The source was home canned potatoes. The potatoes were canned in hot water bath and not a pressure canner. From the CDC:
The attendee who prepared the potato salad with home-canned potatoes reported using a boiling water canner, which does not kill C. botulinum spores, rather than a pressure canner, which does eliminate spores (2). In addition, the potatoes were not heated after removal from the can, a step that can inactivate botulinum toxin. The combined evidence implicated potato salad prepared with improperly home-canned potatoes, a known vehicle for botulism (3).Unfortunately, we hear of too many who still use a hot water bath to can low acid foods. Outbreaks like this remind us of the importance of following scientifically developed canning recipes, especially the use of a pressure canner for low acid foods.
CDC Morbidity and Mortality
Notes from the Field: Large Outbreak of Botulism Associated with a Church Potluck Meal — Ohio, 2015
July 31, 2015 / 64(29);802-803
Carolyn L. McCarty, PhD1,2; Kristina Angelo, DO2,3; Karlyn D. Beer, PhD2,3; Katie Cibulskas-White1; Kim Quinn, MS1; Sietske de Fijter, MS1; Rick Bokanyi, PhD1; Eric St. Germain1; Karen Baransi1; Kevin Barlow4; Gwen Shafer4; Larry Hanna4; Kelly Spindler4; Elizabeth Walz, MD5; Mary DiOrio, MD1; Brendan R. Jackson, MD3; Carolina Luquez, PhD3; Barbara E. Mahon, MD3; Colin Basler, DVM2,3; Kathryn Curran, PhD2,3; Almea Matanock, MD2,3; Kelly Walsh, MPH3; Kara Jacobs Slifka, MD2,3; Agam K. Rao, MD3 (Author affiliations at end of text)
On April 21, 2015, the Fairfield Medical Center (FMC) and Fairfield Department of Health contacted the Ohio Department of Health (ODH) about a patient suspected of having botulism in Fairfield County, Ohio. Botulism is a severe, potentially fatal neuroparalytic illness.* A single case is a public health emergency, because it can signal an outbreak (1). Within 2 hours of health department notification, four more patients with similar clinical features arrived at FMC's emergency department. Later that afternoon, one patient died of respiratory failure shortly after arriving at the emergency department. All affected persons had eaten at the same widely attended church potluck meal on April 19. CDC's Strategic National Stockpile sent 50 doses of botulinum antitoxin to Ohio. FMC, the Fairfield Department of Health, ODH, and CDC rapidly responded to confirm the diagnosis, identify and treat additional patients, and determine the source.