Vaccination—what are we doing and why?

The current debate on Salmonella vaccination strategies (and indeed the role of vaccination in the control of the current Foot and Mouth crisis) has raised many questions, some political, some moral, some commercial and some technical. It has brought vaccination and vaccine use to everyone's attention and left many of us scratching our heads.

The aim of this article is to attempt to explain some of the basic technical aspects of vaccination of our hens. What we do, how it works and why we do it. We all know that we vaccinate our birds, our pets, our children and ourselves. What are we trying to achieve?

Going back to the very early days of vaccination, the observation was made that animals or people exposed to certain diseases or infections, then seemed to be resistant to getting the disease a second time. This led to the concept of exposing animals or man to low (hopefully harmless) doses of a particular infection to mimic this effect and hence protect them. This remains the basis of most live vaccination programmes. Concerns about infecting animals or man with potentially harmful live doses of bugs led to the development of killed vaccines. These hopefully contain the bug in a way that stimulates the animal to respond to it but is not capable of giving it the disease.

So what are we trying to achieve with vaccination of our birds?

* Prevention of diseases which can reduce performance, reduce egg production, increase seconds or increase mortality.

* To act as a 'primer' for other vaccines given later, to boost the birds' response.

* To protect the bird against infection or contamination by bugs of human health significance (eg Salmonella vaccines).

Vaccines stimulate the birds' immune system which reacts to the vaccine as it does to any foreign or unnatural substance the bird comes into contact with. This response is one that has evolved in all animals as an attempt to protect the body against nasty or harmful challenges.

The bird then produces either antibodies in the blood (which are simply proteins) or cells in the body which can fight this challenge. The body 'remembers' the type of challenge and the next time the bird is vaccinated the response is boosted or if it meets the same disease, then the body reacts to kill the infection. This battle can occur in the bloodstream, in the respiratory tract or the ovary depending on the bug involved. Hence, blood tests are done on birds to detect the antibody in the blood produced by the vaccines or the disease itself.

So what options do you, as a producer, have to vaccinate your birds?

Broadly, as discussed above, the choices are live vaccine, killed vaccines or a combination of both to produce a sensible and cost effective vaccination programme. Vaccines can be produced against either bacteria or viruses. It is worth pointing out that all vaccines licenced in UK have gone through a rigorous testing system and are safe to use in your birds, if properly applied.

Live vaccines

Essentially these are modified versions of virulent hot strains or naturally occurring mild strains of the same disease. Virulent strains are made harmless by the way they are grown in the laboratory or through addition of certain chemicals. Common examples are live Infectious Bronchitis (IB) vaccines (eg H120) or Newcastle Disease (ND) vaccines (eg HB1), based on original field isolates. On the other hand, the Mareks vaccine HVT is a live naturally mild strain of the virus originally isolated from turkeys that protects against Mareks disease in chickens without giving them the disease. Live vaccines can be given via the drinking water, spraying by eye drop, or even injection.

The most common techniques of water or spray application enable large numbers of birds to be vaccinated at the same time. These techniques produce rapid protection in birds (if properly applied), although it may be a short lived response and require several boosters. Obviously live vaccines need to be handled with care to prevent them dying before the bird can respond to them. Hence the use of skimmed milk powder to protect the vaccine and the need to use vaccines as soon as possible after they have been reconstituted in water.

Live vaccines are frequently used as primers to prepare birds for the later killed vaccines, although with certain diseases multiple live vaccinations are considered sufficient to give protection through lay.

Killed vaccines

As their name suggests, these are vaccines made from live bacteria or viruses that are then rendered harmless by killing the organisms. This is usually done by a chemical treatment to inactivate the bug. Hence, killed vaccines are also known as inactivated vaccines.

As the vaccine is dead, it will not spread from bird to bird which reduces the likelihood of spreading the disease itself but does mean each bird has to receive its own dose. Hence, killed/inactivated vaccines have to be given by injection on an individual bird basis. Another aspect is that as the organism is dead, the birds have to be 'persuaded' to respond to it to get the best effect. Therefore, all killed vaccines contain an adjuvant or irritant such as mineral oil or aluminium hydroxide. This is why birds vaccinated with these vaccines may be less active or 'sore' for a day or so after vaccination.

This is also why great care must be taken by vaccinators giving these vaccines, as accidental injection into your hand can lead to a severe reaction and swelling, especially fingers where the swelling can be intense. It must be remembered that in the case of oil based killed vaccines, any accidental injection of people requires immediate hospital attention.

However, despite these warnings, killed vaccines given prior to the onset of lay can give lasting protection throughout lay if the birds have been properly primed beforehand.

Fitting a vaccination programme to suit your birds

Most, if not all, vaccines are given during the rearing phase prior to 18 weeks. Vaccination programmes vary depending on conditions local to the rearing site and those in the area where the birds will be placed in lay. Advice should always be taken from your veterinary surgeon. Planning a vaccination programme should be laid down clearly in the Veterinary Health and Welfare Plan.

Producers buying point of lay pullets should liaise at a very early stage with their pullet supplier on the most appropriate vaccination strategy.

It is not possible in this article to suggest the perfect, all embracing vaccination programme. In general terms, the following should be considered:

IB, ND and TRT: Live priming via the water or spray followed by killed vaccine by injection.

IB variants (eg 793B, 4/91): The use of these depends on local conditions but in the absence of a licenced killed vaccine, two live doses may be sufficient to protect birds into lay. A combination of standard IB vaccines may help give a broad protection to a number of IB variants.

AE vaccine: Live vaccine given in the water late in rear to protect against egg drops in lay. This is a bit of a 'forgotten' disease. Important aspects to consider are not to vaccinate too close to the onset of lay and to remember that the vaccine (like many live virus vaccines) is very susceptible to chlorine levels in the water. Switching off any chlorinators whilst vaccinating and the use of skimmed milk powder are obviously good ideas.

Gumboro: One or more live vaccines are given in the drinking water to protect birds from this potentially devastating virus whilst in rear.

EDS: Protection is obtained using the multiple killed vaccination by injection prior to point of lay.

Salmonella: A live vaccine for protection against Salmonella enteritidis is available with several doses being given in the drinking water. Alternatively, two doses of a killed vaccine is available for protection again