Washington pork linked to massive summer Salmonella outbreak
During the summer of 2015, Washington state experienced the largest Salmonella outbreak in the state’s recent history. The outbreak, which was linked to contaminated Washington pork from a single slaughterhouse, is a reminder to consumers and health officials alike that pork is an important source for human Salmonella infections.
Ultimately, public health officials confirmed a total of 192 outbreak cases in five states, with 96% (184 cases) reported in Washington. The youngest outbreak case was less than a year old, while the oldest confirmed outbreak case occurred in a 90-year-old individual. Officials had hospitalization information for 180 cases. Of those 180 cases, 30 (17%) required hospitalization as a result of their illness.
The Washington State Department of Health (WADOH) and Public Health-Seattle & King County (PHSKC) investigated 22 clusters of a specific serotype of Salmonella during June and July of 2015. Despite being the fifth most common serotype of Salmonella in the broader United States, Salmonella serotype I 4,, 12:i-, the serotype involved in this outbreak, is uncommon in Washington state.
On July 29th, as the second month of investigation was coming to a close, WADOH and PHSKC contacted the US Centers for Disease Control and Prevention (CDC) to request assistance. The CDC agreed to help identify the infection source (ultimately determined to be Washington pork), determine risk factors, and make recommendations for prevention.
Investigating an outbreak
When the CDC looks at how an outbreak evolves, it looks at seven distinct stages and how they relate. These steps, in order, are:
- Detecting a potential outbreak;
- Looking for and finding cases that are (or may be) involved in the outbreak;
- Interviewing people who are experiencing or already experienced a case of the outbreak illness and formulating a theory of where the sickness may have come from;
- Testing the theories that public health officials came up with from analyzing interview answers;
- Figuring out what the “outbreak vehicle” (the thing that carried the germs – in this case, the pork) and the “point of contamination” (where the germs got onto or into the outbreak vehicle – this time, the point of contamination was a single slaughterhouse);
- Controlling the outbreak, which can be accomplished using a number of different measures, including recalls and shutdown of the production facility; and
- Declaration that the outbreak is over after no additional outbreak cases are reported for a period of time.
Interviewing outbreak patients to determine the outbreak souce
Based on the answers that infected individuals initially provided to public health officials, agencies developed a supplemental questionnaire that went into more detail — specifically, the questionnaire included additional questions addressing meat and livestock consumption and exposure.
Public health officials and medical personnel interviewed 42% of all confirmed cases – or 80 cases – using the refined questionnaire. Of those 80 patients, 74%, or 59 patients, reported eating pork during the week before they became ill. This number was significantly higher than the number of people that would be expected to eat pork in any given week (approximately 43%, according to a governmental study to establish a baseline to be used for this exact purpose).
Identifying the contamination point for Salmonella-laden Washington pork
Health agencies used data collected during the interview process to investigate and determine the source of the outbreak.
When food prepared in a multiple different kitchens (for example, chicken fingers from many stores of the same chain) or a food bought from multiple stores is linked to an outbreak, the contamination likely occurred somewhere in the production chain before the final kitchen. When that happens, investigators do a “source traceback” to find out where contamination occurred.
Finding the contamination point helps define where contamination occurred and can help to confirm the hypothesis. Investigators ask about who supplied the food that they think became contaminated to stores, restaurants, or cafeterias “where they believe the suspect food was bought or eaten. They then ask food suppliers where they received the suspect food item from, and so on,” according to the CDC. “They study purchase and shipment information to find food items that are most closely associated with the illnesses,” the CDC report continues.
State or local environmental health officials are typically responsible for this phase of the investigation, though for more widespread outbreaks, investigators from the Food and Drug Administration (FDA), U.S. Department of Agriculture (USDA), and CDC may become involved.
In the Washington Pork contamination case, the state investigation into the source of the Salmonella traced the meat consumed by 59% (35 infected people) of the 59 people interviewed back to a pork slaughter establishment in Graham, Washington. The institution, a U.S. Department of Agriculture (USDA) inspected facility that was regularly examined by the USDA Food Safety and Inspection Service (FSIS), distributed whole hogs and pork parts. The meats were distributed primarily from six farms – five in Montana and one Washington – to retailers and and restaurants in Washington, Oregon and Alaska.
Since 21 of the 59 people interviewed did not eat pork directly traceable to the suspected slaughterhouse, public health officials dug deeper. Their efforts paid off: 13 of those 21 people had eaten at one of the two restaurants that served pork from the establishment or shopped at the one market where pork from the same place was served.
One more complication: Antibiotic resistance
Ten isolates of the different outbreak strains from the Washington pork outbreak were submitted to the CDC National Antimicrobial System for resistance testing – in other words, officials wanted to determine whether or not the outbreak strains could have been treated with antibiotics if it had become necessary.
Every single one of the ten samples submitted for testing showed resistance to ampicillin, streptomycin, tetracycline, and sulfisoxazole – some of the most common antibiotics used to treat Salmonella infections. Less than 1.5% of this Salmonella serotype were resistant to these antibiotics in 2009, just 7 years ago. In the course of those 7 years, resistance to antibiotics in this strain jumped 44% to 45.5%.
This is particularly worrisome to officials because infections with antimicrobial-resistant Salmonella strains are associated with an increased risk of treatment failure, bloodstream infection, hospitalization and death.
Controlling the Outbreak: Sometimes, it Takes More than a Recall
Eight of eleven environmental samples collected on July 31st from the slaughterhouse in question tested positive for at least one of the outbreak strains of Salmonella. Another investigation by the USDA’s FSIS conducted from August 10th – August 14th “cited insanitary conditions, supported by isolation of outbreak strains from samples taken before the start of daily operations,” the CDC outbreak recap explains.
The slaughterhouse at the heart of the Washington pork outbreak recalled approximately 116,262 pounds of whole hogs that it had produced between April 18th and July 27th on August 13th. Two weeks later, on August 27th, the establishment expanded the recall. In addition to the initial 116,262 recalled pounds of pork, another 523,280 pounds of pork product that was produced April 18-August 26 due to potential Salmonella contamination.
The slaughterhouse voluntarily ceased operating on August 27th, the date of the second recall.
Learn and evolve: Declaring an outbreak over and post-outbreak analysis
Information source traceback and environmental analysis often suggests ways to control the ongoing outbreak and prevent similar outbreaks from happening in the future.
In the case of the Washington pork outbreak, the CDC concluded its report stating that “best practices in all parts of the pork production industry, from farm to processing plant, can help reduce the risk for future outbreaks. In addition, prevention strategies that include rigorous Salmonella control in pork slaughter establishments in conjunction with food handling education at the wholesaler and restaurant level should be strengthened.”