The Agri-Food & Veterinary Authority of Singapore (AVA) had released a press statement yesterday regarding the recall of rock melons (cantaloupes) from an Australian farm implicated in a nation-wide outbreak of listeriosis.

There were apparently two consignments shipped to Singapore, available for sale at Sheng Siong supermarket outlets and wet markets between 12 February and 2 March. The size of these consignments relative to all rock melons sold in Singapore during this period is unknown.

Food Standards Australia New Zealand (FSANZ) issued a trade recall of rock melons at the end of February, after 10 cases of listeriosis were linked to consumption of the fruit that had come from a farm in the Riverina region in New South Wales, Australia. There are currently 15 cases (3 deaths) from 4 states linked to rock melon consumption, with the earliest case reported on 17 January 2018.

Listeriosis is a term encompassing all diseases caused by Listeria spp. The most common pathogen is L. monocytogenes, but other Listeria spp. may also rarely cause human disease. These bacteria are ubiquitous in soil in the environment, but are opportunistic pathogens and do not cause disease except when ingested in large quantities or by an immunocompromised person. It is a foodborne disease, and human-to-human transmission has not been documented with the exception of maternal-to-fetus transfer.

There is a wide range of incubation periods – 3 to 70 days is the most often quoted, based on a 1987 NEJM paper – although later work suggested approximately 2 weeks for bacteremia/CNS disease and 6 weeks for pregnancy-associated cases. Probably the most prevalent disease manifestation is also the hardest to pinpoint as being caused by Listeria spp. – fever with myalgia (muscle aches) with or without nausea and diarrhoea. In immunosuppressed persons, however, the bacteria can invade the bloodstream and cause severe disease such as septicemia and meningitis. In pregnant women, infection with L. monocytogenes can result in spontaneous abortion, preterm birth and intrauterine death. Infection in neonates (usually first 7 days) has been reported as well, with severe septicemia being the usual presentation, and a mortality rate that may exceed 50%. Here is a nice article on listeriosis in the newborn.

Out of curiosity, I asked the local microbiology blogger how many positive cultures were picked up in his laboratory, and it turned out that there were less than 10 cases each year, meaning that listeriosis is truly a rare disease in Singapore.

Prior to this outbreak, it would not have crossed my mind to check about rock melon consumption with regards to risk of acquiring Listeria spp. infection. But in fact there was a large 28-state outbreak of listeriosis – 147 cases and 33 deaths – in the U.S.A. in 2011 that was linked to cantaloupes from Jenssen Farms in Colorado. There are also a significant number of studies (dozens!) in PubMed looking at cantaloupes and Listeria spp., including the effects of refrigeration, method of cutting (Listeria spp. reside on the surface/rind of the cantaloupes, and are introduced during cutting or if the cantaloupe in question is bruised/damaged).