Pullet chicks were reared in isolation; all except a control group were variously vaccinated against infectious bronchitis. Individual haemagglutination inhibition (HI) antibody responses were measured from 1 day to 38 weeks old when all birds were challenged with virulent infectious bronchitis virus, and egg production recorded for a further 5 week period. In the controls HI titres remained low until challenge: this caused a loss of 15.1 eggs/hen. In birds injected with oil emulsion killed vaccine (OEV) at 3 and 16 weeks old, the serological response was uniformly high but on challenge the loss in egg production was 2.9 eggs/bird. Birds given H120 live vaccine at 3 weeks and H52 live vaccine at 15 weeks old had a low (23%) individual rate of serological response to the latter, and egg production after challenge was 3.63 eggs/hen less than before. In birds given H120 live vaccine at 3 weeks and emulsion killed vaccine at 16 weeks old, 100% serological response to the latter occurred and egg production was unaffected by challenge. A further group also received H120 and H52 live vaccines at 3 and 15 weeks old respectively: however, they were then subdivided into four groups and injected with emulsion vaccine at either 17, 19, 21 or 23 weeks old. Their response to H52 vaccine was variable. The proportion of birds in each sub-group responding serologically to subsequent vaccination with OEV was 45, 65, 73 and 92% respectively. After challenge egg production in these four sub-groups was reduced by 1.92, 1.15, 0.94 and 1.55 eggs/bird respectively. It is concluded that response to oil emulsion infectious bronchitis vaccine can be impaired if it is used within 8 weeks of H52 live vaccine. Best results are achieved where birds are given a primary dose of H120 live vaccine at 3 weeks old followed by emulsion vaccine 12-16 weeks later. Use of the less attenuated H52 strain of live vaccine before emulsion killed vaccine is contra-indicated.

SUMMARYIn three separate and unrelated experiments, in which vaccinated hens were challenged with virulent infectious bronchitis virus, the ability of individual hens to maintain egg production was related to their serum haemagglutination inhibition antibody titre at the time of challenge. It was found that, regardless of the vaccination programme used, the ability of laying hens to withstand infectious bronchitis virus challenge, as measured by the effect upon their egg production, is directly related to individual antibody titre at the time of challenge. In all three experiments, birds with antibody titres of >8 Iog2 (n = 82) did not show a significant reduction in egg production after challenge while those with titres within the range 5-7 log 2 inclusive (n = 126), over a period of 3 or 4 weeks after challenge, showed a significant reduction in their rate of egg lay, viz: 0.38, 0.33 and 0.47 eggs per hen per week, respectively and those with titres <4 log 2 (n = 101) showed, over the same time period, a reduction of 1.0, 0.45 and 1.16 eggs per hen per week, respectively. The ability of different vaccination programmes to stimulate uniformly high antibody responses to infectious bronchitis virus, and hence good overall protection of egg production was compared. It is concluded that the programme of choice is first to vaccinate the birds with a highly attenuated strain of live infectious bronchitis vaccine during rearing (H120), followed by the injection of a potent killed oil emulsion adjuvant vaccine at point-of-lay. The ability of the less attenuated H52 strain of live infectious bronchitis vaccine to interfere with response to killed vaccine was demonstrated in two of the three experiments. In both cases this interference

Commercially-reared laying chickens were challenged at 31 weeks of age with a virulent infectious bronchitis (IB) virus. They showed a sharp drop in egg production, despite having been vaccinated at four and eight weeks old with live attenuated IB vaccines to a recommended schedule. In contrast, similar birds that had been further immunised at point-of-lay with inactivated oil emulsion IB vaccine, or with a combined IB/Newcastle disease (ND) emulsion vaccine, showed no detectable fall in egg production after the same challenge. Unvaccinated susceptible specific pathogen-free birds challenged at the same time stopped laying almost completely. In the birds revaccinated with emulsion vaccine, measurement of haemagglutination inhibition antibody levels to IB showed their geometric mean titres to be raised from less than 5 log2 at the time of vaccination to over 10 log2 four weeks later. Their antibody levels did not rise further followining the IB challenge whereas in the birds that had not been revaccinated antibody rises to nearly 10 log2 were detected after the same challenge. For pullets vaccinated earlier with live IB vaccine, revaccination with inactivated IB or IB/ND oil emulsion vaccine at point-of-lay provides a safe and effective way of protecting their egg production against IB infection.

Chicks vaccinated with live Hitchner B1 Newcastle disease vaccine at 17 days old and subsequently re-vaccinated with an oil emulsion killed Newcastle disease vaccine at either 38 or 52 days old showed high and persistent HAI antibody levels for at least eight months. Re-vaccination of these birds at 17 weeks old caused a further rise in antibody level to log212 which, even at 38 weeks, had dropped only to log210. Chicken primarily vaccinated with oil emulsion killed vaccine at six weeks old developed HAI antibody levels after four to five weeks of log29 which re-vaccination four weeks later increased to log211. Chicken given killed aluminium hydroxide adjuvant Newcastle disease vaccine were serologically HAI negative 13 weeks after vaccination while those given the oil emulsion vaccine still showed an antibody level of log28. Groups of birds inoculated with oil emulsion vaccine and then, at 20 weeks old, challenged with virulent Newcastle disease showed a 100 per cent survival rate. The particular merits of oil emulsion killed Newcastle disease vaccine for laying and breeding birds are discussed.

Haemagglutination inhibition and virus neutralisation antibody responses of chickens given different vaccination programmes were compared. This was followed by a further experiment in which variously vaccinated laying hens were challenged at 30 weeks of age with two strains of infectious bronchitis virus of the "variant" Dutch D207 serotype. Chickens were given primary vaccinations to different strains of infectious bronchitis live virus during rearing and then injected at 16 weeks of age with inactivated oil adjuvanted virus vaccines prepared from either M41, GV101 or both viruses combined (bivalent vaccine). Antibody titres to M41 infectious bronchitis virus were high, and to D207 serotype low. in birds given Mass type vaccines only. In birds given an initial 'priming' with Mass type live vaccine and then 'boosted' with bivalent killed vaccine, high haemagglutination inhibition and virus neutralisation antibody levels against both the M41 and D207 serotypes of infectious bronchitis virus were stimulated. In another experiment, the ability of laying hens vaccinated according to this programme, to withstand challenge with two strains of virulent infectious bronchitis virus of the D207 serotype, was tested. Protection of egg production in vaccinated hens was found to be good and in all groups correlated with the individual hen haemagglutination inhibition titre at the time of challenge. The significance of these results with regard to the use of killed virus vaccines in laying hens and to the necessity to develop live virus vaccines from 'variant' strains of infectious bronchitis virus is discussed.