An investigation into a Listeria outbreak in Australia linked to seven deaths has found the weather played a significant role in contamination.
The outbreak from rockmelons, known as cantaloupe in North America, affected 22 people with six cases in New South Wales (NSW), eight cases in Victoria, seven in Queensland and one in Tasmania between Jan. 16 and April 10 this year. Seven people died and one case resulted in a miscarriage.
Government officials said the outbreak was linked to one grower named as Rombola Family Farms (RFF), based in Nericon in the state.
The strain of Listeria monocytogenes that caused infections was related by whole genome sequencing to isolates from samples taken from 37 rockmelons from the farm. Listeria also was found on the floor around the melon packing area and from a composite swab of washed melons. All Listeria positives were identified as the outbreak strain.
Rockmelons are grown off the ground on plastic sheeting but are in the open farm environment and subject to rain, wind and other natural contamination.
An NSW Department of Primary Industries (DPI) report found hygiene and sanitation practices at Rombola Family Farms were on par with, or better than, most other rockmelon growers in Australia.
Adverse weather events listed as heavy rainfall in December prior to harvest, followed by dust storms were blamed for increasing the organic load and amount of Listeria on rockmelons prior to harvest. Another issue is the netted skin of the fruit making it harder to clean and sanitize.
“Prior to the detection of the outbreak, RFF utilized a one minute and 30-40 second pre-wash and scrubbing step to reduce organic load, followed by a 35 second sanitizing step of 100 ppm chlorine sprays and scrubbing,” said the NSW DPI report.
“It is likely that the 35 seconds at 100 ppm sanitizing step was effective in reducing Listeria monocytogenes to a very low level, but these levels still resulted in illness when rockmelons were consumed by immunocompromised individuals.”
Wash water was not recirculated, sanitizer was constantly monitored and applied through an auto-dosing system, and all water coming into the facility was treated and considered potable, the report says.
Dirty fans used to reduce moisture level on melons after washing and some spongy material on packing tables that was not easily cleaned may have been contributing factors to the outbreak but are not considered to be major underlying causes, investigators reported.
NSW Food Authority inspections found there was an opportunity for introduction of Listeria after washing through contact with surfaces or equipment that may have had traces of the pathogen. This included dust blown from fans to dry the fruit after washing and from porous material on packing tables that was not able to be effectively cleaned.
“During the investigation there was an allegation of raw organic fertilizer use on the rockmelon crop which was found to be incorrect,” the agency report said.
The business resumed harvesting and packing in early April 2018 and no Listeria monocytogenes has been detected on any fruit since then.
Rombola Family Farms has made line modifications to improve cleaning, increased cleaning and sanitizing of the packing line environment and sanitizer concentration during washing of melons.
“The rockmelon pre-wash was dosed with 200 ppm chlorine water during use. Similarly, the concentration of chlorine in the sanitizer wash step was increased to 200 ppm. This gives a total contact time for chlorine of over two minutes,” according to the NSW DPI report.
“The fungicide tank and post wash drying fans have been made more accessible and easily cleaned. Black sponge material on the packing table has been removed and replaced with more suitable material.”
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