Mosquitoes and dengue fever

Summary

Mosquitoes on Koh Samui can transmit dengue, chikungunya and Zika. Dengue is the main risk, especially in and after the rainy season. Day-biting Aedes mosquitoes are the carriers, so repellent and covering up matter all day, not just at night.

Details

Koh Samui has both main dengue vector species, Aedes aegypti and Aedes albopictus, documented in a published entomological study on the island. The island has a history of dengue outbreaks: epidemics in the 1960s and a 1995 outbreak with an incidence of almost 500 cases per 100,000 population. Aedes mosquitoes bite mainly during the day, with peaks in the early morning and late afternoon, and they breed in standing water - flower pots, water jars, construction sites, coconut husks. Mosquito numbers rise in the rainy months (on Samui the wettest period is roughly October to December, with a second wetter spell mid-year), but southern Thailand has mosquito activity year round, so there is no zero-risk month. Dengue causes high fever, severe headache, joint and muscle pain and sometimes rash; severe dengue is a medical emergency. There is no specific treatment for tourists to self-administer - prevention is the strategy: use repellent with DEET or picaridin, wear long sleeves and trousers in the morning and evening, use air conditioning or screened rooms, sleep under a net where rooms are open, and avoid aspirin/ibuprofen if dengue is suspected (paracetamol is preferred) because of bleeding risk. A dengue vaccine exists but is only recommended in specific cases - ask a travel doctor before your trip. Malaria risk on Samui itself is very low; malaria is mainly a mainland border-area disease in Thailand.

Safety and ethics

Use DEET/picaridin repellent day and night; cover up at dawn and dusk; empty standing water around your accommodation; suspect dengue with high fever + severe headache/joint pain - see a doctor and use paracetamol, not aspirin/ibuprofen; severe belly pain, bleeding or persistent vomiting = emergency.

Frequently asked questions

Is there dengue on Koh Samui?

Yes. Dengue is present on the island and past outbreaks have been documented in medical literature. Risk is highest in and after the rainy season, but transmission happens year round.

When do dengue mosquitoes bite?

Aedes mosquitoes bite mainly during the day, especially early morning and late afternoon - unlike malaria mosquitoes, which bite at night.

How can I protect myself?

Repellent with DEET or picaridin, long clothing at dawn and dusk, air-conditioned or screened rooms, and no standing water around where you stay.

Is there malaria on Koh Samui?

Malaria risk on Samui is very low. In Thailand, malaria is concentrated in rural forested border areas, not the Gulf tourist islands.