Syphilis, chlamydia and gonorrhoea surveillance in New Zealand

Syphilis, chlamydia and gonorrhoea surveillance in New Zealand 

Purpose of STI surveillance:

Surveillance is the on-going systematic collection, analysis and interpretation of outcome-specific data for use in the planning, implementation and evaluation of public health practice [1]. Surveillance is an important part of the strategy to reduce the short and long term burden of sexually transmitted infections [2]. New Zealand’s STI surveillance system has five identified purposes [3]:

  • to understand the burden of disease (as an input to planning, policy development, prioritisation and resource allocation),
  • to monitor inequalities in the burden of disease between population groups,
  • to monitor trends in the burden of disease over time,
  • to identify emerging problems, and outbreaks or clusters of disease,
  • to evaluate the effectiveness of policies and programmes.

Since 4 January 2017 syphilis (infectious and congenital) and gonorrhoea have been notifiable on an anonymised basis in New Zealand.

Laboratory-based surveillance for chlamydia and gonorrhoea

Laboratory-based surveillance of gonorrhoea and chlamydia gradually expanded from reporting by a few laboratories in 1998-2004 to all but one laboratory in 2013 and all laboratories reporting from 2015.

Since 2009, improvements to laboratory surveillance reporting standards have enabled the reporting of population based rates of chlamydia and gonorrhoea for many DHBs as well as estimates of national rates.

Since 2013, ESR has worked with laboratories to extend surveillance to enable collection, analysis, and reporting on testing and positivity rates by different key population characteristics including ethnicity.

Laboratories submit data monthly on all patients who are tested via a secure SharePoint portal website. Information collected is limited to demographics and test results.

Syphilis surveillance

From 2013, ESR commenced enhanced syphilis surveillance to collect demographic characteristics, sexual behaviours, and other risk factors for infectious syphilis cases. This system was limited to SHCs until late 2018 when other diagnosing health practitioners were also expected to complete questionnaires to provide the key information described above.

Analytical methods:

Quarterly dashboard updates are based on data submitted at least two weeks before dashboard publication. Any data submitted after this point will be reflected in subsequent quarterly updates to the dashboard.

Calculations for ethnicity use the Ministry of Health prioritised ethnicity definition.(external link)

Case counts

For laboratory based surveillance rates, the numerator is the total number of laboratory-confirmed cases reported after exclusion of repeat tests for an individual within a defined episode period for the specific disease.

For infectious syphilis surveillance, the case counts are those cases classified as confirmed or probable based on the surveillance case definition. (external link)

Where there is insufficient information provided, ethnicity is reported as Unknown.

Rate calculation

Caveats for interpreting rates:

-       Rates will not be presented where there were fewer than five cases in any category due to instability in these rates.

-       We advise dashboard viewers to consider the absolute number of cases that make up the numerator (see case counts) of any rate being interpreted.

  • With few cases in a numerator, rates could change dramatically based on differences of 1-2 cases. Rates with fewer than 20 cases in the numerator should be interpreted and compared to other rates with caution.
  • Sometimes, the highest rates involve few cases and therefore a relatively small proportion of the overall disease burden for New Zealand.

Laboratory-based surveillance rates use the applicable mid-year population estimates published by Statistics New Zealand as the denominators. Population estimates are downloaded from the Statistics New Zealand website(external link) annually and include estimates stratified by age, sex, and ethnicity (prioritised based on Ministry of Health definitions(external link)).