Listeria monocytogenes

In 1926, E.G.D. Murray first reported the isolation of Listeria monocytogenes from rabbits and since then it was considered as an animal pathogen primarily causing “circling disease” in ruminants (cattle, sheep, and goats), pigs, dogs, and cats. The animals walk in a circle and exhibit uncoordinated posture, and are unable to stand without a support. However, in the late 1970s and early 1980s, this organism emerged as a foodborne pathogen causing numerous outbreaks in humans in North America. It is responsible for rare but fatal systemic disease called listeriosis, affecting primarily immune suppressed populations: pregnant women, infants, AIDS patients, and organ transplant recipients. However, in recent years, gastroenteritis cases resulting from L. monocytogenes infection have been reported. L. monocytogenes is a facultative intracellular pathogen and has been used as a model organism to study intracellular parasitism. It produces numerous virulence factors that ensure its survival in a host, thus it has been the subject of many recent investigations for its utility to carry foreign genes for vaccination against other diseases including cancers.

About 2,500 people in the US contract invasive listeriosis each year. The mortality rate is highest of all foodborne pathogens, and is usually 20–30%, but has been reported to be as high as 50%. There is a zero tolerance policy for L. monocytogenes in ready-to-eat foods in the US. Other countries like Canada and some European countries allow a limit of 100 cfu/25 g food. In most recent years, several outbreaks in the US were linked to ready-to-eat meats. A multistate outbreak involving tainted hotdogs affected 22 states in 1998–1999, causing 101 illnesses, 15 deaths, and 6 miscarriages. In 2000, deli turkey meat was responsible for a multistate outbreak resulting in 30 illnesses, 4 deaths, and 3 miscarriages. In 2000–2001, consumption of Mexican-style soft cheese (Queso Fresco) made with unpasteurized milk resulted in 12 cases of listeriosis and 5 miscarriages in a Hispanic community in North Carolina. In 2002, ready-to-eat turkey deli meat was implicated in a multistate outbreak involving nine states resulting in 54 illnesses, 8 deaths, and 3 stillbirths. In 2003, raw milk cheese was responsible for an outbreak in Texas, and in 2005, a multistate outbreak involving consumption of turkey deli meat affected nine states and caused 12 illnesses. L. monocytogenes serotype 4b was implicated in each of these outbreaks. Though L. monocytogenes contamination does not always result in outbreaks, product recalls are routinely initiated to prevent outbreaks. The estimated cost associated with Listeria contamination is approximately 2 billion dollars per year.