Division 4 Differential diagnosis

Preliminary differentiation

As it is important that control measures aimed at limiting the spread of the AI virus must be implemented as soon as possible, each national reference laboratory that has isolated a haemagglutinating virus must be in a position to identify it if it is an influenza A virus of H5 or H7 subtype or Newcastle disease virus. The haemagglutinating fluids must be used in haemagglutination inhibition tests as described in Chapter IX. Positive inhibition, such as a titre within 2 to 3 log₂ of a positive control, with polyclonal antisera specific for H5 or H7 subtypes of influenza A may serve as a preliminary identification enabling the imposition of interim control measures.

Confirmatory identification

Since there are 16 haemagglutinin subtypes and 9 neuraminidase subtypes of influenza viruses and variations occur within each of these it is neither practicable nor cost effective for each national reference laboratory to hold antisera that may allow a full subtype identification of influenza isolates. However, each national reference laboratory must as a minimum:

1. (i)

   confirm that the isolate is an influenza A virus using an immunodoublediffusion test to detect group antigens;

2. (ii)

   determine whether or not the isolate is of H5 or H7 subtype, positive identification requires implementation of the control measures for LPAI of H5 and H7 subtypes;

3. (iii)

   F1immediately undertake confirmation and full characterisation of all HPAI, H5 and H7 isolates.

In addition it is desirable in laboratories with appropriate facilities to:

1. (iv)

   carry out an intravenous pathogenicity index test in six-week-old chickens as set out in Chapter VII. Intravenous pathogenicity indices of greater than 1,2 indicate the presence of virus requiring a full implementation of control measures for HPAI.

National reference laboratories must also consider putting in place expertise and equipment that allow nucleotide sequencing of the haemagglutinin gene to determine whether or not there are multiple basic amino acids at the cleavage site of the haemagglutinin precursor protein for any LPAI H5 or H7 virus. F2...

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