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This article was published in 1998
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R McKinnon, DV Tamworth


An outbreak of virulent avian influenza occurred on three properties near Tamworth, New South Wales during November/December 1997.

Avian influenza (Al) is a type A virus of the family Orthomyxoviridae. The virus is further categorised into subtypes according to antigens of the Haemagglutin (H) and Neuraminidase (N) projections on the surface. There are 14 H and 9 N subtypes. All highly pathogenic Al viruses have been of the H5 & H7 subtypes.

The host range includes all domestic poultry, many species of wild birds and seabirds. In many of the wild birds and seabirds infection is subclinical. Domestic fowl and turkeys are highly susceptible.

This is the fifth outbreak in Australia:-

1976 Melbourne H7N7
1985 Bendigo H7N7
1992 Bendigo H7N3
1994 Lowood H7N7
1997 Tamworth H7N4

AI is highly contagious by direct contact such as contact with faeces, nasal or ocular discharge or other fomites. The disease has an incubation period of a few hours to two or three days. Wildbirds are incriminated as the main spreader of the infection and being the reservoir of infection. Many of the strains are non-pathogenetic or only mildly pathogenetic to poultry. However a virulent strain may emerge by reassortment of less virulent strains or by passage through susceptible species.

The clinical signs can be highly variable from sudden death to text book cases. Basically birds are depressed with a profuse watery diarrhoea. The combs and wattles may be cyanotic and swollen with haemorrhages on unfeathered areas. In the Tamworth outbreak the birds mainly showed signs of watery diarrhoea. As time progressed the birds became depressed with an escalating death rate with some birds exhibiting swollen combs and wattles. Pancreatic necrosis was the main postmortem feature (see Table 1).


On approximately the 16th November an increase in daily mortality rate from 0.25% to 1.3% on a broiler breeder farm was noticed. A bacterial infection was suspected initially with AI being suspected as the mortality rate increased from the 21/11/97 to 24/11/97. This was confirmed on the 25/11/97 by the Australian Animal Health Laboratory (AAHL). The mortality rate in affected sheds ranged from about 40% to 100% over about seven days (see table 3). Table 2 reflects the age structure of the farms.

Table 1: Clinical and postmortem signs from IP1 early (24/11/97), IP2 and IP1 late (26/11/97).
Textbook IP1 early IP2 IP1 late
sudden death sudden death
soft shelled eggs
combs and wattles cyanotic and swollen small % bluish combs combs and wattles swollen
profuse watery diarrhoea watery diarrhoea watery diarrhoea
haemorrhages unfeathered areas
depressed birds depressed birds
ecchymotic haemorrhages throughout the body haemorrhagic tracheitis in some haemorrhagic tracheitis haemorrhagic tracheitis
subcutaneous oedema head, hocks some birds perihepatitis haemorrhages throughout body subcutaneous oedema of head
yellow or grey necrotic foci may be in spleen liver, kidneys, lungs oedematous lungs and viscera fibrinous peritonitis
necrotic lesions in pancreas
flaccid ovaries
ruptured yolks
peracute die with minimal signs
Table 2: Farm structure. There were four farms each having 4 sheds with each shed allocated approximately 8000 birds. Water source was river water.
SHEDS 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
AGE (WKS) 93 92 - 23 23 25 24 23 81 58 57 57 41 41 40 40

Following the confirmation of virulent AI, Commonwealth and State Chief Veterinary Officers agreed that an eradication program would be implemented in accordance with the Australian Veterinary Emergency Plan (AUSTVETPLAN). NSW Agriculture was the lead agency coordinating operations with the assistance of Rural Lands Protection Boards and local emergency operations organisations.

At the start of the outbreak, a 3 km restricted zone and 10 km control zone were declared around the outbreak area restricting movement of poultry and poultry products. All poultry flocks within this area were closely monitored for disease via post mortems of dead birds or serology. Two further infected properties were identified early in December, as a result of intensive surveillance in the area.

All birds on the infected premises were destroyed and buried, litter was removed and buried, and sheds thoroughly cleaned and disinfected. The last of the infected birds was destroyed on the 13/12/97 and decontamination was completed in early January. After decontamination the sheds were left empty for a further 21 days. Surveillance is being carried out on these properties for a further 6 months to ensure that the virus has been successfully eliminated.

H7N4 virus was isolated from all three properties however on one farm the virus had a lower pathogenicity index than the other two.

A total of 310,000 broiler breeders and progeny, 1.2 million fertile eggs, 261 emus and 147 emu eggs were destroyed. All direct costs of eradication will be met under the Commonwealth-States Cost Sharing Agreement for eradication of certain exotic diseases. The laboratory processed 2816 serums, 2577 pancreas and cultured 1687 swabs.

AI probably came in with migratory birds, how the farms were infected is a source of conjecture. All we can say in confidence is that we had it, we got rid of it and we don't want to see it again. The main emphasis for these industries is to avoid contact with wild birds at all cost including securing water and feed sources along with walk ways.

There were many problems encounted at the beginning before the Local Disease Control Centre was set up these included:-

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