Outbreak Linked to Morel Mushroom Exposure — Montana, 2023

Investigation and Findings

Identification of Index Patients and Implicated Foods

Investigation of the two index patients revealed that both persons had consumed a special sushi roll containing salmon and morel mushrooms. Morels were a new menu ingredient and were the only ingredient unique to the special sushi roll, making it an early suspected source of the outbreak. The morels were prepared in various ways during March 27–April 17. On April 8, morels were served partially cooked: a hot boiled sauce was poured over the raw morels, after which they were marinated for 75 minutes. On April 17, the morels were uncooked and cold-marinated before serving. During an inspection of restaurant A on April 18, food samples were collected, including salmon and morel pieces remaining from the original packaging. Multiple violations were identified at the time of inspection, including temperature control issues, improper time control and sanitization procedures, and improper storage of personal items. On the basis of the thorough environmental inspection and collection of food samples, clinical presentation of index patients, and early evidence implicating a specific food item, investigators determined that environmental sample collection was not necessary. On April 19, restaurant A informed GCCHD that it had received four additional telephone calls from patrons reporting illness after consumption of the special sushi roll. Other patrons reported illness directly to GCCHD throughout the investigation.

Clinical Characteristics of Ill Persons

After a May 3 press release providing an update on the outbreak and soliciting additional cases, GCCHD continued to receive reports of gastrointestinal illness related to eating at restaurant A between March 28 and April 17. In total, the investigation identified 51 cases of illness* (including five cases among employees and 46 among restaurant patrons); of these cases, four ill persons visited a hospital emergency department, three were hospitalized, and the two index patients died (including one who sought emergency care but was not hospitalized). Among the 51 ill persons, 45 (88%) reported consumption of morels at restaurant A. Review of medical records for the three persons who were hospitalized and one person who sought emergency medical care without hospitalization revealed that the onset of gastrointestinal symptoms was rapid, with a median symptom onset of 1 hour after the restaurant meal. Vomiting and diarrhea were reportedly profuse, and hospitalized patients had clinical evidence of dehydration. The two patients who died had chronic underlying medical conditions that might have affected their ability to tolerate massive fluid loss.

Testing of Clinical and Food Specimens

MTDPHHS and GCCHD coordinated with FDA for food specimen testing and with CDC’s toxicology and enteric diseases groups for clinical specimen testing.^† Food specimens were tested for multiple substances, including volatile and nonvolatile organic compounds and enteric pathogens.^§ Neither clinical testing nor food testing identified a causative agent.

Traceback of Implicated Food Items

Health department sanitarians traced the morels to a single importer, and a separate distributor that supplied mushrooms to multiple states. The morel mushrooms were cultivated and imported fresh from China. FDA and the California Department of Public Health contacted 12 California facilities that received morels from the same importer during January 1–May 17, 2023, six of which responded to the inquiries and reported receiving no illness complaints from patrons who ate morels prepared and served by those facilities. All six facilities reported cooking or otherwise thoroughly heating the morels before they were served. A CDC Epidemic Information Exchange (Epi-X) multistate call for cases did not result in additional case reports.

Matched Case-Control Study

To identify foods associated with illness and assess a possible dose-response relationship, MTDPHHS and GCCHD, with support from CDC, conducted a matched case-control study. A questionnaire solicited demographic and symptom information and food items consumed.^¶ A case was defined as the occurrence of diarrhea, nausea, vomiting, or abdominal pain in a person any time after eating at restaurant A during March 27–April 18, 2023. Cases were identified through reports received by restaurant A, public health departments, and case-patient interviews. Control participants were identified during case-patient interviews as dining partners who did not report illness after eating at restaurant A. Case-patients (or a proxy) and control participants were interviewed during May 4–12, 2023.

State and county health department staff members collected questionnaire data using Jotform software (version 4.0; Jotform Inc.). Deidentified data were sent to CDC for analysis. CDC personnel conducted matched and unmatched logistic regression analyses using SAS software (version 9.4; SAS Institute). This activity was reviewed by CDC, deemed not research, and was conducted consistent with applicable federal law and CDC policy.**

A total of 41 case-patients^†† and 22 control participants responded to the questionnaire and were included in analyses, representing 29 unique dining parties. Overall, 27 (36.9%) case-patients were female and 31 (75.6%) were aged 20–49 years. A majority reported diarrhea (37; 90.2%), nausea (34; 82.9%), abdominal pain (28; 68.3%), loss of appetite (25; 61.0%), fatigue (24; 58.5%), vomiting (22; 53.7%), and abdominal distension (22; 53.7%) (Table 1).

In logistic regression analyses matched on dining party, the odds of eating the special sushi roll with morels among case-patients were 15.78 times higher than the corresponding odds among control participants (Table 2). In addition, the odds of eating any morels among case-patients were 10.77 times higher than the corresponding odds among control participants. Other foods did not appear to be associated with case status.

When the amount of the special sushi roll consumed was modeled continuously, the association was positive between a one-piece increase in consumption of the special sushi roll and case status in all analyses (Table 2). When the special sushi roll was modeled as a three-level categorical variable (none, one–three pieces of a roll, and four pieces of a roll or more), associations in the unmatched models also suggested a dose-response relationship.^§§

In models stratified by meal date, odds ratios comparing the odds of eating the special sushi roll (yes versus no) among 27 case-patients and 18 control participants were higher among those who ate at the restaurant on April 17, when uncooked morels were served, relative to April 8, when the morels reportedly underwent a partial cooking process (Table 3).