It’s that time of year to talk about the prevention and treatments for Dengue Fever.
As the rainy season is nearly upon us and the health warnings will not only apply to respiratory virus infections. Mosquitoes are much more common at this time of year and so are the diseases that they transmit, most notably dengue fever. To reduce illness spread by mosquitoes we have to make it harder for them to breed and harder for them to bite us.
The Aedes mosquito that transmits dengue and similar viruses, like Zika, chikungunya and yellow fever virus, has white spots and stripes, and she bites during the daytime as well as at the more typical feeding times of dusk and dawn. For this reason, it’s advisable to take precautions throughout the day and not just in the evening.
Dengue is a common and at times severe virus infection transmitted by mosquitoes. There were 98,000 cases of dengue in Thailand in 2019, along with 100 deaths, and yet 3 out of every 4 infections are thought to give no symptoms at all. The question that is asked the most is about the treatment of Dengue fever; Is there one? Like most viral illnesses there is no cure and hospital treatment for severe cases consists of supportive care and hope.
The symptoms of dengue begin a few days after a mosquito bite. Typically, there’s a sudden fever, pain behind the eyes, headache and severe body aches (so bad that in earlier times dengue was known as ‘break bone fever’). In extreme cases there may be bleeding from the eyes, nose and mouth, explaining dengue’s Thai name of ‘khai leuat ork’ (fever blood out).
There is a vaccine against dengue fever but it has some serious drawbacks.There are 4 distinct strains of dengue virus and the interaction between them and the human immune defence system is key to why vaccination can be dangerous. It isn’t licensed for use in people over the age of 45 years and currently isn’t recommended for tourists or expats under 45 years either.
Infection with a dengue virus results in long-term immunity to that strain. For a year or two there is cross-over immunity to the other strains also, but this protection is lost as the antibody levels fall, making it possible to catch dengue again. The strains can be caught in any order, with the second infection usually much worse than the first, third or fourth (whichever strain causes it).
This happens because antibody levels from the first infection pass through a ‘danger zone’ as they fall from high to low levels over time. If the second virus comes along when antibodies are in the danger zone then the immune response goes into overdrive, generating an excessive, harmful and sometimes fatal reaction. Levels above or below the danger zone are safe.
This is why we don’t recommend the dengue vaccine – if you’ve never had a dengue infection and you take the vaccine then you will make a high level of antibodies, the same as happens with a first dengue infection. As those antibody levels fall you will for a while be at a much higher risk for severe and possibly fatal dengue than if you’d never been vaccinated at all.
Checking for antibodies before giving the vaccine would avoid this problem but unfortunately the antibody test for dengue is unreliable because there are false positive results from antibodies to similar viruses (chikungunya, yellow fever and Zika). Interestingly, several Covid-19 patients have had false-positive dengue tests also, perhaps indicating a similarity between the antibodies for dengue and Covid-19. The significance of this observation is unknown.
Preventing Dengue fever is should be a top priority in the absence of a safe vaccination program and the best way to protect yourself against is to eliminate mosquito breeding sites around the home, ie remove or drain anything that may collect small pools of water during the rainy season. We also recommended wearing long sleeves, long trousers and to use insect repellant at this time of year, even during the daytime. Please note that when using sunscreen, it’s better to use insect repellant on top of rather than underneath the sunscreen.