The number of people with listeriosis in mainland China may have been underestimated, according to researchers.

The study reported 562 infections during 2011 to 2017 compared to a previous analysis which identified 229 cases between 1964 and 2010.

Scientists said a comprehensive monitoring system for Listeria infection is urgently needed in China.

“Improvements in methods of tracing Listeria infections, increased awareness of listeriosis among clinicians, and the greater number of databases searched in the present study may have contributed to this increase in number of cases,” said researchers.

No evidence of outbreaks
Outbreaks are rare, and contaminants have not been reported clearly, according to the report. However, the contamination rate of Listeria in retail food seems to be high based on previous research, indicating a potential risk of listeriosis in the Chinese population.

Incidence of listeriosis has been estimated at three to six patients per one million population per year globally. In a study published in 2013, Feng et al. reviewed listeriosis patients reported in China from 1964 to 2010 and found there were 147 sporadic and 82 outbreak cases, with death rates of 26 percent in overall listeriosis and 46 percent in neonatal listeriosis.

A search of three Chinese language databases and three English language databases was performed for articles on listeriosis in mainland China published during 2011 to 2017. A total of 136 articles met the inclusion criteria for the study published in the International Journal of Infectious Diseases.

In total, 562 listeriosis infections were reported from January 2011 to October 2017, including 227 non-perinatal patients, 231 perinatal patients, and 104 non-clustered patients in whom the clinical diagnosis was diarrhea without a description of other characteristics. They all had sporadic infections and there was no evidence of an outbreak. Healthcare-associated infections were a large proportion of cases.

Summer peak
The 227 non-perinatal listeriosis patients had a mortality rate of 23.78 percent. Of the 231 perinatal patients, 32.68 percent resulted in abortion and/or newborn death.

The number increased each year until 2013 and then decreased annually. Although cases were reported in each month, most were in the summer. The capital city, Peking, and coastal areas reported the most patients, which may be due to the higher detection rate of Listeria, dietary habits, and high population density.

Of the 164 non-perinatal patients with gender information, 86 were male. Age information was available for 113 patients and those over 55 years old accounted for 33 percent. Thirty-four of 110 people with an underlying disease died and five of 53 people without an underlying disease also died.

Only 17 patients reported consumption of raw, cold or unclean food, mostly barbecue. In three patients, beef and manufactured meat sold in a supermarket were traced as the sources of contamination by pulsed field gel electrophoresis (PFGE) testing.

The perinatal patients included 23 aborted fetuses and 47 newborns who died. The most commonly reported serotype was 1/2b, followed by 1/2a while 13 MLST subtypes were reported with ST87 being the top isolated subtype.

Meanwhile, another study found a higher Listeria monocytogenes prevalence in meat products from central and northeastern China provincial regions.

The study estimated the prevalence of Listeria in a range of meat products from 24 different Chinese regions. These insights could help guide effective risk management by state and provincial authorities. Results were published in the International Journal of Food Microbiology.

The study found a decline in the overall prevalence of Listeria for sampling data obtained after 2010 compared to those before 2010 but the pooled prevalence level in raw meats, especially prefabricated raw meats and fresh pork, was still relatively high. Sampling period and location were found to influence the prevalence level.

“A better understanding of differences in prevalence levels per geographic region and between meat product sources may allow the competent authorities, industry, and other relevant stakeholders to tailor their interventions to control the occurrence of Listeria monocytogenes in meat products effectively,” said researchers.

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