A recent outbreak of listeria via contaminated rockmelons was a cause for concern among many fruit lovers, including myself.

Between January and April, at least 19 people in Australia contracted listeria, and seven have died.

Rockmelons contaminated with listeria were exported from one farm in Australia to numerous countries, including Singapore.

The authorities here promptly recalled the affected consignments and destroyed the fruits.

On April 16, the Ministry of Health said that two people in Singapore were infected with a strain of listeria similar to the one found in a patient in Australia that is tied to rockmelons.

But the ministry added, however, that there is no further health risk here from the Australian outbreak following the recall of implicated consignments in March.

So what is listeria and should we still be concerned about eating rockmelons from now on?

Listeria is actually a bacterium that is normally found in soil and decayed vegetables. It may infect humans if it manages to enter our food chain.

If a healthy individual consumes food contaminated with listeria, he may have a short episode of gastroenteritis over two or three days, with symptoms like fever, abdominal pain, vomiting, and diarrhoea.

In addition, people infected with listeria often have headaches and body aches. The illness is self-limiting when it occurs in healthy individuals.

But when patients with high-risk features contract the disease, the gastroenteritis may progress to invasive listeria infection, affecting the central nervous system, causing encephalitis or meningitis.

In high-risk individuals, the mortality rate can be as high as 20 per cent to 30 per cent.

High-risk individuals include the elderly and immunocompromised persons such as cancer patients, as well as those on steroids or those with HIV.

Pregnant women form another high-risk group, whereby infections may lead to miscarriages.

In the past, listeria outbreaks typically occurred when the bacterium contaminated farms or food-processing facilities. Ready-to-eat food, such as those found at delicatessens — ham, soft cheese, sausages — are particularly at risk.

The largest listeria outbreak to date took place in South Africa last year, where at least 975 individuals were infected.

Polony, a popular local food which is similar to ham, was found to be the source of infection. Listeria was believed to have contaminated the food processing factories, causing polony to be contaminated.

Listeria outbreaks have also been described in contaminated vegetables and fruits such as rockmelons and fresh bean sprouts.

Why rockmelons? Some scientists believe that listeria could have contaminated the soil in the rockmelon farms.

As rockmelons grow on the soil, listeria could have resided on the surface of the rockmelons.

If consumers do not wash rockmelons properly, a cutting knife could bring the bacteria from the surface into the flesh of the fruit.

Listeria outbreaks have however not been reported in watermelons or honeydew, though both fruits grow on the ground like rockmelons.

It is believed that the rough, irregular surface of rockmelons make it hard for bacteria to be washed off.

According to the Ministry of Health, besides a cluster of cases in 2006-7 where there were nine and six cases of listeria in a year, listeria was hardly detected in Singapore in the last decade.

The most causative agent for food poisoning in Singapore is actually salmonella, with around 1,000 to 2,000 cases annually.

“Common sources of salmonella include undercooked chicken, beef, eggs or unpasteurised dairy products. Salmonella can survive in the environment for several days,” said the National Environment Agency.

The rotavirus and norovirus are two other common viruses that cause gastroenteritis.

All of these bacteria or virus cause gastroenteritis after they enter the food chains of human beings.

In my clinical practice, I see patients with gastroenteritis almost daily. It is not uncommon to see family members falling sick from the same pathogens one after another, probably through cross reaction at home.

The Food and Drug Administration of the United States actually recommends that once we purchase rockmelons and bring them home, we should wash them under running water, and scrub them with a soft brush. The melons should then be dried with another clean cloth before being kept in the refrigerator.

The World Health Organisation (WHO) has a useful set of recommendations on safe handling food and produce as follows:

1.      Buy right: choose product that is not bruised or damaged; when buying pre-cut fruits, choose those that are refrigerated; separate fresh fruits and vegetables from raw meat, poultry, and raw seafood.

2.      Store right: all pre-cut or packaged produce should be refrigerated; store perishable fresh fruits and vegetables at a clean refrigerator at 4 deg C or lower.

3.      Separately properly: utensils such as knives and chopping board for cooked and uncooked food should be separate in 2 sets

4.      Prepare right: wash hands for at least 20 seconds with soap and water before cooking; all produce be washed with running water before use.

5.      Looking back at the whole episode

The current listeria outbreak is not the first, nor will it be the last outbreak of contaminated food.

Besides relying on the Agri-Food and Veterinary Authority (AVA) to screen and detect contaminated food items, we all ought to practise safe food practices to prevent outbreaks of gastroenteritis.

The current listeria outbreak has been declared to be over by the Australian authorities. The contaminated farm has been cleared by the local authorities and production and sale of rockmelons at the affected farms have recommenced.

It is interesting to look at the timeline for the whole episode. The Australia National Focal Point (NFP) notified WHO on March 2 of the outbreak of listeria. On March 3, a Saturday, NFP informed all eight countries, Singapore included, of their imported rockmelons being contaminated by listeria.

On March 5, a Monday, AVA announced that two consignments of infected rockmelons were sold in Singapore at Sheng Shiong supermarket outlets and wet markets between Feb 12 and March 2.

I am personally amazed at the great speed that AVA worked on the problem over a weekend. The whole problem was publicised with great transparency, minimising damages to our public health system.

Close cooperation between food exporters and importers, coordinated by WHO, will be the future model for preventing epidemics.

ABOUT THE AUTHOR:

Dr Desmond Wai is a gastroenterologist and hepatologist in private practice.

An earlier version of this commentary said that according to the National Environment Agency, listeria accounted for less than five cases of food poisoning per year in Singapore over the last 10 years and that the most important causative agent for food poisoning in Singapore is actually salmonella, with more than 3,000 cases discovered yearly locally.  This is incorrect.  According to data from Ministry of Health, which collates such figures, listeria was hardly detected in Singapore in the last decade, and there are about 1,000 to 2,000 cases of food poisoning caused by salmonella.  We apologise for the errors.