Evaluating the Environmental and Social Determinants of Enteric Disease in New Zealand

Abstract

Globally, enteric disease is a leading cause of illness and death. Projected climate change and agricultural intensification is expected to substantially influence zoonoses emergence and resurgence. New Zealand has high rates of enteric zoonoses, intensified livestock production and a considerable impact of regional climate on local weather. This thesis investigates the influence of climate variability, livestock density and social and demographic factors on four important enteric diseases in New Zealand: campylobacteriosis, salmonellosis, cryptosporidiosis and giardiasis.

Section I presents a conceptual framework that describes how linkages between environmental and social factors drive zoonotic disease transmission. In New Zealand, a review of the published literature emphasises the significance of environmental change for enteric zoonoses. A description of reported and hospitalised cases show marked demographic and social variations in disease burden.

Section II focuses on identifying potential risk factors through a detailed evaluation of temporal patterns in these diseases. The Gini-coefficient is used to establish temporal variation in disease incidence patterns globally. A novel seasonality index shows regionally consistent, summer peaks for campylobacteriosis and salmonellosis. Cryptosporidiosis displays spring peaks in in predominantly agricultural countries and in summer elsewhere, whereas giardiasis shows little seasonality. In New Zealand, salmonellosis incidence is positively associated with La Niña linked warmer wetter conditions, while cryptosporidiosis is positively related to El Niño linked cooler, drier conditions. A nonlinear and temporally lagged model describes the positive, but complex association between runoff rate and disease risk.

Cryptosporidiosis and giardiasis are recognised by the WHO as ‘Neglected Diseases’. Section III examines the importance of environmental and social factors in influencing spatial and temporal patterns in these two diseases. Cryptosporidiosis incidence rates and spatial relative risk patterns indicate higher risk in rural areas, with areas dominated by livestock showing recurrent disease clusters in spring. Multivariate space-time analyses show that while dairy cattle density, rural living and being less than four years old are significant risk factors, high population density and temperature appear protective. Giardiasis patterns demonstrate minimal seasonality and negligible associations with climatic variables, with disease clusters detected in urban areas. Multivariate analyses identify rural living and being less than four years old as significant risk factors while dairy cattle density appears protective, although the inter-relationships between rurality and livestock densities makes the interpretation of these findings complex.

Overall, in New Zealand, the influence of climate variability and livestock intensification on enteric zoonotic disease patterns is strongly modulated by human population characteristics. This relationship suggests that projected climate change and agricultural intensification are likely to have a disproportionate impact on vulnerable populations. Cross disciplinary research will be crucial to improve public health responses to projected environmental change. This work may be applied to understand the implications of environmental change for other diseases in New Zealand, and elsewhere.