Vaccine Failure in Poultry Production and its Control Methods: A Review

Poultry production provides different types of animal proteins
in the form of meat and eggs...


Introduction
Poultry production provides different types of animal proteins in the form of meat and eggs. The demand for chicken meat has increased over time because of the increasing demand for quality food in the form of meat and eggs. Poultry birds are prone to is a biological preparation that are administered at the optimal and safe amount to enhances active acquired immunity to a particular disease. They are synthesized using several techniques such as eggbased vaccines, cell-based vaccines, and vaccines produced using investigational-manufacturing (plant, bacterial culture, and insect cell) systems. The immunization of birds is generally accepted as the most cost-effective and sustainable method of controlling infectious fowl diseases. This is mostly achieved by inoculation of non-pathogenic (attenuated or killed forms of the microbe containing its toxins or one of its surface proteins) that have great potential to elicit and boost the immunity of the host cell against diseases [9]. According to different research outputs, chickens are the most vaccinated farm animal species in recent days. For instance, a broiler chicken receives an average of eight different vaccines whereas a layer chicken may receive an average of around twelve different vaccines during their production cycles. Even though vaccines are used extensively in different poultry farms, it has also some limitations in control infectious poultry diseases using vaccination [8].
A vaccination failure arises when birds fail to develop adequate antibody titer levels and/or are at a risk of field disease outbreak.
According to different research outputs, vaccination failure occurs due to inappropriate vaccines schedule (timing), improper handling of vaccine, vaccine quality, vaccine strain/serotype and insufficient amount of antibody titer levels after vaccination that predisposes the chickens to a field disease outbreak. Moreover, immunesuppression, maternal antibodies, stress, and management practices were also known causes of vaccination failure in poultry flocks [10,11]. Vaccines have a significant role in disease prevention and control worldwide. This, in turn, has a great importance in minimizing the emergence of outbreaks at the farm level and contributes a lot to the growth of poultry production. There different type of vaccines and vaccination schedule throughout the world and their efficiency depends on several factors such as the method of production, biosecurity level, disease pattern, maternal immunity, availability of vaccines, costs and potential losses [12][13][14]. Even though different methods have been applied and recommended for controlling infectious diseases in poultry, vaccination was found as the most important tool [4]. Thus, Immunization is the process of boosting immunity using biologically prepared antigen in the form of vaccines and used as prophylactic measures against specific pathogens [5].
The aim of vaccine is to immunize the birds through inoculation of non-pathogenic components of the pathogen in question, or closely related organisms that is nearly similar to naturally acquired immunity. This provides assurance to reduce the clinical and production impacts of the infections rather than the prevention of various infections outbreaks in poultry flocks [10,15]. Proper vaccination also used to develop a very high antibody levels in hens to increase the maternally derived antibodies (passive immunity) in the hatched offspring to protect the chicks against different infectious diseases during the early phase of life [3,4]. Vaccination can be performed using live non-attenuated or attenuated vaccines.
Live attenuated vaccine is a type of vaccine that is contains attenuated viral strains whose virulence is reduced via series of cell culture or embryo passages that induces both cell-and antibodymediated immune responses by stimulating the body's immune response [16]. Newcastle disease, infectious bronchitis, infectious bursal disease, and other bacterial vaccines are commonly used live vaccines against diseases of poultry [17]. Unlike live attenuated vaccine strains, the non-attenuated vaccines have a great potential to cause disease in the vaccinated host especially in chickens with decreased immunity and are not preferred as a first choice [12].
This is due to reversion to virulence during virus replication in the host and inadequate attenuation results in clinical disease and adverse vaccine reactions and this causes great difficulty in controlling the disease [4,6].
The use of a live attenuated vaccine strain has a great advantage since it can be administered through the natural route of infection and enables to induce mucosal and systemic immunity [13,18].
Moreover, it helps with a large number of flocks since it is easy and suitable for administration [12]. Inactivated or killed virus vaccine is considered safer than the conventional live attenuated vaccines since are they are less immunogenic and do not promote the endogenous production of antigenic proteins stimulating the cellmediated immune response [2]. Such vaccines are made inactive by using physical (use of ultraviolet radiations and heat) and chemical (use of formalin) means to inactivate their pathogenicity, but the protein coat structure has been maintained, which acts as immunogenic. Most poultry vaccines were either inactivated organisms that were formulated with an oil-based adjuvant or live attenuated vaccines and were not very effective in many cases. Thus, the discovery of antigen/gene delivery systems has facilitated the development of novel prophylactic and therapeutic veterinary vaccines. Recently, reverse vaccinology is the best approach and uses several bioinformatics algorithms to predict antigen localization and it has been successfully applied to immunize against many veterinary diseases [19].
There are several options for application of the immunization in poultry and these should fit to the conditions of the disease in the locality, the standard of farm biosecurity, and the level of challenge for each type of poultry operation. Inactivated or Killed vaccines confers a long-term immunity for the flocks [20]. So, successful vaccination with inactivated vaccines requires multiple inoculations and the presence of adjuvants to enhance immunogenicity [3,13].
Furthermore, as the inactivated vaccine does not replicate in the host after administration, it is generally considered unsuitable for mass application and needs to be administered by intramuscular injection [4]. During vaccination programs, the overall cost-associated with the service delivery that is cost of vaccine, labour, and equipment, nursing, laboratory testing, and all other related activities [21]. Besides, vaccination or failure to gain immunization even after vaccination is the most common complaint by producers and farmers on the basis of different previous research findings [7]. Accordingly, vaccine failure is caused due to lack of developing adequate immunization of birds after vaccination and exposes them to different infections [1].
In developing countries, the most prevailing problems related with high rates of vaccination failures in poultry flocks are improper transportation, handling, storage, and administration of vaccines [2,6]. Consequently, this paper was organized to enlighten about vaccine failure in poultry and its control measures. Thus, vaccine failure can be caused due to lack of proper handling (failure to properly store the vaccine at a suitable temperature) and improper administration of vaccines that may result in failure in immune response in apparently healthy birds. The potency of vaccines is maintained to a certain period; however, its viability may be lost due to past their expiration date even it is stored under the appropriate temperature. Moreover, the vaccine can be inactivated if it is mixed with different types of residual disinfectant on the syringes and needles [13,23].

Antigenic differences between Existing Vaccine and Field
Strains: Lack of vaccine efficacy due to antigenic variation amongst the vaccine and field strains (such as antigenic drift, antigenic shift) are usually a problem with killed vaccines than modified live vaccines. Accordingly, effective antibody-mediated immunity is developed if the antibodies exactly bind to the specific antigenic strain on the bacterial or viral surface. In contrast to antibodymediated immunity, cell-mediated immunity lacks strain specific immunization. Thus, it is necessary to isolate the circulating field strain and compare it with the existing vaccine strain [3,21].

Inadequate Level of protection:
The vaccine strain should elicit effective and sufficient immune response, and this depends of the antigenicity of viral strain. Eggs are the source of maternal antibodies or passive immunity in poultry and remains effective for the first two weeks. After proper vaccination, most poultry vaccines reach their peak response typically between 2-6 weeks and the protection or immunity level then starts to gradually decline. If birds are typically vaccinated with virulent infectious agents during these periods of time where protection by maternal antibodies reaches high, the vaccine may fail to stimulate immune response due to neutralization [4,8]. Even though birds are frequently vaccinated, the bird may become susceptible to different poultry diseases before they develop their own active immune responses.
It is advised to give booster dose on the recommended schedule to develop the better protection level [6,24].

Factors associated with the Host/Birds
After proper vaccination, the host requires few days to develop an effective immune response. If the birds infected by an infectious agent before and during the time of vaccination, the vaccine fails to produce sufficient antibodies resulting in apparent vaccination failure. Moreover, the following limitations are considered as the common cause for vaccine failure [7].

Stress Factors:
Stress is a non-specific response to change in the environment that poses an excessive demand on the physiological and behavioral adaption abilities of the birds. The body composition of birds normally has inadequate resources to defend itself from diseases and environmental changes. Different predisposing factors cause stress in poultry birds. This includes stress due to poor ventilation, cold, heat, excess humidity, transportation, high stocking density (overcrowding), low spacing, improper sanitation, excess load of litter, under and malnourishment (nutritional deficiency), parasitism, fever, and so on [4,23]. Since poultry birds are very much sensitive to extreme weather conditions, health-related and other management problems and this, in turn, lead to reduced immune response. Accordingly, chickens become generally unresponsive to vaccinations and are susceptible to infection during disease outbreaks. Furthermore, live vaccines can cause huge outbreaks if it is administered to immunocompromised chickens. Hence, stress predisposes chicken to reduced immunity, vaccine failure and economic losses due to low production [1].
Interference with Maternal Antibodies/ Immunity: Poultry are regularly vaccinated against different infectious diseases on the basis of their disease prevalence and the newly hatched chicks are passively immunized due to maternal antibodies in their blood.
According to some research findings, maternal immunity can protect the chickens from various infectious diseases such as Newcastle disease virus and Gumboro (IBD) disease during the first week of birth. However, these maternal antibodies have potential to interact with the vaccine antigens and result in neutralizing antigen. Thus, vaccines such as live vaccines affect the development of immunity production by reducing the level of antibodies in the newly hatched chicken if they are vaccinated for first weeks [13,25].

Immunosuppressive and Coexisting Diseases:
Immunosuppression is caused due to compromised immune response and resulted in increased risk of infectious diseases in flocks of birds such as mycotoxicosis, infectious bursal diseases (Gumboro), infectious anemia, Marek's disease, and others. All this stress conditions resulted in impaired immune response that may also lead to vaccine failure and affect effective immunization [6,13].
This may result in the development of limited protection from vaccination and an excessive vaccine reaction including morbidity and mortality [26]. Vaccines are expected to vigorously stimulate the immune system. Thus, the presence of any concurrent mild local and systemic diseases may result in reduced vaccine efficacy.
Additionally, the effectiveness of vaccine will be reduced if the infected or stressed birds are vaccinated for the same disease since the vaccine antigens are neutralized due to reaction with naturally produced antibodies against infectious pathogens. This causes vaccine reaction in birds and the disease condition may worsen and leads increased morbidity and fatality rate. Accordingly, it is highly recommended and important to secure the health status of birds before vaccination [2,4,6].
Genetic Factor: Response to vaccine varies depends on the type of species of birds. The structure of major histocompatibility complex (MHC) are responsible for the individual variation of birds and this helps to determine the response of birds towards the bacterial and viral antigens. Birds might be more susceptible to pathogen due to the lack some structure in MHC helps to recognize one of the antigens [21,27].

Lack of proper Storage and Vaccine Instability in Cold
Chain: Vaccines should be properly stored, transported, and applied as per the manufacturer's recommendations. In developing countries, the common encountered problems during vaccine storage that led to inactivation of vaccine are shortage of storage equipment, inappropriate storage temperature, lack of functional and effective refrigerators, mixing vaccines with other food items, interaction of vaccine with the disinfecting agents [23]. According to Nelson et al., [28] trying to store or transport the vaccine without proper preservation, handling and following cold chain results in denatured antigen thereby ends with vaccine failure. Antigens are killed when the vaccine is exposed to direct sunlight which leads to reduced antigen concentration in the vaccine and vaccine effectiveness. Therefore, the vaccine should be well placed and protected from direct sunlight for better efficiency [2,6]. it will be resulted in inadequate dosage that causes low level of vaccine titers and inactivation of vaccine [17].

Improper Formulation of Vaccine and Diluent used:
Poultry vaccines should be prepared and properly formulated using specific diluent (saline water) as per the manufacturer's guidelines. Some diluents are specific for a single vaccine (diluents for lyophilized vaccines) and contains some preservatives that can demolish the activity of the other vaccine. Improper dilution and lack standard procedure of vaccines formulation using the diluent in a single syringe may also decrease the potency of vaccines. Thus, during formulating different type of vaccines in a single syringe, the possible outcome after interaction should be adequately examined [6,30].  [4,10]. During the time of vaccination, it is important to consider the age of the bird. Some antigenic receptors are age specific since the develop starting from day old (as early as they hatch of a chick) to old age. In early age, birds start to develop the receptors of Newcastle disease, infectious bronchitis, and infectious bursal disease (IBD) whereas the receptors for infectious bursa disease and fowl pox develop with advancing age. Accordingly, it is advisable to vaccinate birds after development of this receptors in the bodies since lack of receptors for the antigen in the host result in vaccine failure [4,13]. However, low level of antibody titers is caused due to the absence of booster dose that in turn results in failure of vaccine [4].

Lack of Strict Biosecurity: Biosecurity is a compulsory
comprehensive practice in poultry farms to protect the farms both from intentional and unintentional biological threats. Thus, the lack of inclusive biosecurity strategies makes susceptible to different poultry disease outbreaks. This makes the chicken immunocompromised and contributes to vaccine failure as a result of the change of the vaccine strain to pathogenic one [22]. Accordingly, numerous practical aspects affect vaccination and contribute to vaccine failure (Table 1).

Factor Description
Maternal immunity Earlier interfere with antigens of the vaccine Concurrent disease with immunosuppressive agents Birds immune system fail to produce sufficient immune response against the infectious agents Mismatch between field virus and the vaccine (Pathotype of field strain) Lack of antibodies similarity between the circulating viral strain (field virus) and the vaccine strain to produce full protection The quantity of Antigen and immunogenicity of the vaccine An adequate immune response is not induced if a load of antigen is not enough to elicit immune response.
Health status of the chickens When the bird's immune system does not produce a sufficient response Vaccine coverage (the total number of birds vaccinated) Large number of birds must be protected to restrict the transmission Species or breed of bird During preparation, the target species should be considered for optimal response Duration of immunity or Immunocompromised birds are exposed to infection and disease since the immunity declines Quality of administration An optimal and full dose should be administered properly with proper equipment.

The Time or Speed to develop optimum level of protection after vaccination
Any exposure before sufficient development of sufficient immunity may lead to infection Note: Source: [22].

Mitigation Strategies for Vaccine Failure
Poultry vaccines are commonly used in several types of production systems. A cost-benefit assessment should be conducted to identify the suitable approach available before implementing the control options, while considering various situations [8]. For instance, applying stamping out measures to control major poultry diseases poses a great impact on the long-term sustainability of farm production and on the livelihood of smallholder farmers in developing countries, where there are limited compensation measures [13]. Thus, cost-benefit analysis should consider the following conditions such as: the species of birds, the production system, fowl densities, the antigenicity/virulence of the viral strain involved, provision of veterinary facilities, and the impact on trade [4,12,27]. The following strategies are considered as a best option to control the major infectious and contagious poultry diseases.

Proper Formulation of the Vaccines
It is always recommended to follow the instruction provided by

Proper Storage and Maintain Cold Chain Temperature
Poultry vaccines should be properly stored and transported at the optimum temperature as per the manufacturer's standard since temperature variation has a direct effect on the efficacy of the vaccine [4,21]. Besides, during storage the vaccine must be stored separately from other items such as food item, chemical, pathological samples.
For this reason, certain live vaccines like Marek's disease vaccines are easily inactivated due to improper handled and storage [13,23].
The proper cold chain temperature should be maintained to keep the viability of the vaccine. However, there are several factors that affect the cold chain temperature like shortage of electric source and lack of cooling systems (refrigerator), over chilling, and lack of vaccine transporting equipment (ice box). Likewise, the extra chilling of oil-based vaccines results in reduced potency of vaccines due to crystal formation of adjuvants (aluminum salts) in the vaccines [13]. Thermostable vaccines are produced in such a way to cope up with the fluctuation of temperature such as cold and hot environments, while freeze-dried vaccines should be stored in the refrigerator at 4°C and transported using ice blocks/cooling system to keep at the recommended temperatures [13,23]. The use of thermostable vaccines can be an alternative to overcome the difficulties related to cold chain and storage temperature [23,33].

Avoiding Use of Expired Vaccines
Most poultry vaccines that are produced worldwide have

Use of Adjuvant and Stabilizers
An adjuvant is a supplementary substance that are added to the vaccine to increase the bioavailability of vaccines and to enhance immune response to the target antigen [34]. This in turn, helps to increase the immunogenicity of vaccines to confer long-term protection and to reduce the required antigenic dose to produce effective immunity [31]. Adjuvants have a potential effect on inducing a mucosal immune response that help to improve the safety of the vaccine. For instance, vaccines given orally faces a great difficulty due to the microbial barriers in gastrointestinal tract and the antigen must be protected using adjuvants to overcome this problem that helps to activate the immune response [1334].
A combination adjuvant platform is promising and beneficial for suboptimal vaccines and particularly advantageous for vaccines against specific and more susceptible flocks of poultry populations.
In contrast single adjuvants, combined adjuvants act synergistically by stimulating and activating various types of immune cells such as dendritic cells, macrophages, lymphocytes, etc [31].
A stabilizer is a substance that is added to a vaccine in order to and Vital Blue are the best stabilizers. Besides, locally available materials like skimmed milk at the rate of 2 g/L can also be added to vaccine and used as substitute for other stabilizers [21,32]. should be taken after identifying the source of the stress such as reducing the bird density, increasing the number of feeders and drinkers, improving the ventilation and others [8,13].

Controlling Stress and Use of Antimicrobials
In poultry production, Antimicrobial can be used pre and post vaccination and are given to minimize and prevent the development of a high risk of secondary bacterial infections due to opportunistic microorganisms during the periods of stress.
When selecting the antimicrobials compatible with the immune defense system, spectrum of activity, and its potential against the pathogenic opportunistic microbes. For instance, antimicrobial like Erythromycin have a special immunomodulating capacity through stimulating the secretion of interleukins by leukocytes that help to maintain strong immunity for the birds [27,35]. However, the vaccines should not be mixed with the antimicrobials. Thus, there should be sufficient duration of time for the antimicrobials and the antimicrobial should be administered before minimum of two days before and after administering the vaccination [4,8].

Administration of Vitamin and Mineral
Vitamins and minerals should be provided to . [13,36]. According to studies, the supplementation of additional vitamin and mineral is necessary to maximize the resistance to disease since routine poultry feed may not be sufficient to fulfil the demand of normal immunity [37,38].
The combination of vitamins (A, D3, E, B-group water-soluble vitamins), minerals (selenium) and amino acids have a great role in immune modulation and stimulating humoral and cellular immunity and also most effective mechanism to tackle vaccine [18,39].

Correct Vaccination Schedule and Strick Biosecurity
A proper vaccination program should be there for poultry flocks depending on the prevalence of the disease in the area in order to minimize disease outbreaks and financial loss associated with the disease. it is better to vaccinate the birds prior to the incidence of the disease so that the birds develop the required amount of antibody titration. For instance, vaccination against infectious bronchitis and avian influenza should be given before winter period since the outbreak of diseases are commonly occurs in birds during winter [18,21]. Besides, biosecurity is the most important and crucial component of any poultry outbreak prevention and control strategy and should be maintained at a high level to complement vaccine use. In addition, it is advised to follow strict stamping-out (all in all-out) principles to avoid cross-contamination between flocks [4,22].

Conclusion and Recommendations
Infectious Poultry diseases are threat to the poultry producers guidelines for storage, timing, due dates, and administration. Also, chicken farmers and managers should consult veterinarians before vaccine administration, and also, they should monitor the health status of chickens before vaccination.
The goal of vaccination needs to be clearly defined and the farmer must take all the necessary steps to control both the practices used during the vaccination operation and external factors. Furthermore, vaccination helps to enhances the capacity to control the poultry diseases and should be implemented along with other disease control and eradication measures.