Australia's notifiable diseases status, 2002: Annual report of the National Notifiable Diseases Surveillance System - Introduction/Methods

Introduction

Surveillance of communicable diseases is vital to the control of communicable diseases, to identify and assess the relative burden of diseases and to monitor trends over time. It is also required for the guidance of policy making.

Communicable disease surveillance in Australia exists at the national, state and local levels. Primary responsibility for public health action lies with the state and territory health departments and with local health authorities.

The role of communicable disease surveillance at a national level includes:

• identifying national trends;
• guidance for policy development at a national level and resource allocation;
• monitoring the need for and impact of national disease control programs;
• coordination of response to national or multi-jurisdictional outbreaks;
• description of the epidemiology of rare diseases, that occur infrequently at state and territory levels;
• meeting various international reporting requirements, such as providing disease statistics to the World Health Organization (WHO), and;
• support for quarantine activities, which are the responsibility of the national government.

Methods

Australia is a federation of six states (New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia) and two territories (the Australian Capital Territory and the Northern Territory). State and territory health departments collect notifications of communicable diseases under their public health legislation. The Australian Government Department of Health and Ageing (DoHA) does not have any legislated responsibility for public health apart from human quarantine. States and territories have agreed to forward data on a nationally agreed set of communicable diseases to DoHA for the purposes of national communicable disease surveillance.

Fifty-five communicable diseases (Table 1) agreed upon nationally through the Communicable Diseases Network Australia (CDNA) are reported to the National Notifiable Diseases Surveillance System (NNDSS). The system is complemented by other surveillance systems, which provide information on various diseases, including some that are not reported to NNDSS.

Table 1. Diseases notified to the National Notifiable Diseases Surveillance System, Australia, 2002*

* Jurisdictions may not yet have been reporting a disease either because legislation had not yet made that disease notifiable in that jurisdiction, or because notification data for that disease were not yet being reported.

† In the Australian Capital Territory, infections with Murray Valley encephalitis virus and Kunjin virus are combined under Murray Valley encephalitis.

NEC Not elsewhere classified.

The national dataset included fields for unique record reference number; notifying state or territory; disease code; age; sex; Indigenous status; postcode of residence; date of onset of the disease; and date of report to the state or territory health department. Additional information was available on the species and serogroups isolated in cases of salmonellosis, legionellosis, meningococcal disease and malaria, and on the vaccination status in cases of childhood vaccine preventable diseases. While not included in the national dataset, additional information concerning mortality and specific health risk factors for some diseases was obtained from states and territories. The Australian Institute of Health and Welfare supplied hospital admission data for the financial year 2001-02.

Notification rates for each notifiable disease were calculated using 2002 mid-year resident population supplied by the Australian Bureau of Statistics (Appendix 1). Where diseases were not notifiable in a state or territory, adjusted rates were calculated by excluding the population of that jurisdiction from the denominator. As in previous years, we report age-standardised notification rates of sexually transmitted infections (STIs) in Indigenous and non-Indigenous Australians based on data from the Northern Territory, South Australia and Western Australia.

The geographical distribution of selected diseases was mapped using MapInfo software. Maps were based on the postcode of residence of each patient aggregated to the appropriate Statistical Division (Map 1). Rates for the different Statistical Divisions were ordered into six groups -the highest value, the lowest value above zero, those equal to zero, and the intermediate values sorted into three equal-sized groups. The two Statistical Divisions that make up the Australian Capital Territory and the Northern Territory were combined to calculate rates for each territory as a whole.

Information from communicable disease surveillance is disseminated through several avenues of communication. Fortnightly teleconferences of the Communicable Diseases Network Australia provide the most up-to-date information on topics of immediate interest. The Communicable Diseases Intelligence (CDI) quarterly journal publishes surveillance data and reports of research studies on the epidemiology and control of various communicable diseases. The Communicable Diseases Australia website publishes disease surveillance summaries from the NNDSS. The annual report of the NNDSS, Australia's notifiable diseases status, provides yearly summaries of notifications.

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Notes on interpretation

The present report is based on 2002 'finalised' annual data from each state and territory. States and territories transmitted data to DoHA each fortnight and the final dataset for the year was agreed upon in July 2003. The finalised annual dataset represents a snap shot of the year after duplicate records and incorrect or incomplete data have been removed. Therefore, totals in this report may vary slightly from the totals reported in CDI quarterly publications.

Analyses in this report were based on the date of disease onset in an attempt to estimate disease activity within the reporting period. Where the date of onset was not known however, the date of presentation to a medical practitioner or date of specimen collection, whichever was earliest, was used. As considerable time may have lapsed between onset and report dates for hepatitis B (unspecified) and hepatitis C (unspecified) notifications, these were analysed by report date.

Under-reporting is an important factor that should be considered when interpreting NNDSS data. Figure 1 shows the steps necessary for an episode of illness in the population to reach the NNDSS. Each step contributes to under-reporting resulting in only a proportion of notifiable diseases reaching the surveillance system. Due to under-reporting, notified cases can only represent a proportion (the 'notified fraction') of the total incidence. Moreover, the notified fraction varies by disease, by jurisdiction and by time.

Figure 1. Communicable diseases notification fraction

Methods of surveillance can vary between states and territories, each with different requirements for notification by medical practitioners, laboratories and hospitals. Some diseases were not notifiable in some jurisdictions (Table 1). The case definitions for surveillance vary among jurisdictions. In addition, changes to surveillance practices may be introduced in some jurisdictions and not in others, making comparison of data across jurisdictions difficult. To inform the interpretation of data in this report, states and territories were asked to report any changes in surveillance practices including changes in case definition, screening practices, laboratory practices, and major disease control or prevention initiatives undertaken in 2002.

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Postcode information usually reflects the residential location of the case, but this does not necessarily represent the place where the disease was acquired. As no personal identifiers are collected in NNDSS, duplication in reporting may occur if patients move from one jurisdiction to another and were notified in both.

The completeness of data in this report is summarised in Appendix 2. The patient's sex was not stated in 0.5 per cent of notifications (n=468) and patient's age was not stated in 2.3 per cent of notifications (n=3,268). Indigenous status was reported for 41.9 per cent (n=54,243) of notifications nationally. The proportion of reports with missing data in these fields varied by state and territory and by disease.

Discussions and comments of CDNA members and state and territory epidemiologists have informed the present report and their contribution to the accuracy of these data is gratefully acknowledged.