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Recombinant vaccines, a new option in disease control

Vaccination is the most effective strategy for the control of disease in poultry. In-ovo vaccination of embryos at 18 days of incubation is already a method to combat various diseases. For this purpose conventional strains and strains specially developed for in-ovo administration are used. Molecular engineering technology has enabled the development of a new generation of vaccines, called recombinant vaccines that can be applied in-ovo or subcutaneously by injection at day one.

By Dr Aris Malo, MSD Animal Health, Boxmeer, the Netherlands
Recombinant vaccines offer new opportunities for the development of control programs and eradication of diseases in poultry facilities. They consist of a vector (for example a virus or bacteria) which expresses foreign antigens.
The process involves the introduction of genes coding for antigens, usually proteins into the vector genome, and when it multiplies, it expresses the product of the inserted segment. Vaccination with a recombinant vaccine results in an immune response against the vector but also against the antigen included expressed by the vector without using the actual disease agent.
Several recombinant vaccines have recently entered the poultry market offering new opportunities for the sector. These vaccines have proven to be able to offer a level of protection similar to conventional vaccines, stimulating the development of cellular and humoral immunity.
Various vectors
The main viral vectors used for the development of recombinant vaccines are Herpes virus of turkey (HVT) and the Poxvirus among others. These viruses have genomes large enough to accept large inserts.
Examples of recombinant vaccines are:
• Herpes Virus Turkey expressing Newcastle Disease virus protein
• Herpes Virus Turkey expressing Avian Laryngotracheitis virus protein
• Herpes Virus Turkey expressing Infectious Bursal Disease virus protein
• Fowl pox virus expressing Avian Influenza virus protein
• Fowl pox virus expressing Newcastle Disease virus protein
• Fowl pox virus expressing Infectious Laryngotracheitis virus protein
Advantages and characteristics
The main advantage of using recombinant vaccines is that they eliminate the post vaccination reactions typical of vaccines based on live organisms and the local reactions caused by the use of inactivated vaccines. The use of a recombinant vaccine based on a continuous replicating virus (such as Herpes virus for example) could eliminate the need for booster vaccinations with either live or inactivated vaccines in the field.
The use of recombinant vaccines also allows the use of the so-called “DIVA” system (English: Differentiating Infected from Vaccinated Animals) to differentiate vaccinated birds from infected birds. The possibility of recombination of wild and vaccine viruses in the environment will not occur with the use of a recombinant vaccine. Recombinant vaccines eliminate the possibility of interference with the maternal immunity and/or other vaccines and the possibility of vaccine virus from becoming latent in vaccinated birds.
Swollen eyes are a clear symptom of ILT infection in poultry.
Establish good immunity
The development of immunity using a recombinant vaccine will depend on the replication of the vector. This would be the time required for the vector to establish a good level of immunity in vaccinated birds. The lack of a good level of immunity in the initial phase of the bird’s life (for example in high challenge areas) can be complemented if necessary by the use of conventional vaccines.
It must also be taken into account that the use of recombinant vaccines does not result in high levels of antibodies to the infectious agent against which the vaccine is being used, when using conventional serological methods (such as HI or ELISA). If high levels of antibodies are found this would indicate contact with classical circulating virus either of vaccine or field origin.
Example: ILT vaccination
The advantage of using a recombinant vaccine is explained in the following example in which a recombinant vaccine for the simultaneous control of Marek’s Disease and Infectious Laryngotracheitis (ILT) is used. HVT is used as a vector for the insert of the ILT virus.
ILT is an acute respiratory, highly contagious disease caused by a herpes virus and can affect chickens of all ages. It is a disease of great economic importance to the poultry industry because it causes heavy losses in the production of both meat and eggs. Current vaccines against ILT are based on a modified live virus. They are highly effective but are often associated with a number of adverse effects, such as the spread of vaccine strains to unvaccinated flocks, resulting in increased virulence of the virus and the existence of latent carriers, which in turn contribute to the spread of the virus in the field. On the other hand, vaccines based on HVT have been used extensively to prevent Marek’s Disease.
Development of immune response
In the following example the use of a recombinant vaccine developed for the prevention of both Marek’s Disease and ILT is described. The vaccine contains inserts that result in the expression of two glycoproteins of the ILT virus (I and D). These glycoproteins are important for the onset of the infection process and thus also for the development of an immune response and protection against infection. Birds vaccinated subcutaneously with the recombinant vaccine against ILT and unvaccinated birds were challenged and were observed at different ages to determine the presence of clinical signs of ILT.
Lifelong immunity
One dose of the recombinant vaccine administered subcutaneously at day one of age induced excellent protection against ILT in vaccinated birds (Figure 1). The unvaccinated control group showed severe signs of disease.
Lifelong immunity against ILT is expected when using a vaccine of this type since HVT persists in the vaccinated birds.
Taking into consideration the characteristics of recombinant vaccines such as safety, efficacy and ease of administration and the lack of spreading of live viruses from vaccinated to unvaccinated birds, we can conclude that they represent the ideal choice to replace or complement the use of current live attenuated ILT vaccines, thus avoiding the disadvantages associated with them.
* References are available from the author
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Dr Aris Malo

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