D. Chick Anaemia Virus

Our objective is to ensure that all parent flocks have sero-converted to Chick Anaemia Virus (CAV) prior to the onset of lay. Any flocks with a significant number of sero-negative birds in lay will result in Chick Anaemia Syndrome in the progeny, usually over a 6 to 10 week period coinciding with sero-conversion. At the time of writing one live vaccine has received a provisional license in the UK. This vaccine was not in use when the serological data presented here were collected. Further vaccines may appear in due course. In order to evaluate the prevalence of sero-negative samples, we have reviewed the results of the serological assay (an Elisa test supplied by Guildhay Ltd) for all submissions received over a 21-month period (January 1993-September 1994).

In the period in question a total of 671 submissions were received and 6891 assays were performed. Each submission refers to a group of chickens in 1 house, often there will be groups of sera submitted from a number of houses at the same time. In most cases 10 samples were tested from each house. In Table 4 and Figure 3.8 we have classified the results in 4 categories according to the % of the submitted samples which were seropositive. Approximately one quarter of the submissions had a substantial number of sero-negative samples, and one third had some sero-negative samples. In interpreting this it must be kept in mind that any flock with a significant number of negative samples would normally be re-tested to ensure sero-conversion prior to lay. In some cases this process may need to be repeated. This means that approximately 50% of flock/houses have sero-negative birds when first submitted . Most such submissions occur when the birds are between 10 and 16 weeks of age.

Since there were, during this period, no vaccines licensed for CAV in the UK, we must assume that all sero-conversions reported here are the result of natural field exposure.

In order to evaluate any changes in the pattern of sero-conversion over time we have plotted in Figure 3.9 all submissions which included negative sera in the order in which they were received at the laboratory. There appears to be no trend towards an improvement in the situation. This is as we would expect in view of the increased attention to farm hygiene with a view to the control of other infectious diseases.

Negative serology for CAV in broiler parent chickens in rear is still a common phenomenon. Careful serological monitoring continues to be necessary in order to prevent sero-conversion in lay with its consequent serious losses in progeny. One option open to encourage sero-conversion is cross-seeding of litter or birds from positive to negative houses. The risk in doing this may be quite slight when movements occur within one farm. Vaccination is preferable since other diseases can be inadvertently transmitted when birds or litter are moved between farms. The substantial number of samples which we test is a clear indication of the importance which the poultry industry attributes to this problem.