M.J. Jolly, J.R. Roberts and W.Ball
University of New England,
Armidale, NSW
Australia
The eggshell quality of vaccinated hens challenged with infectious bronchitis (IB) virus in lay at 41 weeks of age was found to decline. These effects were observed in hens vaccinated only during the growth phase and not revaccinated during the lay period. T strain virus was found to have a more prolonged impact while the antigenically-different N1/88 strain produced a greater decrease in egg quality parameters. There was an unexpected increase in internal quality of the virus-exposed groups during the challenge period. While the genetic strain of the hen had no significant effects on the response to challenge in terms of egg quality, there were inherent strain differences in egg quality unrelated to the challenge and these are briefly discussed.
Introduction
IB virus has been widely reported as a cause of dramatic declines in egg production and egg quality of laying flocks. The egg quality impacts usually associated with IB infection include a lightening of the shell colour, an increased number of broken eggs and a thinning of the albumen (Cavanagh and Naqi, 1997). All of these potential effects would have a negative impact on the commercial viability of an infected laying flock. Consequently all commercial flocks in Australia are vaccinated against IB virus. The trial described here is one component of a larger project aimed at identifying the impact of IB on the egg quality of vaccinated laying hens and determining the optimum vaccination protocol for laying hens. The current trial was designed to investigate the impact of challenge with two antigenically-different (Wadey and Faragher, 1981; Ignjatovic and McWaters, 1991; Ignjatovic et al., 1997) Australian strains of IB virus on four different genetic strains of commercial laying hen that had not been vaccinated beyond the growth phase.
Methods
Sixty-four pullets of each of four commercial strains of laying hen were obtained from commercial hatcheries as day old chickens. The birds were raised on the floor until they were 14 weeks of age, at which time they were transferred to wire laying cages in large isolation sheds. The birds were vaccinated against IB virus at day old, 4 weeks and 14 weeks with a commercial vaccine administered by coarse spray. One or both vaccines of serotypes C and B were used to produce four vaccination treatment groups that were equally distributed throughout strain and challenge groups. Data for the vaccination treatment group is still being analysed and will be presented at a later date.
At 41 weeks of age (peak of lay) the birds were divided into three groups. One group of 128 birds was inoculated by eye drop with the serotype C (Wadey and Faragher, 1981) T-strain IB virus; the second group (118 birds) received the serotype L (Ignjatovic and McWaters, 1991) N1/88 IB virus via the same route and the last group (128) birds was not challenged (the control group). Both viruses were obtained from Dr. Jagoda Ignjatovic, Australian Animal Health Laboratory, Geelong.
Egg collections were made immediately prior to challenge with the virus and then at weekly intervals for five weeks, ten eggs per group per week. Eggs were analysed for shell deformation, breaking strength, shell reflectivity, egg weight, albumen height, Haugh units, yolk colour, shell weight, percentage shell and shell thickness.
Analysis of variance was used to test the effect of challenge, bird strain and vaccination group on the egg quality parameters measured, with significance indicated by P<0.05. Fishers protected LSD was used to distinguish between means when significant effects were seen.
Results
Statistically significant effects of the challenge treatment across all collection times were observed in egg weight, shell weight, percentage shell, shell thickness, albumen height, Haugh units and yolk colour (Table 1). Egg weight, shell weight and yolk colour score were all significantly lower for both groups of challenged birds. N1/88 exposed birds had significantly lower shell thickness and percentage shell. Unexpectedly, albumen height and Haugh units were found to be significantly lower for the control groups. There were no significant effects of challenge group on shell reflectivity, breaking strength or deformation.
There were significant interactions between time after challenge and challenge group for some egg quality parameters (Table 2). Egg weight was significantly lower than the controls for both challenge virus groups at 1, 3 and 4 weeks post exposure, and for the T birds only at 2 and 5 weeks post challenge. When compared to the other treatment groups at the same time after infection, deformation of the shell was significantly lower for the T group only at 1 week after challenge, then higher for the N1/88 group at three and five weeks post exposure. The group that received N1/88 virus had significantly lower percentage shell at week three after challenge, while the T group had a lower percent shell at 5 weeks. Shell weight, albumen height also indicated significant interactions, mainly as a result of changes in egg weight and Haugh units respectively. Shell thickness and breaking strength were not significantly different between the challenge treatment groups across the time frame of the trial.
There were no significant differences among the measured parameters of any of the four strains of hen, challenged with different antigenic strains of IB viruses. However, there were some differences among the strains for each of the egg quality parameters that were consistent across time, challenge group and vaccination group (Table 3). The ISA Brown and HiSex strains had significantly darker, stronger, heavier, thicker shells that constituted a greater percentage of the egg and the HiSex strain had a significantly more flexible shell. The tinted egg layer the Hyline gray, had significantly smaller and paler eggs. Both Hyline strains had significantly higher albumen height and Haugh unit measurements. The yolk colour of the ISA Brown and Hyline Brown strains was significantly higher than the other two strains of bird.
Discussion
Challenge at 41 weeks of age after not having been vaccinated since 14 weeks of age had negative effects on the egg quality of the birds. Over the trial period as a whole, the birds exposed to N1/88 had a greater drop in shell quality parameters. This strain of IB virus had deleterious effects on shell thickness and percentage shell in addition to the declines in egg weight, shell weight and yolk colour that were also significant in the birds challenged with T-strain.
When the egg quality parameters are compared at each of the collection times (before and weekly after challenge) the timing of the declines in shell quality can be seen. In general the egg weight for the T group declined at 1 week and stayed low for the duration of the trial, while the N1/88 group had a decline to the first few weeks after challenge and then a return to the level of the control group. At weeks three and five post exposure, the group that received the N1/88 virus had higher deformation than the other treatment groups, as well as a lower percentage shell and, while not statistically significant, the shell thickness tended to be lower. These factors together probably indicate that the thinner, lighter shell was more flexible, however, the declines in the quality of these parameters has apparently not affected the breaking strength of the shell at any of these collection times.
One of the signs classically associated with IB is a decline in internal quality of the eggs (Cavanagh and Naqi, 1997). Unexpectedly in this trial, the albumen height and Haugh units of the control group was lower than that of the infected groups. This apparent increase in Haugh units due to challenge is due largely to the significantly higher values for the N1/88 group across almost all collections. The reason for this increase is unclear, although a steady but slight decrease in the Haugh units of the control group has possibly contributed more to this difference than an increase in the infected birds. However, either way, it can be said that the lowest mean Haugh unit recorded for the control birds was 91.35 reported in the second week after the other groups were exposed. This value is still at a very high level well above the USDA AA grade level of 80.70 (Jones et al., 2002).
In general, there were declines in egg shell quality with challenge at peak lay, with T strain having a more prolonged effect, but N1/88 having the more severe reduction in quality.
There were no significant differences between the reactions of the different strains of laying hen to challenge with either of the IB strains used in this trial. There were however, consistent differences between the performances of the layer strains after having been raised to this age under identical environmental conditions.
In general the shell quality of the ISA Brown and HiSex strains was superior to that of the Hyline strains, while the internal quality as measured by Haugh units and albumen height was better for the two Hyline strains. The higher mean yolk colour score for the ISA Brown and Hyline Brown birds is probably largely due to these two strains being the largest in body size and therefore eating more pigment-containing feed, per egg laid.
The significant shell quality and Haugh unit effects are probably indicative of inherent differences among the four strains, giving each layer strain its advantages and disadvantages in terms of egg quality parameters.
References
Cavanagh, D. and Naqi, S.A. (1997). Diseases of Poultry. Ed. B. W. Calnek, H. J. Barnes, C. W. Beard, L. R. McDougald and Y. M. Saif. Ames, Iowa State University Press: 511-526.
Ignjatovic, J. and McWaters, P. G. (1991). Journal of General Virology 72: 2915-2922.
Ignjatovic, J., Sapats, S. I. and Ashton, F. (1997). Avian Pathology 26: 535-552.
Jones, D. R., Tharrington, J.B., Curtis, P.A., Anderson, K.E., Keener, K.M. and Jones, F.T. (2002). Poultry Science 81: 727-733.
Wadey, C. N. and Faragher, J. T. (1981). Research in Veterinary Science 30: 70-74.
From Proceedings of the "2004 Australian Poultry Science Symposium", New South Wales, Australia.
