Research and analysis

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

Published 28 August 2025

Disease or pathogen	Chikungunya
Location	France
Reporting date	27 August 2025
Summary	Between 1 May and 26 August 2025, 228 locally acquired cases of chikungunya were recorded in mainland France. This is an increase of 74 cases since the last report on 20 August 2025. 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 NaTHNaC country information page: France

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 human case of paralytic poliomyelitis 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 are rare in the UK and the last case of paralysis due to polio was reported in 1984. A travel-associated case 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 Green Book Chapter NaTHNaC country information page: Papua New Guinea

Disease or pathogen	Dengue
Location	Samoa
Reporting date	26 August 2025
Summary	On 17 April 2025, the Samoa Ministry of Health declared a dengue fever outbreak following a substantial increase in the number of recorded dengue cases. Between 1 January and 24 August 2025, 10,982 clinically diagnosed dengue cases (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, 496 confirmed dengue cases 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 NaTHNaC country information page: Samoa

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 declared an outbreak of legionnaires’ disease in London, Ontario, Canada. Between 1 January and 27 August 2025, 100 cases of Legionnaires’ disease (including at least 4 deaths) 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 44.5% of all legionnaires’ disease 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 cluster of Legionnaires’ disease in Central Harlem, United States. Between 22 July and 29 August 2025, 114 cases  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 declared over. 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: Canada and the United States

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