A. Avian Rhinotracheitis (ART)

Figures 3.13 and 3.14 show the pattern of mean titres according to broiler age for one test which has been available for some years (CVS) and a second which only became available in mid-1994 (Svanova). The fact that the regression line for the CVS test is very low is a reflection of a high proportion of negative results. If ART is a significant component of respiratory disease in broilers we would expect a definite trend towards rising titres at the end of the grow-out. While there is a significant proportion of strongly positive groups of samples in this series the large proportion of negatives suggested that ART was, at best, only a minor component of the problem. Some of the sera tested at 14 days of age were quite strongly positive. Some of these reactions seemed to be too strong to be of maternal origin.

When the Svanova test became available we took the opportunity to test 20 groups of broiler sera which had been negative to the CVS test. All except 1 group was positive on the new test. Mono-specific SPF sera for IB and ND were negative in the test. Similar findings were obtained with broiler parent sera (see accompanying case history number 4). Most of the data represented in Figure 3.14 is derived from routine submissions received from July 1994 onwards. There are many sero-negative groups and overall 43% of individual sera are negative. However there is clear evidence that a response to ART virus is detectable by this test and that it increases with broiler age. This supports the view that ART is a significant part of current broiler respiratory disease problems.

Up to the end of 1993 the CVS kit had been identifying a high proportion of groups of sera as sero-positive. Some negative groups were associated with broiler production areas whose staff were adamant that there were definite signs of "Swollen Head Syndrome" in the birds. The frequency of positive groups of sera from broilers seemed to decrease drastically from early Spring 1994. In order to demonstrate this the mean titres from all non-negative groups is plotted against year/month in Figure 3.15.

Live vaccination against ART became fairly widely available under ATC at about the time we stopped seeing positives with the CVS test. In most cases the vaccine is administered by coarse spray at 10 - 14 days of age. We have had little success in demonstrating a serological response to this vaccination when samples are tested at 28 days of age.

These results suggest that the introduction of live vaccination may have resulted in an antigenic shift in the population of field ART viruses. The serological reaction to the current field strains seems to be poorly detected by one commercial kit (CVS). The second kit seems to be able to detect a response both to the "classic" ART challenge and to the current field challenge. Neither kit has been very effective in detecting a response to live field vaccination.