These are three mosquito-borne viruses, which have been spreading into new parts of the world in recent years. Dengue and zika are closely related, but chikungunya is not. However, they are all transmitted by the same two species of day-biting mosquitoes, Aedes aegypti and the cold tolerant, Aedes albopictus, which are both exquisitely adapted to the human and urban environment. These mosquitoes can breed in small pools of water, e.g. in car tyres or in the saucers of houseplants.
The diseases can present with similar symptoms. A blood test is usually needed to tell them apart. This may need to be repeated two weeks later if at first negative.
Dengue is now the most common mosquito-borne disease worldwide by far (at least 10x the number of cases of malaria). It is an increasing risk in over 100 countries, mainly in Latin America, Asia and Africa (where it is very often mistaken for malaria), with occasional outbreaks in Southern Europe and the Southern USA.
Dengue infection can essentially lead to three different disease outcomes:
There are four main serotypes of dengue virus worldwide, DENV-1, 2, 3 and 4, although two more have recently been described from Malaysia and Brunei. If you are infected with one serotype, knowingly or unknowingly, you will make antibodies and become immune to that particular type thereafter. However, if you are infected with a second serotype a few years later, you may then develop severe dengue due to a cross-reaction with the antibodies you already have.
The ‘take home message’ is therefore to try not to get infected the first time by using effective insect repellents such as Mosiguard Natural® or DEET, particularly around the ankles and feet.
There is one dengue vaccine in routine use for public health purposes in Brazil and the Philippines, and a couple of others in development. However, none is yet suitable for use in travellers.
What to do if you think you might have dengue:
Chikungunya is the most common of several, related alphaviruses that can cause arthritic symptoms. The name derives from the kiMakonde language of East Africa and literally means: “the bending disease”, referring to the stooped posture of chronic sufferers.
Since its discovery in 1952, there have been several large-scale epidemics of chikungunya in Africa, Asia and the Indian Ocean. An outbreak starting in St Maarten/St Martin in the Caribbean in December 2013 quickly spread through the Americas, affecting over 2 million people over the next three years. To date, over 60 countries have been affected. The typical pattern of a chikungunya outbreak is to spread rapidly through a population and then to largely burn itself out (particularly on islands), once herd immunity has been established.
The degree of onward transmissibility, and of resulting clinical symptoms, may vary considerably, depending on the particular strain of virus associated with an outbreak. As a result of a single mutation, one strain has become much more transmissible via the cold tolerant, Asian tiger mosquito (Aedes albopictus), which is now increasing its range in Europe. There have been recent outbreaks in 2017 in Italy and France, and one on Zanzibar island in 2018. The majority of cases reported so far in 2019 have been in Brazil, Republic of Congo, Sudan, Thailand and Ethiopia.
An up-to-date awareness of local outbreaks in certain countries and bite avoidance during the day with insect repellents (e.g. Mosiguard Natural® or DEET) are the current mainstays of prevention.
A new and effective, nanoparticle vaccine is in development and should be on the market in a few years.
Symptoms and treatment
The commonest presenting symptoms of chikungunya are: abrupt onset of fever and joint pain +/- muscle pain headache, nausea, fatigue and rash. A classic sign is tenosynovitis (not a feature of dengue or zika).
The joint pain can vary from mild to be very debilitating, and may last from a few days to many months or years, depending on the strain.
There is no cure, only supportive treatment. Non-steroidal anti-inflammatory drugs can be used to provide pain relief, but only if/once dengue has been excluded.
Unlike dengue, you can only be infected once with chikungunya: thereafter most people develop life-long immunity. In about 1 in 1000 cases, the disease can be fatal.
Zika virus was first discovered in monkeys and mosquitoes in Uganda in the late 1940s and later found to be present in many other African and South Asian countries in the decades that followed. It was not thought to have much clinical significance in humans until 2015, when a large number of congenital birth defects were noticed in association with a large, de novo outbreak of zika infection in NE Brazil.
The strain of the zika virus that had struck the Americas, and Western Pacific two years earlier, was determined to be of the Asian lineage of zika virus. This strain, first seen in Malaysia, has since been found to have undergone two key mutations, rendering it: a) more transmissible from humans on to mosquitoes, and b) more pathogenic, targeting neural tissue in the developing foetus. A range of congenital birth defects have since been reported in babies born to mothers who were pregnant when infected, from microcephaly through to deafness, difficulty swallowing, brain damage and eye defects.
The risk of congenital birth defects if infected with Zika virus during pregnancy is approximately 30x higher than the background risk associated with a ‘normal’ pregnancy*.
By contrast, there has been no similar association so far detected with pregnancies infected with the African lineage of zika virus, despite large numbers of people infected (e.g. 6% seroprevalence in Western and NW Provinces of Zambia in 2015). One plausible theory is that this may be due to the African strain more commonly leading to early miscarriage, rather than to full term births.
Fortunately, the pandemic of zika in the Americas and Caribbean has now largely subsided and the risk is now considered to be comparable to that in Africa and Asia.
Zika virus is transmitted via mosquito bites, but also via sex and from mother-to-child across the placenta. Women are considered to be safe to conceive once 8 weeks or more back from a zika affected region, whereas in men unprotected sex is now considered safe from 3 months.
As with any mosquito-borne disease, awareness of the risk in a particular country is therefore important. Up-to-date risk country lists are available via the following web link: Fitfortravel.
If at risk of pregnancy, while or shortly after travelling, bite avoidance should be practised, including postponing a non-essential trip to a ‘high’ or ‘moderate risk’ country, if need be. Other measures, such as insect repellents, air-conditioning and safer sex when travelling should also be employed. Mosiguard Natural® and Para’kito® are both safe and effective in pregnancy.
Finally, if you do find that you are pregnant, or planning to be pregnant, within the timescales listed above, a blood test to see if you are infected would be a sensible precaution. These can be arranged via Dr Bond (please see our Post-travel page).
The aim of Zika testing is essentially to reduce the risk of congenital birth defects down to mathematically negligible above the background risk of a ‘normal’ pregnancy at a given age (see above*).