Latest figures from the U.S. Centers for Disease Control and Prevention (CDC), say since 1st August 2006, 628 people in 47 states have been infected with salmonellosis after eating peanut butter contaminated with the Tennessee strain of Salmonella.
Fortunately none of those affected by this outbreak have died and experts say the outbreak now appears to be tailing off following the product recall in mid February.
Following an investigation that began in November 2006, the large, widespread outbreak of salmonellosis was found to be caused by contaminated jars of Peter Pan or Great Value peanut butter with a product code beginning with 2111.
The affected batches of peanut butter were made in the same factory in Georgia, belonging to food manufacturer ConAgra Foods and the manufacturer subsequently voluntarily recalled the products, destroyed any remaining batches they still had, and suspended further production while tests were carried out.
In previous years, incidents of salmonella were occurring about one to five times a months, but in October 2006, 30 cases were reported.
When experts looked closer at samples of the strain they noticed that three of the pulsed-field gel electrophoresis (PFGE) patterns of the Salmonella Tennessee strains isolated from many affected patients were closely related, which indicated a common source of contamination.
In January 2007 more patients were interviewed using a more probing questionnaire containing about 200 items; this revealed that 48 per cent of patients had eaten turkey (excluding delicatessen-sliced turkey), and 85 per cent had eaten peanut butter in the week before falling ill.
The CDC then conducted a case controlled study to prove that the source of contamination was the peanut butter, involving 65 patients over 18 years of age who had been infected with the outbreak strain of Salmonella Tennessee and had symptoms of diarrhea.
They were matched with 124 randomly selected controls who were about the same age and lived in the same areas as the patients.
The study showed that patients were more far more likely than the controls to have eaten peanut butter, and to have eaten either Peter Pan or Great Value peanut butter.
The CDC then notified the Food and Drug Administration (FDA), who quickly issued a public health alert on the 14th February 2007.
The case study enabled authorities to track down the source of the outbreak and identify which jars had been contaminated but how and exactly where in the plant the peanut butter became contaminated, remains unclear and the FDA is still investigating the plant.
The affected jars came from 13 states, Arkansas, Florida, Georgia, Illinois, Indiana, Iowa, Kansas, Minnesota, New York, Oklahoma, Pennsylvania, South Carolina, and Tennessee.
There are about 2,500 strains of Salmonella that cause the infection called salmonellosis, the symptoms of which include diarrhea, fever, and abdominal cramps and usually appear 12 to 72 hours after exposure.
Salmonella Tennessee infection is rare, and the source is often not known and according to the CDC, only one other outbreak of the strain where the food source is known, has been identified; this was in contaminated milk powder.
Salmonella can contaminate peanuts at any stage from when they sprout in the ground, through growth, harvesting and processing; the bacteria is able to withstand high temperatures, even those above 70 degrees C (158 F), the temperature at which peanut butter is heat treated.
The bacteria could also get into the peanut butter in the factory, from animals such as rats, or from other sources, including humans not washing hands or attending thoroughly to hygiene measures.
The CDC says the outbreak shows the need to be scrupulous about surveillance, safety and hygiene at all stages of the manufacturing process and demonstrates the potential for widespread illness from a broadly distributed contaminated product.
It is estimated that every year, 76 million cases of foodborne illness occurs in the U.S., resulting in 325,000 hospital admissions and 5,000 deaths.
The CDC says anyone who still has a jar of Peter Pan or Great Value peanut butter with a product code beginning with 2111 should throw it away immediately.
An update of the situation is reported in the CDC's Morbidity and Mortality Weekly Report (MMWR).