Cat & Dog - Antibodies and Antibody Titres

What are antibodies?

Antibodies are specialised proteins (immunoglobulins) that circulate in the blood.  They are produced by a type of white blood cell called a plasma cell.  This cell produces specific antibodies in response to stimulation from foreign proteins (antigens).  These are found on bacteria, viruses, toxins etc.

Antibodies can react with these foreign proteins and render them harmless.  It is in this way that antibodies provide protection against the diseases caused by bacteria, viruses, etc.

How long do antibodies last?

Antibodies do not last for life.  They gradually diminish in the body.  Therefore the length of effective immunity varies depending on various factors, including the type of infection (antigen).  It is therefore useful if the level of immunity (i.e. amount of circulating antibodies) can be measured.

What is an antibody titre?

Serum is the liquid part of the blood after red and white blood cells and substances such as fibrinogen, an essential component for the clotting process, have been removed.  Although the plasma cells have been removed the antibodies they produced are still present in the serum.  The antibody titre is the highest dilution of serum that reacts against a fixed amount of any particular antigen (so if you can dilute it a lot and it still reacts to the antigen, there must have been a lot of antibodies in the sample).

What do antibody titres indicate?

Antibodies are only produced after the body has reacted to the foreign material, (antigen).  Thus a positive result at any dilution indicates exposure to that antigen.  Most antibodies tend to become weaker over time, therefore if there is a reaction at a very high dilution it means that the animal has a very good immunity, i.e. there are a high number of circulating antibodies against that particular disease.

Does that mean that if there is a high antibody titre to parvovirus, that my dog has had parvovirus?

No, antibodies last for a variable time in the circulation but the majority become reduced over time.  A high parvovirus antibody titre indicates either that your dog has been exposed to parvovirus or has been vaccinated against the disease relatively recently.

My dog had a blood test for leptospirosis.  It came back negative.  What does that mean?

A negative result (a lack of antibody titre to a particular organism) in this case Leptospira, can indicate either:

1. Your dog has not been exposed to leptospirosis and is not suffering from that disease.
2. Exposure to the disease occurred so recently that there has been insufficient time for antibodies to be produced in sufficient quantities for them to be detected.   

In the latter case it is important to obtain another blood sample in two or three weeks to see if the antibody titre has risen.  If there is no increase in antibody titre on the second test then leptospira is clearly not the cause of the illness.

Are antibody titres used in the diagnosis of many diseases?

Yes. This test is used in the diagnosis of many infectious diseases.  Distemper, hepatitis (canine adenovirus), leptospirosis, parvovirus, feline enteritis (panleucopaenia) and feline immunodeficiency virus (FIV) together with many recently imported diseases are all common conditions for which antibody tests are useful for diagnosis.  They are also routinely used to establish whether immunity following vaccination is adequate or whether a booster should be administered.

Why are antibody titres used to diagnose diseases?

Many diseases will share similar clinical signs, therefore antibody titres will give an indication of the presence of a specific disease.  Antibody titres are also frequently used in the case of zoonotic diseases.

What is a zoonotic disease?

This is a disease that can be transmitted between animals and people.  Quarantine was instituted in Britain originally to control rabies, probably the best known zoonosis.  These regulations have now been relaxed so that certain pets can enter or re-enter the UK without having to undergo quarantine, provided certain stringent requirements are met.  This, however, has resulted in the appearance of other 'tropical' diseases in the UK, some of which have zoonotic potential.  If on return the animal becomes ill, sometimes antibody titres are useful to pinpoint the cause and, therefore whether or not there is a likely risk to people.