We defined a case of rhabdomyolysis as any person with elevation in creatine kinase (CK) value plus clinical manifestations of myalgia or limb weakness (10,11). We defined a Haff disease case as illness in any person with acute onset of rhabdomyolysis after ingestion of freshwater fish or seafood within 24 hours in Anhui Province during June–August 2016. We searched for physician-diagnosed rhabdomyolysis cases from the NFDSS, an internet-based, passive surveillance system for foodborne diseases searchable by food source in China. We also reviewed the outpatient and inpatient medical records in hospitals in Wuhu and Ma’anshan during the outbreak period to search for potential rhabdomyolysis cases.

Local Anhui Province CDC staff or Chinese Field Epidemiology Training Program trainees interviewed all rhabdomyolysis case-patients, either in-person or by telephone, using a structured questionnaire. Information collected included age, sex, date of onset, disease duration, clinical symptoms, potential risk factors (e.g., food, drugs, alcohol consumption, intense exercise, allergy history, underlying chronic illness), and the quantity of crayfish consumed. The researchers also obtained laboratory test findings from hospital medical records. They applied the Haff disease case definition to rhabdomyolysis cases to identify Haff disease cases and collected blood and urine specimens from Haff disease case-patients for further analysis.

In total, 673 rhabdomyolysis cases were identified in Anhui Province during June–August 2016. Of these, 99.9% (672/673) were compatible with the definition of Haff disease. All but 1 patient consumed cooked crayfish before symptom onset. The patient who did not eat cooked crayfish was a steelworker who had been working in the factory before onset, suggesting his illness might have been caused by heatstroke.

Although nearly all Haff disease case-patients ate cooked crayfish, we did not know what percent of persons who did not become ill also ate meals containing crayfish, given that crayfish were widely available during June–August 2016. Therefore, we conducted a matched case–control study to assess the association between eating crayfish and Haff disease. Cases in this study were persons who met the definition for Haff disease, had shared a meal with someone else before symptom onset, and consented to participate in the study. We identified >1 control per case; controls were selected among persons who shared the suspected meal of exposure with the case-patient before symptom onset. Controls had no clinical symptoms compatible with rhabdomyolysis and consented to participate in the study. Persons with other illnesses (e.g., fever, cold, injury, etc.) were disqualified as controls. In total, 67 cases and 108 controls were enrolled in the case–control study. Trained investigators conducted telephone-based interviews August 7–15, 2016, using a standardized questionnaire.

For all Haff disease cases, we conducted a traceback investigation for the source of the implicated food by interviewing case-patients, restaurant owners, fishermen, and crayfish sellers. We conducted an environmental investigation of potential contamination along the distribution chain or unusual events during the outbreak period. We investigated the restaurants where case-patients had a meal before onset to find out where the crayfish came from and how they were cooked. We also visited crayfish farms, settings where crayfish were caught, and factories along the Yangtze River to identify whether the implicated crayfish or the environment in which the crayfish were raised had been contaminated.

We performed statistical analysis using SPSS Statistics 20 (IBM, https://www.ibm.com). We compared cases and controls by χ² test. Significant risk factors (p<0.05) in the χ² tests were included in a multivariate Cox proportional hazard model to determine the odds ratio (OR) and 95% CI for the potential risk factors associated with Haff disease. All of the p values were 2-sided, and p<0.05 was considered significant.

In total, we verified 672 Haff disease cases in Anhui Province during June–August 2016. All cases occurred in 7 cities along the Yangtze River in Anhui Province; 83.3% (560/672) of the cases occurred in Wuhu (334 cases) and Ma’anshan (226 cases). We focused our investigation on the cases that occurred in Wuhu and Ma’anshan. Of the 560 case-patients in Wuhu and Ma’anshan, 495 (88.4%) completed the questionnaires; all 495 had consumed crayfish within 24 hours before symptom onset. The epidemic curve suggested a continuing common-source outbreak (Figure).

The outbreak started at the end of June, peaked in mid-July to early August, and lasted through August 17. Of the 495 case-patients, 197 (39.8%) were hospitalized; mean length of hospital stay was 7.3 ± 3.2 days. No deaths were reported. The mean age of the case-patients was 38.7 ± 13.5 years; 323/495 (65.3%) patients were female. Although cases were widely distributed in the 2 cities, 87.7% (434/495) were in residents from urban areas close to the Yangtze River. Each case-patient consumed a mean of 11.6 ± 6.1 crayfish pieces. The mean incubation period was 6.2 ± 3.8 hours.

All 495 case-patients experienced myalgia that was local or diffuse, involving the back, waist, whole body, neck, limbs, and chest (Table 1). A total of 271/495 (54.7%) experienced muscle weakness. Additional symptoms included brown urine, dyspnea, vomiting, abdominal pain, dizziness, and headache. Symptoms of nerve paralysis and fever were rare. Acute renal failure was not observed.

We reviewed the laboratory test findings of blood and urine for some cases. The mean value of myoglobin was 330.0 ± 121.2 ng/mL, and mean CK level was 5,439.2 ± 4,765.1 U/L. In >80% of the cases, the levels of muscle-type CK and aspartate aminotransferase were abnormally elevated. In addition, 50.0% of case-patients were positive for urinary occult blood and proteinuria (Table 2).

In the case–control study, 100% of the 67 cases and 93.3% (101/108) of controls ate crayfish during their shared meal (OR = ꝏ, 95% CI 0.92–ꝏ). We observed a significant dose-response relationship between the number of pieces of crayfish eaten and Haff disease (χ² = 29.225; p<0.001) (Table 3). Further analysis showed that eating crayfish liver was associated with increased disease risk (OR = 4.0, 95% CI 1.2–12.7).

Wuhu and Ma’anshan are located in the middle to lower reaches of the Yangtze River. Crayfish is a popular dish for residents of these 2 cities. Before the Haff disease outbreak, Anhui Province experienced heavy rainfall, which caused the largest flood disaster in decades. Consequently, rain or floodwater was retained in irrigation ditches and detention ponds for an extended time, and the amount of crayfish caught on the shores of the Yangtze River or its connected ditches was 5–10 times more during the outbreak period. However, no industrial or chemical contamination along the Yangtze River was reported.

The only common risk factor for all cases was eating crayfish, which were cooked thoroughly. We conducted a traceback investigation of the source for the implicated crayfish in Ma’anshan and Wuhu by interviewing persons in markets, restaurants, fisheries, and settings where crayfish were caught, as well as fishermen; we were able to trace 50.1% (248/495) of the implicated crayfish to their sources. Of these, 96.8% (240/248) were wild crayfish caught on the shores of the Yangtze River or its connected ditches. When we consulted with crayfish biologists, we found that the species of crayfish implicated during this outbreak was Procambarus clarkii.

Local governments issued a warning about the dangers of eating crayfish. In addition, public health departments instituted continuous surveillance and investigation of the outbreak.