Pathogenesis of egg infections by Salmonella and the implementation of preventive measures

Second Part

Early protection has been demonstrated after primary live vaccine administration, early post-hatch, but this effect is serotype-specific, while cross-protection between some serotypes has been demonstrated after booster immunisations. Vaccines can only be efficient when biosecurity is optimal. A variety of nutritional strategies can be used to reduce Salmonella colonization, also for broilers. It is a utopia to eradicate Salmonella from chickens and the environment, but one should try to aim for appropriate levels of protection, and thus low flock prevalence and within-flock prevalence, and low individual colonization levels.

Vaccination to reduce Salmonella

A lot of experimental vaccines have been produced for chickens, and also a variety of commercial vaccines are available on the market. These comprise both live and inactivated vaccines. The currently available live vaccines are produced by chemical mutagenesis or are selected on culture media as slow growing natural mutants (metabolic drift mutants).

In general, it is believed that live vaccines induce better protection because they stimulate both cell-mediated responses and antibody responses, while inactivated vaccines mainly induce antibody production, but both methods are in use, singly or in combination in vaccination regimens. Triple dose vaccination schemes are common for layers and breeders, and also combinations of live and inactivated vaccines are given.

Live vaccines are mostly administered in the drinking water (or using a coarse spray) and inactivated vaccines need to be administered parenterally. Autologous vaccines are used in some countries, made by killing a strain isolated from the flock where the vaccine is administered. Cross-protection is shown to be occur but it is believed that intra-serotype and intra-serogroup protection is more pronounced. For example, Eeckhaut et al. (2019) showed that live Enteritidis vaccines significantly reduce Salmonella Infantis colonization in layers.

Vaccines have been used extensively in laying hens and should a) reduce or prevent the intestinal colonisation resulting in reduced faecal shedding and thus egg shell contamination and b) prevent systemic infection resulting in a decreased colonisation of the reproductive tissues, in this way reducing internal egg contamination. Inactivated vaccines are often used in parent flocks. Parenteral administration of inactivated Salmonella vaccines to breeder birds will induce a strong production of antibodies. These antibodies will be transferred to the progeny. The maternally transferred antibodies persist for a few weeks but, although there seems to be some protective effect against disease in the early post-hatch period, there is little effect on intestinal colonisation by challenge strains. There is a report on the efficacy of inactivated vaccines in prevention of egg contamination in layers. Gantois et al. (2006) showed that oral vaccination with live vaccines at day 1, week 4 and week 16 decreased internal organ colonisation, including reproductive tract colonisation, and egg contamination. Although it is very difficult to prove reduction of egg contamination following vaccination under field conditions owing to the low and variable percentage of contaminated eggs laid, a European baseline study showed that vaccinated layer flocks were less frequently contaminated by Salmonella as compared to non-vaccinated flocks (4% vs 12%). In theory, an ideal live vaccine strain should possess following characteristics:

Induce a high degree of protection against systemic and intestinal infection. Protect against a variety of important serovars (serogroups). Show adequate attenuation for poultry, other animal species, humans and the environment. Be easy to administer without animal welfare issues. The inactivated and live vaccines should not affect growth of the animal. Vaccine strains should not be resistant to antibiotics (or contain resistance genes). Vaccines have markers facilitating the differentiation from Salmonella wild-type strains. Application of vaccines should not interfere with Salmonella detection methods. Humoral antibody response after vaccination should be distinguishable from a Salmonella wild-type response to allow the use of serological detection methods.

Multiple scientific groups have reported a phenomenon, in which oral administration of Salmonella wild type and attenuated strains can confer resistance to infection by a virulent Salmonella challenge strain within 24 h of administration. This ‘competitive exclusion’-like phenomenon is called colonization-inhibition. These data suggest that it might be possible to administer live Salmonella vaccine strains to newly hatched chicks such that they would colonize the gut extensively and very rapidly, inducing a profound resistance to colonization by other Salmonella strains of epidemiological significance, which may be present in the poultry house or may also have arisen from the hatchery. Colonisation of the gut by the colonisation-inhibition strains would prevent gut colonisation by virulent strains, while invasion in the gut tissue would evoke an inflammatory response that would prevent invasion to the internal organs by virulent strains. This means that live vaccines can thus also be used in broilers to control gut colonisation and shedding. An issue is to administer the strains as early post hatch as possible to the birds; this is not ideal using drinking water applications but can be done using coarse sprays. It is difficult to speculate about the nature of future vaccines but good methods are available to rationally design live vaccines that have defined mutations so that both detection methods and safety aspects are highly controlled. These are, however, genetically modified organisms and their use is still under debate although some are already marketed.

Many research groups have designed genetically modified live vaccines with a very good safety and efficacy profile, and with markers that are differentiating the strains and the serological response from wild type strains and serum responses, respectively. In relation to emerging phenotypes and the variety of Salmonella phenotypes in broilers, developing vaccines against other serotypes can become a need, but the registration process is long, hampering development of these vaccines. To be complete, one needs to mention that, in addition to vaccines, other methods are available and a multiple hurdle approach is needed. Biosecurity is crucial, and it is evident that rodent and insect control and general hygienic and biosecurity measures are a prerequisite for keeping Salmonella out of the farms. In addition, many drinking water and feed additives are being used, including organic acids such as butyrate, prebiotics, probiotics and phytochemicals, amongst others. This is not within the scope of this paper but reviews can be consulted: Van Immerseel et al. (2002), Micchiche et al., (2018), Clavijo and Flores (2018) and many more. It is a utopia to eradicate Salmonella from chickens and the environment, but one should try to aim for appropriate levels of protection, and thus low flock prevalence and within-flock prevalence, and low individual colonization levels.

References are available on request From the Proceedings of the 2020 Australian Poultry Science Symposium