Where in Thailand should I be cautious about malaria?

In Thailand they tend to put fish in any standing water - even fountains and small potted water plants (like water lotus which are popoular here) have guppies in them, so many lavae are eaten. Rivers and canals still breed mosquitos though and the risk of being bitten anytime of the year is quite high - especially at night. However, the risk of malaria is really very low unless you are planning a jungle trek near the borderlands. They have made some effort to dredge canasls and rivers since last years floods, so that should help a lot this rainy season.

Most cases of malaria outside borderland in Thailand are contracted in and around the borderlands (or abroad). There are some quite detailed information kept on this (I looked through some of it and at wall charts etc a few years ago in a very rural clinic). I live in Thailand, as do my two pre-teen daughters. I have spent more than 15 years living in and visiting Thailand - I know many foreigners here - and I have known only one person with malaria in all that time (he caught it in Cambodia whilst serving in the Thai Navy).

There is a bigger chance of Dengue Fever - which the last two years has seen a large rise in numbers (year before last my local hospital had to start turning away patients as there were so many). This is akin to malaria as it is mosquito bourne, but only has a 10 day or so cycle. After which you should be fine. It is quite uncomfortable though with big headaches, fever, lack of energy and sweats - aircon helps tremendously and sleep - no cure. There are rumours that catching Dengue twice (I beliebve this must be another strain) can be fatal - though I have had it twice and neither time was worse than headaches etc for a few days, both times.

This is the CDC's travel advisor site for Thailand: http://wwwnc.cdc.gov/travel/destinations/thailand.htm

Their advice on areas with malaria Malaria, which echoes the advice here:

Areas of Thailand with Malaria: Rural, forested areas that border Burma (Myanmar), Cambodia, and Laos. Rural, forested areas in districts of Phang Nga and Phuket. None in the cities of Bangkok, Chiang Mai, Chiang Rai, Koh Phangan, Koh Samui, Pattaya, Phang Nga, and Phuket. (Updated September 2, 2011)

While some strains of malaria in Thailand are resistant to treatment, an experienced travel doctor should be able to figure out what treatments they are not yet resistant to. The CDC's map of Mefloquine-resistance is below:

Their recommended treatments are Atovaquone-proguanil or doxycycline. Be aware that Doxy, while dirt cheap and effective, does give you a sensitivity to sun exposure, so if you're anticipating spending a lot of time on the beach with exposed skin, etc. it's probably not the best bet.

Make sure other immunizations you have are up to date for your kids - things like measles are quite prevalent at times, and other more "exotic" vaccines may be suggested by the CDC or your travel doctor - Hepatitis A and B, Typhoid, etc.

Dengue Fever is probably a bigger deal, as it is an actively circulating disease in all parts of Thailand. Unfortunately, there is no vaccine against Dengue, as it's an extremely complex disease to design a vaccine for, so insect repellant, loose, covering clothing and other measures to protect against bites are the only ways to means of preventing infection.

Many of the side effects from malaria drugs such as malarone are not at all fun to deal with, and may in fact lead to some long-term effects if taken for months at a time. I was advised to take malarone before flying in to Thailand to work for a year....As it turned out, the CDC "hot zone" they referred to----my work area, near Mae Sot---- was nowhere near as bad as they made it out to be, and school admin and doctors in the area emphasized that taking meds for it was not only unnecessary but in fact may not even work anyhow. I decided not to take the malarone I had brought from the USA. I have been in Thailand for 3 yrs now, which is two more than the original contract time I signed on for. I do not believe taking malarone for 3 yrs would have been healthy, even if there were no noticeable side effects. I now live in the coastal area of the southern half of Thailand, where dengue, if anything, is the greater threat to public health. I cannot stress enough that MDs, whose advice often hangs on every word of the CDC, are not the last word for a balanced perspective on good health. Listen to them, but don't take their words as the gold standard of preventive medicine if you are planning on travelling abroad. Get advice from seasoned locals, friends and relatives who travel, and health pros in the area where you intend to go, before you start pill popping what your US MD sells you....
Bring good bug spray, wear sensible clothing, use a mosquito net at night, and avoid drinking so much (since alcohol lowers your immunity), and your stay will [likely] be a lot more hazard-free than it would be if you'd put all your shillings into Mother's Little Helper.....