Dengue fever on Koh Samui: risk and prevention

Dengue is the main mosquito-borne disease risk for visitors to Koh Samui. It circulates in Thailand all year, including on the islands, and reported case numbers typically climb during and just after the rainy months; Thailand recorded more than 2,000 new cases per week at points in 2025. There is no widely available traveller vaccine and no specific cure, so avoidance and early recognition are the strategy. The Aedes mosquito that spreads dengue bites mostly during daylight, especially early morning and late afternoon, unlike the night-biting malaria mosquito, so protection is a daytime habit. Prevention: apply repellent containing DEET or picaridin to exposed skin whenever outdoors, reapplying after swimming or sweating; wear loose long sleeves and trousers at dawn and dusk; choose accommodation with screens or air-conditioning; and reduce standing water around your room or villa where practical. Symptoms usually appear 4-10 days after a bite: sudden high fever, severe headache, pain behind the eyes, joint and muscle pain, and sometimes a rash. Most cases are mild and pass with rest, fluids and paracetamol, but important: avoid aspirin and ibuprofen with suspected dengue, because they increase bleeding risk. Warning signs needing urgent medical care include severe abdominal pain, persistent vomiting, bleeding gums or nose, blood in vomit or stool, and extreme tiredness as the fever falls; severe dengue is dangerous but treatable with prompt care. If you develop a fever during or within two weeks of your trip, tell the doctor you have been in a dengue area.

Summary

Dengue is present across Thailand, including Koh Samui, year-round, with case numbers typically rising in wetter months. The dengue mosquito bites mainly during the day. No vaccine is routinely available for travellers, so prevention is repellent with DEET or picaridin, covering skin, and avoiding bites.

Key facts

• Dengue circulates in Thailand year-round, rising in wetter months
• Aedes mosquitoes bite mainly by day, especially dawn and dusk
• Use DEET or picaridin repellent; cover skin; use screens or air-conditioning
• Symptoms: high fever, severe headache, pain behind eyes, joint pain, rash
• Take paracetamol, NOT aspirin or ibuprofen, for suspected dengue
• Warning signs (abdominal pain, bleeding, persistent vomiting) need urgent care

Local tips

• Apply repellent every morning before going out, not just at night
• Buy DEET-based repellent locally at any pharmacy if you forget yours
• Ask for a room with screens or air-conditioning
• See a doctor promptly for high fever with headache or rash, and mention Thailand