A rapid method to detect human noroviruses in food has been proposed in Finland.
The findings were part of a doctoral dissertation comparing four published extraction methods for detection of norovirus in lettuce, ham, and frozen berries.
Maija Summa, a specialist in veterinary medicine, environmental health and food control, is employed at the virology unit of the Finnish Food Authority (Ruokavirasto).
Noroviruses are classed into seven genogroups and human noroviruses (HuNoVs) belong to genogroups I (GI), II (GII), and IV (GIV).
Challenge to detect viruses in food
HuNoVs are responsible for a large number of acute human gastroenteritis cases globally in all age groups every year and cause food-related illnesses in countries including Finland by outbreaks through a variety of vectors that include contaminated water and foodstuffs.
Based on reports of the European Commission, in Europe and Finland the most common foods causing HuNoV outbreaks are shellfish, berries — especially frozen raspberries, vegetables, and mixed foods, which most likely become contaminated by a sick food handler.
In most foodborne norovirus outbreaks the causative agent is identified only in fecal samples of patients and food handlers and only rarely in food samples. One challenge in detecting viruses in foods is the viruses are often present in food samples at low levels.
Other methods for detecting HuNoVs in food have been developed but many are time-consuming and sensitivity has been highly variable, according to the dissertation.
Four compared methods
The four compared methods included one based on an ultrafiltration method, a commercial NoroCheck IMS kit based on immunomagnetic separation, the third was based on an ultracentrifugation method and the final one was a combination of two PEG precipitation-based methods.
The method based on alkaline elution and polyethylene glycol (PEG) precipitation was the most reliable for all three food matrices tested. Recovery efficiency of the method with frozen raspberries was on average 28 percent.
Two rapid methods for detection of HuNoV in frozen raspberries were also presented. The rapid method based on direct RNA extraction yielded the same recovery levels (32 percent) as the PEG precipitation one. It detected HuNoV in naturally contaminated berry samples linked to outbreaks so is a relevant alternative to the standard method, especially in outbreak situations when results of virus analysis are needed quickly.
Treatment with a chloroform-butanol mixture or dilution of food samples for the reverse transcription polymerase chain reaction (RT-PCR) reaction was efficient in reducing the effect of PCR inhibitors. The same impact was achieved with PEG as a supplement in the food samples.
“The use of PEG as a supplement was found to reduce inhibition of the RT-PCR reaction in the two rapid methods, and therefore, the commonly used chloroform-butanol treatment, which easily loses viruses during processing, could be omitted,” according to the dissertation.
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