Impacts of poultry vaccination on viruses of wild bird
Introduction
Global poultry production is increasing rapidly, with chicken meat forecast to become the largest meat sector worldwide from 2020 [1]. The potential for viruses in wild birds, which are often considered reservoirs of infection, to ‘spillover’ into poultry and cause disease has long been recognised. Thus biosecurity programmes in poultry operations have been developed to help prevent the direct or indirect transmission of pathogens from wild birds to poultry [2]. Virus transmission in the other direction (from poultry into wild birds) lacks the immediate economic consequences that viral incursions into commercial poultry can have and has not been as thoroughly considered. Consequently, fewer biosecurity measures are in place to prevent such transmission events. This is despite the massive scale of modern commercial poultry production which can enable large-scale contamination of the environment with infectious material through practices such as disposal or re-use of poultry litter [3] or through airborne dispersal of infectious agents from infected flocks [4]. Furthermore, an increased interest in free-range poultry production allows increased direct contact between wild birds and farmed poultry [2].
A developing awareness of the concept of ‘one health’, which recognises connections between animal, environmental and human health, including at the livestock-wildlife interface [5, 6] has served to highlight the risks that poultry viruses may pose to the environment and to wild birds. The potential for spillover exists when the natural host range of a virus includes a domestic poultry species and a wild bird species. As viruses in wild birds are much less studied than those in poultry, the natural host ranges of many avian viruses are not well understood. Similarly, the degree of host susceptibility to disease caused by different viruses is often not known. However, some poultry viruses are known to have broad host ranges (e.g. avian paramyxovirus-1, also called Newcastle disease virus, NDV) and are thus a potential risk to a wide range of wild bird species [7]. Other poultry viruses have narrow host ranges (e.g. some avian herpesviruses). These may only be a risk to small number of wild bird species that are closely related to farmed poultry species, such as other Galliform or Anseriform birds [8, 9]. Without appropriate surveillance programmes in wild bird populations, spillover events from poultry into wild birds are unlikely to be detected.
Vaccines, along with biosecurity programmes, are critical to the control of viral diseases in poultry. Table 1 summarises vaccines used for this purpose and also lists the genus and family names of each virus. These vaccines typically reduce clinical signs of disease but do not prevent virus infection [2]. Vaccination can have a profound influence on virus populations within commercial poultry flocks, and, by extension, viruses that may spillover into wild bird populations. Poultry vaccines may have an impact on wild bird viruses through the transmission of attenuated vaccines from poultry to wild birds; the transmission of virulent viruses that have evolved in response to the use of poultry vaccines; or through undetected viral contaminants within commercial vaccines being transmitted to wild birds. New, vectored poultry vaccines also have the potential to have an impact on wild bird viruses. These vaccines use a virus vector to express heterologous (foreign) proteins from other poultry pathogens. These four mechanisms through which poultry vaccines may affect wild bird viruses (Figure 1) will be explored in further detail below, with a focus on recent contributions to this field and consideration of the potential risks to wild birds.
Section snippets
Attenuated vaccine viruses transmitted from poultry to wild birds
Spillover of vaccine viruses from poultry into wild birds can only occur with live vaccines. Attenuated vaccines are commonly used in poultry (Table 1) as they are relatively inexpensive to produce and administer, particularly when they facilitate mass administration by drinking water, aerosol or spray. These vaccines also often induce more effective immunity than inactivated vaccines [2]. Prior to their registration, the safety characteristics of attenuated poultry vaccines are examined
The influence of vaccines on the evolution of poultry viruses
Vaccines can influence the evolution of viruses within poultry flocks. Selection for viruses that ‘escape’ vaccine-induced immunity can occur when there is incomplete cross-protection between viral serotypes or antigenic types. This has been reported for a number of poultry viruses, particularly avian influenza virus (AIV) [16•, 17], but also other viruses, including IBV [15•] and NDV [18]. The viruses that persist in poultry despite (or due to) the use of vaccines can then be transmitted to
Viral contaminants of poultry vaccines
Contamination of attenuated vaccines with viruses used to generate inactivated viruses in the same facility, or with viruses circulating in flocks used for production of embryonated eggs for vaccine production, has the potential to contribute to dissemination of virulent viruses into wild birds through poultry vaccines. In addition, inadequate inactivation of virulent viruses used to produce inactivated vaccines may also result in dissemination of viruses that pose a risk to wild birds.
New generation (vectored) vaccines
A recent dramatic expansion in the use of viral vectored poultry vaccines may have impacts on viruses in wild birds. Most vectored poultry vaccines use double-stranded DNA viruses as vectors. Herpesvirus of turkey (HVT) and FPV are used commercially as vaccine vectors to express genes from other poultry pathogens, including AIV, IBDV, NDV and infectious laryngotracheitis virus (ILTV) (Table 1). Fowl adenovirus (FAdV) and a number of avian herpesviruses (ILTV, MDV-1 and duck enteritis virus)
Conclusion
Vaccines are crucial for achieving high standards of animal production, health and welfare in farmed poultry, but many of the impacts of poultry vaccines on wild bird viruses are incompletely characterised. This is not unexpected given the difficulties involved in studying viruses in wild birds and the historical focus on wild birds as a potential source of infection for farmed poultry. As the scale of poultry production grows it will become increasingly important to better characterise these
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
Joanne Devlin is supported by a Future Fellowship from the Australian Research Council (FT140101287).
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