Two other viruses which are very similar to Zika virus – the chikungunya and the dengue viruses – are also known to cause neurological complications and may have contributed to the recent increase in neurological diseases that I’m studying here in Brazil.
Although the international collaboration I’m working with is called “ZikaPLAN”, our group recognises the importance of these other two viruses and we’re actually studying chikungunya and dengue with just as much interest as Zika.
If you’re interested in learning more about the Zika virus itself, I’ve written a basic introduction to the Zika virus as well as some posts on the ‘story’ of the Zika virus: where it came from, where it is now and what we expect it to do next.
All three of these viruses – dengue, chikungunya and Zika – are closely related and are members of the same ‘family’ of viruses: the ‘flaviviridae’.
All three viruses are transmitted by the same types of mosquito – the Aedes mosquitoes – and therefore tend to affect similar areas and occur at similar times of the year. Indeed, over the period of the Zika virus outbreak (2015-2016), all three were circulating in Brazil.
Worldwide, we are doing pretty badly in our fight to control the Aedes mosquito. As a result, both dengue and chikungunya are on the rise and this is probably at least part of the reason why Zika went from being a relatively obscure virus confined to Africa to causing a worldwide outbreak. It has even been suggested that over the next few years, dengue and chikungunya may become the leading cause of encephalitis (a neurological disease involving inflammation of the brain) .
All three viruses also produce a similar range of symptoms (fever, rash, headache, muscle and joint pains), although each virus has a predilection for causing some of these symptoms more so than the others. In addition, the timing of the symptoms is slightly different with each virus.
Because of this overlap in symptoms, it can sometimes be difficult to tell from a patient’s story alone which of these viral illnesses they have had.
If that wasn’t frustrating enough, because the viruses are so similar in structure, our diagnostic tests struggle in distinguishing them. For example, if a patient has dengue in the past and developed antibodies against dengue virus, their blood tests may return a ‘false positive’ result for Zika as well, even if they’ve never previously been exposed to the Zika virus.
Dengue fever is a leading cause of illness and death in the tropics affecting around 100 million people a year and causing 20,000 deaths. According to the WHO, the number of cases are on the increase as the disease spreads to new areas.
In most people it causes a relatively mild flu-like illness, but a small group suffer ‘severe disease’. There is an official definition of severe disease based on the presence of ‘warning signs’ such as severe abdominal pain, persistent vomiting, bleeding gums or blood in the vomit. Of the 100 million cases a year, around 2 million are severe disease.
Dengue tends to occur in outbreaks, meaning some years have far more cases than others. 2016 was a particularly bad year for Latin America. Brazil alone had 1.5 million cases, around three times higher than what the number in 2014. However, so far this year in Brazil, the number of dengue cases has thankfully been very small (as of July 2017).
Developing a vaccine for dengue proved quite challenging, but, following many years of research, a vaccine for dengue was released in 2016. Although this represents a huge step forward for global health, the vaccine is not without its problems and it is only recommended in areas which have particularly high rates of dengue infections.
Chikungunya virus, was first discovered in 1952 in Tanzania. Its name originally comes from Swahili (the local language), and means “the one who walks with the arched back”.
It causes a similar range of symptoms to Zika and dengue, however, the pain in the joints is particularly intense and people are often completely incapacitated by the pain (hence the Swahili name). Swelling of the hands and feet is also a particularly characteristic feature.
Similar to Zika, chikungunya remained a relatively obscure virus that few had heard of until an outbreak on Réunion island in 2006, during which almost a third of whole population became infected and 203 people died.
Chikungunya didn’t reach Brazil until 2010, when the first case was identified in Rio de Janeiro. The first reports of transmission within Brazil occurred in 2014, with an outbreak which started in Bahia, in the North-East of the country.
Why is it important to study all three viruses at once?
Some researchers have suggested that flavivirus co-infection (having two or more of these viruses at the same time) results in a higher risk of either birth defects or neurological complications. This is one of the things that the research project I am working on is trying to clarify.
Another complicating factor is that having had an infection with dengue in the past may make a Zika virus infection more severe – although this is just a hypothesis as the moment. The long-lasting immunity the body develops against dengue may make it easier for Zika to slip past our body’s defences. This is known as ‘antibody dependent enhancement’ and is something I will explain further in a future blog post.
Finally, let’s not forget that these other viruses are important global health problems and worth studying for their own sake. Although Zika took the world by storm in 2015-2016, the other two viruses have been going around for longer, affect huge numbers of people overall and deserve our attention just as much as Zika!