Surveillance testing during a velogenic Newcastle disease outbreak at Mangrove Mountain, NSW, during 1999 resulted in the isolation of a previously unidentified strain of Newcastle disease virus in the Somersby area.
This virus was isolated from a farm which was more than 3 kilometres from the nearest disease outbreak but which had been under closer surveillance because of a reported increase in daily mortalities.
The farm was a contract broiler grower operation. There were 4 sheds on the farm. Each shed had been stocked on 29 March 1999 with 23300 (sheds (1-3) or 23821 (shed 4) day old chickens per shed.
Daily mortality figures were reported for each shed by the grower. After the first fortnight, mortalities settled down to generally less than 20 per shed per day (Figure 1).
At day 29 daily mortalities began to rise, initially in sheds 1 and 2 (55 & 35 respectively) with shed 3 increasing on day 30. On day 32 approximately 10,000 birds were removed from each shed. Mortalities increased markedly in all sheds and then began to decline steadily in shed 1-3 but continued to increase in shed 4.
On day 45 all sheds were inspected by a Veterinary Investigations team. The birds in sheds 1 and 4 appeared healthy. The birds in shed 2 and to a lesser extent shed 3 were lethargic with an apparent increase in the number of birds "squatting". The grower reported a slight increase in coughing in shed 2. There were two birds in shed 2 seen with torticollis.
Histopathology on brains collected at the time showed varying degrees of inflammatory change with some perivascular cuffing but no changes strongly suggestive of Newcastle Disease.
Newcastle disease haemagglutination inhibition tests on blood samples collected at the time showed no seroconversion in shed 1, partial seroconversion in shed 2 (titres: 8x negative, 1x4, 1x8); strong seroconversion in shed 3 (titres 1x negative, 1x4, 2x8, 2x16,
2x32, 2x64); moderate seroconversion in shed 4 (1x negative, 3x4, 2x8, 4x16).
Daily mortalities for all shed declined to approximately 20 per day per shed.
Sheds 3 and 4 were emptied for processing on day 48 and no further investigations were undertaken.
Sheds 1 and 2 were processed on day 52. Inspection of the birds on day 51 did not reveal any apparent disease. Blood samples collected at the time showed moderate conversion in both sheds. Shed 1 had 2x negative, 1x2, 1x4, 2x8, 2x16, 1x32. Shed 2 had 1x2, 1x4, 3x8, 4x16, 1x32.
If the timing and strength of seroconversion relate to the time of infection in any shed it would appear that shed 3 was infected first, then shed 4, then sheds 1 and 2.
The clinical changes in shed 2 on day 45 together with the increase in seroconversion by day 52 would indicate that it is likely that a Newcastle disease virus was active in the shed at the time. The lack of observable changes and the level of seroconversion in sheds 3 and 4 would indicate that they had been infected in the past.
No increased mortality was observed during the clinical episode in shed 2. This indicates that the virus is essentially avirulent.
The increase in mortalities in shed 4 at about the time that the infection is likely to have been active in the shed could be due to the stress of thinning. This stress increased the mortalities in all sheds, but in shed 4 it would seem to have increased the effects of the infection to the extent that the mortalities increased for several days.