Steps to help avoid the mosquito-borne bug
- Infection largely confined to tropical regions of the world
- Tokyo cases likely to be result of mosquito biting infected traveller
- Most effective prevention is to minimise the risk of being bitten
Over recent weeks, the rising number of cases of dengue fever contracted in Tokyo has been widely reported in the media. This news has understandably caused many more people than usual with unexplained high temperatures to come to my surgery for examination.
At this time of year, there are probably very few Tokyo residents who haven’t picked up at least one mosquito bite within the previous fortnight, so concern is understandable.
Worldwide, dengue is a common infection. The World Health Organization estimates that 50–100mn cases occur yearly, but these are largely confined to tropical regions where mosquitoes are a year-round presence.
Cases in temperate countries are regularly detected in travellers returning from the tropics, while there is the occasional occurrence of local outbreaks—presumably the result of an infected traveller being bitten by a mosquito, which then infects people in a normally dengue-free area.
The mosquito most commonly responsible for transmission (Aedes aegypti) typically lives for a month, and has a preference for urban areas where it has access to both standing water and a large human population. It is, therefore, easy to see how outbreaks occur in large international cities.
In 2010, 63 cases were reported in Key West, Florida, and in 2008–9, over 500 cases occurred in a similar local outbreak in Cairns, Australia. Both these areas normally are regarded as dengue free.
In this context, the current Tokyo outbreak is not so surprising, and fits the pattern of outbreaks driven by increasing international travel, urbanisation and climate change.
In many people, dengue is a mild infection and a number of the infections lead to symptoms so mild that infected individuals don’t seek medical care.
Other sufferers get a high fever, generalised muscle aches, headaches and, sometimes but not always, a widespread measles-like rash that typically comes on 3–10 days after the ailing individual has been bitten by an infected mosquito.
But severe cases do also occur. At the extreme end of the condition’s spectrum, the virus can suppress the body’s production of platelets, which are essential for normal blood clotting. This can lead to haemorrhagic shock, a life-threatening fall in blood pressure due to uncontrolled bleeding, which is sometimes fatal.
There are five different sub-types of the dengue virus. More serious infections are usually caused by serial infections, when a person who is infected with one strain of dengue is then infected by another.
This scenario is unlikely to play out in Tokyo, where the reported cases of dengue fever probably represent a single strain from one infected individual.
At present, there isn’t any cure for the fever. In both mild and severe cases, rest and treatment of the symptoms are advised, but there are currently no drugs or vaccines with which to treat the virus or block infection.
Given that dengue cannot be treated, and isn’t always diagnosed in infected people, the most effective way of controlling outbreaks is to target the mosquitoes that carry the virus.
To this end, Tokyo park ponds linked to the infections have been drained. Similarly, taking steps to avoid leaving standing water around your residence will deny mosquitoes the breeding grounds that their larvae need.
It’s also a good idea to minimise your risk of being bitten by covering up with long sleeves and using insect repellent, as preventing bites breaks the cycle of infection between human and mosquito.
Historically, reducing the number of mosquitoes has been the most effective way of limiting the infections that they carry.
During the Middle Ages, malaria was endemic in the UK, but was eradicated before the parasite that caused it had been identified. This wasn’t a result of medical treatment but, rather, because of such changes in land use as the draining of marshes.