Severe weather events linked to outbreaks of crypto in Aotearoa – New study

Cryptosporidium are microscopic parasites that can cause gastrointestinal pain, diarrhea, vomiting and fever with symptoms usually lasting one to two weeks. They are common cause of waterborne outbreaks of infectious intestinal disease in NZ.¹ 

In NZ, Cryptosporidium infections (cryptosporidiosis) in people have been linked to rainfall^2-4 and temperature.^{2 3 5} Heavy rainfall events can increase surface runoff of Cryptosporidium in the environment,^6-9 and such events have been linked to higher Cryptosporidium pathogen loads in waterways and increased infection rates.^10-12 Runoff due to heavy rainfall could increase the risk related to the consumption of untreated drinking water, and higher infection rates have been reported in areas with untreated or inadequate drinking water supplies in NZ.¹³ In addition, Cryptosporidium are resistant to conventional water treatment techniques¹⁴ and increased pathogen loading due to heavy rainfall events can overwhelm drinking water and wastewater infrastructure and lead to disease outbreaks.¹⁵ Recreational water contact, such as swimming, has also been identified as an important risk factor for infection.⁵ The Government is planning to weaken protections for water, including drinking water. In an announcement in April 2024 on changes to the Resource Management Act, the Government reiterated its plan to deprioritise drinking water protections by changing Te Mana o te Wai framework.¹⁶ 

Clusters of Cryptosporidium infections in NZ

In our just published study,¹⁷ we identified a total of 15,822 cases of cryptosporidiosis that were reported in NZ from 1997 to 2015. Sixty-five clusters were detected using statistical methods during that period, and 38 (58.5%) of those clusters were statistically significant, meaning that they were unlikely to have occurred by chance. Approximately 4% (645/15,822) of reported cases from 1997 to 2015 were part of significant clusters. The number of cases in the significant clusters ranged from three to 83 (mean = 17.0, median = 10). 

Comparison to severe weather events

The average incubation period for cryptosporidiosis is around seven days.^18-21 However, Cryptosporidium can survive in the environment for more than 12 weeks.²² Therefore, we searched the National Institute of Water and Atmospheric Research (NIWA) Historic Weather Events Catalogue for severe weather events with increased rainfall in the 21 days before each of the significant detected clusters. We found that around one third (34.2%, 13/38) of the statistically significant clusters occurred at places and times that aligned with severe weather events from NIWA's Catalogue. Figure 1 shows the locations of statistically significant clusters of cryptosporidiosis that aligned with severe weather events in NZ.

Figure 1. Clusters of cryptosporidiosis in NZ (1997-2015) at the census area unit (CAU) level that align temporally and spatially with severe weather events. The orange regions (with red dots at the CAU centroids) are CAUs with statistically significant clusters (p<0.05).

Nearly half (46.2%, 6/13) of the 13 clusters that aligned with severe weather events occurred in the spring, three occurred in the winter, three occurred in autumn, and one occurred in late summer and early autumn. Clusters detected during spring months may be related to spring calving and lambing, because newborn livestock can be an important source of cryptosporidiosis.^23-25 

Research and policy implications

This study found that heavy rainfall and flooding events occurred before several detected clusters of cryptosporidiosis in NZ. However, additional research is required to better determine the role of extreme weather events on rates of infectious intestinal diseases. There is an urgent need for such research as rainfall intensity is expected to keep rising in NZ due to climate change.^{26 27} 

This study also supports the need to further protect drinking water supplies from contamination with pathogens, especially from agricultural runoff and sewage. Cases and outbreaks of cryptosporidiosis, as well as other infectious intestinal diseases, have been regularly linked to contaminated water supplies.^28-30 For example, the 2023 waterborne Queenstown cryptosporidiosis outbreak was likely due to human faecal contamination of water supplies and resulted in at least 72 confirmed cases.^{31 32} Additionally, the 2016 waterborne Havelock North campylobacteriosis outbreak was attributed to the contamination of two untreated bore wells with sheep faeces, and resulted in approximately 7570 cases and four deaths.³³ Such outbreaks highlight the need to strengthen source protection of water supplies and for a strong regulatory framework that prevents water pollution from livestock agriculture and inadequate water infrastructure (which was a major conclusion of the Government Inquiry into the Havelock North Drinking Water³⁴). 

Conclusions

This study provides additional evidence that extreme weather events can play a role in outbreaks of cryptosporidiosis in NZ. Additionally, 46.2% of the clusters that aligned with severe weather events occurred in the spring and may be linked to calving and lambing. Additional research is needed to better assess the role of heavy rainfall and other environmental factors, such as livestock farming and inadequate water infrastructure, in cryptosporidiosis cases and outbreaks, especially as rainfall intensity is expected to keep increasing in NZ due to climate change. This research highlights the need for drinking water safety and water quality to be prioritized in policy to protect public health. The Government should be discouraged from its plans to weaken protections for water. 

Author details: 

Assoc Prof Leah Grout, Southern California University of Health Sciences, USA 

Prof Simon Hales, Department of Public Health, University of Otago Wellington, and Public Health Communication Centre

Prof Michael Baker, Department of Public Health, University of Otago Wellington, and Public Health Communication Centre

Prof Nigel French, Distinguished Professor of Infectious Disease Epidemiology and Public Health at Massey University

Marnie Prickett, Department of Public Health, University of Otago Wellington, and Public Health Communication Centre

Prof Nick Wilson, Department of Public Health, University of Otago Wellington, and Public Health Communication Centre