Research highlights potential of climate change to boost cryptosporidiosis in New Zealand swimming locations

Research published in the international journal Epidemiology & Infection - the first study to compare clusters of outbreaks of cryptosporidiosis to severe weather events in Aotearoa – has highlighted the potential of climate change to boost cryptosporidiosis in New Zealand swimming locations and drinking water.

Cryptosporidium is one of the most common causes of waterborne gastrointestinal illness, with almost 16,000 cases of cryptosporidiosis notified in Aotearoa between 1997 and 2015. Most people are infected either by swimming in contaminated waterways or drinking contaminated water.

As extreme rainfall events become more frequent in New Zealand, higher levels of the diarrhoea-causing parasite will be washed into waterways, public health researchers warn.

The researchers studied clusters of cryptosporidium outbreaks around the country between 1997 and 2015 and found 13 coincided with severe weather events.

One of the researchers, Professor Simon Hales from the Department of Public Health at the University of Otago, Wellington, says the study found 38 'statistically significant' clusters, unlikely to have occurred by chance. The 13 that coincided with severe weather events included 55 cases that occurred after heavy rain in Kaikoura in March 1999 and 22 cases following a countrywide weather bomb in October 2000.

Professor Hales says runoff from livestock is likely to be heightening the risk of disease outbreaks. Nearly half of the 13 clusters that aligned with severe weather events occurred in the spring, suggesting a link to calving and lambing times, with newborn livestock a known source of the parasite.

Professor Hales says cryptosporidium is resistant to conventional water treatment techniques and higher levels of the pathogens in heavy rain can overwhelm drinking water and wastewater infrastructure, causing disease outbreaks.

"Cases and outbreaks of cryptosporidiosis, as well as other infectious intestinal diseases, are often caused by contamination of water supplies."

He says the 2023 Queenstown cryptosporidiosis outbreak when more than 72 people got sick was likely caused by human faecal contamination of water supplies. The 2016 Havelock North campylobacteriosis outbreak, in which 7,500 people got sick and four died, was the result of water bores being contaminated with sheep faeces.

"These outbreaks highlight the need to strengthen source protection for water supplies, and for a strong regulatory framework to prevent water being polluted by runoff from livestock farms.

"Our findings show how important it is to protect our drinking water supply, and the places people swim, from contamination from agricultural runoff and sewage leaks from broken pipes."

Professor Hales says there is a pressing need for more research to be done on the links between waterborne diseases and extreme weather events as severe rainfall events become more frequent with climate change.

Image. Cover of NZ Ministry of Freshwater 2023 report. Lake Tarawera. Credit: Fraser Tebbutt/ truestock

Ministry for the Environment and Stats NZ in its 2023 report notes “our freshwater environment is under pressure from our activities on the land and in the water, and from a changing climate. While some of our freshwater bodies are in a reasonably healthy state, many have been degraded by the effects of excess nutrients, pathogens, and other contaminants from land. 

“The effects of our historic and contemporary activities on our freshwater environment have impacts on many of the things we value as individuals, communities, and as a nation, such as our iconic and taonga species and being able to swim and practice mahinga kai without risk of illness.

“This report has been produced at a particularly poignant time, in the immediate aftermath and initial recovery from a number of severe weather events, notably, Cyclone Gabrielle. The effects of these events have made the combined pressures of climate change, land use, and human modifications to waterways more evident than ever before.