Research and analysis

Outbreaks under monitoring: week 34 (week ending 24 August 2025)

Published 28 August 2025

Disease or pathogen	Chikungunya
Location	France
Reporting date	20 August 2025
Summary	Between 1 May and 19 August 2025, 154 locally acquired cases of chikungunya  were recorded in mainland France. For the first time ever, locally acquired cases were reported in the departments of Grand Est, Bourgogne-Franche-Comte and Nouvelle-Aquitaine. 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 1
Location	Israel
Reporting date	20 August 2025
Summary	On 20 August 2025, the World Health Organization (WHO) reported a circulating vaccine-derived poliovirus type 1 (cVDPV1) outbreak in Israel. According to the WHO, 9 cVDPV1 isolates were detected in environmental samples collected from 7 sampling sites across Israel between February and July 2025. Whole-genome sequencing revealed that the viruses were genetically linked to each other and to multiple Sabin-like viruses isolated from environmental samples since October 2024. No human cVDPV1 cases have been reported since the detection of environmental positive samples. Poliovirus cases are rare in the UK and the last locally acquired case 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: Israel

Disease or pathogen	Crimean-Congo haemorrhagic fever
Location	European Region
Reporting date	18 August 2025
Summary	On 18 August 2025, the European Centre for Disease Prevention and Control (ECDC) reported 5 Crimean-Congo haemorrhagic fever (CCHF) cases in the European region in 2025, in Greece (2 cases) and Spain (3 cases). The CCHF cases reported in Greece are the first since 2008. Both cases were reported from Larissa, Thessaly region, which is unexpected as CCHF has not previously been detected in Thessaly or surrounding regions. CCHF is not present in the UK, nor have there been any reports of established Hyalomma tick populations, which are the main vectors of the CCHF virus. Only travel-associated CCHF cases, with no onward transmission, have been reported in the UK.
Further information	Crimean-Congo haemorrhagic fever: origins, reservoirs, transmission and guidelines HAIRS risk assessment: Crimean-Congo haemorrhagic fever NaTHNaC country information page: Greece and Spain

Disease or pathogen	Legionella
Location	United States
Reporting date	19 August 2025
Summary	On 25 July 2025, the New York City (NYC) Health Department announced a community cluster of Legionnaires’ disease in Central Harlem, United States. Between 22 July and 19 August 2025, 108 cases  and 5 associated deaths have been reported. On 4 August 2025, the NYC Health Department reported that 11 cooling towers tested positive for Legionella bacteria and have been remediated. In the UK, reported legionnaires disease cases follow a seasonal pattern, with a peak number of cases reported between June and October. Between 2017 and 2023, 3,287 cases of legionnaires disease were reported in the UK. In 2023, 212 of 604 confirmed cases of Legionellosis diseases (35.1%) had exposures during foreign travel.
Further information	Legionnaires’ disease: guidance, data and analysis NaTHNaC country information page: United States

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