Articles
30 Jul 2025
There are two main reasons we use serology:
To see if a bird has had a disease.
To check if a vaccine worked like it should.
What happens during an immune response?
When something foreign (like a vaccine, bacteria, or virus) gets into a bird’s body, the immune system kicks into gear. That foreign substance is called an antigen. In response, the body starts producing antibodies, and this process is called seroconversion.
Vaccines can trigger different types of immune responses:
One kind is where the bird makes antibodies that float around in the blood — that’s a humoral immune response, and it’s what serology tests pick up. The antibody level is called a titre.
The other type is called cell-mediated immunity, which trains the bird’s white blood cells to find and kill infected cells. This type doesn’t always show up strongly in serology tests — so don’t panic if those results are low after certain vaccines.
Timing of serology testing matters and the result is influenced by how long it’s been since exposure or vaccination, how long the birds were exposed to a disease and at what level. Antibodies can also stick around long after the bird had the disease or vaccine.
Submitting blood samples
Each bird’s blood gets tested separately. If you don’t send in enough samples, the results won’t be very reliable. Ask your vet or the lab how many samples they need so you don’t have to go through the hassle of re-sampling.
Using serology to check vaccination success
Many rearers do serology at around 16 weeks to check if their vaccines worked. If the results show weak seroconversion, it's time to relook their vaccination procedure – there are many steps at which issues can creep in. Keeping track of seroconversion records is a good idea in case a veterinarian asks for them during a disease investigation.
Some farms have their own ongoing serology monitoring plan, designed with a vet. This helps build a “baseline” of what’s normal for that flock, so you can spot problems more easily later on.
When monitoring vaccine uptake, consider the following:
Timing: Wait at least 2–3 weeks after vaccination before testing. If you bleed the birds right after vaccinating, you won’t see antibodies yet.
Vaccine type: A live vaccine triggers a strong and rapid response; a killed (inactivated) vaccine produces a gradual, weaker response.
Natural variation: Not all birds respond the same way. Random sampling helps even things out.
Understanding the lab results
The lab will send back a simplified report to the farmer and a more detailed report to the vet. The reports may contain the following information:
Number positive: How many birds had a positive result (strong or weak). For most vaccines, over 80% positive is what you want. But if most of those positives are weak, it might not be good enough.
Individual titre levels: Different vaccines have different expected titre ranges. For some, 2000–4000 is great. Others can be as low as 1000 or go up to 10,000 or more and still be normal.
Mean titre: This is the average of all the titres. A mean between 2000–4000 is usually considered good. But watch out — one or two super high or super low results can skew the average. So it needs to be looked at alongside the other numbers.
%CV (Coefficient of Variation): This shows how even the antibody response is across your flock. A low %CV (under 40%) means the birds responded consistently. A high %CV means the results vary a lot, which could mean uneven vaccine uptake.
VI (Vaccination Index): This is a combo of mean titre and %CV. A high VI usually means a strong, even response. A low VI might suggest something went wrong with the vaccination.
Using Serology to Check for Disease
Once a bird has been exposed to disease, it takes a few days for the body to recognise the antigen and release antibodies. Sometimes the clinical signs will have resolved, but the antibodies are still present. Serology gives a snapshot of the birds’ antibody levels at a specific time and often only tells us that there had been exposure to a disease. The presence of antibodies doesn’t necessarily mean there’s a disease outbreak — it could be due to vaccine antibodies or a harmless field strain.
When trying to figure out what disease might be causing trouble in your flock, antibodies alone aren’t enough. A few things need to be considered, for example:
How strong the antibody response is
How consistent the response is
What vaccines the flock has had
The age of the birds
What’s been happening on the farm
What you’re seeing clinically and at post-mortem
Your vet will use all this info to narrow down the likely causes and other tests like PCR or bacterial cultures. It is really important to have a vet who is familiar with your farm and flocks and to reach out to your vet as soon as there is any cause for concern.
At Hatch House, we work alongside farmers to design practical, farm-specific serology plans, interpret results in context, and troubleshoot vaccination success. Whether it’s building a baseline, advising on sampling, or digging deeper into post-mortem findings, our vets bring real-world experience to your side of the gate.
If you're ready for a vet who knows poultry, understands serology, and cares about getting it right, contact us through our contact page.
