Listeria monocytogenes infections in humans generally
occur via ingestion. Clinical disease is of most significance
in pregnant woman/neonates, the elderly and the immunosuppressed.
In the U.S., approx. 2500 people become ill and approx. 500 people
die each year because of listeriosis.
Clinical presentations may include:
- mild flu-like illness
- fever and gastroenteritis
- fetal infection
- abortion, still birth, neonatal septicemia or meningoencephalitis
- Fetal infection can occur even if the mother is simply an
asymptomatic carrier!
- meningoencephalitis in the elderly or immunosuppressed
- various foci of granulomatous inflammation
Foodborne transmission:
Listeria monocytogenes has relatively recently been
recognized as an important foodborne pathogen. Foodborne infection
has been documented in association with soft cheeses, coleslaw,
fresh vegetables, meats and milk.
- In 1998-99, 50 people across 11 states were sickened after
consuming contaminated hot dogs.
- In March 2000, Listeria bacteria were detected in
pre-prepared cheeseburgers sold to convenience stores and vending
machines, and in cold smoked fish products in the U.S.
- From May-November 2000, 29 people across 10 states in the
U.S. were sickened by eating contaminated deli turkey.
- Seven people died of listeriosis during an outbreak in France
in 1999-2000.
- Nearly 400,000 pounds of luncheon meats were recalled in
the United States in November 2001 because of Listeria
contamination.
- Over 27 million pounds of ready-to-eat meats were recalled
during another Listeria outbreak in the U.S. in 2002.
Several factors promote the ability of this organism to
be transmitted via food or milk:
- Listeria monocytogenes is both heat and cold tolerant.
- Listeria monocytogenes can survive pasteurization
if bacterial counts in the milk are >1,000 bacteria/ml. (which
is not very high), and an FDA survey in 1997-1998 of 700 U.S.
milk plants found that 2% of pasteurized milk samples contained
Listeria organisms. Ultrapasteurization, however, can produce
sterile milk.
- Listeria monocytogenes can also replicate at refrigerator
temperatures in contaminated products.
