With the rain comes the mosquito

It also marks the return of mosquito-born diseases, however, with countless sites for breeding. Thailand has three main diseases affecting humans that are transmitted by mosquitoes. The best known is malaria, courtesy of the plasmodium parasite. The others are dengue fever and Japanese encephalitis, caused by arthropod-borne viruses – arboviruses for short.
All three induce roughly similar flu-like signs of various intensity, such as fever, chills, headache, intense fatigue, low appetite, vomiting and pain in the muscles and joints.
Some symptoms are nevertheless quite specific to each disease. Malaria often brings high fever with shivering, profuse sweating and headache. Japanese encephalitis, in its severe forms, leads to neck stiffness, stupor and disorientation. Dengue fever typically induces severe pain, especially retro-orbital pain and back pain, which explains its nickname, “break-bone fever”.
The severe form of dengue, called dengue haemorrhagic fever, is responsible for internal bleeding. The first symptom to be recognised is haemorrhagic rash, with small red spots appearing anywhere on the skin.
The three diseases arise in different locales. Malaria and Japanese encephalitis do not usually occur in rural areas. Malaria is present mainly along the Cambodian, Lao and Myanmar borders, where adventure trekking risks infection. Dengue, on the other hand, is just as prevalent in urban districts as in rural areas, and that’s why it’s the most common of the three among expatriates.
The mosquitoes transmitting these three diseases are of different genera and, since they prefer biting at different times of the day, the risk of infection continues day and night.
Treatment varies with the type of disease.
Dengue infestation can deplete one type of blood cell, the platelets, which are required for the clotting process. With too few platelets, bleeding can occur in any organ. Haemorrhagic complications occur in 10 per cent of dengue victims, among which 5 to 10 per cent suffer life-threatening bleeding. They need to be in intensive care to prevent cardiovascular collapse.
There is no specific treatment on the dengue virus itself. As vaccine will hopefully be available in a few years, and meanwhile prevention depends on minimising exposure to mosquito bites, especially for children, who are more prone to the disease’s complications.
Most Japanese encephalitis infections are asymptomatic. Less than 1 per cent of victims develop complications like acute encephalitis, but if they do, this brain infection is severe – death occurs in 20 per cent of the cases. There is, however, an effective vaccine that’s recommended for children in Thailand.
Malaria complications mainly involve the mass destruction of red blood cells. Diagnosis is easy, using a simple blood test, and oral medicine is effective on the parasite itself. A vaccine for malaria is not yet available, but there are many promising trials ongoing in Asia and Africa.

Dr Gerard Lalande is managing director of CEO-Health, which provides medical referrals for expatriates and customised executive medical check-ups in Thailand. He can be contacted at [email protected].