Climate-sensitive diseases

This section describes statistics on climate-sensitive diseases and health conditions in Aotearoa New Zealand.

A changing climate will have several effects on health

Climate change has been described as “the biggest global health threat in the 21^st century” [1]. As well as impacting human health directly (eg. through changes in temperature), climate change can also change the frequency or severity of certain diseases by providing conditions that are more favourable to their transmission.

Examples of how a warming climate may worsen health problems include:

• gastrointestinal infections: Rates of the gastrointestinal infections campylobacteriosis, cryptosporidiosis and giardiasis are affected by rainfall patterns. Rainfall washes giardia and cryptosporidium cysts, or campylobacter bacteria, into waterways, where they can contaminate drinking water sources. Drought conditions can lead to a greater cyst concentration in groundwater and surface water sources (Britton et al 2010; Lal et al 2013)
• infectious diseases: Increasing temperatures can change the geographical distribution of some mosquitoes, which carry infectious diseases such as malaria and dengue fever.
• respiratory problems: Increasing temperatures bring a longer pollen season and increased fire risk, associated with increases in respiratory problems.
• cardiac (heart) problems: Heat is linked to the worsening of heart problems and to an increase in overall death rates (Hales et al 2007; McMichael et al 2003).
• drinking water supply: Severe drought can reduce the quality and the amount of drinking water available. New Zealand’s populations that rely on rainwater tanks for their drinking water supply can be particularly affected. Conversely, extreme rainfall and flooding may lead to chemicals or pathogens contaminating drinking water sources.
• effect on mental health: Drought can significantly affect mental health, particularly for those in rural areas who rely on rain for their livelihoods.

Gastrointestinal diseases are sensitive to rainfall and temperature

Studies show that cryptosporidiosis and giardiasis (gastrointestinal ‘tummy bug’ infections) are affected by rainfall patterns. Cryptosporidiosis and giardiasis can be caught by drinking water that has been contaminated with cryptosporidium and/or giardia cysts. High rainfall events can wash cysts from fields into water sources [4]. Drought conditions can lead to a greater concentration of cysts in rivers due to low water flow and volume [3].

Higher temperatures are linked to an increase in salmonellosis notifications. Salmonellosis can be contracted by eating or drinking food and/or water that has been contaminated by humans or animals with the Salmonella bacteria. An increase of 1°C in monthly average temperatures has been associated with 15% more salmonellosis notifications in that month [5].

Studies have also shown a positive association with higher temperatures and heavy rainfalls increasing the risk of campylobacteriosis outbreaks, such as in Havelock North in 2016 [6].

Leptospirosis benefits from a tropical climate and heavy rain or floods 

Leptospirosis is a serious bacterial infection that causes flu-like symptoms (eg. fever, headache, or muscle aches) that can last for months. In some people, it can cause kidney and liver damage, meningitis, and even death. Infection can go unrecognised if symptoms are mild, mistaken for flu or absent. Leptospirosis can be transmitted to humans indirectly, through contact with contaminated water, soil or food. The extent to which infection is transmitted depends on many factors, including climate.

Leptospira can survive for weeks to months in moist soil or water, prefer warmer conditions and spread rapidly after heavy rain or flooding. Outbreaks have been reported overseas following extended periods of hot, dry weather and following flooding (ECDC, 2014-2023). Flooding is expected to become more common in New Zealand due to the increases in rainfall that climate change will bring to certain parts of the country.

The leptospirosis notification rate was approximately five times as high for males (2.5 per 100,000) as for females (0.5 per 100,000). The leptospirosis notification rate in rural areas was over 32 times as high as in major urban areas (standardised rate ratio = 32.29, 95% CI 20.19–51.64). When accounting for age, most leptospirosis cases were in people aged 25­–44 years, with 30 notifications (39.0%), and 45–64 years, with 24 notifications (31.2%) in 2021.

Some jobs increase the risk of leptospirosis. People working directly with animals or involved in their slaughter are at increased risk (eg. farmers and meat workers). Of the 77 leptospirosis notifications in 2021, 93.5% recorded an occupation, with 43 (59.7%) people working in at-risk occupations.

Ongoing monitoring of the health effects of climate change is needed

Climate change trends occur over many years, so correspondingly long-term health data are needed to show changes in disease occurrence and severity due to climate change.

However, it is useful to monitor diseases that are sensitive to climate change, in anticipation of possible increases and changes in the future. Monitoring data on aspects of the environment and vulnerable populations is also needed to sit alongside monitoring of health effects, as is climate data at the regional level.

Information about the data

Specific change over time corresponding with climate change cannot be shown as the common baseline period in climate change science is 1960-1990, for which comparable data is not available.

Notifications of gastrointestinal diseases

Source: EpiSurv, ESR

Definition: This indicator presents campylobacteriosis, salmonellosis, giardiasis and cryptosporidiosis notifications. Notifications where the person was overseas during the incubation period have been excluded. Notifications only cover those people who visited a GP or hospital for treatment, and may therefore underestimate the disease rate. Rates presented are per 100,000 people (or 100,000 people per year for combined data over 10-year periods). Age-standardised rates have been presented where possible, to take into account the population age structures of different population groups.

Notifications of leptospirosis

Source: EpiSurv, ESR

Definition: Notifications of confirmed leptospirosis, excluding cases known to have been overseas during the incubation period, as they were unlikely to be infected in New Zealand. Notifications of diseases may underestimate the actual number of cases, as not everyone will go to a doctor when sick.