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Salmonella and Considerations for Longer Lay

Richard Turner MA VetMB MRCVS, St David’s Poultry Team

The recent move towards older birds in lay has brought our attention back to an old enemy, salmonella. With reduced staff in some official laboratories due to COVID planning, there has been issues with testing and the time it takes to define the strain of salmonella. This understandably has led to some very stressed clients. It is timely therefore to look at the testing regimes and how we can best develop immunity and protect farms against a salmonella infection.

The introduction of vaccines in the 1990s for salmonella was a steep change in the control of salmonella infection in layers, and subsequent licensing of live vaccines made the process of vaccinating birds a lot easier with less side effects. Over the last 20 years the old layer houses have been redeveloped into colony and barn houses and the salmonella challenge on site has been greatly reduced. The fear of a salmonella positive flock with subsequent loss of egg sales and cleaning costs is hugely reduced, and the control systems adopted under schemes such as Lion Code has been very successful.

Cases of Salmonella Enteritidis (SE) and Salmonella Typhimurium (ST) are thankfully at a very low level, other than the occasional case in old birds. The so called exotic strains of salmonella that are not included in the control plans can still be found, and if too many cases occur there is always the risk that regulators and customers will want to see more strict controls. It is essential when choosing the laboratory to use for salmonella testing that it can at an early stage rule out both SE and ST in any positive findings. If they cannot or do not do this extra identification, maybe due to having a cheaper test, then the farmer will have to wait for maybe 12 days or longer for official tests to be carried out. With a last test in rear at 14 weeks and often a planned move quickly after this age, it is therefore essential that if a positive salmonella test is found at 14 weeks, the SE and ST strains are quickly ruled out. There is often some confusion about what happens with a positive salmonella test in birds in lay. The important thing to remember is that as the rules stand now, the eggs are still allowed to be used until an official test confirms the identity of the salmonella.

So why are we talking about salmonella now? The issue is relatively simple. We have started to keep birds longer which is a positive plan for many farms due to the improved nutrition and genetics available to the farm, but what we have not managed to do is to plan for a longer immunity. Most of the available vaccines have a data sheet immunity of between 55 and 60 weeks. This does not mean that the immunity drops off a cliff after 60 weeks, nor that the vaccine programmes are inadequate. What it does mean is this is the age which the vaccine was tested to at the time of licencing, and it doesn’t take into account the effect of any local challenge to boost immunity.

Salmonella strains are present in many other species including rodents, dogs and cats, as well as being transmitted in feed. The level of local challenge is often not recognised due to the inability to quantify it correctly, so as we move to older birds we need to be careful and assume that their immunity is dropping. Longer acting vaccines are underway, but they will come with other issues, so we need to revisit the challenges and how we best immunise the bird now.

Testing is carried out according to legal requirements and in approved laboratories, but the issue is often not the test so much as the sample. The testing relies on sufficient salmonella in the sample size to allow for the selective culture and multiplication of any salmonella present to be detected. This might sound simple, but if for example the level of contamination is low on a site, it is possible to have a false negative test. Boot overshoe samples are widely used, but there are cases where one boot sample is positive whilst another in the same house is negative. Dust samples of course can be contaminated by non-bird species if there is a rodent issue for example. Worryingly, we also see samples from chick boxes which are positive in one lab and negative in another.

The vast majority of testing is carried out well and there should be no concern that the legal testing is failing, but it is possible for a very small number of tests to be incorrect. The need to assure the customer that the product is safe has led to enhanced testing of older birds, and it is great to report that of many samples taken only a handful per year are positive to salmonella. The problem comes for those positive farms to get back to their status and be able to sell eggs from subsequent flocks. Most flocks are insured as the cost of killing a flock and cleaning a positive site is very large. Only authorised and audited laboratories can carry out salmonella testing, but as with many areas of food production where profit is low there has been pressure on laboratories to reduce costs. Whilst I am not saying that cheap testing is bad, I am sure that quality costs more.

We must not expect any vaccine to give 100% protection, a subject which is no doubt taxing the government at present! The birds need to be fit and healthy and in the right environment to allow for a satisfactory immune response to a vaccine. It is clear for example that various bacteria, which are the first colonisers of the young chick’s intestine, effect the development of the gut immune system so there are a range of poorly understood factors which might alter the immunity of a birds after correct vaccine administration. The first colonisers of the intestine are from the chick’s environment, as well as from its mother, and there is no easy way to measure or optimise their effect. 

It is still essential for any farmer to look at the whole site and ensure that the possible sources of salmonella are reduced. In some other farmed species such as pigs there is more salmonella found, and therefore any link between a pig and a poultry farm should be controlled. This extends to the wheat that might have had a non-poultry manure applied to it, through to having wild birds such as starlings contaminating poultry yards. Every farm making their own feed should have a full salmonella testing regime and have controls in place including where necessary the correct and most efficacious anti-salmonella additives.

It is now possible to add various nutraceuticals to the feed of young birds in rear to improve their immune system development and whilst this added cost is difficult in practice to define a cost benefit, the theory is good and the added cost is low, making it a serious consideration. In the older bird we have to consider that as long as the site is clean, which most are, the immunity to salmonella will be reduced. In this case the use of various short and medium chain fatty acids in water or feed will reduce the chance of any consumed salmonella both colonising and multiplying in the lower intestine. Some of these products properly fed will also have positive effects on the gut health and improve late egg shell quality, reducing seconds.

Feeding the older bird for immunity, health and better production should now be a priority for all farmers planning to extend the lay, and this change in feeding should start in rear.



As originally written for Ranger Magazine

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