The detection of antibodies against infectious agents (antigen) in body fluids, primarily serum, but also saliva or milk samples, is referred to as serology and is one of the classic examination procedures in infection diagnostics.

The most common method is the ELISA test (enzyme-linked immunosorbent assay), in which one component of the antigen-antibody reaction - usually the antigen - adheres to a plastic surface and the other component - usually the antibody - is contained or sought in the sample (indirect ELISA).

The procedure is technically very robust against interfering factors and usually takes only a few hours. The detection limits and the specificity are - of course depending on the choice of antigen and the immunological characteristics of the disease - usually good to very good.

Since serological methods (indirect ELISA) often do not detect the pathogen itself, but the immune reaction of the infected animal to the pathogen contact, this is referred to as indirect pathogen detection.

Depending on the question, this has advantages or disadvantages for diagnostics.

A positive immune reaction is - again depending on the respective disease - only possible between approx. 5- 21 days after contact with the pathogen - after the so-called seroconversion. After that, the pathogen itself no longer has to be present for positive detection, which extends the period of detection of disease for pathogens with a very short or no septicaemia/ viraemia phase.

This detection period is again disease-dependent and can last from a few weeks to many months or even years.

The limits of the method are set by the specificity of the antigen. It is not always possible to detect certain serogroups or serotypes of a pathogen equally well or, if necessary, to distinguish them from each other, as the antibodies in the animal are also subject to a high degree of variability, especially in the early stages of a disease. In some diseases, the serological test results are influenced by vaccination. Knowledge of the vaccination status is therefore of particular importance for the interpretation of serological results. Since the pathogen is only detected indirectly by means of antibodies, no pathogen material is available for further characterisation of the pathogen, a resistance test or even for the production of a stall-specific vaccine. If this is desired, methods of classical cultural bacteriology must be used.

The detection of infectious agents by means of serology is generally particularly suitable for herd diagnostics. The antibody profile of a herd, e.g. by sampling different age groups or animals in different phases of the disease, provides insight into the course of the disease. Serological testing is equally suitable for regular monitoring of the herd to monitor herd health or confirm the health status of a farm.

Indirect detection is also a useful option for diseases caused by viral pathogens with very short or no viraemia (influenza) or for bacterial diseases where direct detection may otherwise only be possible from organ material or where cultivation of the pathogens is technically complex (e.g. Lawsonia intracellularis, Chlamydia) and lengthy (Leptospira, M. paratuberculosis). 

The IVD GmbH has therefore compiled some serological screenings that taking important differential diagnostic pathogens in case of respiratory problems or reproductive problems and abortions into account.

For example:

• Respiratory screening "Ahlem" for indirect detection of Actinobacillus pleuropneumoniae, Mesomycoplasma hyopneumoniae, Influenza A Virus and PRRSV in blood serum.

• Reproductive screening "Ery + Parvo" for indirect detection of Erysipelothrix rhusiopathiae (erysipelas), Leptospira, Porcine Parvovirus and PRRSV in blood serum.

For further questions please contact:

Dr. med. vet. Sebastian Fischer
(Head of serology and specialist for veterinary microbiology)
Phone: +49 511 220029-22 or -0
e-Mail: fischer@ivd-gmbh.de

Our diagnostic offers for various diseases and animal species can be found in our list of products and services as well as on the diagnostic request forms.