The downside of broiler vaccination

Vaccination has, without any doubt, contributed very significantly to the health and welfare of man and animals including birds in poultry farming. However, vaccination may have negative side effects, which are often underestimated and should be counterbalanced against the benefits.

Frequently, broiler chickens are vaccinated with live attenuated vaccines against three viral diseases, i.e. Newcastle disease (ND), infectious bronchitis (IB) and infectious bursal disease (IBD). Generally, birds are vaccinated only once against each of these diseases as repeated vaccination is considered to be too expensive.

Although the efficacy of single live attenuated vaccines in small scale laboratory experiments as a rule is satisfactory, in daily practice they show a great variation in protective immunity, leaving flocks insufficiently protected against field challenge in the majority of cases (de Wit personal communication 2012; van Eck & van de Rijt, non-published results 1992–1993) (Goren 1979; Voeten et al. 1987; de Wit et al. 2010). For example, this became notably apparent during the epizootic of ND in the Netherlands in 1992, when the paramyxovirus of concern was able to spread rapidly also in broiler flocks, which had been compulsory vaccinated against the disease once. Thus, a false sense of security is provided, while the downside effects of live vaccines remain.

Insufficient effectiveness is due to bird- and vaccination-related factors. As bird-related factors the following should be considered: (1) Broilers are in most cases vaccinated at an age at which the birds lack a mature immune system and still possess maternal antibodies. (2) There is always an immunologic gap between vanishing maternal antibodies and the development of a protective immune response after vaccination. (3) Due to the short life span of broilers, the period during which the birds enjoy protective immunity is very short. Vaccination-related factors affecting effectiveness are amongst others: (1). The (lack of) meticulousness with which the vaccine administration is performed. (2) The vaccine virus loss during mass application, which may result in an insufficient vaccine dose per bird (Corbanie et al. 2008; Huyge 2012).

Vaccine viruses replicate in the intestinal (ND, IB and IBD vaccine viruses) and/or respiratory tract (ND and IB vaccine viruses), which may result in a number of downside effects. Vaccinations with these viruses may induce growth retardation, which is not always redeemed by compensatory growth. Moreover, IBD vaccines induce various degrees of immunosuppression paralleling the extent of attenuation of the vaccine strains used (Luckert and Saif 2003). Due to replication in the respiratory tract ND and IB vaccines provoke respiratory distress and ND vaccines even sometimes suffocation, but most importantly, both vaccines significantly increase the susceptibility of broilers to colibacillosis. The latter is a disease with great clinical and economic relevance to the broiler industry worldwide and in part responsible for the large scale use of antibiotics. No significant difference in the ability to increase the susceptibility to colibacillosis was found between IB vaccine and virulent virus (Matthijs et al. 2003); and the same proved to be true for mild and more virulent ND vaccines (van Eck and Goren 1991). Both, ND and IB vaccines are known to exacerbate Mycoplasma gallisepticum and M. synoviae lesions in the respiratory tract as synergism between these bacteria and named vaccines exists (Kleven et al. 1972; Villegas et al. 1976; Rodriguez and Kleven 1980; Hopkins and Yoder 1982).

The true significance of vaccine virus replication in the digestive tract is hardly known and merits investigation as it could also contribute to the downside of broiler vaccination.

Considering IB, it is worth mentioning that the difference in virulence between the vaccine and virulent field virus is relatively small. In fact, IB vaccination might be considered more or less as ‘planed exposure’. Moreover, the regular emergence of variant strains of IB virus hinders a successful control of the disease.

In view of what has been mentioned previously it is advisable to vaccinate broilers following a ‘switch-on, switch-off’ system, i.e. vaccinations against ND, IB and IBD should only be performed in case of a serious threat of infection with virulent field virus. These vaccinations should then be carried out in such a way that vaccinated flocks can withstand a challenge with virulent field virus and the spread of infection is halted. In order to achieve this, vaccinations should be performed at least twice, whereby follow-up vaccinations should be done with less attenuated vaccines and/or by a mass application method inducing a stronger immune response. Only then a positive balance between effectiveness and the downsides of vaccinations can be expected.