Snapshots of Salmonella Serotypes

In 1885 while studying swine diseases, microbiologist Theobold Smith and veterinarian Daniel Salmon discovered the first type of Salmonella and found that it caused a serious illness in pigs. Salmonella Choleraesuis, named for Dr. Salmon, is not included in the Atlas because it is so rare.

Human cases of infection with this serotype are unusual in the United States and typically occur in adults who live in rural areas and work with pigs. The average age of people infected with serotype Choleraesuis is around 40. Few infections occur in children. Salmonella Choleraesuis is unusual—although not unique—because it often invades the bloodstream. Unlike many more common Salmonella serotypes, over half of the serotype Choleraesuis infections are invasive. Outbreaks caused from eating contaminated pork are extremely rare.

The reports from sources other than humans are almost all from pigs. It remains an important cause of disease in pigs to this day.

At a Glance: Salmonella serotype Choleraesuis:

• Some consider this to be the first type of Salmonella discovered
• Important cause of disease in pigs but is rare in humans
• More than half of the infections diagnosed in humans are invasive
• Outbreaks caused from eating contaminated pork are rare

Salmonella serotype Enteritidis [PDF - 15 pages]

Like the majority of other serotypes affecting humans, Salmonella Enteritidis (SE) affects people of all ages. Infections increased in the early 1980s in the Northeastern states, spread to the mid-Atlantic region by the end of the decade, then catapulted to California and gradually, the rest of the country. At the same time, it spread around the world. More recently, SE has also appeared in “broilers” (chickens that we eat). Now, SE is the most common serotype in the United States and in many other countries.

Six different maps capturing SE from 1968-2011 (pages 6-11)  [PDF - 15 pages] show the spread graphically.

• Figure 9 (1968-1975), Salmonella Enteritidis is scattered at low levels throughout the country.
• Figure 10 (1976-1983) shows a new hotspot in the extreme Northeast.
• Figure 11 (1984-1991) shows how the hotspot has spread down the Eastern coast to New Jersey, Pennsylvania, Maryland, and into the Midwest. This was at least partly related to the unknowing transfer of infected chickens.
• Figure 12 (1992-1998) shows the sudden emergence in California and the West.
• Figure 13 (1995-2005) shows some of the first impact of control efforts in many states.
• Figure 14 (2006-2011) shows there is more work to be done to prevent this infection.

Stealth invader:  Imagine a healthy looking, but infected, chicken that can lay eggs with contaminated yolk. Frightening, but true. Salmonella Enteritidis, one of the most common serotypes of Salmonella reported worldwide, often linked to eggs. If a fertile egg is contaminated with Salmonella Enteritidis, it infects the new chick. If the egg is for eating, and someone eats the egg raw, or undercooked, that unlucky person may be infected with Salmonella Enteritidis.

Since the 1990s, infection rates have decreased as the egg industry voluntarily placed controls on egg flocks and consumers have eaten fewer raw eggs. Still, large outbreaks have occurred. The US Food and Drug Administration (FDA) enacted the Egg Rule in 2010 to control SE on egg farms; they also established safe handling and labeling requirements for shell eggs. 

  At a Glance: Salmonella serotype Enteritidis:

• Infects yolks of seemingly healthy chickens
• Recently spread to “broilers,” chickens that we eat
• Most common Salmonella serotype in the US and many other countries
• FDA established the Egg Rule to control Salmonella Enteritidis on egg farms

Salmonella serotype Agona [PDF - 15 pages]

Forty years ago, infections with serotype Agona were rare and seen only in people traveling to other parts of the world. Then in 1971, an outbreak at an Arkansas restaurant spotlighted its arrival in this country. When traced back, the outbreak was linked to local chickens that were fed chicken feed made from, among other things, anchovies ground into fish meal. The meal was contaminated with Salmonella Agona during the drying or grinding process. It was used in chicken feed around the world, and Salmonella Agona suddenly appeared in many countries. In the US, it first appeared in chickens and turkeys, then in other animals, and in people eating them. While infections have decreased some, it is not likely to go away.

At a Glance: Salmonella serotype Agona:

• No longer rare
• Initially linked to animal feed made with fish meal.
• Infections first appeared in animals and rapidly spread to humans
• Infections have decreased but unlikely to disappear