Avian immunology

Why do we vaccinate? The reason for vaccinating poultry is to prevent infection and disease caused by the more commonly encountered pathogens. Vaccination is never 100% effective, and the level of protection varies from vaccine to vaccine, and reflects, to some extent, the way in which the virulent pathogen– whether this is bacteria, virus, or parasite– stimulates the immune system.

It is important for all livestock keepers to know that vaccination does not compensate for the lack of hygiene and biosecurity and will not compensate for increased infection levels where biosecurity is poor, such as might be found in some backyard farms in Brazil, for example, as shown here on the left, or with badly kept free-range chickens, as shown here on the right. Vaccination is a mechanism to protect the flock rather than individual birds and will be much more effective where hygiene is good. Backyard flocks can never reach the high levels of biosecurity seen with some large commercial flocks, as you can see here, but they can aspire to it.

Most of the diseases against which vaccines are used in commercial farms are respiratory and/or viral aetiology. See week 3. This applies to both large commercial farms and backyard flocks. What is different is the commercial value of both and the approach to protecting the birds. Although vaccines are expensive, the cost of losing a large commercial flock ensures cost effectiveness. This may not be the case for small backyard and hobby flocks.

It is unusual to vaccinate birds which are kept in back gardens and yards, both because of cost effectiveness and also because vaccines are produced with large commercial flocks in mind. And vaccines in small numbers of doses appropriate for three to 20 birds are simply not available. One feels that there is a market waiting to be exploited here. Rehomed hens from commercial farms will have already been well vaccinated and which may continue to protect for several months, albeit at a reduced level.

One benefit of vaccinating hens is that antibody in the blood of hens is transferred to the yolk, which is absorbed by the developing embryo so that the hatched chicks may be protected until the antibody levels wane, after two to three weeks.

I’m indebted to Professor Paul Wigley for the slide. Protection against infection by pathogenic bacteria, viruses, or parasites is afforded by several natural mechanisms innate to the bird itself and which do not require prior exposure to infectious agents. This can be augmented by stimulating immunity, so-called adaptive immunity to specific pathogens by vaccination. Adaptive immune responses are slower to initiate but offer a much more specific response that is often effective in preventing infection and reinfection. It is this memory response that is the basis of vaccination.

The components of innate immunity include antimicrobial secretions produced in mucus, the low pH, high acidity in the proventriculus and gizzard, and the natural inhibitory effect against colonisation by harmful pathogenic bacteria, which is produced by the highly complex intestinal microflora, the microbiome, where there may be more than 10,000 million commensal and harmless bacteria in every gramme of contents of the ceca.

The innate immune system also involves a number of cell types amongst the white blood cell, the leukocyte class. One class of cells are granulocytic, highly phagocytic cells, which are very effective at killing bacteria. These are called heterophil granulocytes and stain appropriately. They are the equivalent of neutrophil granulocytes of mammals. See the image on the left. Other major leukocytes include macrophages, the image in the centre, and dendritic cells, which are similar to macrophages but with long dendrites for sampling the environment, the image on the right. These are present in the tissues and blood, and in mammals also situated in lymph nodes, and liver, and spleen. Birds, which are essentially feathered dinosaurs, do not have lymph nodes.

Other cells associated with the early innate immune response include gamma delta T cells, natural killer cells, and thrombosis, which are also linked to blood clotting. But we will not discuss these here. Macrophages and dendritic cells are phagocytic and are also key initiators of adaptive immunity. This slide is a very simplistic representation of how the immune response works. Macrophages and dendritic cells, on the left, ingest, and chew up the pathogen, and present small pieces of it on their surfaces, which serves to stimulate other types of leukocytes called T lymphocytes and B lymphocytes. Protein signals produced by these cells also direct the T cells in the type of response that they should make.

T cells function via cell mediated immunity, by killing intracellular bacteria and viruses, and B cells produce antibodies, which can attach to and, in some cases, inactivate viruses. Different types of T lymphocytes have different functions, which we need not go into detail here. Suffice it to say that some promote the killing of intracellular bacteria. Some are induced by bacteria themselves to inhibit the killing effect so that the bacteria can persist. Some are able to kill virus infected cells. And some induce infected cells to kill themselves. It is a complex and really quite amazing effect.

Although the immune response prevents and controls many infectious pathogens, some of them can undermine the immune response and modulate it for their own benefit. These include Salmonella pullorum, which alters the immune response enabling it to persist in spleen and liver for months from where it enters the reproductive tract with transfer to the eggs and progeny. Other pathogens such as infectious person disease, IBD virus, and chicken anaemia virus, CAV, suppress the immune response by damaging components of the immune system as an aspect of their pathogenesis. This can open up the way to secondary infection by other pathogens, especially bacteria.