Chikungunya in the Indian Ocean region

Acknowledgement
This report is being published on behalf of the GeoSentinel network. GeoSentinel is a cooperative agreement U01CK000632-01-00 between the Centers for Disease Control (CDC) and Prevention and the International Society of Travel Medicine (ISTM).

Authors
Ralph Huits, MD, PhD, Emilie Javelle, MD, PhD

Key facts

• Reunion Island is experiencing a large outbreak of chikungunya virus (CHIKV). This outbreak is reminiscent of the emergence of CHIKV as a pathogen of global importance, when an East-Central-South African (ECSA) CHIKV lineage spread in 2004 from Kenya to the islands in the western Indian Ocean in 2005 and 2006, and infected millions of people in countries in South and Southeast Asia bordering the Indian Ocean, commonly referred to as the Indian Ocean Region (IOR).
• The first case in Reunion was diagnosed on 23 Aug 2024, but the numbers started increasing exponentially in January 2025, with an estimated number of 160 475 cases as of 30 Apr 2025, with 44 000 cases confirmed. Some strains implicated in the current outbreak carry the E1-A226V mutation that was identified during the 2005-2006 outbreak (and which is associated with more efficient transmission by Aedes albopictus) and have therefore been identified as an Indian Ocean lineage (IOL) CHIKV.

Travel-associated cases and geographical distribution

• Between 01 Aug 2024 and 27 Apr 2025, 51 cases from the IOR were diagnosed at GeoSentinel clinics (unpublished data). 17 cases were acquired in India where CHIKV transmission is endemic, but GeoSentinel identified 13 cases among travelers returning from Réunion, and 21 from additional IOR destinations, including Sri Lanka (n=5), Indonesia (n=2), Maldives (n=2), Mauritius (n=2), Philippines (n=2), Cambodia (n=1), Malaysia (n=1), Pakistan (n=1), Kenya (n=1), Mozambique (n=1), Somalia (n=1), Sudan (n=1) and Uganda (n=1).
• Most diagnoses were classified as probable, based on clinical presentation, epidemiological risk factors, and CHIKV-specific antibody detection assays in single samples obtained from travelers after returning to their home countries. GeoSentinel also captured 22 cases confirmed by reverse transcription polymerase chain reaction (RT-PCR) from Reunion (n=10), India (n=7), Mauritius (n=2), and Indonesia (n=1), and evidence of seroconversion in paired sera (India (n=1) and Sri Lanka (n=1).
• The increase in travel-acquired cases from Sri Lanka, where dengue is co-circulating, is particularly noteworthy, because a surge in chikungunya cases from Sri Lanka was recently reported in local media. On 30 Apr 2025, the authorities in the Western Province of Sri Lanka established a task force to eliminate chikungunya and dengue from the Province.

Virological considerations
Different lineages likely caused cases from countries in the western region of the IOR than those in the eastern IOR. CHIKV genotypes from IOR countries other than La Réunion have not been determined.

Immediate actions
The observed trends prompted GeoSentinel to reach out to the network for enhanced vigilance and rapid reporting of chikungunya cases from the IOR on 25 Mar 2025. We anticipate reporting on the molecular epidemiology of the ongoing CHIKV outbreaks shortly.

References

1. Public Health France. Chikungunya in Reunion Island. Bulletin of April 30, 2025. 30 Apr 2025 (in French)
2. Health officials sound alarm as chikungunya infections surge across island. Lanka News Web. 10 Apr 2025
3. ‘War on Dengue’ Task Force to eliminate Chikungunya & Dengue. Newswire. 30 Apr 2025