Three people have died from a severe bacterial infection leaving health officials scrambling to find the source.
A further two were left badly ill following the listeria outbreak, a health investigation found.
All patients were aged over 60 and all of them had a compromised immune system before contracting the bacteria in Washington state, the Washington Department of Health said in a statement Friday.
Four of the patients were treated in Pierce County, near Tacoma, and one was treated in neighbouring Thurston County.
Genome sequencing helped investigators conclude the infections most likely came from the same place but they are yet to trace the outbreak to a common food source.
Listeriosis is usually caused by eating food contaminated with Listeria monocytogenes.
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The deadly bacteria can be found on food preparation surfaces, fresh, unpasteurized cheese, leafy greens, and cold cuts, as well as in raw milk.
It can be killed by heating food to 74C or higher.
According to the Centers for Disease Control and Prevention: “The disease primarily affects pregnant women, newborns, older adults, and people with weakened immune systems. It’s rare for people in other groups to get sick with Listeria infection.”
Symptoms can include fever, flu-like symptoms, such as muscle aches and fatigue, headache, stiff neck, confusion, loss of balance and seizures.
An outbreak last year that left 18 people in hospital was connected to leafy greens.
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In May this year, health officials in 19 states prevented the sale of raw milk amid fears the product can be “contaminated with harmful germs,” including listeria.
However, Iowa joined dozens of others on July 1 in allowing small producers and farmers to sell unpasteurized milk from cows, sheep, and goats to consumers.
Governor Kim Reynolds signed off on the bill which states raw milk must be sold within seven days after its production and have containers stating the contents were not subject to public health regulations.
Cheese, yoghurt, ice cream and other raw milk products can also be sold.
This is despite public health authorities warning against the practice, claiming raw milk can be tainted with dangerous bacteria including E.coli, salmonella, and listeria.