Investigation, detecting & identifying Salmonella

Published on : 30 Aug 2022

Public Health England are the government department tasked with improving the nation’s health.  Part of this remit includes investigating food poisoning outbreaks, working with Food Standards Agency (FSA) to identify common causes of wide scale ill health and working across the supply chain to control the risk.

Salmonella remains a common cause of food poisoning in man, and of all the Salmonella species which can cause vomiting and diarrhoea, S. Enteritidis and S. Typhimurium top the list.  Of course, Salmonella may be associated with non-poultry foodstuff; pork, lamb, beef or unpasteurised milk.  Indeed the poultry meatand egg sector has an excellent record in identifying cases of Salmonella and working with FSA to prevent infectious material entering the foodchain through pasteurisation or heat treating of meat or eggs, or through on-farm slaughter.

These interventions come at a significant cost to the unfortunate producer where Salmonella is identified, however a zero tolerance to Enteritidis or Typhimurium infections has helped the poultry industry to grow consumer confidence in the meat and eggs which we produce.  Enhanced biosecurity alongside vaccination of laying hens has meant that Salmonella infections are mercifully rare in the poultry sector.

As you will be aware chickens and turkeys are subjected to a legal requirement to conduct Salmonella testing prior to slaughter for meat birds or periodically through the laying phase in laying hens.  The nature of the sampling required, and the test methodology, is laid down by European regulations, however the devil is in the detail.  Careful sampling and accurate completion of submission forms are important to ensure that we have confidence in negative results and are able to assure ourselves, and our customers that flocks are free of Salmonella.  In order to ensure that “negative” really means “negative” it is vital that sampling instructions are carefully followed and that samples are submitted to an approved and accredited laboratory as soon as possible in suitable packaging.  Whilst it may not seem self-serving to take additional measures to enhance the sensitivity of your testing, there are a number of compelling arguments for ensuring that operator Salmonella samples are conducted accurately:

• Alongside operator samples laying flocks are also subjected to “official samples” conducted by personnel who have been trained to follow sampling procedures which maximise the chances of detecting Salmonella.  Where official samples detect Salmonella which has not been detected by recent operator samples questions may be raised as to quality of operator samples which may affect relationships with packers and retailers.
• Approved, accredited labs can only conduct effective testing on valid samples.  Where samples have not arrived at the lab within 4 days of sampling, where paperwork is incomplete or where samples appear to have been “tampered” the laboratory is unable to test under their UKAS approval and as such any resulting result certificates will not carry the UKAS logo.  These tests will not be considered as valid by inspectors and auditors.
• As assurance schemes and retailers strive for ever higher levels of food safety they are likely to demand deeper transparency around laboratory procedures.  Interrogation of interim laboratory results could be used to support or dispute the quality of operator sampling.
• Genetic profiling of Salmonella organisms can be used to link human cases of food poisoning to a farming enterprise with a high degree of certainty.  If infection is missed in a laying flock due to poor on-farm sampling the infection may be further disseminated leading to more cases of Salmonella
• Where cases of Salmonella are not identified early there is a significant risk that the infection could spread to nearby flocks.

There are 2 golden rules to conducting accurate operator Salmonella testing.  The first is to make sure that you do not accidentally find Salmonella which is not really infecting your hens.  The Salmonella testing procedure is very sensitive and can detect just a few Salmonella organisms.  It is vitally important that the bootswabs or hand swabs are sterile before they enter the poultry house to be tested and only come in contact with the bird area to be tested.  Sampling material must be kept in sealed, fly and rodent proof packaging and only handled with clean hands.  Swabs must be carefully transported into the specific biosecure area and care must be taken to ensure that no cross-contamination occurs between houses.  Swabbing is best conducted from the interior of the house away from pop-holes and entrances.  Furthermore bootswabs must only be put onto clean boots on top of plastic disposable overboots.  Remember, without a sterile overboot, the inside of the bootswab is sampling bacteria from the sole of the shoe which may have become contaminated by flies or rodents or by being worn outside of the specific biosecure area.

The second golden rule is that the goal of testing ought to be to find Salmonella if it is present (for all of the reasons listed above!).  To this end swabs should be thoroughly moistened prior to sampling (pay attention to the cleanliness of the equipment used to pre-moisten the swabs).  A sufficiently large area of the house should be sampled.  For bootswabs this should be 100 “shuffle steps” per pair of bootswabs, care should be taken to ensure the whole house is walked and sampled during this process.  For muck belts in multi-tier units hand swabs should be sufficiently large (900cm2 each) and should sample a large area from at least 3 areas on the belt ensuring that the swabs are thoroughly coated with faeces on both sides.  

Once the swabs have been taken they should be sealed within a plastic bag before leaving the specific biosecure area.  Prior to posting it is worth double checking that the correct number of bootswabs have been conducted (2 pairs- ie 4 individual bootswabs per airspace for flat deck.  1 pair- ie 2 individual bootswabs AND 2 hand swabs per airspace for multi-tier).  Paperwork should also be checked to ensure that you supply all of the necessary information including the house number, age of the flock, name of sampler, date of sampling, declaration that the birds have not been treated with antimicrobials, establishment number.  It is also important that the bag containing the swabs is also marked so that it can be linked to the paperwork should the two get separated.

Swabs should be stored out of direct sunlight and kept cool.  The swabs should be received as soon as possible by the testing laboratory.  Samples should be delivered by hand or sent by recorded or guaranteed delivery.

As food producers across all livestock sectors adapt to an increasing requirement to assure food safety it is important that the egg sector upholds the high standards we have set, remaining ahead of other livestock sectors in providing nutritious, safe and healthy food.