Epidemiology of Giardia infection in New Zealand and the risk in children

Abstract

Background: Giardia is a leading cause of human gastrointestinal illnesses globally and is the most commonly notified waterborne disease in New Zealand. The national incidence rate of 46.6 per 100,000 is thought to be one of the highest among developed countries, peaking in the 1-4 and the 25-44 age groups. Risk factors for infection among vulnerable groups have not been explored systematically in New Zealand, although environmental factors and person-to-person transmission have been suspected. The true burden of Giardia infecion in the community has been difficult to estimate due to suspected under-notification of the disease. An important component of disease surveillance and the validation of disease incidence rates is an estimation of the degree of undercount. AIMS: To describe the epidemiological patterns of Giardia infection in the Auckland region and in New Zealand, compare them with local and international patterns of infection, and explore environmental links. To estimate the level of completeness of giardiasis notification in the Auckland adult population by using a simple capture-recapture method. To identify risk factors for giardiasis among Auckland children under 5 years of age. Methods: Analysis of Auckland data: Anonymised giardiasis notification data from Auckland Regional Public Health Services (ARPHS) for the period of July 1996 to June 2000 were analysed by person, place and time. Infection rates and relative risks were calculated and compared with national and international information. Analysis of New Zealand data: A study of national surveillance data utilised anonymised information for 7818 notified cases throughout New Zealand between July 1996 and June 2000. A weighted average of drinking water grades was estimated using the Community Drinking Register. Pearson's coefficient was used to measure the correlation between average drinking water grades and notified cases. Daily climate data were plotted against daily case notifications and modelled, using Poisson's regression, to predict any influence of climate on infection. Data were presented by age, gender, ethnicity and area using statistical and spatial methods. Estimation of under-notification: The capture-recapture technique is now being used in many countries to evaluate the completeness of disease ascertainment. Comparison of disease ascertainment involves two or more datasets. Two independent datasets of giardiasis cases aged 15 years or over were generated. Of them one was generated from the demographic information of cases recruited during the Auckland Giardiasis Study in 1998-99 and the other from giardiasis cases notified to the ARPHS for the same period of time. The area of residence of cases was geo-coded, mapped and overlaid by water distribution zones. Cases were matched and under-notification was estimated using a two-sample capture-recapture method. Case-control study: A case-control methodology was used to analyse the exposure history of 69 cases and 98 controls under 5 years of age in Auckland. Significant risks of infection were estimated and their attributable risks. Results: Giardiasis in Auckland: Auckland had a significantly higher rate of Giardia notification (58/100,000) than New Zealand (46/100,000) as a whole. Notification rates, which peaked during February-May, were significantly higher in Pakeha/Europeans and Asian/others, compared with Maori/Pacificans. Adjusted notification rates were higher for residents of North Shore and Auckland cities than for other areas of Auckland. The crude regional and national notification rates were almost 6 times the rate of laboratory identification of positive isolates in the UK and 4 times US reported rates. Giardiasis in New Zealand: At the national level, most cases occurred in the 1-4 year age group followed by the 25-44 year age group, and most cases were Pakeha/European. Ethnicity was not known for 18% of cases, affecting demographic calculations. Infection rates were high for a number of Health Districts (West Coast, Wanganui, Waikato and Tauranga) compared to the national average. Over 50% of the population received Aa-graded drinking water. No correlation between infection and the weighted average grades for water treatment plants (r = -0.12) or the reticulation systems (r = -0.11)) was found. A significant correlation with the mean daily maximum (r = 0.05) and minimum (r = 0.06) temperature was observed. Poisson's regression modelled minimum-temperature (chi2 = 5.40, p<0.05) and relative humidity (chi2 = 5.37, p<0.05) as predictors of a significant number of Giardia infections on a given day. Under-notification of giardiasis: The estimation of under-notification during the 12 month period, compared 413 cases who were notified to the ARPHS with 199 cases who had participated in a case-control study over the same period of time. North Auckland had slightly higher notification rates and also study participation rates. The giardiasis notification rate was higher in un-reticulated water zones (72.4/100,000 population). The capture-recapture calculation indicated that only 49% of cases were notified. Risk of giardiasis in children: In the case-control study, 95% of cases and 86% of controls used water from the Auckland Metropolitan Mains (AMM) supply for domestic purposes, 44 cases and 42 controls swam, and 59 cases and 54 controls wore nappies. Children wearing nappies were at significantly increased risk of the disease (OR=3.0, 95%CI 1.01-8.9), as were those from households which had more than one child wearing a nappy (OR=6.5, 1.8-23.4). The AMM water supply was associated with a reduced giardiasis risk compared to other drinking water sources. Significantly increased risks were associated with drinking water consumed away from home (OR=4.7, 2.2-10.1), swimming at least once a week (OR=2.4, 1.1-5.3) and travelling in side New Zealand (OR=2.5, 1.03-6.0). Conclusions: The higher rates of giardiasis observed in Auckland and New Zealand, in comparison with other developed countries, may be related to environmental or social factors. Missing ethnicity information precludes clear interpretation of variations in notification rate by ethnic group and suggests a need for improvement in data collection. There are opportunities to investigate the influence of risk factors on seasonal changes in notification rates both locally and nationally. Reported improvements in some areas could be due to local health measures or to random variation. Time-trend analysis suggests a seasonal pattern. The weighted average drinking water grading is a novel and readily available measure, and may not be truly representative of local supplies. Modelling of climate data showed an association with giardiasis but was inconclusive. Giardia notifications in Auckland are under-notified by half. This has obscured the true burden of Giardia infection. This has implications for estimates of the burden of disease in New Zealand. Notification rates vary inversely with socio-economic conditions and the presence of a reticulated water system. The case-control study identified vulnerable groups and modifiable risk factors for Giardia infection. Nappy-wearing was an independent risk factor for infection. Further study is advocated to ensure better protection of public health, especially for children. These studies identified vulnerable groups and major data-gaps. The risk of infection in children needs further attention for effective infection control measures to be developed for this disease. Recommendations for improvements in disease surveillance and data quality are discussed. GIS (Geographical Information System) is a useful tool for disease monitoring. Capture-recapture techniques are useful tools in evaluating the completeness of disease surveillance.