurred in French Polynesia affecting approximately 11% of the population and later spread to three other Pacific Islands. These outbreaks indicated a possible association between Zika infection and congenital malformations, neurological and autoimmune complications.

In 2014, small clusters of Zika began to appear in North East Brazil and were confirmed a year later in mid-2015 by which time the disease had been circulating for months. It is thought the virus was introduced to Brazil during major sporting events that brought thousands of international travellers together in close proximity, e.g. the 2014 Soccer World Cup and the World Canoe Championships.

Within a year Zika had become a major problem in Brazil and early in 2016, the Ministry of Health estimated that between 497,593 and 1,482,701 infections had occurred, which is likely to be an underestimation due to minimal reporting systems. Since the outbreak began in Brazil the disease has continued to spread and transmission has been reported in 33 countries and territories of the Americas. By April 2016, Brazil had experienced 4.908 cases of microcephaly and other congenital abnormalities, with smaller numbers occurring in Panama, Columbia and Cabo Verde. Six countries have also reported locally acquired infection through sexual transmission – Argentina, Chile, France, Italy, New Zealand and the US.

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In an effort to try and contain the aedes mosquito Brazil mobilised more than 200,000 soldiers who worked their way through backyards, garages and people’s bathrooms trying to eliminate standing water where female aedes lay their eggs. Overall, however, the campaign did little to control the mosquito population. There is little doubt that the aedes mosquito has beautifully adopted to living with humans. They hide in curtains, cupboards, underneath beds, in cars and in rubbish in backyards.

Understandably, many are fearful of the implications for an international event like the upcoming Olympics in Brazil. Despite Brazilian officials saying the risk of transmission will be reduced due to fewer mosquitoes being present in the winter months when the games are on, do we really know enough about the breeding cycles of the Aedes mosquito or indeed the living conditions of millions of Brazilians to definitively say what the risk for the human population will be. Given what we know, it is still difficult to determine how far reaching the implications for locals, tourists and athletes will be. It is also important to consider if there are more pertinent threats to visitors to Brazil than Zika such as the dengue virus?

In May of this year, 150 medical professionals called for the games to be moved or postponed. They rightly argued that if the recommendations are for people to consider delaying travel to areas with active Zika virus transmission that athletes will be placed in a difficult position having to choose between risking disease and competing for an event they have trained years for. Recently, many of the top golfers have publicly pulled out citing Zika as a reason - this is the first time golf has re-entered the Olympics since 1904 and its worth considering that perhaps golfers are in better position than other types of athletes to make the decision not to partake with various other available tournaments to compete in. If you’re a sprinter or a long jumper, the Olympics really is the pinnacle of competition.  There is also no doubt that some tourists have already been deterred from visiting Brazil, particularly the most vulnerable – those wishing to start a family soon or already pregnant women.

Combating the continuing threat of Zika certainly requires more knowledge – for example it is still unknown why only some babies are left with microcephaly whilst some are spared. There is no known treatment for Zika virus and experts have estimated a vaccine is at least a decade away. Technologies used for diagnosing the virus are slow and expensive and more resources are needed. There is also some evidence to suggest that the aedes mosquito is beginning to appear in Europe. It is a disturbing scenario and perhaps it is just a matter of time before diseases like Dengue and Zika begin to affect a wider audience.

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About the Authors

Peter Curson is Emeritus Professor of Population and Health in the Faculty of Medicine and Health Sciences at Macquarie University.

Alexandra Bhatti is Associate Lecturer in Public Health, Faculty of Medicine and Health Sciences at Macquarie University.

Other articles by these Authors

All articles by Peter Curson
All articles by Alexandra Bhatti

Creative Commons LicenseThis work is licensed under a Creative Commons License.

Photo of Peter CursonPeter CursonPhoto of Alexandra BhattiAlexandra Bhatti
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