Australian notifiable diseases case definitions - Appendices C and D

These appendicies to the Australian national notifiable diseases case definition document, detail the epidemiological linkage of cases and a notice regarding detection of IgGs.

Page last updated: 24 December 2003

Appendix C - Notice regarding detection of IgGs

Wherever possible when a serological diagnosis is made, recent infection should be shown to have occurred by demonstrating a significant change in IgG between acute and convalescent sera. It is particularly important for infections which either fail to produce a measurable IgM response (eg influenza) or where the IgM response persists for extended periods (eg flavivirus infections). Usually an interval of 10-14 days is sufficient though for some infections (eg legionellosis) the antibody may rise may take up to 4-6 weeks. Significant changes in IgG may be shown by either:

• Seroconversion: Change from IgG negative to IgG positive between acute and convalescent samples. This may be used for confirming recent infection using tests that do not quantify the antibody levels. That includes most enzyme-linked immunosorbent assay, particle agglutination, immunofluorescent antibody and latex agglutination tests as performed routinely.

• Significant increase in antibody level or titre: This is generally confined to tests which use titrations in two-fold dilutions, in which a four-fold increase is regarded as significant. For enzyme-linked immunosorbent assays that are not titred, it may be possible to establish changes in absorbence that may be regarded as significant.


Appendix D - Epidemiological linkage

General description of an 'epidemiological link'

An epidemiological link is established when there is:

1. Contact between two people involving a plausible mode of transmission at a time when:

a) one of them is likely to be infectious;

AND

b) the other has an illness which starts within the incubation period after this contact;

AND

2. At least one case in the chain of epidemiologically linked cases (which may involve many cases) is laboratory confirmed.

Notes and examples of epidemiological linkage

• To be notified, epidemiologically linked cases must also satisfy the clinical criteria.

• Cases may be identified/reported in a different order to that in which they became ill.

• If the linked case became ill after the laboratory confirmed case, then the link is prospective (Figure A). If the linked case became ill before the laboratory confirmed case then the link is retrospective (Figure B). A chain of epidemiologically linked cases is established when further cases are either retrospectively or prospectively linked to those already linked to the laboratory confirmed case (Figure C).

Figure A: Prospectively linked case

Laboratory
confirmed case

Epidemiologically
linked case

Time

If the linked case became ill after the laboratory confirmed case, then the link is prospective (Figure A)


Figure B: Retrospectively linked case

Laboratory
confirmed case

Epidemiologically
linked case

Time

If the linked case became ill before the laboratory confirmed case then the link is retrospective (Figure B)



Figure C: Chain of epidemiologically linked cases (example)*

 

 

 

A chain of epidemiologically linked cases is established when further cases are either retrospectively or prospectively linked to those already linked to the laboratory confirmed case (Figure C).
 Time

 

* One case in the chain needs to be laboratory confirmed

E= exposure period, I= infectious period





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