A brief history of serological monitoring in poultry

The first report of the use of a serological test in the diagnosis and control of a disease of poultry was made by Jones in 1913. This referred to the use of a tube agglutination test for Pullorum disease. The causal organism had been discovered 14 years earlier (Rettger,1900), but it was the subsequent recognition of the importance of vertical transmission that stimulated the search of a satisfactory means of identifying infected hens. The tube agglutination test was used in the first eradication programmes. This stimulated a great deal of work on the standardization and refinement of such testing which led to the introduction of a rapid plate serum agglutination test (Runnels et. al. 1927) and a stained antigen whole-blood test (Schaffer et.al. 1931). The development of a serological test for Mycoplasma gallisepticum infection in chickens followed a similar but relatively abbreviated course. The causal agent was first cultivated in chick embryos by Delaplane and Stuart (1943) and a stained antigen rapid serum agglutination test was described by Adler in 1954.

The methods discussed above are only applicable to relatively large particle antigens. An analogous procedure can be used to detect antibodies to viruses by demonstrating the formation of precipitates. Double-immunodiffusion in an agar gel (DID) is applicable to the detection and identification of a wide variety of viral antigens and antibodies against them. This technique was initially described by Oudin (1946) but is more commonly attributed to Ouchterlony (1948). The first report of its use in the U.K. was for the detection of Infectious Larynogtracheitis antigen (Jordan and Chubb, 1962). The use of this test in poultry requires much higher levels of salt in the agar than for the test carried out in mammals (Woernle, 1966). DID has been the main test used for checking SPF poultry flocks for a wide range of poultry pathogens. The usefulness of the test is somewhat reduced by the difficulty experienced in quantifying antibody, although some laboratories have routinely used it to quantify Infectious Bursal Disease antibody (Faragher,1971, Cullen and Wyeth, 1975).

Another type of serological test became available with the discovery of the haemagglutinating ability of Newcastle Disease Virus (NDV), (Burnet, 1942). This phenomenon allowed the detection and quantification of specific anti-NDV antibody by their ability to inhibit the agglutination of red blood cells by an antigen known to contain a high titre of NDV. In the early days this test was generally performed using an "alpha" procedure in which the antigen was diluted and added to a constant dilution of serum. In recent years the "beta" procedure, in which the antigen is constant and the serum is diluted, is more commonly used. The procedure has also been improved over the years by its adaptation to micro-titration methodology. Perhaps the first report of the value of a population approach to poultry serology arose from the use of the HI test to evaluate response to NDV vaccination in the UK (Phillips, 1973). A standard method for carrying out this procedure was published by Allen and Gough in 1974. The same technique can be used to detect and quantify antibodies to other haemagglutinating viruses such as Avian Influenza virus and the adenovirus which causes Egg Drop Syndrome (EDS-76). The discovery that treatment of the Infectious Bronchitis virus with an enzyme caused it to become haemagglutinating (Alexander and Chettle, 1977) opened the door to the use of a similar HI test to detect and quantify antibodies to IB.

The next serological testing method to be widely taken up by the poultry industry was the enzyme-linked-immuno-sorbent assay (Elisa). This assay system had been initially used to measure immuno-globulins of various classes (Engvall and Perlmann, 1971). Five years went by before the first reports of the use of this technique to measure antibody response to specific infections (Voller et alii, 1976). The first report of its use in avian medicine covered its use in detecting antibodies to Mycobacterium avium (Thoen et alii, 1977). The Elisa technique was, initially, used by scientists working in university and government laboratories, generally producing their own reagents. However within a remarkably short period of time kits were being made available commercially. This allowed relatively simple laboratories to carry out a range of serological tests for important poultry pathogens. The range of kits available continues to grow, albeit now rather more slowly than previously. A later section provides a table indicating the range of Elisa tests which have been developed.\par \par It could be said that serum agglutination was the main serological test up to the 1950's, immunodiffusion the test of the 60's, while haemagglutination-inhibition was much improved and applied to new diseases in the 70's. The 1980's and early 90's saw the dissemination of Elisa's. Each new addition did not automatically lead to its widespread use or substitution of tests already in use. Serum-agglutination, for instance, continues to be widely used in monitoring Mycoplasma status.

Many other tests such as serum-neutralization, immunofluorescence, complement fixation and so on, have also played an important part in the development of our understanding of infectious disease in poultry. They have not, however, been applied widely in the monitoring of vaccinal response or disease in poultry. This brief history of serological monitoring does demonstrate the speed and effectiveness with which the poultry industry takes advantage of new testing methods which are sufficiently robust and cost-effective to apply on a wide scale.