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There is still no lead on the spate of listeria cases, which has claimed over 30 lives, as attempts to identify the source of the outbreak carry on.

This is according to the National Institute for Communicable Diseases (NICD) and the Health Department.

Every lead followed up

The investigation has seen various directorates with the Department of Health join forces with the departments of Trade and Industry and Agriculture, Forestry and Fisheries, as well as private sector organisations.

“Every lead suggestive of the potential source is being followed up. Case patients are being interviewed to determine detailed food histories for the period preceding their illness and as yet no patterns have been identified. In addition, case patients are being followed-up so that we can collate more complete outcome information,” the NICD and Health Department told Health24.

Listeriosis caused by the bacterium, Listeria monocytogenes can contaminate animal products and fresh produce, such as fruits and vegetables.

The foodborne illness has spread across the country with more than 500 cases detected in the last 11 months and 37 deaths reported.

Interestingly, most of the cases have been caused by a single strain of listeria, said clinical microbiologist and head of UCT’s medical microbiology division Professor Mark Nicol.

“This suggests that there is a common source for these infections. The fact that this is a national outbreak (and not local or regional) implies that the most likely source is a food product that is distributed nationally. At present, it is not clear what the source is.”

Professor Nicol explained that it isn’t completely understood why some individuals develop mild disease, such as gastroenteritis while others develop severe illness, including bacteraemia – bacteria in the blood stream and meningitis.

More virulent strain of bacteria

What is known is that people at the extremes of age such as newborn infants and people older than 65 years of age are at increased risk of severe illness.

“Similarly, people who have a weakness in the cellular immune system, such as people living with HIV and those with malignancy or receiving chemotherapy, are also at increased risk,” said Professor Nicol.

“Pregnant women are particularly at risk, since the bacterium can spread to the placenta and infect the unborn child, causing premature delivery, stillbirth or infection in the young baby.”

However, Professor Nicol said a concerning feature of the current outbreak is that a number of cases have occurred in young, otherwise healthy, adults.

“This is unusual and may be related to the strain of bacterium causing the outbreak, which may be more virulent and therefore more likely to cause severe disease.”

Dr Jo Barnes, emeritus senior lecturer at Stellenbosch University’s faculty of medicine and health sciences, said tracing the source of the infection is proving to be quite difficult due to some factors associated with it.

Lacking laboratory services

The first one is that the time between acquiring the infection and showing symptoms can be as long as 70 days.

“That means that by that time any traces of the food source that most likely transmitted the disease are long gone. Thus determining the source of the infection becomes a search for a needle in a haystack.”

The second complication, according to Dr Barnes, is that sometimes the symptoms can in the beginning resemble flu-like illness or a stomach upset.

“The availability of laboratory services to determine whether the patient really has a listeria infection can be lacking in many health service treatment facilities,” she warned.

In these facilities, such patients are simply treated symptomatically and they are never diagnosed and counted as infected with listeria, Dr Barnes said.

“It is therefore likely that the counts [recorded] are under-counts and not representative of the real situation.

“It is also possible that in some remote areas such infected persons can die without being diagnosed and entered into the formal statistics,” she said.

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Adiel Ismail