The effect of climate variation on infectious diseases in humans in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Masters in Veterinary Studies in Epidemiology at Massey University, Palmerston North, New Zealand

Abstract

The emergence and spread of infectious disease are a major issue associated with environmental change. Contributing to this is the effect climate variability and change may play in altering disease risk. The aims of this study were to investigate the association between climate and infectious diseases in humans throughout New Zealand from 1997 - 2007, then use the identified associations to project the burden of disease in 2015, 2040 and 2090 with respect to future climate change scenarios. The four infectious diseases selected and investigated were campylobacteriosis, cryptosporidiosis, influenza hospitalisations and meningococcal disease. The association of weather variables and other confounders with the incidence risk (IR) of disease were explored using a quasi-Poisson generalized linear model, indicating that weather variables were significantly associated with disease risk. These results, along with expert opinion on epidemiological plausibility, were used to select confounders for the past association models. Climate variation was associated with the IR of campylobacteriosis and cryptosporidiosis in New Zealand from 1997 - 2007. Campylobacteriosis notifications were found to be positively associated with the weekly absolute humidity. Additionally, campylobacteriosis notification risk factors were increasing beef and dairy density, intermediate and poor drinking water quality, being over 65 years of age and identifying with Pacific Island or Asian ethnicity. Protective factors were being less than 4 years of age and identifying with Maori ethnicity. Cryptosporidiosis notifications were found to be positively associated with the weekly average temperature and negatively associated with the weekly average rainfall. Risk factors for cryptosporidiosis notifications were poor drinking water quality, being less than 4 years of age and living in rural areas. Protective factors were identifying with Maori ethnicity and unknown drinking water quality. Influenza hospitalisations and meningococcal disease notifications were not significantly associated with past climate variation. Identifying with Maori ethnicity was found to be a risk factor for influenza hospitalisations, with no protective factors identified. Risk factors for meningococcal disease notifications were an increasing social deprivation index (SDI) score, being less than 4 years of age and identifying with Maori ethnicity. Identifying with Asian ethnicity was a protective factor for meningococcal disease. The projection calculations of the change in disease incidence from the study period to 2015, 2040 and 2090 were carried out under a combination 3 Intergovernmental Panel of Climate Change (IPCC) climate scenarios (A2, B1 and A1B) and 12 downscaled global climate models. The projected change in campylobacteriosis and cryptosporidiosis suggested increases in the rate of notifications, and a small to no decrease in influenza hospitalisation and meningococcal disease notifications.