The largest-ever listeria outbreak is occurring in South Africa: What is the source?

Current status

An outbreak of listeria was declared in South Africa on Dec. 5, 2017. As of Jan. 17, 2018, up to 748 confirmed cases of listeriosis have been reported in the country, making it the world’s largest-ever listeria outbreak. Emerging as a simple flu-like illness, listeriosis may cause diarrhea, fever, general body pains, vomiting and weakness. The condition becomes severe and life threatening when it spreads to the blood stream (septicemia) or the brain (meningoencephalitis).

In South Africa, the number of fatalities has already reached 67. The Gauteng province remains the hardest hit area, with more than 442 laboratory-confirmed cases. The Western Cape ranks second, with 18 deaths confirmed among the 92 infected to date. Up to 90% of the infected cases in the Western Cape are reported to be concentrated in the Cape Metro area. KwaZulu-Natal ranks third in the list of hard hit regions in South Africa (at 7%; 51 confirmed cases to date).

Tracing the source

The South Africa Health Ministry has warned that among the high number of new cases emerging in the last two months, only a few could be traced back, an effort made even more challenging with the rising death rates. The bacterial incubation period may range from six hours to 70 days, which in itself poses a significant challenge when tracing the infection source. Of the total number of confirmed cases, only 134 patients have so far been traced in the country.

The last widespread Listeria monocytogenes food poisoning occurred across 28 U.S. states in 2011, causing 30 deaths among 147 confirmed cases. The high number of fatalities reported that year, during the period of July 31 to December 8, were traced back to the consumption of contaminated cantaloupes linked to a Colorado farm. Similarly, the source of the current outbreak in South Africa needs to be identified as soon as possible, so as to cut the transmission route of the listeria bacteria and curb further spread of the infection.

Previous infection sources

Listeriosis, though carrying serious consequences, is a treatable and preventable disease. L. monocytogenes, a ubiquitous Gram-positive bacterial pathogen, is able to contaminate foodstuff at every level in the food chain. The bacteria is widely distributed in nature, present in soil, water and vegetation. Following contamination, the bacteria is transmitted to humans via the consumption of mainly ready-to-eat types of food, among others.  Meat and meat products, fresh produce (fruits and vegetables) and dairy products (unpasteurized and pasteurized) can be contaminated with the bacteria, being responsible for sudden outbreaks of the disease.

Risk from unpasteurized milk products

For detailed historical data on listeriosis, the South Africa Health Ministry can turn to the U.S. Centers for Disease Control and Prevention (CDC) which has been tracking listeria outbreaks since the 1970s. Two and three deaths, respectively, were caused by listeria infections in March 2017 and March 2015, in the U.S., via the consumption of soft raw milk cheese and ice cream products contaminated with the bacteria. An outbreak of listeria from cheese was reported in 1985, in California, resulting in 142 confirmed cases and 52 deaths (including 19 stillbirths and 10 infant deaths), making it the deadliest foodborne illness outbreak in the United States, to date.

In all these incidents, identifying the source of contamination helped to break the infection cycle. If unpasteurized dairy products are widely consumed in South Africa, these should be screened on a regional/area basis for L. monocytogenes and an outbreak reporting system be established to provide a survey of hospitals and laboratories for infection data in the country.

Even if dairy consumption in South Africa is low, bacterial infections associated with unpasteurized dairy products may have risen in recent years. In the U.S., the past 15 years have seen an increase in outbreaks associated with unpasteurized dairy products – be it contamination with Campylobacter spp., Salmonella spp., Shiga toxin – producing Escherichia coli (STEC) and L. monocytogenes (Costard et al., 2017). U.S. outbreak data extracted from the National Outbreak Report­ing System (NORS) for the period 2009 to 2014 reported a total of 87 outbreaks causing 750 laboratory-confirmed illnesses and 215 hospitalizations. Although unpasteurized products were consumed by a minority of the U.S. dairy consumers, they were responsible for 96% of these illnesses, with an illness risk of more than 800 times evident among consumers of unpasteurized milk products than for consumers of pasteurized dairy products. For all the pathogens, the overall mean incidence rate ratio (IRR) of illnesses was 838.8 (0.4 from L. monocytogenes and 7,601 from Campylobacter spp). The rate of hospitalization was higher for unpasteurized dairy consumers than for pasteurized dairy consumers (mean IRR 45.1), with an IRR of 0.5 for L. monocytogenes.

These results suggest a detailed screen of the unpasteurized milk products in circulation in worse-affected areas in South Africa, as well as efforts to reduce the use of such products.

Editor’s note: This is an adaptation of the whitepaper by Shyama Ghosh of the same title, “The largest-ever listeria outbreak is occurring in South Africa: What is the source?” To download the full report, which also includes discussion of the population at risk in a listeria outbreak, genetic analyses, integrated surveillance systems to monitor listeriosis, full references and more, click here.

Also, for further information on Listeria check out the Disease Summary at the Incidence and Prevalence Database from Clarivate Analytics.