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Differential diagnosis of negated avian notifiable disease cases in Great Britain


This focus article has been prepared by David Welchman, veterinary lead of the APHA Avian Expert Group, Rowena Hansen, veterinary lead for avian virology at APHA Weybridge, and Alex Schock, team leader of the diagnostic and consultant avian pathology services at APHA Lasswade

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Legislation requires that, in Great Britain, any suspicion of the two notifiable avian diseases of poultry, avian influenza and Newcastle disease, must be reported to the APHA. Suspicion of disease may be based on suggestive clinical signs in poultry or gamebirds, or changes in production; for example, reduced egg production in laying hens or changes in feed or water intake in broilers.

The clinical signs and pathological findings associated with notifiable avian disease (NAD) have been summarised by Irvine.1-3

Not all suspect cases will be confirmed as notifiable disease because clinical and pathological findings in NAD are not patho-gnomonic. Some cases are negated on clinical grounds by the investigating APHA veterinary inspector, and others are negated following official testing of statutory samples.

The numbers of NAD report cases in poultry and gamebirds received between 2014 and 2018, and the outcome of the investigations, are shown in Table 1. In the majority of negated cases, a diagnosis is neither achieved nor recorded as part of the outcome of the case.

Table 1:

Notifiable avian disease (NAD) report cases in Great Britain and their outcome, 2014–2018

Testing to exclude

Since 2014, the option of testing for exclusion of NAD has been available for chickens and turkeys in Great Britain.4 This applies in cases where there are insufficient grounds for suspecting NAD but where NAD cannot be excluded from the differential diagnosis.

The service allows samples to be submitted to APHA Weybridge if, after discussion with the APHA, it is agreed that NAD is not suspected and is very low on the private veterinary surgeon’s differential diagnosis list. This may lead to notifiable disease either being negated or detected within the affected flock.

In the five years of the scheme, 31 cases have been tested, three of which have been confirmed as NAD.

Differential diagnosis

There is now a new option available to private veterinarians when suspect NAD cases have been negated. This is for the APHA to conduct differential diagnostic testing in an attempt to ascertain the cause of the clinical signs or production loss in the flock.

The differential testing can be undertaken on further samples (such as additional postmortem examinations) taken from the flock, once any statutory restrictions that may be in place on the premises have been lifted, or on existing samples.

Postmortem examinations of fresh or frozen carcases can be undertaken at APHA Veterinary Investigation Centres, APHA Lasswade or at partner postmortem examination provider sites and encompass a full range of diagnostic tests.

Differential diagnostic testing can also be undertaken on swabs or carcase material submitted to APHA Weybridge as part of statutory testing, which may have the advantages of greater timeliness of sampling in relation to when the clinical signs were seen in the birds, and avoidance of the biosecurity risk of samples infected with a NAD being stored outwith a containment laboratory. The scope of this testing will be more limited but can include virus detection techniques (and histopathology if tissues are available).

The purpose of this differential testing is to achieve a diagnosis, allowing appropriate veterinary advice to be given to address the issue in the flock. It also enables a greater understanding of the disease conditions that can present as suspect NAD, and is a potential means of detecting new or re-emerging disease conditions that may pose a threat to poultry in Great Britain.

Further information

More detail on the testing for exclusion scheme is available on the APHA Vet Gateway at

Private vets wishing to use the service for the differential diagnosis of negated NAD cases can contact the veterinary lead of the APHA Avian Expert Group via SIU{at}

This service is applicable to all poultry and gamebird species and will be chargeable at the APHA’s Disease Surveillance Test price list rate (see


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