Everything You Need to Know About Salmonella in Eggs

Grandmothers across the globe chase kids away from bowls of raw cookie dough, and pregnant moms avoid fresh mayonnaise. Why? So, they don’t get sick.  

Eggs aren’t inherently bad or dangerous – in fact, according to the CDC, they are one of the most cost-effective and nutritious foods available.   

Unfortunately, however, Salmonella sometimes finds its way into eggs.  The CDC offers some great advice on how to reduce your chances of getting a Salmonella infection: 

• Keep eggs refrigerated at 40°F (4°C) or colder at all times. Only buy eggs from stores and suppliers that keep them refrigerated.          
• Discard cracked or dirty eggs. 
• Consider buying and using pasteurized eggs and egg products, which are widely available.              
• Cook eggs until both the yolk and white are firm. Egg dishes should be cooked to an internal temperature of 160°F (71°C) or hotter.  
• Make sure that foods containing raw or lightly cooked eggs, such as hollandaise sauce, Caesar salad dressing, and tiramisu, are made only with pasteurized eggs. 
• Eat or refrigerate eggs and foods containing eggs promptly after cooking. Do not keep eggs or foods made with eggs warm or at room temperature for more than 2 hours, or 1 hour if the temperature is 90°F or hotter.
• Wash hands and items that came into contact with raw eggs—including countertops, utensils, dishes, and cutting boards—with soap and water.   
• Listen to your Grandmother... do not taste or eat any raw dough or batter, such as cookie dough and cake mix made with raw eggs. Bake or cook raw dough and batter before eating.

How Does Salmonella Get Into Eggs?

There are three ways Salmonella contaminates eggs: 

1. Shell contamination by fecal matter through a shared passage.
2. Egg contamination post-lay through Salmonella absorption via shell pores. 
3. Egg contamination within the chicken’s reproductive organs.

Infected birds lay contaminated eggs, but how exactly does Salmonella get into parent flocks and production birds?

Salmonella has been shown to be present in a host of inputs, from water and finished feed to bedding and feed ingredients, as well as being present in the environment, in dust, in vermin, insects etc and on other mechanical vectors such as human boots and vehicle tires.  The industry invests significant time, energy and money addressing Salmonella at source, and on interventions such as vaccines designed to prevent pathogens from accessing poultry. 

One route is of particular concern because of its ability to support pathogens for long periods and reach right across production facilities.  Salmonella can enter the feed chain at the source; it is naturally occurring in soils, and growing crops are a magnet for wild birds, rodents, insects and Salmonella-carrying wild animals.   Harvested crops are gathered and processed in communal transport and facilities, spreading the risk.   Certain raw materials are statistically more likely to present salmonella risk, including oilseeds and vegetable proteins. 

Processed feed ingredients cover huge distances over long periods in transit to feed mills for further processing.  There’s ample evidence, however, that a dry feed matrix provides the perfect environment for pathogens to survive and thrive for long periods.    

Risk doesn’t end when raw materials reach the feed mill.  Plant managers who actively monitor the biosecurity of their feed mills consider dust in intake and outload bays to be the most accurate source material for Salmonella analytics. 

When it comes to feed pathogen control, heat is the miller’s friend.  Steam in the pelleting process knocks back levels of pathogen load in feed.  It is possible to kill some types of Salmonella with heat if temperatures are high enough (86°C) for long periods (upwards of 6 minutes).  But recontamination is a real threat.  Pelleted feed exits the production process via a cooler into silos and trucks, where the environment has been proven hospitable for Salmonella colonies. 

The challenge of presenting Salmonella-free feed to birds at the point of consumption isn’t easy.   And because feed from a central mill rolls out to multiple complexes and birdhouses, the risk quickly spreads across entire operations.  Biosecure operations use control tools that sanitize feed and provide protection against recontamination for 14 days and more; long enough to ensure feed is free from Salmonella when it arrives in the feeder pan.

Where feed biosecurity programs fail, Salmonella can enter the live production process through feed.  Once in the bird, it colonizes the gastrointestinal tract and then cycles through fecal matter, reaching more birds and contaminating the shell. Specific serotypes, such as S. enteritidis, can be absorbed through shell pore and replicate within the albumen and yolk.  S. enteritidis can also spread from the colonized intestine throughout the chicken to other organ systems, such as the reproductive organs. One study found that hens inoculated with S. enteritidis laid 2.47% of the eggs laid between 7-21 days post-inoculation were contaminated. 

How does Salmonella move from an infected reproductive system to a table egg?  One literature review found that Salmonella deposited within yolk membranes becomes encapsulated as the egg matures within the oviduct. Thus, allowing S. enteritidis to spread to the albumen, shell membranes and eggshell before lay. 

It is important to note that egg contamination isn’t just a problem in laying hens; it is also a serious challenge in hens responsible for genetics and generation of broiler stock.  Salmonella transmission in genetic birds can have direct financial consequences as it can generate Salmonella-positive chicks. 

How to Prevent Salmonella in Eggs:

The poultry industry has a big role to play in ensuring poultry meat and eggs are safe for human consumption.   

Genetics companies take responsibility for keeping hens Salmonella -free so that broiler and commercial layer flocks aren’t passing salmonella infection through the breeding lines.  Feed mills are increasingly biosecurity aware; treatment of raw materials and finished feed is growing as supply chains prioritize clean, safe feed. Some countries have specific programs for Salmonella prevention in poultry like the National Poultry Improvement Program (NPIP) in the US and the Lion Program in the UK.  Their customers, the broiler and layer farms, take preventative measures to keep feed-source pathogens out of production birds, as well as putting in place a host of other salmonella controls.  As well as the food safety issues, it also helps them improve productivity and the welfare of their flocks.  There’s a role for consumers too, to ensure proper handling of meat and egg products.

For more information on what feed sanitizers can do for you, click here.   If you're ready to take action immediately, you can contact a feed biosecurity specialist here.