The US Listeria outbreak
I’ve had a couple of students ask me about the current outbreak of listeriosis that has caused a number of deaths over the past few weeks in the United States. As of the end of September, 2011, the outbreak has been responsible for 15 deaths across 8 states, with illness due to the outbreak in 19 states. The outbreak is due to infection by Listeria monocytogenes, a Gram positive bacterium that is found many places in the environment. Human infections occur by consumption of contaminated foods, and uncooked fruits, vegetables, and dairy products are the primary routes of introduction into the diet. The current outbreak has been traced back to cantaloupes originating from a farm in Colorado. Those that read to the end will find a special bonus opportunity.
Infections by Listeria are for the most part relatively innocuous in most individuals with a competent immune system, and the most significant symptoms for them include minor gastrointestinal discomfort. In individuals with immune dysfunction such as those undergoing chemotherapy, in newborns and the elderly, or women who are pregnant, the situation can be far more severe. In these situations, Listeria can cause septicemia or meningitis which can have a very poor prognosis without treatment, with a fatality rate of around 25%. Treatment involves antibiotics, and the disease does typically resolve well if properly diagnosed. Women who are pregnant are not themselves in significant danger, however the organism can pass into the developing fetus and is a cause of spontaneous miscarriage and stillbirth.
Listeria monocytogenes is an interesting microorganism in its own right, and possesses very few virulence traits aside from one very interesting one: Listeria is able to reside inside our cells, and move from cell to cell without being exposed to the humoral immune system. It is this property that allows the organism to pass across the placenta to the developing fetus, and to pass across the blood-brain barrier to cause fatal meningitis. Fortunately, our basic defenses are typically able to control infection, and it is primarily the high risk groups above that the bacterium is able to thrive in.
So what are our realistic risks for acquiring this infection at present? The source of the current outbreak has been identified, and control measures are in place. However, the CDC reports that there are around 1600 cases of listeriosis annually, with decreasing annual trends over the past decade as consumer awareness is heightened. Still, the organism is ubiquitous throughout the environment, and therefore we should all be aware of the potential for infection from a source outside of one such as seen in this outbreak. At risk groups, especially pregnant women, should thoroughly wash all fresh fruits and vegetables, and avoid unpasteurized dairy products.
BONUS: For a special Bonus Point Opportunity (lowest quiz grade over the past week raised to a 5), add in the comment thread any organism that you can find in the textbook that is able to evade our immune system. This offer is good through up to the next midterm exam.