Salmonella Enteritidis - The New Zealand Outbreak

Salmonella is an important group of bacteria that can affect poultry and people. Chickens may carry Salmonella without appearing sick, which is particularly important because the bacteria can potentially contaminate eggs, meat and the environment.

For backyard chicken keepers, good hygiene and flock biosecurity are therefore about more than keeping the birds healthy. They also help protect the people handling them and eating their eggs.

What Is Salmonella?

There are many different types of Salmonella. Some are particularly important in poultry and human food safety, including Salmonella Enteritidis.

New Zealand has historically had a relatively favourable status for S. Enteritidis, but detections in poultry have demonstrated why continued surveillance and biosecurity remain important.

Poultry are not the only potential source. Rodents, wild birds, insects, other animals, contaminated feed, equipment and the environment can all contribute to the movement of Salmonella.

What Does Salmonella Look Like in Chickens?

One of the challenges with Salmonella is that infected chickens do not necessarily look sick.

Some birds can carry and shed the bacteria while appearing clinically normal.

When disease does occur, signs can include:

• Lethargy or depression
• Reduced appetite
• Poor growth
• Weakness
• Diarrhoea
• Dehydration
• Increased mortality, particularly in young birds

These signs are not specific to Salmonella, so laboratory testing is required to determine whether it is actually involved.

Why Does Salmonella Matter to People?

Some Salmonella strains are zoonotic, meaning they can pass between animals and people.

People can become infected through contaminated food or through contact with infected animals and their environments.

Human salmonellosis commonly causes diarrhoea, abdominal cramps, fever, nausea and vomiting. Disease can be more serious in young children, older people and people with compromised immune systems.

This is why basic hygiene around backyard poultry is so important, even when the chickens themselves appear perfectly healthy.

Eggs and Food Safety

Eggs should be handled as a raw animal product.

Simple precautions include:

• Washing your hands after handling chickens, eggs or poultry equipment
• Keeping nesting boxes clean and dry
• Avoiding heavily soiled or cracked eggs for human consumption
• Preventing cross-contamination between raw eggs and other foods
• Refrigerating eggs appropriately
• Cooking eggs thoroughly when preparing food for higher-risk people
• Keeping kitchen surfaces and utensils clean

Children should also be supervised around poultry and encouraged to wash their hands thoroughly afterwards.

Flock Biosecurity

Good biosecurity is one of the most useful ways to reduce the risk of Salmonella entering or circulating within a flock.

Control Rodents and Wild Birds

Rats, mice and wild birds can contaminate feed and housing with droppings.

Store poultry feed in secure containers, clean up spills and make feeders as inaccessible to rodents and wild birds as practical.

Keep Equipment Clean

Feeders, drinkers, transport crates, footwear and other equipment can move contaminated material between birds and properties.

Regular cleaning and appropriate disinfection can substantially reduce environmental contamination.

Be Careful When Introducing New Birds

New birds can introduce pathogens without showing any signs of disease.

Quarantine new arrivals and source chicks, pullets and hatching eggs from reputable suppliers with good flock-health and biosecurity programmes.

How Is Salmonella Diagnosed?

Because healthy-looking birds can carry Salmonella, clinical signs alone are not enough for diagnosis.

Testing may involve bacterial culture or PCR using samples such as cloacal or faecal material, environmental samples or tissues collected during post-mortem examination.

The type of Salmonella identified matters. Detecting a Salmonella organism does not automatically tell us its significance without considering the strain, clinical picture and flock circumstances.

Should Salmonella Be Treated With Antibiotics?

Antibiotic treatment of Salmonella in poultry is not as straightforward as simply treating an infection and assuming it has disappeared.

Treatment may improve clinical disease in some circumstances, but it does not necessarily eliminate carriage from the flock. Antimicrobial resistance and food-safety considerations are also important.

For this reason, antibiotic treatment should be based on veterinary assessment, appropriate diagnostics and the circumstances of the individual flock.

What About Probiotics and Competitive Exclusion?

The intestinal microbiome plays an important role in resistance to colonisation by pathogens.

Competitive exclusion involves establishing beneficial intestinal microorganisms that compete with organisms such as Salmonella for nutrients and attachment sites within the gastrointestinal tract.

Certain strains of Lactobacillus, Bacillus, Bifidobacterium, Pediococcus and other beneficial bacteria have been investigated for their ability to reduce Salmonella colonisation in poultry.

However, probiotics should be viewed as one component of flock health and biosecurity, not as a treatment for salmonellosis or a substitute for appropriate testing, hygiene and disease control.

Choosing a Poultry Probiotic

Not all probiotic products are equivalent.

When comparing products, look for:

• Clearly identified bacterial strains or species
• Evidence supporting their use in poultry
• Multiple complementary beneficial organisms where appropriate
• A clearly stated viable bacterial count, usually expressed as CFU
• Appropriate storage and handling instructions
• A practical method of administration

The number of bacteria in the bottle is only useful if those organisms remain viable and reach the bird in sufficient numbers.

The Bigger Picture

Managing Salmonella requires several layers of protection.

Good flock hygiene, rodent control, appropriate feed storage, sensible sourcing of new birds, food hygiene, diagnostics and gut-health management all contribute to reducing risk.

Most importantly, a chicken does not need to look sick to carry or shed Salmonella.

That makes prevention and good everyday biosecurity far more valuable than waiting until disease appears.

Written by Dr Sam Hurley, BSc, BVSc, MAHM