Avoiding soft cheeses, deli meats and dressings like mayonnaise is well known to pregnant women to avoid contracting the bacterium, Listeria monocytogenes. But rockmelons are less well known for causing listeriosis outbreaks.
One of the largest outbreaks due to Listeria monocytogenes on rockmelons was in the US in 2011, where 33 people died. In Australia between January and April 2018, there were 22 human cases of listeriosis associated with consumption of rockmelons linked to a single NSW farm. The cases of listeriosis occurred across New South Wales, Victoria, Queensland and Tasmania and led to seven deaths and a miscarriage. The NSW Food Authority report on the investigation into the outbreak concluded that adverse weather events were the major contributors to the outbreak, with heavy rains in December and subsequent dust storms significantly increasing the organic load and amount of Listeria on the fruit. Some other factors were also noted as potentially contributing to a lesser extent.
The incidence of listeriosis (the bacterial infection caused by eating food contaminated by Listeria) is comparable in Australia to other western countries – we have around 60-70 cases per year. We had an outbreak linked to cheese in late 2012-early 2013 and that was our last large-scale Listeria incident. Listeria, along with Salmonella, is responsible for more deaths in Australia than any other foodborne disease, each accounting for about 15 deaths per year.
Listeria is generally only contracted by a fairly small, vulnerable group of people – the elderly, unwell and pregnant women – but it is fatal for about 1 in 5 of those who contract it. Compared to other pathogens (microorganisms that make humans sick or is fatal) like Salmonella or E coli, Listeria has a high mortality rate among the risk groups. The three people who have unfortunately passed away this week from infected rockmelons were over 65.
Listeria is abundant in the environment and therefore it’s difficult for food producers and processors to control it. It’s commonly found in the soil which, of course, is where horticultural crops like rockmelons happen to grow. Additionally, the ‘netted’ nature of the rockmelon skin makes it particularly hard to clean and sanitise. Listeria can go from the soil to the melon skin and although it doesn’t grow on the skin, when we slice the melon to eat it that can introduce it into the flesh. Then, we mightn’t eat it all at once but a few days later cut it up some more and put it out to eat at a picnic, and the bacteria has grown even more. Or if you leave it out of the fridge, it can grow faster. Listeria grows at four degrees but keeping it in the fridge will help slow the growth.
Because rockmelon is less acidic than say tomatoes, there’s more chance of microbes surviving.
So, should we wash rockmelons at home before eating them? It shouldn’t be necessary because they’ve already undergone a cleaning process. Australia’s food safety procedures are among the best in the world but there’s no harm in giving them a scrub, especially if obvious dirt can be seen on the fruit.
Rest assured, the rockmelon industry has taken significant steps to even further strengthen their food safety procedures since the incident including one-on-one food safety consultations with all Australian rockmelon growers and packing sheds and developing a best practice toolbox.
What can we do at home to reduce our risk of food-borne illness? Key food safety tips include washing your hands, storing food that is meant to be chilled (like salad bags) at 5 degrees or colder, cooking food properly and keeping raw and cooked foods separate during storage and preparation. An additional outcome of the 2018 outbreak is that health authorities are now recommending that vulnerable persons do not consume rockmelons, along with other higher risk foods, to minimise their risk of exposure to Listeria (see more in the NSW Health Listeriosis fact sheet).
This article was first published 2 March 2018 and updated 5 December 2018.
12th March 2018 at 2:54 pm
How is it diagnosed and what is the treatment, what are the symptoms and what symptoms if any, are exclusive to listeria?
11th March 2018 at 2:42 pm
Why not tell the real truth ? harvesting rockmelons during rain or even before 24 hours of the rain stopping, poses a massive risk of food-borne illness I have tried to get the fresh produce safety centre, Freshcare , HIA,ProVeg,HARPS,Sydney University and Growcom, to inform and enforce harvesting rules that can save lives instead of dollars. Since 2015 there has been scientifically based evidence about the heightened risk of foodborne illness for ANY fresh produce .
Here are easy to find, links all published and available 2015.
11th March 2018 at 2:07 pm
I wonder if there is any connection between the quality of the watering system used to grow the rockmelons and any Listeria outbreak..
For example the water source could be re-cycled or the growing medium could contain contaminants, maybe even treated sewage?
15th March 2018 at 8:24 am
Contaminated water and/or the growing environment could introduce pathogens to fresh produce. In the case of Listeria monocytogenes soil itself is a natural environment where this bacterium can be found. This highlights the difficulty in controlling cross-contamination of this bacteria. The practices that may have contributed to the recent outbreak are not publicly known so these are general comments on things that potentially can go wrong.
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11th March 2018 at 1:41 pm
Nobody has mentioned the quality of the water that was used for irrigating the rock melons on the farm where the outbreak occurred. Perhaps it was poorly-treated sewage? The Environment Protection and Heritage Council, the Natural Resource Management Ministerial Council and the Australian Health Ministers’ Conference prepared the “Australian Guidelines For Water Recycling” in November 2006 at http://www.agriculture.gov.au/SiteCollectionDocuments/water/water-recycling-guidelines-health-environmental-21.pdf. Does anyone remember these? Maybe the document is too long-winded, and scientists hadn’t caught up with Listeria in those far-off days?
15th March 2018 at 8:21 am
The practices that may have contributed to the recent outbreak are not publicly known so we can only comment on things that potentially can go wrong and use of contaminated water, whether incorrectly recycled water or other contaminated sources, is one thing that can introduce pathogens to fresh produce.
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9th March 2018 at 4:17 pm
Can I ask is it safe now to purchase cantalope and consume it ???? All the supermarkets are not stocking at the moment but the local green grocer is but now I am scared to eat it !!!!
15th March 2018 at 8:22 am
The NSW Food Authority have stated on 28 February that: “affected product is being removed from the supply chain, so consumers can be assured rockmelons currently available on shelves are not implicated in this outbreak.” The grower in question ceased production on 23 February to investigate the contamination with the Authority. Other rockmelon growers haven’t been implicated. Australia has one of the safest food chains in the world and there’s no reason to expect similar problems with the currently available rockmelons. http://www.foodauthority.nsw.gov.au/news/newsandmedia/departmental/2018-02-28-listeriosis-outbreak-link-to-rockmelon
We hope this helps!
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