Between late 2011 and spring 2012, dozens of people across 20 U.S. states and Canada were falling ill with apparent Salmonella infections all coming from the same source. For nearly six months, the illnesses slowly cropped up around North America, with health investigators unable to connect the dots about how they were being caused. It wasn’t until April 2012 that the puzzle finally came together, when the Michigan Department of Agriculture and Rural Development performed a routine test on a retail product that came up positive for Salmonella. When they checked the exact strain against a federal disease database, they realized that the food had been sickening people for half a year. But the food in question was not something like raw chicken or leafy greens – it was dry dog food. More specifically, it was Diamond Naturals Lamb Meal & Rice produced at a Diamond Pet Foods plant in South Carolina. Soon after, the Ohio Department of Agriculture found another contaminated bag of a different formula. And then the U.S. Food and Drug Administration (FDA) found more when inspecting the South Carolina facilities. The plant-wide contamination resulted in one of the largest pet food recalls in recent history and actually encompassed nine brands names, including Canidae and Natural Balance. The company expanded the recall eight times – eventually including cat food – and FDA inspectors found additional contamination at another Diamond plant in Missouri. Ultimately, 49 humans tested positive for Salmonella from the pet food. But the actual number of those who became ill could have been closer to 1,500. (For every person who actually tests positive for Salmonella, another 30 are estimated to have been infected, according the U.S. Centers for Disease Control and Prevention.) That outbreak was a reminder that contaminated pet food poses a threat to not just dogs and cats, but their owners as well. A few years earlier, in 2007, at least 62 people became ill in a Salmonella outbreak linked to pet food manufactured by Mars Petcare, which owns brand names such as Pedigree and Whiskas. When a new pet food outbreak makes headlines, readers often ask how humans end up getting sick. Pet owners don’t need to eat kibbles to become sickened by contaminated food. Most people who fall ill from pet food do so by handling contaminated food or having contact with infected animals. Thorough hand washing after serving pet food or touching pets is always recommended to avoid potential pathogen transmission. Of course, foodborne illness outbreaks can work both ways. Among the patients testing positive for Salmonella in the 2008-2009 peanut butter outbreak was one dog. Because dogs and cats are almost never tested for foodborne pathogens such as Salmonella or E. coli, it’s impossible to know how many are sickened when big outbreaks strike a pet food product. Only two dogs were tested positive for Salmonella in the Diamond outbreak, for instance. When dogs or cats do become infected with a foodborne illness, they typically suffer the familiar symptoms such as diarrhea (sometimes including blood or mucus), vomiting, dehydration and lethargy. But some pets may serve as carriers without showing any symptoms, shedding the pathogen in their stools or harboring it on their fur or in their saliva. Parents are often advised to take extra precautions with pets around young children because children have developing immune systems that are especially susceptible to pathogenic transmission. Of the patients in the Mars Petcare outbreak, 39 percent were younger than one year old. It’s possible that children could crawl on floors where pets have been eating contaminated food or treats, or simply come into contact with a pet that has fecal contamination on its fur. These pet-to-human contamination scenarios are one of the many reasons FDA is proposing to overhaul safety rules on pet food manufacturing as part of the Food Safety Modernization Act. Read more on Food Safety News about the changes FDA is proposing, the reactions FDA has received, and 10 changes that have been recommended by experts. The pet food safety series on Food Safety News is sponsored by ABC Research, a company that conducts testing on pet food products. Read more about ABC Research pet food testing on the company blog.