Margaret Hardy BVet Med MRCVS, St David’s Poultry Team
Serology is the means by which we can evaluate the response a bird’s immune system mounts to a specific invader or pathogen. The pathogen may be viral or bacterial and is called an antigen. The substances produced by the bird’s immune system are released into the blood stream and are called antibodies. Antibodies have many functions but they basically bind with the antigens to remove them from the body and thus remove the disease challenge.
We can use different chemical methods to detect both the presence and the level of antibody production. This antibody production may be due to a real or ‘field’ challenge or in the form of a vaccine. The basic response is the same. This enables us to use serology to evaluate both vaccination status e.g. whether the bird has been vaccinated, the level of ‘success’ of the vaccine administration and also whether the bird has been exposed to the disease challenge.
We are essentially interpreting what the response of the birds tells us and different bird types respond differently in similar situations. Therefore, it is really important that any serology results are accompanied by comment from a veterinarian who is familiar with the vaccination regime, the bird type and the disease challenges on the individual farm. It is also very important that serology test type and timing are tailored towards what we need to know. Poorly selected tests and poor timing can lead to false impressions and false conclusions.
Serology can be used to monitor both live and inactivated vaccines. Live vaccines are, as the name implies, alive and the aim is to ensure that they are still alive and efficacious when administered and that every bird gets as close to a full single dose as possible. This sounds easy, however it is anything but. Live vaccines (usually viral) are not identical to the virus which can cause overt disease but have been slightly moderated (or attenuated) which means the immune system can still recognise them as the disease causing virus and therefore produce antibodies but the attenuation process means disease does not develop. It is relatively easy to change the structure of a virus as is done here but there are small segments which must remain unchanged – these are the pieces of the virus that the immune system reacts to. These are called epitopes.
The next challenge is to keep the vaccine viable and alive until it reaches the administration stage Viruses are killed by a multitude of things; heat, freezing, UV light, sanitisers, disinfectants, dirt, chlorine and heavy metals. So storage in transit and on farm is really important, as is hygiene during preparation of vaccine for administration. The titre levels we achieve are a good indication of how well the vaccine has been stored and handled prior to administration. It is important to protect vaccine from chlorine in the water and also from heavy metals which occur in all bore well water by using a binder to remove them. If the vaccine has been killed prior to administration our titres will be zero and if it has been damaged we will see lower titres than anticipated. It is essential to know which vaccine has been used before embarking on titre analysis, as some live vaccines are ‘hotter’ than others and therefore produce higher antibody levels. For example, different Gumboro vaccines (IBD) will produce different responses and if the type of vaccine used is not taken into account it can be difficult to differentiate between good vaccination and actual challenges on the farm. The same is true for different Infectious Bronchitis vaccines.
When administering to large numbers of birds, it is challenging to ensure that every bird gets vaccinated and that every bird receives as close to a full dose of vaccine as possible. Live vaccines may be administered either via the drinking water or by coarse spray. The route of administration is usually determined by the body system we are trying to protect e.g. respiratory vaccines are administered via course spray. Course spray ensures that the particles do no penetrate deep into the respiratory system but rather target the areas which have specific receptor sites. The receptor sites are mostly in the nose and throat. The vaccine particles bind to these receptor sites and therefore block the possibility of disease causing ‘field strains’ to attach. In the absence of being able to attach, these pathogenic viruses pass through the bird and are excreted. The vaccine also ‘primes’ the immune system so that when it meets the virus again it is ready to produce large quantities of protective antibody.
The titres we achieve using live vaccines are lower because it is really just step one in the process of preparing the bird to withstand viral challenge. Most of the live vaccines are in the rearing period but they may be used in lay as well to help boost protection by blocking attachment sites and producing local antibody responses. It is important to stress again here that different types and strains of vaccine produce different titre levels and need to be analysed with that in mind. Short lived birds such as broilers receive only live vaccines. Non-respiratory vaccine are administered via the drinking water and produce a similar effect in receptor sites in the intestines e.g. IBD (Gumboro disease) vaccine.
Both spray and drinking water administration are difficult to achieve uniformly in large populations so anyone involved must be properly trained. The success of administration throughout a population can be indicated by the CV or coefficient of variation which is really just a measure of the spread of the titre levels within population. If it is very wide (e.g. 60 or over) it means that some birds have received a much larger dose than others. To get uniform protection in a flock we need all birds to get close to 1 dose as possible. If CVs are very narrow (e.g. 20 or below) it may indicate that the flock has suffered a disease challenge.
Once the immune system has been primed we now have the opportunity to stimulate it to produce high levels of circulating antibodies. These antibodies remain in the bird’s bloodstream and are ready to ‘mop up’ any antigens. If we consider IBV (Infectious Bronchitis virus), the main effect it will have on a laying flock is reproductive rather than respiratory. It will cause production drops and associated egg shell and internal egg quality issues. Antibodies in circulation will act to remove the virus before it reaches the oviduct and protect the flock from production losses.
Stimulation of the immune system is by the administration of inactivated vaccine by intramuscular injection. This vaccine usually contains a number of viruses and contains an immune system stimulant such as oil. Again, it is a challenge to ensure that every bird receives a full dose of vaccine but with careful handling, penetration and injection it should be much easier than live administration. We evaluate the success of this procedure by looking again at the titre levels which should now be higher and more uniform (i.e. the CV should be lower – usually 40 or below). Analysis should be done no earlier than 4 weeks post vaccination.
Some companies will routinely monitor vaccination ‘success,’ whereas others are interested only in investigating disease challenge. Both are valid uses of serology but it is really important to have a base line ‘normal’ for individual flocks to compare potentially elevated disease titres against so taking bloods post vaccination and storing without testing costs very little but helps in investigations. There are a number of methods available to measure antibody levels and the selection of test method is important to ensure you get the result you need. The basic premise is that the antibodies and antigens bind together to form a clump. The formation of the clumps tells you that both antigen and antibody are present. The usual method is to add serum to a pre-prepared plate which is coated in a known antigen and wait for a clump to form (or not!).
In some diseases, especially those that can be vertically transmitted via egg to the chick, we only need to know that the laying hen has antibodies in her blood which will then remove the antigen and prevent the chick becoming infected. In these cases we do not measure clump density, we have a simple yes/no answer but in most cases we learn more from measuring clump density by adding a fluorescent substance and using a machine to measure the level of fluorescence. More fluorescence = more density of clump = more antibody present. This is the basis of the Elisa test.
Laboratory submission forms can be extremely daunting so you should always feel able to discuss your requirements with the technical team at the lab who are best placed to help you with test selection. As we have said above, the analysis of results is very complicated so again you should receive analysis from your vet and feel able to discuss the results with them to make the best decisions for your birds.
St David’s Laboratory Services, 20 Roskeen Rd, Moygashel, Dungannon BT71 7RG
Telephone: +44 (0)28 8772 2225