Arenal has few mosquitoes in town and two questions for the travel clinic.

Some, though visitors mostly describe La Fortuna as light on mosquitoes, and the dusk biters people remember from night walks are larger, horsefly-like insects. The planning items come from the CDC. Arenal is in Alajuela province, which the map for the CDC's chikungunya notice shades in full, and Alajuela is one of the two Costa Rican provinces where the CDC lists malaria prevention medicine.

Chikungunya and malaria in Alajuela

The CDC's Level 2 notice, last reviewed July 30, 2026, says "There is an outbreak of chikungunya in northwest Costa Rica," and the CDC's map for the notice shades all of Alajuela, La Fortuna and the Arenal area included. By mid-September the Pan American Health Organization counted 539 lab-confirmed chikungunya cases in Costa Rica for 2026, 132 of them in Alajuela, second only to Guanacaste. The notice says "Vaccination is recommended for travelers who are visiting an area with a chikungunya outbreak," and it advises pregnant travelers to reconsider travel to the affected areas, particularly close to delivery.

Malaria is the second item. The CDC's Costa Rica page says malaria is "Present in the provinces of Alajuela and Limón" and lists prevention medicines for both. Everywhere else, including Guanacaste, Monteverde and Manuel Antonio, it recommends bite precautions only.

Dengue is running above 2025 nationally, and Costa Rica is on the CDC's global dengue list. The mosquito behind dengue and chikungunya bites by day around town, hotel gardens and pools, so repellent goes on over sunscreen at breakfast. The CDC's Costa Rica page also lists New World screwworm, an infestation of wounds by fly larvae, and advises keeping cuts clean and covered.

Town, hot springs and the forest after dark

In La Fortuna itself, visitors report few mosquitoes, and hot-spring resorts such as Tabacón and Baldí draw little complaint in the evening. Trails tend to bite more right after rain.

The forest is where the exposure is. Volcano trails, hanging-bridge parks such as Místico and guided night walks put you under the canopy at dusk and after dark, when the horsefly-like biters come out and malaria mosquitoes and phlebotomine sand flies bite. The sand flies spread leishmaniasis, a skin infection, and the CDC Yellow Book names Costa Rica among its most common sources in travelers. They are small enough to pass through ordinary mosquito netting. Wear permethrin-treated long sleeves, trousers and socks with closed shoes. Permethrin goes on fabric at home, never on skin.

A humid winter

December to April is the drier, busier stretch, but Arenal stays humid through the winter, so bites continue in a way they don't on the dry Guanacaste coast. In October 2026 the Pan American Health Organization also warned that a strengthening El Niño could bring outbreaks outside the usual seasons. Check the CDC notice again the week before you fly.

Where you're going

Scores are for a December–April trip.

Our Costa Rica briefing covers the whole loop. Heading to the coast next? The Guanacaste and Manuel Antonio briefings cover two stops outside the CDC's malaria provinces.

What travelers report

Paraphrased from TripAdvisor threads and reviews and linked so you can read them in full. None gives a year, and one describes a late-July visit, outside the December–April window.

• Arenal / La Fortuna

  Several visitors had few or no mosquito bites in La Fortuna town and on night hikes. One said the worst bites came from large horsefly-like flies around dusk. TripAdvisor ↗

• La Fortuna (night hike) · late July

  Asked about mosquitoes in late July, a poster who had visited around then said they were not much of a problem; the few bites came from larger, horsefly-like insects at dusk on a night hike. A forum regular said visitors would see only a few mosquitoes. TripAdvisor ↗

• Arenal hot springs · evening

  Posters said they had little or no mosquito trouble at the hot springs in the evening. A regular still advised repellent because mosquitoes are most active at dusk, and said the warm water and gases seem to keep them down. TripAdvisor ↗

• La Fortuna Waterfall · after rain

  Under the title 'Bring your bug spray and water shoes', a reviewer said they were bitten during a picnic right after rain. TripAdvisor ↗

One rule outlasts the trip. Any fever or joint pain during or after the trip means a doctor visit, and you tell the doctor you were in Alajuela, Costa Rica. A bite sore that doesn't heal is worth a visit too.

Traveler questions