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Establishing the source of a foodborne disease outbreak takes a combination of teamwork and technology. And a healthy dose of luck.

All of these elements were indispensable to the detection and definition of the Salmonella Infantis outbreak that is linked to contaminated dry pet food manufactured by Diamond Pet Foods in Gaston, South Carolina.

We know the toll of confirmed human illnesses caused by contaminated dry pet food from Diamond Pet Foods’ Gaston facility – 22 cases as of June 13, 2012. These have been documented by CDC and the Public Health Agency of Canada.

Thirteen states are reporting outbreak illnesses, all of them caused by a single strain of Salmonella Infantis. The US reports come from Alabama (2), California (1), Connecticut (1), Illinois (1), Michigan (1), Missouri (3), North Carolina (3), New Jersey (1), New York (1), Ohio (2), Pennsylvania (2), South Carolina (1) and Virginia (1). Canada has confirmed two outbreak cases, one each in the provinces of Quebec and Nova Scotia.

Distribution of confirmed US outbreak illness reports

Outbreak-related illnesses began as early as October 8, 2011; the most recent illness onset was May 11, 2012. Infected individuals cover the entire age spectrum – from less than one year old to 82 years old. Two-thirds (68%) of the  patients are female, and at least six of the outbreak victims were hospitalized.

In response to the outbreak and to the finding of Salmonella Infantis in samples of three different Diamond Pet Foods products by Michigan, Ohio and FDA, the company recalled a range of products that were manufactured in its Gaston, South Carolina facility between December 9, 2011 and April 7, 2012.

Let me repeat (in case you missed it). The outbreak began, as far as CDC is concerned, with an illness that developed on October 8, 2011. The earliest production date that was recalled was December 9, 2011.

I’ve been wondering about this disparity for quite some time. Yesterday, I had the opportunity to put the question to Dr. Casey Barton Behravesh, of the US Centers for Disease Control and Prevention (CDC) during a telephone interview. Here is what I learned:

The outbreak first came to light as a result of a routine retail-level pet food sampling program conducted by the state of Michigan. On April 2nd, the Michigan Department of Agriculture and Rural Development detected Salmonella in an unopened bag of Diamond Naturals Lamb Meal & Rice dry dog food, which had been collected March 14, 2012. The state profiled the Salmonella strain using Pulsed Field Gel Electrophoresis (PFGE), often referred to as “genetic fingerprinting.” As is routine, Michigan reported the PFGE result to CDC.

Whenever CDC receives this type of report, the agency looks back at recent reports of human illnesses – reporting human Salmonella illnesses to CDC is mandatory – and checks for matching PFGE profiles. In this instance, CDC found several reports, dating back to the beginning of 2012. The next step was to contact the people who had been infected, and interview them to determine whether an outbreak was underway. These interviews, most often, are carried out by state or local health authorities.

There are two dates that are highly relevant to CDC in this type of investigation. One is the date of onset – the first day that a person experiences symptoms. The other is the date that the microbe was actually recovered from the patient. Typically, these two dates are fairly close in time. But this outbreak wasn’t typical.

When the state or local health authorities contacted one of the people for whom the isolation date was reported as January 2012, they were told that this person actually had begun to experience symptoms on October 8, 2011. The individual also reported “dog contact,” but could not name a specific brand of dog food that was involved. The combination of “dog contact” AND the fact that the individual had been infected with the outbreak strain of Salmonella Infantis, convinced CDC to include this individual in the case count.

What was the rationale for choosing December 9, 2011 as the earliest production date to include in the recall? I didn’t ask this question of Dr. Barton Behravesh, as recalls come under the auspices of FDA, not CDC. I suspect, though, that there was not enough firm evidence to convince Diamond Pet Foods (and, perhaps, FDA) that the October victim had been infected through handling a Diamond product. But, why December 9th, rather than January 3rd, which was the earliest production date of a batch that was proven to contain Salmonella?

I believe that the answer lies with the second outbreak victim, whose symptoms began on January 4, 2012. This person would have been using a batch of food that was manufactured prior to that date – perhaps several weeks prior to the January 4th illness onset date. It’s even possible that the victim was able to report the actual production code to investigators.

And what about reports of sick and dead pets? CDC doesn’t track those (unless they occur in a household where a human outbreak case is confirmed). Dr. Barton Behravesh referred me to FDA’s Center for Veterinary Medicine (CVM) for an answer to this question. She also declined to provide a specific answer to my question about which brands of Diamond Pet Foods products were used in households where outbreak cases had been confirmed – saying only that all of the brands reported by outbreak victims were included in the various Diamond recalls.

Veterinarians and pet owners are encouraged by CVM to report pet illnesses that may be related to pet food, treats or medications. Unfortunately, the data generated by these reports are not readily accessible to the public, and not easily searched, even within FDA. Thus, as has happened several times in the past, my request for statistics on pet illnesses related to Diamond Pet Foods brands was met with “You will need to submit a Freedom of Information Act request.”

Fortunately, someone beat me to the punch. I learned earlier today from an eFoodAlert reader that several “Incident Reports” were posted on the website of the VIN News Service on June 6th. According to these reports, various Diamond-manufactured brands were blamed by pet owners for causing the deaths of five dogs and one cat. Most of the deaths occurred in the first half of April. There is no way to confirm the cause of these deaths – or of many anecdotal reports of animal illnesses – because Salmonella testing was not carried out on most of these animals. Previously, FDA acknowledged two animal deaths – both dogs – in a household where a human outbreak case was confirmed.

As for the single production batch of Diamond Naturals Small Breed Adult Dog Lamb & Rice Formula Dry Food that was manufactured in Meta, Missouri and recalled by the company on May 18th, Dr. Barton Behravesh assured me that CDC has checked its database of PFGE Salmonella profiles, and no illnesses have been linked to this batch. Only one individual was reported ill with the same PFGE strain; however, that person was traveling outside the USA during the entire incubation period and is thought to have become infected while away from home.

Finally, Dr. Barton Behravesh urges pet owners and people who come into contact with companion animals to follow CDC’s advice to pet owners, below:

Advice to Consumers and Families with Pets

CDC offers this advice to pet owners:

  • Consumers should check their homes for recalled dog food products and discard them promptly. Consumers with questions about recalled dog food may contact Diamond Pet Foods at telephone number (800) 442-0402 or visit www.diamondpetrecall.com.
  • Follow the tips listed on Salmonella from Dry Pet Food and Treats to help prevent an infection with Salmonella from handling dry pet food and treats.
  • People who think they might have become ill after contact with dry pet food or with an animal that has eaten dry pet food should consult their health care providers. Infants, older adults, and persons with impaired immune systems are more likely than others to develop severe illness.
  • People who think their animal might have become ill after eating dry pet food should consult their veterinary-care providers.
  • Read Additional Information for Pet Owners to learn the signs and symptoms of salmonellosis in dogs and cats, and to understand how to deal with possible Salmonella illness in your pet.

Also, be aware that dogs may be infected with Salmonella – and may shed the bacteria in their stool – without showing any outward symptoms of illness. If your pet has consumed a Diamond Pet Foods dry dog food, be especially careful to wash your hands after handling it, and supervise closely any interaction between children and your pet.

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