The case definitions for this investigation were established as the following (1): suspected cases were those who had onset of systemic or local muscle pain, fatigue, brown urine, and other symptoms related to rhabdomyolysis syndrome with unknown specific-causes; clinically-confirmed cases were those with levels of CK at least five times higher than that of the normal range upper limit. The investigation team conducted case searches door-to-door in Ping’an village.
Five clinically confirmed cases were found, with an incidence of 5.1% (5/98). The main symptoms were onset of muscle soreness 5/5), vomiting (4/5, 1−3 times), brown urine (4/5), and fatigue (3/5). Clinical examination showed that CK increased more than 5 times (5/5, value is 1,389−4,073 U/L), leukocyte count increased (5/5), and myoglobin increased (4/5).
The first case occurred at 19:30 on November 7, and the last case occurred at 01:00 on November 8. The event lasted for 5.5 hours. The epidemiological curve’s distribution suggested a point source exposure. There was no history of joint exposure except for dinner on the evening of November 7 for the 5 cases, which was then confirmed as the exposure event. The median incubation period from consumption to onset of symptoms was 5 hours (range: 0.5–6 hours).
On November 7, 2016, a total of 6 persons from two families consumed the hotpot containing carp, vegetables, and dumplings together. Patient A caught the carp in his own pond and prepared the carp while retaining the testes, eggs, and swim bladder, which are often consumed. Patient A then fried the parts of the carp with oil and then boiled them in a pot of water for ten minutes. Afterwards, the vegetables and dumplings were added to the pot to be boiled for five minutes. One person only consumed the vegetables and dumplings and did not fall ill, while the five others who consumed parts of the carp fell ill. In addition to these two families, patient A also shared the remaining parts of the carp with two neighbor families consisting of nine addition people, but the neighbors prepared the carp separately and consumed only the fish’s flesh. These additional nine people exhibited no related symptoms or disease.
Using a retrospective cohort study design, eating carp testes (RR=10.99, 95% CI①: 1.70–71.28) and carp eggs (RR=6.00, 95% CI: 1.69–21.26) were found to be risk factors for the disease (Table 1). To study the relationship between CK value and food intake, the investigation team took photos of various carp meat samples weighing 10 g, 30 g, and 50 g to help patients recall their level of consumption before the onset of symptoms. The results showed that CK levels were positively correlated with total recalled intake of carp testes and eggs (Pearson correlation coefficient r=0.98, p=0.04) (Figure 1).
Part of carp Consumers Non-consumers Attack rate (%) RR (95% CI) Fisher’s exact test Case Total Case Total Consumers Non-consumers Flesh 5 12 0 3 41.7 0 Undefined p=0.510 Swim bladders 2 4 3 11 50 27.3 1.83(0.46−7.25) p=0.560 Testes 4 4 1 11 100 9.1 10.99(1.70−71.28) p=0.004 Eggs 3 3 2 12 100 16.7 6.00(1.69−21.26) p=0.020
Table 1. Risk analysis between consuming different parts of carp and rhabdomyolysis syndrome in Ping’an village, Gongcheng Yao Autonomous County, Guangxi, 2016. (N=15)
Figure 1. Analysis of the correlation between creatine kinase value of rhabdomyolysis syndrome patients and total recalled consumption of carp testes and eggs in Ping’an village, Gongcheng Yao Autonomous County, Guangxi, 2016.
Further investigation of the external environment, i.e. the pond where the carp was raised, showed that the water levels were extremely low and had accumulated high levels of algae. In addition, a large quantity of abandoned bait, fishing materials, pesticides, fertilizer, and other packaging bags were found in the vicinity. Samples from the pond and from the hotpot leftovers were collected, but due to a lack of hypothesis on pathogenic factors and direction for the investigation, a determination could not be made. However, the above samples were stored by Gongcheng County CDC for possible future testing.