When the SGH Microbiology blogger (or his team) posted notice on Facebook of the following paper by The Chinese University of Hong Kong investigators, it reminded me of a conversation with that contemplative microbiologist a couple of years ago about hepatitis E. This work from Hong Kong was published in Journal of Clinical Microbiology in February this year. The investigators had compared hepatitis E viruses (HEVs) from human patients with HEVs from food items – pig liver (1.5% of market samples found contaminated by HEV), pig intestine (0.4%) and oysters (0.2%) – and found that those from humans and pigs belonged to the same genotype (HEV-4) and moreover were very similar, sharing over 95% identical nucleotides. The obvious conclusion is that pig products – especially pig livers – are a major source of hepatitis E in humans in Hong Kong.

Screenshot from the Journal of Clinical Microbiology website: HEV paper from Hong Kong.

The link between pork products and human HEV infections is not limited to Hong Kong. In Europe, for instance, HEV is believed to be predominantly transmitted by the consumption of pork (including paté and sausages) and wild boar, or else exposure to these animals. In South Korea and China (behind a paywall), similar links to consumption of undercooked pork or swine exposure have also been made. In countries where HEV is endemic, including India which is perhaps the country of origin of the virus, transmission is primarily faecal-oral (contamination of water seems to be the most common).

Few of these studies were as comprehensive as the Hong Kong work, but largely arise from the understanding that of the four genotypes of HEV, genotypes 1 and 2 infect only humans, while genotypes 3 and 4 primarily infect other mammals (like pigs), but can occasionally infect humans. The latter (genotypes 3 and 4) are the most common genotypes in parts of the world where HEV is not endemic, whereas the former (genotypes 1 and 2) are far more prevalent in parts of Asia and Africa where the virus is endemic.

How about Singapore? Hepatitis E is a notifiable disease here, and has been the most common cause of acute viral hepatitis in Singapore for a number of years, with hepatitis A coming a close second. It is nonetheless relatively uncommon, with less than 90 cases notified each year. An epidemiological report (behind a paywall) on cases notified between 2000 and 2011 showed that more than half of the 540 laboratory-confirmed cases were imported (majority from India and Bangladesh), but that the prevalence among indigenous cases was increasing over the time period. Patients with Chinese and Indian ethnicities were also over-represented among the indigenous cases. The report found no epidemiological links between indigenous cases, nor was any common food item found (although apparently pork liver was investigated).

Chart from the Singapore paper on HEV published in Journal of Infection, 2013.

In a separate paper, investigators from the Singapore General Hospital typed 18 human HEV isolates obtained between 2009-2013 and found that the majority (14 of 18) belonged to genotype 3. This was also supported by work by the National Public Health Laboratory, which found that all indigenous HEV infections where the virus could be genotyped (so only 12 of 25 cases) in 2014/2015 were caused by HEV genotype 3.

It will be interesting to confirm – although the results would be unsurprising – that pork liver and other pork products are also significantly associated with HEV infections in Singapore. I will be abstaining from undercooked sausages and porridge with blanched pork liver in the interim…(*note however that cooking – apparently at 70 degrees Celsius for 10 minutes on average – would be sufficient to inactivate the HEV).