An investigation into what could have caused the the salmonella outbreak in lettuce prepackaged at a Victorian farm could show results as soon as this week.

The outbreak has left 62 people sick in Victoria, with worries more may be coming forward.

There are reports the outbreak might also be linked to illness in Queensland and South Australia. Authorities across the country have recalled products with best before dates leading up to and including 14 February.

Processed foods are a staple of the modern Western, time-poor diet, but are also blamed for increasing obesity rates and childhood allergies. By contrast, prepackaged salads are appealing as they are considered healthy and natural.

But the recent outbreak highlights that raw agricultural products – when simply washed and marketed in bags – aren’t always as good as we may think.

Bacteria and salads

Salmonella is a common food-borne pathogen often associated with poultry. This bacteria is found in many animal species though and can easily contaminate non-animal food products.

Illness from prepackaged salads isn’t uncommon. The current salmonella outbreak in Australia comes at the same time as an outbreak, of another bacteria, listeria monocytogenes, in prepackaged salad in North America.

In 2006, an outbreak of Escherichia coli in prepackaged spinach killed three people and made more than 200 sick in the United States.

And in 2012 about 300 cases of Cryptosporidium parvum were linked to ready-to-eat salad in the United Kingdom. Then in 2013, prepackaged watercress salads were associated with 19 cases of E. coli in the UK.

Salmonella is a common food-borne pathogen often associated with poultry. from shutterstock.com

Early speculation has linked the Australian salmonella outbreak to fertiliser of animal origin, such as one sourced from chickens, though this is yet to be proven.

How salads are processed

Leafy vegetables can become contaminated by bugs, either directly from animals, agricultural run-off water or fertilisers.

The later processing of these vegetables includes operations such as breaking up lettuce heads and chopping leaves. This increases the surface area of the produce and provide new niches for bacteria (and potentially other pathogens) to adhere to and hide. This step may actually minimise the effectiveness of subsequent steps producers use to clean the product – such as washing with chlorinated water.

Then, packaging the leaves into bags means that single point sources of microbial contamination are spread from one point to several. This could turn an isolated case of food poisoning into an outbreak.

The current outbreak seems to support this model of dissemination, as it originated from a single supplier and has affected several brands of prepackaged lettuce mixes sold at Coles and Woolworths.

What you can do

Suppliers have a legal requirement to ensure their products are free from harmful microorganisms; but occasionally the checks and balances can fail. So how do we protect ourselves from illness when eating salads?

Using whole, unprocessed vegetables and washing them thoroughly will reduce the risk of poisoning.

Good food handling practises will too. These include washing and drying hands thoroughly before food preparation, appropriate storage of foods, and separation of raw foods (particularly meat) from foods that have already been cooked or don’t require cooking.

Consumers may choose to rewash bagged leaves. The jury is out on this, with some suggesting it might actually increase the risk of food-borne illness.

But the increases in risk are largely based on an assumption of inadequate kitchen hygiene and food handling practices. With good practises in place in your home an additional wash is unlikely to be detrimental to your health.