4. Yolk Extract

Just as mammals pass on a "package" of specific antibodies to their progeny via the first milk or colostrum, birds achieve the same result by secreting a large amount of antibody into the yolk of their eggs. For this reason yolk extracts can be used as a substitute for serum samples for serological monitoring. The first recorded use of this approach was for the detection of Newcastle Disease HI antibodies (Schmittle and Millen, 1948). A series of more recent papers has examined the relationship between serum and yolk antibody in some detail. The references are listed here according to the specific antibody which was investigated:

In general these studies confirmed that antibody titres in yolk are highly correlated with those in serum (correlation coefficients ranging from 0.84 and 0.97). The correlation te nds not to be as good during the first 1 to 2 weeks after vaccination or challenge (Piela et alii, 1984). This may be due partly to the fact that IgG is the main antibody deposited in yolk, while IgM is the dominant antibody at the beginning of the immun e response. The other factor to consider is that yolk is laid down in concentric rings, mainly over the week prior to ovulation, hence if serum antibody titre is increasing rapidly there may be a gradation of antibody concentration across the various lay er s of yolk. This has led to a common recommendation that the yolk be thoroughly homogenized prior to the sample being collected. Individual hens have been identified as having a low ability to deposit antibody in yolk in some studies (Kramer and Cho, 1970 ). Two main methods of extraction have been reported. In the simpler version the yolk is diluted 1:1 with PBS and centrifuged or allowed to sediment. The supernatent is then used for testing. The second method involves the addition of chloroform to th e aqueous extract (usually 1:2) to remove the fat. After agitation and allowing the mixture to stand for 30 minutes the sample is centrifuged and the top-most, clear, aqueous extract is used for testing. There are no reports of successful use of yolk extra cts in rapid plate agglutination tests. There is a suggestion that chloroform-extraction is especially beneficial when it is intended to conduct HI tests (Piela et alii, 1984).

The balance of advantages and disadvantages of yolk extracts as serological samples may be stated as follows: Advantages:

1. Ease of collecting sample

2. Samples ready packaged

3. Eggs easily stored

4. Eliminates bird handling and stress

Disadvantages

1. Sample extraction according to published methods can be labour intensive.

2. Extracts cannot be used for rapid plate agglutination tests

3. Samples cannot usually be identified to individual bird

4. Results are not always directly equivalent to serum

The available information suggests that yolk extracts can have a useful place in routine surveillance of antibody responses in poultry. Perhaps the area in which this type of sample is most relevant is in the evaluation of maternal-antibody transfer. Care should be taken in the choice of the eggs to be used according to the purpose of the testing programme. Eggs with shell defects are likely to come from birds with physiological or pathological abnormalities. If such birds are not the specific tar get of the testing then normal eggs or those not suitable for incubation for "accidental" reasons may be more appropriate.