Johannesburg - It’s one of the world’s most virulent food-borne pathogens, is widely found in nature and has killed at least 208 people in South Africa in the past 15 months.
And scientists have now warned how hot weather extremes, altered rainfall patterns and water scarcity from climate change could fuel more listeriosis outbreaks in South Africa in the near future.
So far, however, climate change has been overlooked in the discourse surrounding the world’s worst listeriosis outbreak from Enterprise’s food processing plant in Polokwane, say the scientists from Wits University and the SA Medical Research Council (MRC).
Product recalls, closing implicated processing plants and steps to enforce environmental health standards are clearly an immediate priority to halt such outbreaks, they write in an editorial in the latest issue of the SA Medical Journal.
“However, it’s also important to pay attention to factors relating to the longer-term structural environment in which such outbreaks unfold, and which may contribute to an increased frequency of cases in the near future.”
Such factors like climate change, argue Matthew Chersich, Fiona Scorgie and Helen Rees of the Wits Reproductive Health and HIV Institute, and Caradee Wright of the Environment and Health Research Unit at the MRC.
“The wide-ranging environmental effects associated with global climate change markedly alter the epidemiology of food-borne diseases, including L. monocytogenes. Even though Listeria species are ubiquitous within the natural environment, several features of the epidemiology and characteristics of the microbe make it especially climate-sensitive.
“Spikes in ambient temperature and high summer temperature peaks, for example, have been linked to the occurrence of listeriosis, as with most diarrhoeal pathogens.
"Hot weather extremes that become more common with climate change augment the replication cycles of L. monocytogenes and could cause breakdowns in food cooling chains, with rapid rises in numbers of the bacteria on food products. But, aside from temperature increases, altered rainfall patterns and lengthened dry seasons may influence Listeria transmission.”
L. monocytogenes is associated with the food chain, during pre-harvesting and processing, and at retail level.
“Water scarcity can compromise hand hygiene, as well as cleaning and sanitising operations in the food products industry. Cleaning hands with sanitisers, increasingly the norm in drought-affected areas, is less effective than washing with soap and water.”
More importantly, however, they write, in food processing plants water scarcity may hamper efforts to clean machines used for slicing, chopping or related processes.
“Intensive, deep cleaning is required to prevent persistence of L. monocytogenes on such machines, given that the bacterium can tolerate high salt and nitrate concentrations, desiccation, moderate heat and both acidic and alkaline conditions.
"With incomplete cleaning, especially of machines that have 'unhygienic' designs, or are damaged, L. monocytogenes can persist in cracks, niches or other hard-to-reach places. The organisms can adhere to all food contact surfaces, forming biofilms, which are hard to eliminate.”
In a study in Gauteng, for example, the microbe was isolated from stainless steel surfaces in food plants after they had been cleaned and disinfected using a range of cleaning methods. “As could be expected, several studies have detected L. monocytogenes in food samples of street vendors, who have limited access to water and cleaning equipment.”
The bacterium, they write, has been detected in delis in Joburg - in 10% of cleaning cloths. “These levels of contamination will possibly rise as water scarcity, which threatens much of the country, further reduces personal and industrial cleaning.”
There are other ways climate change affects Listeria’s spread. “When supplies of potable water become limited, both subsistence and commercial farmers resort to using surface water for irrigation, which often naturally harbours Listeria species.”
They cite a study of rainwater tanks in villages in three provinces, which found 22% of samples contaminated with L. monocytogenes, possibly from bird faeces and debris on rooftops.
Long-term water scarcity can influence cleaning practices and alter water sources in ways that favour the persistence of Listeria in food processing plants, but also in retail outlets and domestic settings.
Changes in precipitation patterns “wrought by large-scale climate disruption” also impact on Listeria dispersal.
“Rainfall occurring in short bursts of five to 10 minutes favours the dispersal of Listeria and other pathogens from the soil on to plants as with fresh produce, run-off water may contaminate the water in fish farms, an effect especially noticeable during summer months.”
Ultimately, the authors write, infectious disease outbreaks, which may become more frequent with rising ambient temperatures and water scarcity, are the “proverbial canary in a coal mine”. “They serve as but one reminder of the devastating effects of climate change presently unfolding in South Africa.”
The World Health Organisation has warned how changes in infectious disease transmission patterns are a likely major consequence of climate change.
South Africa’s high levels of carbon emissions, especially its reliance on coal for power, may well worsen the impact of climate change, say the authors. “Without concerted action to prepare for the health effects of climate change, and in the absence of efforts to reduce further environmental degradation, South Africans may face many more large outbreaks of infectious diseases in years to come.”