Spotlight on avian pathology: Salmonella – new wine and old bottles

ABSTRACT

Salmonella enterica remains an important avian and human pathogen. Control has been effective in some countries but the hygiene and biosecurity required may not be possible everywhere. Antibiotic resistance is an increasing problem for both veterinary and human medicine. This short review commentary highlights existing and potential new control measures including legislation, hygiene and biosecurity, use of live and inactivated vaccines, and bacteriophages to tackle intestinal colonization, reduce the prevalence of antibiotic resistance and improve carcass decontamination.

KEYWORDS:

• Salmonella
• biosecurity
• hygiene
• control
• antibiotics
• antibiotic resistance
• vaccines
• bacteriophage

Salmonella enterica subsp. enterica remains a major cause of food-poisoning in much of the world, arising from disease-free intestinal colonization of domestic fowl and ducks by most of the more than 2,500 serovars. It is also still an important cause of morbidity and mortality in backyard poultry flocks and where open-sided housing is normal as a result of high ambient temperatures.

The 1980/1990s pandemic involving a small number of phage types of Salmonella Enteritidis led to a few of countries, mainly in the EU, developing a legislative platform to counter the problem, involving routine testing of breeding and laying flocks to reduce transmission to progeny and reduce infection via table eggs, and improved testing and quality control of feed components (De Smet & Mäkelä 2013). This has been tightened and broadened with risk assessments and management targets and includes turkeys in addition to domestic fowl (https://ec.europa.eu/food/safety/biosafety/food_borne_diseases/salmonella_en), and upcoming serovars such as Infantis, Harar and Virchow, in addition to Typhimurium and its monophasic variant and Enteritidis. This approach, implemented gradually over more than 20 years, has resulted in the virtual elimination of food-poisoning serovars from the poultry industry of some countries. Clearly, such an approach may not be directly applicable in countries where the potential for biosecurity may be limited, and considerable thought is therefore needed to develop and adopt the most appropriate approaches to Salmonella infection control.

In addition, there is increasing concern over increasing levels of antimicrobial resistance (AMR) in strains of Salmonella, Escherichia coli and other related bacteria as a result of unregulated and routine prophylaxis and growth promotion (Monroe & Polk 2000; O’Neill 2014; Ventola 2015). This includes resistance to some of the antibiotics of critical importance, the so-called last chance antibiotics reserved for infections caused by multi-resistant pathogens. Resistance to one of these, colistin, used to result from a rare chromosomal mutation but it has recently become plasmid-mediated and has now spread globally (Liu et al. 2016; Apostolakos & Piccalilli 2018). Tightening restrictions on antibiotic use generally, and complete withdrawal as growth-promoters, is essential but one wonders whether “the horse has bolted” already given the slow rate with which antibiotic resistance declines after cessation of use (Smith & Lovell 1981). New approaches are clearly demanded (WHO 2014; 2017).

Where housing biosecurity is limited, a level of protection is possible with the use of a number of live vaccines all of which generate a degree of protection. However, vaccination on its own is never enough and implementation without additional measures will be much less effective as seen in East Germany in the 1980s with the use of a live vaccine which reduced levels of Typhimurium in poultry and pigs, but only when used together with improved hygiene and biosecurity (Meyer et al. 1993). Oral administration of live vaccines to newly hatched chicks can in theory also generate early protection by a competitive exclusion effect and by stimulation of cross-protective innate immunity in the intestine (Van Immerseel et al. 2005).

Live vaccines will always be more protective than inactivated vaccines but it might be possible to improve the latter. The inactivated Salenva vaccine (Woodward et al. 2002) was generated by culturing the bacteria under conditions of Fe restriction such that Fe-chelators and binding proteins, which are important during infection, would be highly expressed. In addition to Fe, Mn and Mg restriction are important in vivo and different carbon sources have also been shown to be important in infection (Eriksson et al. 2003), so in vitro cultural conditions can probably be improved to generate more effective inactivated bacterins.

Synergistic effects between vaccination and other approaches and genetic resistance (Kaiser et al. 2008) have not yet been explored and may be a useful avenue for improved infection control.

Improved biosecurity also has negative implications. Hatching chicks in hatcheries, rather than with the hen, reduces the risk of transmission of pathogens but also eliminates the transfer of protective intestinal flora (Kubasova 2019). This has been exploited over many years by the administration of pathogen-free gut flora preparations to newly-hatched chicks to produce, within 24 h, the full resistance possessed by the hen, the so-called “Nurmi” effect (Nurmi & Rantala 1973).

Innovation is always important in science and technology, and the importance of imagination has been highlighted recently (Barrow 2019). Bacteriophages (phage) are increasingly of interest in human and veterinary medicine, mainly as a result of the antibiotic resistance crisis and the need to find alternative approaches for infection control.

Phages have been shown to be able to reduce disease (Hong et al. 2013) and colonization of the intestine by Salmonella (Atterbury et al. 2007) and also Campylobacter (Connerton et al. 2011). The development of resistance to phage during infection is always a potential concern but can be mitigated by using phages that target surface virulence determinants such that phage-resistant mutants that develop will generally be avirulent (Smith & Huggins 1982; Barrow et al. 1998). Using a similar strategy, we have used phages, that attach specifically to the sex pili produced by self-transmissible AMR plasmids (Kotilainen et al. 1993; Jalasvuori et al. 2011; Ojala et al. 2013), to reduce colonization of chicks by AMR S. Enteritidis with the added consequence that most phage-resistant mutants lost their AMR plasmid (and almost completely replaced the parent strain) or became non-transmissible (Colom et al. 2019). As with all new technologies, there remain a number of new questions arising from the work but it is at least an exciting new opportunity!

When applied topically at a high concentration, phages have also been shown to be able to eliminate Salmonella and Campylobacter from poultry carcass skin (Atterbury et al. 2003; Goode et al. 2003). Whether this is a practical approach, given the logistical issues associated with producing large volumes of phage, remains to be seen but it might be useful where there are severe limitations on the ability to control Salmonella at the level of production.

Finally, it is important to remember that one key factor in controlling Salmonella is the education of staff of the poultry unit so that they understand fully why they are being asked to do what is required to ensure implementation of measures to improve hygiene and biosecurity. I found short courses of instruction to be enormously beneficial with poultry farmers in Wales and it is important for all poultry-producing countries, especially those where environmental constraints may be an issue.

Measures to contain and control Salmonella infections in poultry are thus already available in many parts of the world and several interesting new approaches remain to be explored further.