It delivers vaccines with great accuracy, allowing the embryonic chick to develop its immune response, before being exposed to any threat of disease post-hatch.

However, the process and technique used to administer in-ovo vaccines is critical and determines to a large extent the effectiveness of the method.

Where to deliver the vaccine

To achieve effective in-ovo vaccination, the actual site of injection is of utmost importance. To fully understand that, we have to look at how the embryo is positioned in the egg – which functions both as a protective shelter and as a source of nutrients, which the embryo absorbs during its development.

Between the amnion and the shell, the allantois functions as a sort of waste bag, storing waste products that are formed during the embryo’s growth, including metabolic water. The weight loss of eggs during incubation comes mainly from evaporating this metabolic water through the shell.

Vaccine must be injected deep enough into the egg to be delivered into the amnion fluid, where it can be absorbed by the embryo as it takes up the nutrients contained here. If the vaccine is injected too deep, the needle will hit the developing embryo directly, and although the embryo will be vaccinated, there is a risk of damage to the developing chick. If the vaccine is not injected deeply enough, it will be delivered into the allantois, or waste fluid. This vaccine will not be utilised by the embryo and the vaccination will be ineffective.

Injection

With in-ovo injection, we target the needle to a fixed depth under the shell, positioned for each individual egg. If the embryo is in the correct position, the egg will be injected correctly and the embryo successfully vaccinated. However, the position of the embryo depends on its stage of development. If the embryo is too small, it will not sit high enough in the egg, the needle will not penetrate through the allantoic and the vaccine will be delivered into the allantoic fluid and wasted. If the embryo is too big, the needle will go through the amnion and hit the embryo directly.

Gauging the size of the embryo

The size of an embryo depends mainly on the speed of its development and time since incubation started.

To inject the embryos at the optimum size, we must vaccinate in relation to speed of development. If the embryo is not adequately developed, it is simply a matter of waiting slightly longer before we vaccinate. Speed of development depends on the embryonic temperature inside the egg, which is not equal for each machine, each breed, each egg size and each egg within a machine.

To help identify optimum timing, we can inject the eggs with a dye periodically during incubation, to check where the dye is being delivered. We can also adjust the timing of vaccination by measuring the length of the embryo at 18 days of incubation.

Achieving uniform protection

We often find that the embryonic temperature (inside the egg) is not equal at every spot in the machine. This is dependent on a number of factors, including air temperature, air velocity and the volume and path of water spray, for example.

A spread in embryonic temperature, will indicate a spread in development – and with that, if we do not gauge optimum timing for vaccination, a percentage of the embryos will not be vaccinated correctly and will have no protection when placed in the field, especially when injected relatively early.

To achieve optimum results with in-ovo vaccination, it is therefore, more important to monitor the size of the embryos closely, rather than time elapsed during incubation. To achieve uniform protection, all embryos must be as close to the same stage of development as possible, and of the right size to deliver the vaccine into the amnion.

August 2007