Malaria pills needed for jungle trip in Chiang Mai?

Personally I would be far more worried about Dengue, as you are traveling during the rainy season. Many more people catch Dengue than Malaria, partially because there is no preventative medicine (and Dengue is no fun, I speak from experience).

The Thai government does record a number of cases of Malaria each year, but in terms of percentages and risks, it is basically not an issue.

Your risk for Dengue is actually greater in the city rather than while trekking, as Dengue needs to go from the host to the victim in a very short period of time, so outbreaks tend to be concentrated within a few block area.

The best approach to both diseases (as well as Japanese Encephalitis) is to avoid being bitten by mosquitoes. Using bug spray when you are trekking, as well as when eating outdoors in the city especially in garden settings, is a good start. The mosquitoes carrying Dengue are daytime biters, the mosquitoes carrying Malaria and JE tend to be dawn and dusk feeders.

I lived in Chiang Mai for 2.5 years until 2010 or so and never considered malaria a problem. Nor did anyone I knew.

But, drug resistant malaria is becoming a problem in Thailand:

http://m.bbc.com/news/health-31533559

That said, taking drugs to prevent drug-resistant malaria is pointless, too.

I recently visited Thailand and got lots of in-depth information about it.

It REALLY depends on what you're doing, when you're doing it, where you're doing it, how long you're doing it there, how well-protected you plan to be, and how much risk you're willing to take.

Malaria is more of a problem in northern Thailand. There are more mosquitos during the rainy season. There are lots more mosquitos in the jungle than in the city. If you camp in the jungle, make sure you use mosquito netting while you sleep. During the day, use bug spray on your clothes, especially near your neck, ankles and wrists.

Quoting from elsewhere on the CDC site:

When traveling in Thailand, you should avoid mosquito bites to prevent malaria. You may need to take prescription medicine before, during, and after your trip to prevent malaria, depending on your travel plans, such as where you are going, when you are traveling, and if you are spending a lot of time outdoors or sleeping outside. Talk to your doctor about how you can prevent malaria while traveling. For more information on malaria in Thailand, see malaria in Thailand.

If you are traveling in the jungle for more than a day or two—especially right near the rainy season, I would absolutely take malaria medicine. Better safe than sorry.

Also, be aware of the other infections diseases in the area, such as Japanese Encephalitis, Hepatitis A & B, Yellow Fever and others.

Quoting the CDC's Infectious Diseases Related To Travel / Thailand page,

Areas 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.

Estimated relative risk of malaria for US travelers: Low.

So it sounds like the CDC does not think you need a malaria vaccine for Chiang Rai.

Don't forget to drink quinine - often in 'tonic water'. I understand this to be very beneficial with treatment if you do happen to get malaria.

Seems to make sense to have quinine tabs on hand, although WHO recommends quinine as secondary treatment now.