Salmonella – what’s in a name?

Published on : 30 Aug 2022

Salmonella was named after a US pathologist, Daniel Elmer Salmon, who was working for the US Department of Agriculture in the 1800s, so nothing to do with fishy salmon! In fact records suggest that there was a plague of typhoid fever in Ancient Greece due to Salmonella as long ago as 400 years BC! Mr Wikipedia has even blamed a catastrophic Salmonella outbreak as a contributor to the collapse of the Aztec Empire in the 16th Century, aided by Spanish invaders!

Salmonella are a group of bacteria, bugs, or germs, some of which are capable of causing food poisoning in people. There are estimated to be over 2300 different types, sometimes referred to as strains, types, serotypes or serovars. Some can cause infection in animals and an even smaller number can cause disease or illness in poultry. Each strain has its own name, sometimes associated with the animal it was first isolated from, sometimes the disease they are associated with and most often where they were first isolated – hence some exotic sounding place names.

Salmonella Pullorum and Salmonella Gallinarum are two very specific “avian” strains which were historically able to cause high mortality in poultry flocks. As a result they were at the heart of the development of our Poultry Health Scheme which was designed to remove these two strains from our country flock. These infections are now extremely rare, except occasionally being detected in some backyard flocks (another reason for avoiding contact with backyard flocks!).

Although potentially any strain can cause human food poisoning common things are common, and hence there only a handful of strains that the boffins get excited about. Some come and go having been associated with specific and localised outbreaks. The most common human associated strains, and hence those that cause our industry most grief these days in terms of control and further action, are Salmonella Enteritidis (SE) and Salmonella Typhimurium (ST). If these particular strains are isolated from our flocks that is when we are likely to be affected by legislation and control to help reduce human infection. This is why the cornerstone of the poultry industry control measures depends on robust vaccination specifically against these two types, coupled with biosecurity measures to prevent their access to flocks. As a result of this, fortunately these infections are rare in our industry, even though every flock is tested regularly.

The testing we do is designed to pick up any Salmonella that may be present and then more specific typing can tell us whether we are dealing with something more sinister. UK Government compile and report all the results of testing annually for each different poultry type (layers, breeders, broilers and turkeys) and since the National Control Programmes have been in place, our layer industry has had a good record. The most recent figures suggest that sampling shows positives for any sort of Salmonella is running at less than 0.8% and for the important types (SE or ST) less than 0.2%, way below an EU target of 2%.

If SE or ST are detected then strict control measures are put in place (see other articles for more details on this).

From time to time some other strains might be isolated from our flocks but as they are very unlikely to be passed in or on eggs to affect people and our flocks don’t suffer any ill effects then there are no specific controls necessary.

So, when you submit routine NCP samples you are likely to get one of two results reported to you. By far the majority will be reported as “not detected” or “negative” and this report can then be filed away until the next test is required.

If there is a suspicion of a positive your testing laboratory will let you know that they have a “suspect” and should report to you the Salmonella Group detected. 

The Groups in poultry most usually involve one of Groups B, C, D, E, or G. Once you have been informed of this (usually within a week of the sample being received for testing) the laboratory has a legal responsibility to report this to Defra and to submit the isolate for confirmatory testing by APHA who are the reference laboratory for confirming the more specific serotype. This can be an anxious time for producers. As a preliminary guide the two Groups to raise most concern are Group D (which includes Salmonella Enteritidis) and Group B (which includes Salmonella Typhimurium). Groups C, E and G isolates tend to involve the less exciting strains.

So, if your testing laboratory gives you a preliminary report of a Group D or a Group B then the sample will be fast track tested by APHA to rule out (or sadly confirm) Enteritidis or Typhimurium. Fortunately, there are quite a number of harmless Group Bs and a few Group Ds that are not Enteritidis so hence the anxious wait for several days until APHA can confirm the identity.

As the above explains, the groups, names and identity of Salmonella types can be useful in helping us understand where they have come from and hence help with control measures. Salmonella Agona and Salmonella Agama (both in fact Group Bs) have been isolated from badgers, foxes and other wild mammals and if they pop up in your routine NCP testing may suggest some wild animal contamination on your range. Others (usually from Groups C or G) with a range of exotic names may be associated with feed raw ingredients. The less common and less exciting strains tend not to concentrate in birds and therefore may be present on one sampling but then disappear from a flock and are just noted with “interest” and often a little anxiety until the full typing result is known!

When considering Salmonella Enteritidis or Salmonella Typhimurium these can persist longer in flocks and their environment and hence can be more problematic (see other articles in this series). Salmonella Typhimurium can be associated with contact with pig herds or vermin appearing or being resident on site. Due to their potential to cause human illness SE and ST do arouse interest and control. 

New tests are being developed all the time and can now even begin to link specific flock isolates to specific human outbreaks through a technique called Whole Genome Sequencing (or WGS). This may help deflect the blame away from poultry or may help identify exactly where are problems are.

Our industry can be justifiably proud of our record on Salmonella – we vaccinate every flock, we take biosecurity seriously and we test every flock in accredited laboratories. It means that we can be totally transparent – the flip side is that it will always highlight any chinks in our armour and we will publish all our issues openly. However, on the basis that “forewarned is forearmed” we can always stay one step ahead of the game in identifying those issues and can demonstrate our proactive approach to consumers to give them confidence and safety in our products.