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In 1998 there were 85,096 notifications to the National Notifiable Diseases Surveillance System; slightly lower than in 1997 (89,579). The number of measles cases remained low, and well below the number reported in the outbreak years of 1993 and 1994. Rubella notifications further decreased and remained low in 1998. The Measles Control Campaign from August to November 1998, did not impact significantly on the number of measles or rubella cases reported for 1998. Notifications of Haemophilus influenzae type b reached a record low since surveillance began in 1991, and appeared to have stabilised at a low rate since the introduction of the conjugated vaccine in 1992. The previously reported outbreak of pertussis in 1997 tapered off in early 1998. Foodborne disease, or detection of disease, appeared to be on the rise with an increase in notification rates of campylobacteriosis and salmonellosis. Notifications of hepatitis A decreased, correcting the previous high number of notifications in 1997. Sexually transmissible diseases (STDs) increased. Notifications for chlamydial infection were the highest for all sexually transmitted diseases and third highest for all notifiable diseases. Notifications of gonococcal infection also continued to rise and have doubled since 1991, whilst notifications for syphilis increased slightly after falling steadily over recent years. Arbovirus infections of concern in 1998 were dengue outbreaks in Far North Queensland and the first case of Japanese Encephalitis for mainland Australia, highlighting the importance of surveillance of arboviruses and vectors for their detection and management. Commun Dis Intell 1999;23:277-305.

Introduction

Surveillance of communicable diseases is an important public health activity. It allows the detection of outbreaks and the appropriate investigation and control measures to be instigated. It also allows for the monitoring of baseline trends and considers the impact and effectiveness of interventions to control the spread of diseases. Surveillance systems exist at national, state and local levels. State and local surveillance systems are crucial to the timely and effective detection and management of outbreaks and in assisting in the effective implementation of national policies. The national surveillance system combines some of the data collected from State and Territory-based systems to provide an overview at a national level. Specific functions of the national surveillance system include: detection and management of outbreaks affecting more than one jurisdiction; monitoring of the need for and impact of national control programs; guidance of national policy development; and description of the epidemiology of rare diseases for which there are only a few notifications in each State. It also assists in quarantine activities and facilitates agreed international collaborations such as reporting to the World Health Organization.

The National Notifiable Diseases Surveillance System (NNDSS) was established in its current form in 1991, under the auspices of the Communicable Diseases Network Australia New Zealand (CDNANZ). The CDNANZ monitors the incidence of an agreed list of communicable diseases in Australia and New Zealand; currently only Australian data are regularly published in Communicable Diseases Intelligence (CDI). This is achieved through the national collation of notifications of these diseases received by health authorities in the States and Territories. More than forty diseases or disease categories are included, largely as recommended by the National Health and Medical Research Council (NHMRC).¹ At present the list of notifiable diseases and categories is undergoing review and revision. Information collected on notifiable diseases has been published in the Annual Report of the NNDSS since 1991.^{2,3,4,5,6,7,8}

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Methods

Notifications of communicable diseases were collected by the States and Territories under their current public health legislation. These were collated and analysed fortnightly by the Department of Health and Aged Care and published on the Internet Website fortnightly and in CDI every four weeks. Final data sets for cases reported in 1998 were provided by the States and Territories by September 1999. Missing data and apparent errors were corrected where possible, and duplicate records deleted, in consultation with the States and Territories. For the purposes of the NNDSS, where a patient being treated in one jurisdiction was diagnosed in another, notifications were made according to the State or Territory of the diagnosing medical practitioner.

An established national data set included fields for: a unique record reference number; the disease; age, sex, Aboriginality; postcode of residence of the case; the date of onset of the disease and date of report to the State or Territory health authority; and the confirmation status of the report. Aboriginality was not included in the analyses due to incomplete reporting of this information.

Data are also collected and reported separately by organisations/groups on Haemophilus influenzae type b infection,⁹ tuberculosis^10,11 and non-tuberculosis mycobacterial infection, and HIV and AIDS surveillance.¹² Surveillance of gonococcal and meningococcal infections, typing of the organisms and antimicrobial susceptibility is coordinated by national programs.^13,14 National HIV and AIDS surveillance is conducted by the National Centre in HIV Epidemiology and Clinical Research.¹² Data from the Australian Childhood Immunisation Register (ACIR) were used to calculate vaccination coverage estimates for children aged 12 months and for children aged 24 months using a method described previously.¹⁵

Analyses were based on date of notification in 1998. The data included some notifications with onset dates before 1998, and excluded notifications with report dates in 1999 (even if the onset date was in 1998). For analysis of seasonal trends, notifications were reported by month of onset. Population notification rates were calculated using 1998 mid-year estimates of the resident population supplied by the Australian Bureau of Statistics. An adjusted rate was calculated where a disease was not notifiable in a State or Territory using a denominator which excluded that population. The data were analysed in Excel.

Maps were generated using Map Info based on the postcode of residence of the case and allocated to Australian Bureau of Statistics Statistical Divisions (Map 1). The two Statistical Divisions that make up the Australian Capital Territory were combined, as the population for one division is very small. Notifications for Darwin and the remainder of the Northern Territory were also combined to calculate rates for the Northern Territory as a whole. For South Australia, data for sexually transmissible diseases were combined for the whole State. In general, notification rates for Statistical Divisions were depicted in maps or discussed in the text only where the number of notifications was sufficiently large for these to be meaningful.

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Notes on interpretation

The notifications compiled by the NNDSS may be influenced by a number of factors that should be considered when interpreting the data. Due to under-reporting, notified cases are likely to only represent a proportion of the total number of cases which occurred. This proportion may vary between diseases, between States and Territories and with time. Methods of surveillance vary between jurisdictions, each with different requirements for notification by medical practitioners, laboratories and hospitals. In addition, the list of notifiable diseases and the case definitions may vary between jurisdictions.

Postcode information usually reflects the postcode of residence. However, the postcode of residence may not necessarily represent the place of acquisition or diagnosis of the disease, or the area in which public health actions were taken in response to the notification.

Duplication in reporting may occur if patients moved from one jurisdiction to another and were notified in both because checking between the State data sets was not possible. Data from those Statistical Divisions with small populations (Map 1) may result in high notification rates even with small numbers of cases.

Limitations of the currently collected data include the absence of risk factor information other than age, sex, and postcode of residence. Some additional risk factor information may be found from supplementary data sets reported separately such as for Haemophilus influenzae type b infection,⁹ tuberculosis and non-tuberculosis mycobacterial infection,^10,11 and HIV and AIDS.¹²

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Results - Surveillance notifications and reports

There was a total of 85,096 communicable disease notifications for 1998 (Table 1). The number of notifications was similar (slightly lower) compared with 1997 (89,576). Notification rates per 100,000 population for each disease by State or Territory are described in Table 2. Comparative data for 1998 and the preceding four years are shown in Table 3.

Data were missing in the field for sex for 0.9% notifications (775), age for 0.9% (807), and postcode of residence for 6.4% (5,445). The proportion of reports with missing data in these fields varied by State or Territory, and also by disease.

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Author affiliations

1. National Centre for Disease Control, Department of Health and Aged Care, PO Box 9848, Canberra, Australian Capital Territory, 2601

2. MAE Scholar, NCEPH, Australian National University, Canberra, 0200

3. National Centre for Immunisation Research and Surveillance of Vaccine Preventable Diseases, Royal Alexandra Hospital for Children, Westmead, New South Wales, 2124


This article was published in Communicable Diseases Intelligence Volume 23, No 11, 28 October 1999.