FACTORS WHICH INTERFERE WITH VACCINE EFFICACY
Paul McMullin
Proceedings of the 1st Sta. Catarina Poultry Symposium pp10-20 (1985)

SUMMARY

Factors which interfere with immunization of commercial poultry can be divided into three main groups. They are: factors associated with the vaccine itself, those of vaccine administration, and those which are endogenous to the bird. When investigating suspected efficacy problems in the field all alternatives must be taken into account. The most serious problems occur when two or more of these factors are involved. Specific examples can be given of how many of factors affect vaccination under practical conditions. Possible reasons for an incorrect diagnosis of low vaccine efficacy should also be considered.

(key words)POULTRY, VACCINE FAILURE, NEWCASTLE DISEASE,INFECTIOUS BRONCHITIS, INFECTIOUS BURSAL DISEASE

Suspicions about the efficacy of an immunization program in commercial poultry usually arise from the occurence of one or more of the situations shown in table 1. If serological surveillance is practiced problems may be detected by unusually low titres indicating that the vaccine did not induced the normal level of circulating antibody. However it may also be detected by unusually high serological titres indicating that the vaccine did not provide adequate protection against field challenge. In order to be able to detect immnunization problems with serology it is absolutely necessary that a normal range of serological response be estabilished first (MC MULLIN, 1984). Clinical disease is an obvious motive for suspecting vaccine failure, but altered productivity or unusual vaccine reactions are also good rasons to check the imunization program.There are also a number of factors which can lead us to erroneously suspect vaccine efficacy:

1. Non-vaccination - It could be said that a significant cause of vaccine failure is non-vaccination! At least this possibility should be kept in mind when there are isolated occurences. The flock may not have been vaccinated due to oversight, or delierately because of intercurrent disease, fear of vaccine reactions, etc.
2. Early and/or excessive challenge - Immunization requires a period post-vaccination to reach peak resistance. The degree and nature (local/systemic) of resistance and the length of this periods differs among the different types of vaccine used in poultry. If a vaccine fails to fully protect against a disease because the birds were infected prior to or soon after vaccination this is only an apparent vaccine failure.Very early exposure to marek's disease virus or infectious bursal disease virus, for example, can result in disease because of this factor.
3. Variation in methodology used to measure efficacy - Vaccine efficacy may be measured by occurence of clinical disease, productivity parameters, serology and resistance to artificial challenge. Whichever method of methods used the possibility that an apparent variation in response to vaccination is simply due to variation in the method should be kept in mind. Variation in serological titres among operators and laboratories can occur (THORNTON & ALLAN, 1982) and may lead to erroneous conclusions about vaccine efficacy.
4. Unrealistic expectations of efficacy - Vaccines cannot reasonably be expected to project 100% of the flock under commercial poultry conditions. The actual protection obtained will be determined by the sum of all the factors which can affect vaccine efficacy. However it will never be greater than the maximum obtainable under experimental conditions for a given vaccine.Table 2 shows a check list of factors which should be considered when dealing with a suspected problem of low vaccine efficacy. It seems appropriate to look at each of these factors individually and give some examples of how they affect immunization.

1. The vaccine itself

All of The factors associated with the vaccine itself tend to be closely inter-related. A deficiency in one can be partially compensated by another. A vaccine of moderate-to-poor titre may give satisfactory results if very carefully applied, while it may be a disaster if poorly applied.

I. Titre and Stability
It is self-evident that the live-virus vaccines must have an adequate titre and this titre must have sufficient stability so that under normal conditions it can cause an infection of appropriate intensity. Stability of live-virus vaccines is affected by the success of lyophilization and the temperature under which it is stored. Periods of validity must me strictly followed, or the vaccine re-titrated.With the viruses which replicate in the respiratory tract titre can influence the degree of vaccine reaction and generally speaking this is proportional o degree of protection (BEARD & BRUGH, 1975).

II. Serotype and Biotype
It is possible to encounter considerable variation among commercial vaccines in the type of virus used. Nagi et al. (1980) studied 3 commercial vaccines against infectious bursal disease and found marked differences, both in titre of antibody produced, and in persistence of the virus in the tissues. Muskett et al. (1979) in a similar study found one "vaccine" to be highly pathogenic in young chicks. Winterfield and fadly (1972) also demonstrated considerable variability in response among 6 different commercial vaccines against infectious bronchitis.The biological characteristics or strains used in live-virus vaccines have a great influence on the process of immunization. As general rule, the greater the invasiveness of an organism the greater will be the immunity produced - if the bird survives! It is frenquently necessary to achive a compromise in terms of efficacy-reaction. By using a different serotype vaccine virus it is possible, in some cases, to circumvent maternal immunity and produce an earlier vaccine response. King et al. (1981), have shown that the SB-1 strain of Marek's disease vaccine can be used in substitution for the convencional HVT strain to this effect. However this approach can only be expected to be useful where the new serotype continues to protect against the virulent serotypes present in the field. Occasionally field problems occur as the result of the emergence of a pathogenic strain which is serologically distinct from the standard strain (the new strain is often called a variant strain). The natural serological heterogeneity of infectious bronchitis virus lends itself to this type of occurence. A variant strain of coronavirus has been shown to be involved in facial cellulitis or "swollen head" in broilers (MORLEY & THOMPSON, 1983). This virus is serologically distinct from infectious bronchitis virus.To control this problem a vaccine was developed by attenuating a field strain of virus by serial passage in embryos.

III. Inactivation and Adjuvant
These factors have similar importance for inactivated vaccines as do liophilization and titre for live vaccine. Type and quality of emulsion can influence the serological response to oil-adjuvant vaccines.

2. Administration of the vaccine to the bird.
I. Route
The route by which the vaccine is administered can affect the outcome of vaccination.The superior protection against Newcastle disease afforded by aerosol administration of live-virus vaccines is well documented (BEARD & EASTERDAY, 1967). Winterfield et al. (1980) have shown that aerosol vaccinated birds are also more resistant to infection with virulent virus than those vaccinated only with inactivated vaccine by the intra-muscular route. However aerosol administration of some mild vaccine strains to day-old chicks from nonimmune parents can kill (BEARD & BRUGH, 1975). In a study designed to compare different routes of Newcastle disease vaccination with B1 strain in day-old chicks, Eidsen and Kleven (1976) found the aerosol route to provide the best protection, followed by the ocular route.
II. Uniformity
In ultimate analysis it is essential that the antigen present in the vaccine is uniformly distributed within the flock. The use of "mass vaccination" (drinking water and aerosol) tends towards less uniformity in application than individual application, and need considerable operator care in order to control this tendency.Even with individual application, problems of uniformity of application can occur due to poorly adjusted syringes, faulty droppers, etc.Training and periodic checking of the operator can avoid pitfalls and improve uniformity of application. Live vaccines which allow some lateral spread of the immunizing virus among birds reduce the necessity for uniformity at time of application.

III. Association. Adiministration of certain combinations of live vvirus vaccines may affect the response to each virus, especially when they contain viruses which have the same target tissues (BEARD & BRUGH, 1975). It should be remembered, however, that in industrial poultry production the aim is maximum productivity an not necessarily maximum protection against a given virus. Compromises must be reached. There are a number of live-virus combinationscommonly used in practice. A combination Marek's disease/Fowl pox vaccination at one day of age has been shown to provide acceptable protection against both diseases (HEREDIA, 1977). Care must be taken with the choice of the pox vaccine used. Use of a high virulence and/or high titre embryo-adapted strain can cause serious lesions in inoculated chicks and considerable mortality.

IV. Programs. Immunization against infectious disease is rarely dependent on a single inoculation. In the case of those diseases for which only one or a few applications are made, efficacy usually relies on the persistence of the vaccine agent for a long period in the bird (eg. Marek's disease vaccine). The importance of the vaccination program lies in the immunological phenomenun called the "anamnestic response". This refers to the ability of the lymphoid tissues to recognize and respond to antigens to which they have already been exposed. This responsen is usually more prompt and greater than the response which occurred when the bird was first exposed. To achieve hyper-immunization of breeder hens against infectious bursal disease it is necessary to prime them by exposure to a live virus, with later vaccination using an inactivated vaccine in adjuvant (WYETH & CULLEN, 1979). In some cases systemic administration of an inactivaed vaccine may permit an anamnestic response inlocal antibody when challenge occurs at a mucosal surface. This has been should to be possible with nasal infection with infctious bronchitis (HOLMES, 1973).

V. Diluent. The diluent used for live virus vaccines is very important to ensure that an adequate titre of virus actually reaches the birds. The classical problem of administering live-virus vaccine in chlorinated drinking water is well know, but less extreme.

3. Bird (endogenous factors)
I. Previous exposure. The importance of adequatre priming by prior exposure to the agent has been discussed above. It could be added that repeated exposure over too short a period may not be advantageous. Common practice dictates that the same vaccine should not be re-applied to a flock within 14 days.

II. Passive protection. Circulating antibody may effect the response to vaccination, even independently from the previous factor, i.e. when it is not produced by the bird itsel. This may come about in two ways. Hyperimmune antiserum may be injected to provide passive protection but this is rarely used in commercial poultry today. The commonest source of passive protection is that transmitted from the breeder bird to her chick via the yolk. The baby chick has circulating antibodies in similar concentrations to those found in the breeder at 1-3 days of age. They fall to undetectable titres by 14 - 30 days (depending on the method of detection used). There is no doubt that maternal antibody can influence the response to vaccination during the first weeks of life. The type and degree of this effect is no always what might be expected. Aerosol vacciantion of day-old chicks from immune parents against infectious bronchitis produces immunity as good as birds vaccinated at 15-20 days (DAVELAAR & KOUWENHOVEN, 1977). These authors suggest that viral replication in the harderian gland plays a significant part in this phenomenon. With infectious bursal disease, on the other hand, maternal antibody plays an important part in avoiding infection and the adverse effects of field virus, but unfortunately it also prevents immunization with live-virus vaccines (MUSKETT et al 1979). The broiler productivity gains obtained by breeder bird vaccination would seem to make this the method of choice for routine control for early exposure to infectious bursal disease, at least in broilers (WYETH et al. 1979,1981). One large-scale field study indicated that use of a live vaccine in chicks from breeders which had received oil-emulsion vaccine against infectious bursal disease adversely affected the productivity and condemnation parameters (HENRY & WILLIAMS, 1980). This seems to indicate that a live-virus vaccine may reduce the efficacy of protection provided by maternal antibody, at least in the case of infectious bursal disease.

III. Immunosuppression
Stress of any sort is well known to reduce disease resistance and can also be expected to affect response to vaccination. Exceptionally poor environmental conditions could contribute to vaccination failure under some circumstances.The three infectious agents most associated with the immunological system and most capable of producing immunosuppression are infectious bursal (Gumboro) disease virus, Chick Anaemia Virus and Marek's disease virus. The latter can produce serious immunosuppression when it infects susceptible chicks early in life, but its other effects make it fairly easy to recognize.In a large series of samples (40 flocks) taken in 1980 from both normal flocks and laboratory submissions, in the state of Sao Paulo, all flocks had birds serologically positive for infectious bursal disease. All flocks were over 45 days of age, thus this antibody was not maternal in origin, and vaccination was not widespread at that time. Infection with Gumboro disease seems to be very cammon in broilers.Infectious bursal disease virus is capable of causing a marked deplention of bursa-dependent lymphocytes and some depression of thymus-dependent lymphocytes. This effect is very marked in birds infected at one day of age and is relatively small in chicks infected at 3 weeks of age (SIVANANDAN & MAHESWARAN, 1980). It has been well documented that early infection with infectious bursal disease virus in susceptible chicks can produce long-lasting immunosuppression against many diseases, The following examples may be cited: Newcastle disease (MUSKETT et al 1979), Marek's disease (JEN & CHO, 1980). infectious anaemia (YUASA et al 1980), and pullorum disease (LUCIO & HITCHNER, 1980). Some strains of infectious bursal disease virus which have been used as vaccines have been shown be capable of producing significant immunosuppression when administered to young chicks (MUSKETT et al. 1979). The fact that most broiler flocks are exposed Gumboro virus does not mean that most are suffering from immunosuppression. Serious immunosuppression problems are usually associated with infection of chicks (whose maternal antibody titre is low) during the first and second week of life. Based on the daa presented by Muskett et al. (1979) affected broiler flocks can be expected to have many chicks with a bursal weight/bodyweight ration of 1g/kg or less.Mycotoxins are another possible cause of poor antibody response. Aflatoxin in the diet at levels as low as 0.625 ppm have been shown to significantly decrease the weight of the bursa of Fabricius and delay antibody production (THAXTON et al. 1974).

TABLE 1. Reasons for doubting vaccine efficacy.
A. Low or high serological titres.
B. Clinical disease.
C. Reduced productivity.
D. Abnormal "vaccine reactions".

TABLE 2.
Factors which interfere with vaccine efficiency.
1. Vaccine I. Titre and stability
II. Serotype and biotype III.
Inactivation and adjuvant 2. Administration
I. Route II. Uniformity
III. Association IV. Program
V. Diluent 3. Bird (endogenous factors)
I. Previous exposure II. Passive
protection III. Immunosuppression

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