The importance of the vaccination for the poultry industry

Currently, the poultry industry faces many microbiological challenges that can affect animal health and, consequently, lead to a negative economic impact in poultry farms. This is partly due to the intensification of the production in the recent decades, that,  together with an extremely high genetic pressure, has caused the musculoskeletal system to develop faster than the immune one, making the birds prone to suffer from several diseases.

To prevent these diseases, vaccination programs against viral, bacterial, or even parasitic diseases have been implemented. Probably, the viral ones are the most important in terms of vaccination since there are practically no effective antivirals. In addition to vaccination, it is necessary to establish health and management protocols that take into account risk factors related to the vaccine and the vaccination process itself, and factors related to the animal and the pathogen, in order to guarantee the efficacy of the vaccination and prevent it from failing.

Types of vaccines and techniques of application

The direct objective of the vaccination is the prevention of different diseases, however, keep in mind that the indirect objective is to maintain the performance, because this will determine the productive success of the farms.

Types of vaccines

There are different types of vaccines depending on the disease to protect:

• Live non-attenuated vaccines: This type is not quite common, since there is a risk that the disease will reproduce and spread. As an example, there are some vaccines of this type with herpes turkey virus to vaccinate birds against Marek disease.
• Live attenuated vaccines: These are the most frequent ones in the prevention of viral diseases. The virus is attenuated by repeated passages in in vitro cultures to reduce its virulence. The attenuated virus will replicate in the animal and generate both a humoral and cellular immune response.
• Inactivated vaccines: These vaccines contain, as an antigenic component, a virus that is inactivated by either a thermal or a chemical treatment. In this case, viruses do not replicate and, therefore, these vaccines are less immunogenic. They produce a less intense immune response than the previously described vaccines and, in addition, they only generate a humoral response, for they usually require adjuvants. However, this type of vaccines have certain advantages since they produce long and uniform immunity, require less revaccinations, have less interferences, which allows the use of combined vaccines, and there is no risk of spreading infectious agents since they have been inactivated. As the previous type of vaccines, inactivated vaccines are frequent in the prevention of certain viral diseases.
• Vector-based vaccines: This group includes both DNA and recombinant vaccines and are the most recently developed ones. In these vaccines, the DNA of the pathogen antigen is inserted into a vector that can be either a virus or a plasmid, so that, once the animals have been vaccinated, antigenic proteins that trigger cellular and humoral immune responses.

It is important to know the properties of each vaccine, to make sure it is properly designed, preserved and administered, and avoid vaccine failures.

Application techniques

There are several techniques for the vaccine administration. They are chosen depending on different factors such as management, the type of immune response to be obtained, the type of vaccine, the age and species of birds, among others.

The most frequent techniques are:

• Through drinking water: This technique is frequently used in the case of live attenuated vaccines, such as Gumboro disease, infectious bronchitis, laryngotracheitis or Newcastle disease. It is important to consider multiple factors such as water quality and the presence of other polluting components, as well as the water temperature. These factors can complicate a technique that is, probably, the simplest one. The objective of this technique is to obtain an intense local immune response, both cellular and humoral.
• Parenteral: This is mainly used for inactivated vaccines and is frequent in the rearing phase of laying hens. Furthermore, some live attenuated vaccines, such as Marek disease, are also administered by intramuscular or subcutaneous injection. A generalized immune response is obtained when using this technique.
• In ovo: A modern technique. Birds are vaccinated on the 18^th day of incubation, mainly against Gumboro and Marek diseases. Vaccine is applied in the amniotic fluid.
• By spray: This technique is used for the vaccination of large populations of birds, either in the incubator or in the fattening farm. It is frequently used for infectious bronchitis and Newcastle disease, and intense humoral and cellular immune responses are obtained.
• Aerosol: This is a method commonly used in revaccination and is also the method of choice for to vaccinate against Mycoplasma gallisepticum.
• Eye drop administration: It is the most accurate method because it allows to administer an exact dose of vaccine. This technique is common in the vaccination against infectious bronchitis, infectious laryngotracheitis, Newcastle disease, and pneumovirus. A fast and optimal immune response is obtained.

The choice of the type of vaccine will depend on the disease to be vaccinated, and the route of administration will depend on the type of vaccine and the immune response to be obtained. It is especially important to know the characteristics of each type of vaccine, as well as the routes and techniques of administration, since an adequate vaccination will avoid vaccine failures related to the procedure. Furthermore, keep in mind that vaccinations cause stress and generate several adverse reactions, including fever or decreased feed intake, and even the death of the bird. Using the proper vaccination technique will minimize these problems.

Evaluation of the efficacy of the vaccination

The objective of the vaccination is to reproduce an attenuated infection of a specific disease, so that birds increase their defensive capacity. This is a controlled infection to obtain an immune response that, through the production of antibodies, will ensure that the birds are protected against a future natural infection.

The vaccines generate a humoral-type immune response, based on the production of antibodies that will serve as a defense for birds in subsequent infections. Some vaccines also induce a cellular-type immune response, mainly mediated by T lymphocytes.

Depending on the productive aptitude or the age of the birds, and type of disease, the vaccination programs can differ from relatively simple, as in broilers, to very complex plans, which include vaccinations and revaccination for multiple diseases, as in laying and breeding hens. In both cases, it is important to evaluate the efficacy of the different vaccines applied.

The efficacy of the vaccination is indirectly assessed by evaluating the generated immune response. The most common is to measure the antibody titers generated thanks to the vaccinations, through serological tests such as ELISA, virus neutralization tests, or immunofluorescence tests. Routinely, one of the most common techniques globally is ELISA, which is characterized by an immobilized antigen that is detected by an antibody bound to an enzyme that allows us to observe some detectable change, such as a colour change. Using antibody detection techniques, we can evaluate the production curve of antibodies and, thus, evaluate the efficacy and protection provided by a vaccine. To do this, it is necessary to check the antibody titer prior the vaccination and control the protection at day 21 by repeating the analysis of around 20 animals per group. Additionally, in breeders, it is recommended to evaluate the uniformity of the titers around 20 weeks after the vaccination, using the coefficient of variation. Thus, it can be concluded whether the vaccination administered in the growth stage has been correct. In general, the antibody level is maximum at 4-6 weeks after the administration of the vaccine.

Types of vaccine failures

As described in the previous section, vaccination aims to obtain an immune response that generates specific antibodies to prevent birds from suffering from a certain disease under farm conditions. Therefore, a vaccine failure is considered when a disease appears despite the birds have been previously vaccinated.

There are several factors involved in vaccine failures related to the animal, the pathogen, the vaccine or its application procedure.

Animal-dependent factors

• Interference with maternal immunity: Chicks sometimes receive high levels of maternal antibodies for a certain disease. In this case, if they are vaccinated in the first weeks of life for the same disease, the vaccine can be neutralized by these antibodies already present in the chicks.
• Immunosuppression: Immunosuppressed animals cannot generate an adequate immune response related to the vaccination and they will not generate sufficient protection.

Pathogen-dependent factors

• Agent transformation: Some pathogens show high genetic variability due to their mutation and recombination capacity. They change over time and vaccines stop being effective.
• Excessive infection pressure: Sometimes, despite the vaccination and the development of a correct immunity, if the infective pressure is very high, it can exceed the defensive capacity of the immune system, making the vaccine insufficient for the animal to be protected.
• Early appearance of outbreaks: If the animals are vaccinated but exposed to a pathogen before the immunity develops, the vaccination will not be effective.

Factors dependent on the vaccine and its administration method

• Management issues: Improper storage or using vaccines after the expiration date can cause vaccine failures.
• Por design: This happens when the chosen antigen is incorrect, insufficiently attenuated or, on the contrary, virulence is increased during the passages, or when there are contaminations during the process.
• Administration failures: Vaccine failures can occur when the route of administration is inadequate, age is not optimal, if the vaccine is not applied homogeneously, and when factors dependent on the animal itself or the group are not considered.

Prevention through immunostimulant pronutrients

As described previously, there are several factors that can lead to vaccine failures. It is necessary to identify them and apply a control to minimize the risk. Although some factors, such as the vaccine-dependent ones, can be avoided with good training and management at the moment of the vaccination, most factors are very difficult to control and vaccination will need additional support.

In this sense, immunostimulant pronutrients have a very positive effect in improving vaccination plans and decreasing the risk of vaccine failures.

These are active natural molecules, capable of improving the innate and adaptive immune responses, as well as the specific immune response generated with the vaccines. This effect is achieved by increasing the synthesis of antibodies when these molecules are administered in association with the vaccination, which leads to obtaining more protected birds against the main poultry diseases.

This effect has been evaluated in several trials against the main poultry diseases, such as infectious bronchitis (IBV), Gumboro disease (IBD), or Newcastle disease (ND). The graph below shows the results of a trial carried out in breeders where the immunostimulant pronutrients were administered 5 days before and 5 days after the vaccination. Thanks to the use of these pronutrients, between 22.41% and 45.15% more antibodies were obtained for infectious bronchitis, between 43.46% and 78.95% more for Gumboro disease, and between 23.49% and 40.45% more antibodies against Newcastle disease.

The increase in the synthesis of antibodies thanks to the use of immunostimulant pronutrients also increases the percentage of animals protected against the diseases that are vaccinated against, which significantly reduces the risk of vaccine failure.

On the other hand, this greater immunity is reflected in an improvement of the productive parameters, which leads to a positive economic impact for the farm.

Conclusions

As described throughout this article, there are several factors that can lead to vaccine failures. Some of them can relatively be kept under control, but others, such as the variability of some pathogens, are extremely difficult to control and lead to vaccination failures. For this reason, although the vaccines are essential in the prevention of many diseases, they need an extra support that increases their effectiveness against the main poultry diseases.

In this sense, immunostimulant pronutrients effectively increase the antibody titer and the percentage of animals protected against the main diseases, and improve the productive parameters as an indirect result of the positive effects on bird’s immunity. Therefore, a strategy that combines this type of natural molecules with a proper vaccination program is especially advisable to avoid vaccine failures.