Hepatitis A cases could rise in rainy season, virologist warns

Yong Poovorawan, a virologist at the Centre of Excellence in Clinical Virology, Faculty of Medicine, Chulalongkorn University, has urged the public to remain alert to a growing outbreak of hepatitis A, particularly in eastern Thailand, as cases are expected to rise during the rainy season.

Posting the warning on his Facebook page on Sunday (April 26), Yong said this year had seen a relatively large outbreak of hepatitis A.

Hepatitis A is caused by a virus in the enterovirus group, which is transmitted through faeces and can contaminate the environment, food and drinking water. This can lead to large outbreaks, especially among people who do not yet have immunity.

Although hepatitis A has several genotypes, it has only one serotype. This means that a person who has been infected once will generally develop immunity that can protect them for life.

Yong said hepatitis A spreads very easily. Most Thais under the age of 40 currently have no immunity against the virus unless they have been vaccinated. Among people aged 40-50, almost 50% are believed to have immunity, while most people over 60 have immunity from natural infection in the past.

He explained that food and drinking-water hygiene today is much better than in the past, meaning younger generations are less likely to have been exposed to the virus naturally and therefore have to rely on vaccination.

In contrast, people who grew up in an era when families commonly drank water from the same dipper often already have immunity. Those born 50 or 60 years ago are therefore more likely to have immunity and may not need vaccination.

Most outbreaks usually occur at the start of the rainy season. However, Yong said this year’s outbreak had already begun during the hot season, raising concern that the number of patients would certainly increase during the rainy season.

He said that whenever an outbreak occurs, the ministry usually inspects the affected area and often blames contaminated drinking water.

In reality, however, inspections are often carried out in the later stages, when the virus has already spread widely and viral loads are high. As a result, drinking-water tests often detect the virus.

Yong said the true source of an outbreak is often difficult to identify. However, based on available data, he believes outbreaks often begin with migrant workers carrying the virus into the country, before it spreads in clusters in factories.

This is difficult to verify, but he said it was not unusual for outbreaks to occur in factories where large numbers of people work together.

He stressed that hygiene is especially important in food-related factories and among restaurant staff. Handwashing after using the toilet must be strictly observed. Where possible, vaccination should also be provided to workers in these groups to help reduce the spread of the disease.

Another major concern is seafood. Yong said the virus can survive in seawater, and many types of shellfish can take in the virus while feeding on plankton or other substances. If shellfish are only briefly blanched or cooked at an insufficient temperature, the virus may survive.

He said the virus generally requires temperatures above 85 degrees Celsius to be destroyed. Food that is fully cooked or heated above 100 degrees Celsius should therefore be safe. However, lightly blanched cockles or raw oysters may not reach a high enough internal temperature and could become a source of transmission.

Yong cited the hepatitis A outbreak in Shanghai 30-40 years ago as an example, saying it caused several hundred thousand infections.