Mosquito Related Illnesses

The main mosquito derived illnesses are:

  • Dengue (relatively high)
  • Malaria (very low)
  • Japanese encephalitis (very low)

Mosquito bites are a nuisance anywhere. Anyone who lives in the tropics, however, should become familiar with mosquito borne diseases that so far have tended to be located in the tropics.

Thailand is not a particularly mosquito infested country, and tends to be pretty average for a tropical country. Mosquito borne illnesses exist here, but are not a common infliction upon expats, especially in urban areas. Decades ago, things were different, and government eradication efforts are mainly to credit for this turnaround. Nonetheless, there are still occasional reports of an expat or traveller getting a mosquito borne illness, especially near the borders with Cambodia and Burma.

Malaria is very rare in the main population centers, but dengue fever is on the rise and I've known a few expats who have gotten dengue, as well as some Thai people. I haven't known anybody yet who got Japanese Encephalitis or malaria in Thailand.

A rough calculation states that if you live in Thailand for 20 years with no extra precautions, then you have a 1% chance of getting Dengue. It's still much less than cancer and other risks, but is significant. If you get it, you will have fever, very achy joints, loss of energy for several weeks, and other symptoms. There is no vaccine and few treatments other than rest. This is discussed below in this article.

The chances of getting malaria in a few decades is a small fraction of 1% but you must get medication for malaria.

Symptoms of Japanese Encephalitis is extremely rare, but you should be aware of it.

Minimizing Mosquito Bites

Things you can do to minimize mosquito bites:

  • Apply anti-mosquito spray or lotions. Tests by consumer magazines show that many popular ones do not actually last long. The best are DEET and icaridin. You can find little DEET spray bottles in convenience stores for cheap. I buy these often if I plan to be outside in a place with many mosquitoes, at least just to drive away nuisance mosquitoes.
  • Cover your body with clothes, including thick socks, even if you wear sandals.
  • It's said that mosquitoes are more attracted to dark colors, so wear white. Also, you can see the mosquitoes better if you're wearing a light color. I wear thick clothes which they cannot bite through, but even light white clothes might help.
  • Do not apply perfumes and colognes, which attract mosquitoes (and don't hang out with somebody who does).
  • At restaurants, request mosquito coils and place them upwind. (I don't use them because I don't like breathing the smoke, but they can help. If there is a constant breeze, then I try to keep the smoke away from my face.)
  • At home, cover any standing water (such as the urns), deal with any places that hold rainwater, and for any ponds with lotus flowers and the like you should add some small fish that eat mosquito larvae.
  • Get a mosquito net for travelling with.
  • Notably, tests show that many electronic mosquito killers and repellers for sale do not kill nor repel a significant fraction of the mosquito population.

The best ways to kill mosquitoes in your home are with your hands, insecticide spray, or electronic rackets (like tennis rackets) sold at superstores which give you the equivalent of much bigger hands to swing at them. Insecticide spray can have side effects such as headaches, especially for children. I have read a lot about DEET and I am not very concerned about it, but some people are sensitive to DEET. I can usually kill mosquitoes with my hands, but with hands or a racket, you should aim a little bit low because many mosquitoes instinctively dive when they sense anything heading their way, be it a predator or your hand.

Most mosquitoes are most active around sunrise and sunset, which is why you see a peak in activity around these times, but some mosquitoes are active all times of the day and night. There are several species active in Thailand. Some species seek shelter during the heat of the day.

Most mosquitoes travel along the ground and are fairly energy conserving. People are usually aware of seeing only mosquitoes near eye level or on their arms, but most mosquito bites are on exposed lower extremities. I wear sandals almost all the time, and always wear thick socks (light colored socks are best but you may choose to wear dark brown or black to match your clothes, but whatever you wear, make sure they're too thick for mosquitoes to bite thru). I have never been bitten thru a thick sock. If only thin socks are available, then I will put on 2 or 3 pairs, but just 1 thick sock is enough. The problem is that thick socks that are easy to find in ordinary stores in northern countries are hard to find in Thailand, except at some sporting shops that attend to mountain hikers, so I stock up on socks whenever I travel to a northern country. The best place in Thailand to buy great thick socks is a shop chain called Green Cotton. Thick socks do not make me feel hot, and there are better ways to keep cool.

I advise a lot of people this way, but most people choose to ignore it, so when they complain about bites or mosquitoes around their legs or keep slapping at them, I just remind them that I can lead a horse to water but I can't make it drink. (Socks also protect your feet and keep them clean against anything else down on the ground.) It can get a bit annoying to hang out with people who are constantly slapping at their feet or legs or complaining about mosquito bites.

Mosquitoes eat flower nectar and some other things. Only female mosquitoes bite humans, as they need the protein to make their eggs.

I believe that a main source of mosquitoes is sewers. I have noticed higher concentrations near sewer openings. There are probably few predators of mosquitoes in sewers. I have seen inside some sewers a pool of water with hoardes of mosquito larvae. Maybe a good business may be mosquito nets to cover sewers, letting the drain water in but not letting the mosquitoes out. However, such nets would need to be cleaned periodically. They may also catch a lot of small plastic pollution which gets into streams and eventually into the ocean.

At the sewer drain in front of my house, which has a small hole for water to drain into, from time to time I've seen a frog hang out right at that hole, consistently, night after night, just eating things which fly out, presumably mosquitoes. I've seen that frog grow. Each time, the frog eventually disappears (maybe goes to another sewer), and sometimes I've considered designing and building a frog house to keep him around.

Dengue Fever

The disease "dengue fever" is carried by a particular species of mosquito, the Aedes aegypti mosquito, which is active all day. It is on the rise to becoming one of the top public health problems in the tropics.

The mosquito which carries this virus is most common in urban areas around human dwellings, and is most active during the daytime. The Aedes mosquito breeds in clear water.

Symptoms usually start to appear between 5 and 7 days after being bitten. The symptoms include severe headaches, high fever which may fluctuate, bone aches (hence the slang name), joint and muscle pains, nausea and vomiting, and a rash of small red spots a few days after the onset of fever.

Dengue fever is a very serious illness, and you should go to a good doctor immediately for treatment. It can become fatal if the symptoms are allowed to progress over time to DHF (Dengue Hemorrhagic Fever), which is characterized by heavy bleeding, though DHF is thought to affect mainly longterm residents who have picked up more than one strain of the virus. Aspirin should be avoided, because aspirin reduces blood clotting and thus makes hemorrage more likely.

There is no vaccine against dengue fever, and no specific treatment. You'll have to follow the doctor's orders. Dengue fever is often mistaken for malaria, the flu (influenza) or something else. Malaria can be excluded by a blood test, and the indication of dengue can be inferred by a blood test. Recovery will take time, with tiredness sometimes lasting several weeks.

There are actually several types of dengue fever (currently listed as four), and it's also known as breakbone fever. In Thailand, the dengue type has shifted from predominantly "breed one" to majority "breed two" as of early 2008.

Research revealed that the Aedes aegypti mosquito that carries dengue does not travel far from its breeding place, unlike other species of mosquito, which explains why dengue outbreaks tend to occur in localized areas of a few hundred meters radius. It is believed that dengue spread by infected human migrant workers more than migrant mosquitoes. Therefore, when an outbreak can be determined in a particular location, the authorities may come and spray. However, given that urban dwellers tend to travel around the city and symptoms start around 5 to 7 days later, it's not always clear where the infection occured. Nonetheless, if someone living near you gets dengue, and especially if another person living near you also gets dengue, then maybe it's time for a vacation. However, rarely have two people of the same family gotten dengue, so don't impulsively run like it's the plague.

It is projected that in 2008, there will be around 25,000 to 30,000 reported cases in Thailand. If we set it at 35,000 out of a population of 70 million, that would mean about 1 in 2000 people will be infected. (It's fairly similar to HIV / AIDS statistics.)

At these rates, if you live here for 20 years then you run a 1% chance of getting an infection over those 20 years. However, that assumes you are like the vast majority of people in Thailand in that you don't take any precautions. If you want to reduce your chances substantially, then follow the recommendations near the top of this page.

Malaria

The mosquitoes that transmit malaria are found in remote areas, not any of the major cities. These mosquitoes generally bite around sunset and sunrise. Malaria transmitting mosquitoes are usually found near the Cambodian border (especially near the seaside) and the Burma border.

Symptoms of malaria include: sudden high fever and chills, muscle aches and headaches. They usually show up after an incubation time of 7 to 30 days.

To be properly diagnosed with malaria, you must undergo a blood smear examination, which all major hospitals and many clinics can (and of course should) administer. There are drug-resistant strains of malaria, and it's important to tell the doctor where you've been because the particular drug resistance varies by region.

Japanese Encephalitis

Japanese encephalitis (JE) has been a fatal disease or caused serious neurological damage to humans, and is spread by some mosquitoes and ticks, especially in rural areas. It is of concern more in some parts of northern Thailand but there have been cases in central and southern Thailand, too. It's rare in humans, but there have been many infections in Thailand which led to Japanese encephalitis, especially in the distant past. There is a vaccine, and many people ask me whether they should be vaccinated, especially Americans (maybe since doctors there are more aggressive in recommending treatments...).

The infection rate in humans is low, and most infections have no symptoms. Many infections result in only mild symptoms. However, about 1 in 250 infections progress to serious symptoms. In that small minority, it's fatal to around 30% (so a death rate of roughly around 1 out of 750 infections) and about 20 to 30% of survivors suffer serious permanent brain damage (say, another roughly 1 out of 750).

Japanese Encephalitis infects pigs, too, so proximity to pigs such as for people living in rural areas near livestock can raise your risk. Backpackers in nature may want to get the vaccination, at least to reduce worry.

The risk of being infected with Japanese Encephalitis in southeast Asia has been recently estimated at somewhere between 1 in 1,000,000 to 1 in 30,000, so maybe we might guess 1 in 100,000. Statistics are not good because most infections have no symptoms.

In Thailand in particular, the number of cases reported per year has been between 300 and 400, roughly, out of a population of around 70 million (plus a few tens of millions of tourists of various lengths of stay, but let's forget the tourists for calculations). If we divide the 70 million population by ~350 cases reported, that's roughly 1 in 200,000 people getting serious symptoms which can be attributed to a Japanese encephalitis infection.

Newer vaccines have much lower risk of side effects. There are different vaccines. I'm not going to address the topic of particular vaccines here as I'm afraid this page may become obsolete.




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