The health of chicks is the bedrock of successful poultry production, and evidence is growing to support the role of in ovo vaccination in bolstering early immunity against disease risks both broilers and layers face in the field.
Birds have both innate and adaptive immunity, helping them fight a wide range of pathogens that can circulate in production systems, even with strong biosecurity efforts. These can include respiratory viruses, bacteria and molds.
Innate vs. adaptive immunity
Innate immune responses are immediate and work in a similar way in any situation, using physical barriers, cellular components and chemical signaling processes. They are not specific to any antigen and pave the way for adaptive immunity.
“The innate immune response is very important because it allows birds to later develop effective adaptive immune responses,” Isabel Gimeno, PhD, DVM, from North Carolina State University, explained. “The adaptive response takes a bit longer, responds to specific antigens and has memory, so on repeated exposure to an antigen, the immune response becomes a lot stronger.”
Most of a chick’s immune system develops during incubation. Innate immune response components develop shortly after embryonation (around embryonation day [ED] 7-10). T cells, which coordinate the overall immune response, are present around ED 11. T cells are formed in the thymus, and as they mature, they migrate to the bird’s secondary lymphoid organs. Antibody-producing B cells, produced in the bursa, are present at around ED 12 and become functional by ED 18.
“At this point, the immune system is not functionally mature; it’s still quite rudimentary. There are a lot of things that need to develop later, but it does mean that we can vaccinate in ovo, because the key players are there to mount an immune response,” Gimeno said.
Early threats, long-term effects
Given that B cells take around 2 weeks to produce antibodies and 4 or 5 weeks to produce a full immune response, and T cell immunity takes 1 week after hatch and is optimal at around 6 weeks, there is a window where pathogens can inflict considerable damage on bird health and productivity.
Birds are exposed to pathogenic viruses from the first day of their lives, and when combined with management-related stressors, this can cause problems that persist over the life of flocks. Research has shown that early infection with viral conditions such as infectious bursal disease (IBD) or chicken anemia can wipe out immature B and T cells, leading to immunosuppression throughout the birds’ lives.1
Tools for early protection
Added to the picture of innate and adaptive immunity is the role of maternal antibodies – chicks’ temporary, ‘borrowed’ immunity from the hens. Ensuring immunocompetence in the early stages of birds’ lives involves vaccination of both hens and embryonic chicks, Gimeno explained.
“For IBD, you vaccinate the hens and you also vaccinate the chick embryos. This way, you get a balanced transition between the time when maternal antibodies go down and the time you see an active immune response because of the vaccine you put in the progeny,” she said.
Vaccination stimulates both innate and adaptive immunity at hatch.2 The vaccine options for producers looking to tackle IBD in the face of maternal antibodies are recombinant HVT products, which use a harmless vector to deliver IBD virus genes, or immune complex vaccines, which carry live virus mixed with neutralizing antibodies. For Marek’s disease, developing active immunity as early as possible is even more crucial because the virus associated with the condition infects birds very early.
In ovo vaccination brings early advantage
Considerable research has demonstrated the positive impact of in ovo vaccination on birds’ responses to pathogens. This, along with the practical advantage of making it possible to vaccinate many birds at once, has paved the way for its adoption by most of the US broiler industry. In the case of Marek’s disease, in ovo interventions mean birds develop an immune response 3 days before they hatch, offering a “head start” on the virus.3 But the advantages don’t stop there.
Work from Gimeno’s research group has shown that vaccinating with HVT vaccines in ovo also speeds up the maturation of birds’ immune systems,4 to the point that at the time of hatch, a chick can mount an immune response like a bird that is 2 weeks old. Genetic differences between birds in production mean that in layers, humoral, innate and cellular immunity are activated, whereas in broilers, immune activation is mainly innate and cellular.
This accelerated maturation is linked to the “adjuvant” effect of HVT – its ability to not only protect against Marek’s disease but also to stimulate broader immune activation. For decades now, Gimeno’s lab has tried to optimize this effect of the vaccines, leading to the conclusion that HVT, when administered at the proper doses (not too high and not too low), results in a very strong activation of the innate and adaptive immune responses.
Even well-known vaccine adjuvants did not surpass the effect of HVT. While the addition of other Marek’s disease vaccine strains, including CVI988 and SB-1, did not have a negative effect, they did not increase the adjuvant effect of HVT. Gimeno reported that a novel chimeric vaccine (CVI-LTR) is the only vaccine that, when administered with HVT, results in an even stronger adjuvant effect than HVT alone.
How robust immunity boosts production
Early immunocompetence has lifelong benefits for birds in production, especially in an era of heavily reduced or even eradicated antibiotic use. Biosecurity and vaccination are the cornerstones for controlling disease throughout birds’ lives and ensuring they grow optimally, but for vaccination to succeed, robust innate immunity is essential.
Without this immunity, Gimeno said, responses to vaccines are likely to be poor, and attenuated live vaccines may even cause disease. The practical impact of this is that broilers will use a lot of energy fighting disease, which reduces growth, while for longer-living birds such as layers and breeders, a poor response to live vaccines early in life is likely to affect their response to inactivated vaccines later in their lives. This can have consequential effects for the progeny, which may end up with reduced maternal antibodies.
Thymus focus can advance understanding
Gimeno hopes that further research on this vaccine-induced immunocompetence, with a particular focus on what happens in the thymus of birds a few days after vaccination, will help support the development of more targeted and optimized products.
“Understanding the pathogenesis in the thymus and the early development of T cells is a critical point for both the maturation of the immune responses and for later protection,” she said.
“Most of the studies that you see use the spleen and not the thymus, simply because it’s easier. The thymus is a lot more complicated to work with, but we have to switch gears and focus on the thymus really early. It has the answer to a lot of the unknowns around vaccines and viruses, not just for Marek’s but for many other diseases.”
References
1 Sharma, J.M., Kim, I.J., Rautenschlein, S. and Yeh, H.Y., 2000. Infectious bursal disease virus of chickens: pathogenesis and immunosuppression. Developmental & Comparative Immunology, 24(2-3), pp.223-235.
2 Negash, T., Al‐Garib, S.O. and Gruys, E., 2004. Comparison of in ovo and post‐hatch vaccination with particular reference to infectious bursal disease. A review. Veterinary quarterly, 26(2), pp.76-87.
3 Boone, A.C., Gaghan, C., Fares, A., Browning, M., Cortes, A.L., Mohammed, J., Villalobos, T., Esandi, J., Kulkarni, R.R. and Gimeno, I.M., 2026. Ability to accelerate innate and cell-mediated immune responses in meat-type chickens by in ovo vaccination with monovalent and bivalent Marek’s disease vaccines. Vaccine, 69, p.128001.
4 Boone, A.C., Käser, T., Cortes, A.L., Kulkarni, R.R., de Juan Abad, B.A.L., Villalobos, T., Esandi, J., Perozo, F., Lemiere, S. and Gimeno, I.M., 2020. In ovo vaccination with herpesvirus of turkey enhances innate and cellular responses in meat-type chickens: Effect of vaccine dose and strain. Vaccine, 38(31), pp.4837-4845.
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