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Cases of gastrointestinal illness Giardia in England reached its highest level last year since 2016, according to the latest data from the UK Health Security Agency (UKHSA).

The number of cases increased from 5,450 in 2024 to 6,069 in 2025 and the UKHSA said last year’s figure was “the highest since 2016 and is above pre-pandemic levels”.

Giardia is an infection caused by the parasite Giardia duodenalis and diarrhoea is the most common symptom. It is often accompanied by abdominal cramps, bloating, belching, flatulence and weight loss. Research has shown asymptomatic infection occurs in up to 25 per cent of cases.

London recorded the highest rate with 17.3 per 100,000 population, followed by the South-West (14.3), North-West (13.3), South-East (10.1) and West Midlands (9).

Most infections were found in adults aged 30 to 39, making up 20 per cent of total laboratory reports, and most of those, 64.2 per cent, were male.

“It is likely that changes to testing, particularly the introduction of multiplex gastrointestinal infection polymerase chain reaction testing, are contributing to recent increases in giardia,” the UKHSA said.

“But other potential factors are being investigated including travel, climate and other environmental changes.”

The UKHSA said: “Giardiasis is treated with nitroimidazoles and combinations of antiparasitics if the infection does not respond to standard treatment.

“Infections which persist despite standard treatment (refractory infections) are increasingly common in the UK in both asymptomatic and symptomatic individuals, particularly among children seeking asylum and travellers returning from India.”

The UKHSA said “multiple factors” such as protozoal drug resistance and “re-infection within families or close contacts” could be contributing to drug resistance although it stressed more research is needed.