In 2015, a specific strain of a germ called Salmonella heidelberg made 56 people sick in 15 different states. Germs in the Salmonella group are not uncommon causes of illness in people. The CDC estimates that 1.2 million cases of salmonellosis occur every year – the vast majority of them being food-related. These illnesses come with symptoms such as diarrhea, fever, and abdominal cramps. Most people recover after 4-7 days with no treatment. However, a small percentage of people with Salmonella infections (less than 2%) have severe enough illness to be hospitalized, and around 450 people die from these infections each year.
The Salmonella heidelberg illnesses in 2015 were particularly severe, however. In contrast to the average Salmonella infection, over a third of the affected people needed to be hospitalized. Furthermore, these illnesses were especially prevalent in children under 5 years of age, with more than a third of the cases affecting that age range. Read more about the outbreak at the Centers for Disease Control and Prevention.
But instead of having a food source as an origin, this Salmonella strain was linked to contact with animals – specifically young dairy calves. Severe illness and death losses in young dairy bull calves due to Salmonella heidelberg became more commonly recognized in many Midwestern states, including South Dakota. Infected calves became ill quickly, with high fevers, diarrhea, lethargy, and refusal to eat. Aggressive treatment with antibiotics and fluids usually were not enough to save these calves; in fact, most of these strains were extremely resistant to most available antibiotics. Most of the human S. heidelberg cases in this outbreak were associated with direct or indirect contact with these ill calves.
While virulent Salmonella germs (particularly Salmonella dublin) have been associated with severe disease in calves, Salmonella heidelberg had always been predominantly associated with poultry: chickens and turkeys. How could this germ apparently come out of the shadows in another species to cause such severe illness in calves and people? A research team led by the SDSU Animal Disease Research and Diagnostic Laboratory’s (ADRDL’s) Dr. Joy Scaria, along with personnel from the Wisconsin Veterinary Diagnostic Lab, was able to dive into the genomics of these and many other strains of S. heidelberg. Their analysis of over 600 different bacterial isolates found that these outbreak strains were likely present in the Midwest well before the 2015 outbreak and were part of a genetic cluster of bacteria that developed the ability to infect species other than birds. The acquisition of a gene type that helped the bacteria adhere to the gut of different species likely was the culprit. The investigators were also able to document an increased level of antibiotic resistance in these more recent S. heidelberg isolates compared to ones found years ago.
In addition to Dr. Scaria, ADRDL investigators included pathologists Dale Miskimins and Melissa Behr, researchers Jane Christopher-Hennings and Eric Nelson, and graduate student Linto Antony. The work was supported by the South Dakota Beef Industry Council, the SD Governor’s Office of Economic Development, and the FDA’s GenomeTrakr project.
This work is an example of the ability of Dr. Scaria’s lab and the SDSU ADRDL to use genomic sequencing techniques to better understand the emergence of new strains of germs such as S. heidelberg. Read the complete work in the journal Gut Pathogens.