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Research and analysis

Outbreaks under monitoring: week 35 (week ending 31 August 2025)

Updated 31 December 2025

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Disease or pathogen Chikungunya
Location France
Reporting date 27 August 2025
Summary Between 1 May and 26 August 2025, were recorded in mainland France. This is an increase of 74 cases since the . The reported number of locally acquired chikungunya cases in France is the highest since 2010.
In the United Kingdom (UK), an increase in travel-associated chikungunya cases has been reported in 2025. A total of 73 travel-associated chikungunya cases were reported between January and June 2025, compared to 27 cases reported during the same period in 2024. Most cases reported travel to Sri Lanka, India and Mauritius. No locally acquired cases of chikungunya have ever been reported in the UK.
Further information Chikungunya: transmission, epidemiology and guidance
Chikungunya: epidemiology in England, Wales and Northern Ireland
Disease or pathogen Circulating vaccine-derived poliovirus type 2
Location Papua New Guinea
Reporting date 28 August 2025
Summary On 28 August 2025, the World Health Organization (WHO) reported the first confirmed from circulating vaccine-derived poliovirus type 2 (cVDPV2) in Papua New Guinea. The case is a 4 year old unvaccinated male from Lae, Morobe Province.
On 11 August 2025, Papua New Guinea launched an immunisation campaign targeting children under 10 years with the novel oral polio vaccine (nOPV2).  As of 26 August 2025, the national coverage is 41.2%. A second round of the immunisation campaign is scheduled from 29 September to 17 October 2025 and will include both nOPV2 and Inactivated Polio Vaccine.
Poliovirus cases and the last case of paralysis due to polio was reported in 1984. A was reported in 1993. Vaccine-derived poliovirus type 2 has been detected in sewage samples in the UK.
Further information Polio: guidance, vaccination, data and analysis
Environmental surveillance for polio
Disease or pathogen Dengue
Location Samoa
Reporting date 26 August 2025
Summary On 17 April 2025, the Samoa Ministry of Health following a substantial increase in the number of recorded dengue cases. Between 1 January and 24 August 2025, (including 3,379 confirmed cases) and 6 deaths have been reported in Samoa. 72% of cases have been reported in children aged under 15 years. Dengue virus serotypes 1 (91% of cases) and 2 (9% of cases) have been detected in this outbreak. In comparison, between November 2023 and 11 August 2024, were recorded in Samoa.
There is no current evidence of local transmission of dengue virus in the UK and there is no evidence of established populations of the mosquito responsible for dengue transmission. Most reported UK cases have been related to travel abroad.
Further information Dengue fever: guidance, data and analysis
Travel-associated infections reports
Disease or pathogen Legionella
Location Canada and the United States
Reporting date 26 to 29 August 2025
Summary On 8 July 2025, the Middlesex-London Health Unit of Legionnaires’ disease in London, Ontario, Canada. Between 1 January and 27 August 2025, of Legionnaires’ disease () have been reported in the ongoing outbreak. The outbreak was initially declared over on 5 August 2025 and later redeclared on 26 August 2025, following the report of additional cases. The cases reported in this outbreak represent cases recorded in Ontario between 1 January and 27 August 2025 (227 cases).
On 25 July 2025, the New York City Health Department announced a community in Central Harlem, United States. Between 22 July and 29 August 2025,  and 7 associated deaths have been reported. This is an increase of 6 cases and 2 deaths since the last update on 19 August 2025. The outbreak has been .
In the UK, reported Legionnaires’ disease cases follow a seasonal pattern, with a peak number typically reported between June and October. Between 2017 and 2023, 3,276 cases of legionnaires’ disease were reported in the UK. In 2023, 212 of 604 confirmed Legionella exposures (35.1%) were related to travel abroad.
Further information Legionnaires’ disease: guidance, data and analysis
NaTHNaC country information page: and the

For more information on the background and methods used in this report, see Outbreaks under monitoring: background and methodology.

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