Listeria sandwiches

(Source unknown)

It's a lousy disease, but Radio New Zealand's headline has to be one of the more amusing headlines I've seen in ages: "Listeria sandwiches on sale at hospital cafe":

Listeria has been found in packaged sandwiches sold by a cafe at Middlemore Hospital, Auckland.

Food contractors to Counties Manukau District Health Board, Spotless Services, informed the DHB of the situation on Friday.

The contamination was found in Naturezone Thai chicken sandwiches, sold by the Aviary Cafe on Monday, 3 March. (Radio New Zealand)

So what's this Listeria stuff about, anyway? This material is stolen from . . . well, it's stolen from me. I used to teach this stuff so I went back and rustled up some of my lecture notes. I'm going to skip a lot. The particular lecture I took this from had 78 powerpoint slides, but a lot of them aren't relevant for a quick look, so this is the short version.

While we hear a lot about Listeria, as an emerging foodborne illness, it was described as far back as 1926 in rabbits. Until recently it was thought of as a veterinary disease. There were some human cases, but they were sporadic and in people who had contact with animals. One of the features of the rabbit version was that their blood had an increased number of one kind of white blood cell, the monocyte. This gave the organism its name, Listeria [for Joseph Lister, the father of surgical antisepsis] monocytogenes [for the appearance of increased monocytes in the rabbit's blood].

It's not a rare organism. On the contrary, it's all over the place, in numerous domestic and wild animals, soil, silage and the general environment. It also grows at an unusually wide range of temperatures, from 0 degrees Centigrade (32 degrees Fahrenheit) to 42 degrees C. (107 degrees F.). The important part of this is the lower end: it grows at refrigerator temperatures. Moreover it can grow in a range of acid conditions (pH 5 and higher) and high salt (10 - 12%). This means that a lot of ready-to-eat foods (RTE foods) are good growth media for Listeria.

Listeria can also make you very sick -- very sick, as in dead. Clinical infection occurs in two forms, a mild gastroenteritis and a potentially life-threatening form (general system failure). But it also has a very long incubation period, which can be a month or more. This makes recognizing its connection with a particular meal more difficult. Do you remember where and what you ate a month ago? While Listeria can be found in a variety of foods, dairy products and RTE foods are most commonly implicated. It is also much more common in the summer and you will note that it is now summer in New Zealand where the hospital "Listeria sandwiches" are. Identified cases are fairly rare -- about 5 per million population per year, compared to 20 for E. coli O157 or 177 for Salmonella (these numbers came from my lecture notes and are still correct in relative terms but newer data is probably available). So if it's so rare, why worry about it?

The main reason is because it is much more serious than most other foodborne illnesses. In the 1998 data it caused higher rates of hospitalization than any other foodborne pathogen and caused nearly half reported deaths from foodborne illness. It is also difficult to control and has the potential to cause large, multi-state outbreaks. And most ominously, it is a special threat to fetuses and newborns. This is the reason that pregnant women are counseled not to eat RTE foods like deli coldcuts.

Listeria has attracted the attention of scientists because it has a fascinating and unusual life cycle. I don't have room to go into the details here, but here are a couple of points to pique your interest (there's lots of research info on the internet, including movies (e.g., Theriot lab, try second from top) of Listeria moving around cells; use Google). On arriving in the intestine Listeria is confronted by professional immune cells, the phagocytes, whose task it is to engulf pathogens and then digest them internally. Listeria takes advantage of this by producing a toxin that breaks down the bag of digestive enzymes inside the phagocyte. It's sort of like being taken into custody and then taking over the police station. Once inside the phagocyte, after having broken open the digestive bag meant to kill it, the Listeria bug starts to grow, with a doubling time of about an hour. After 8 hours that means each Listeria inside the phagocyte is now 256 Listeria. At ten hours there are over a thousand. You take it from there.

And they don't just sit there, either. Far from it. In fact what they do next is one of the reasons scientists are so fascinated by them. Inside the cell there is a delicate network of actin protein filaments, forming an internal cytoskeleton. It is always breaking and being extended and it is also sort of a highway for moving things around within the cell. The Listeria organism commandeers the actin construction network. Anchoring to the cytoskeleton network at one end the organism adds new units between itself and an extending actin tail. This pushes the organism forward, a sort of actin propulsion system. It can move pretty fast, somewhere in the range of .02 to 1.4 microns per second. That doesn't sound so fast, but remember it is only 1 - 2 microns long and half a micron wide, so relative to its own size it is really moving. It doesn't take long for it to reach the edge of the cell it is inhabiting and when it does the actin motor pushes it right through the cell membrane and into whatever neighboring cell is next door (I'm simplifying a bit, but this is the general picture). It is thus moving from cell to cell via its hijacked propulsion system and all the while it is hidden from the immune system, which is outside the infected cells.

Here's a summary pic:

(Source unknown)

The Listeria bug attaches and is engulfed at #1, where the phagocytic cell thinks it will digest it in the little bag of enzymes within which it is enclosed. But it is indigestible and breaks free into the cell at #2. It multiples and starts zooming around at #3. At #4 it motors into the next cell and starts again. This is how Listeria spreads from cell to cell in the body. Pretty neat. And potentially deadly.

It takes more than a couple of cells for this strategy to succeed and the probability of tissue invasion depends on the number of organisms ingested (the size of the inoculum, in technical talk). There are different subtypes and strains of Listeria, too, and the immune status of the host also plays a major part. Many people seem to carry Listeria around in their gut without serious consequences. So host factors are extremely important determinant of susceptibiiity. The most susceptible are people with immature or incompetent immune systems, hence fetuses and newborns and pregnant women (whose immune status is suppressed so as not to interfere with the foreign body growing in their uterus). Pregnant women (and their fetuses and offsrping) are about 20 times more likely to get Listeriosis than others. So host status is clearly important. But massive ingestion from contaminated food where large amounts of Listeria have grown up can overwhelm defenses. Outbreaks result. There are likely many mild outbreaks that are never recognized. The ones that are noticed usually involve serious disease.

Fetuses can be infected across the placenta or newborns in their passage through the birth canal. Newborn infections can be early onset or late onset. Early onset has 30 - 50% mortality with premature birth, respiratory distress and circulatory failure. Late onset appears from a week to a month after an apparently healthy full term birth, from mothers with uneventful pregnancies without illness. Often no Listeria can be isolated from the mother but the babies are infected, often presenting with meningitis. Listeriosis is is the third most common cause of meningitis in newborns.

We know something but not enough about the epidemiology of Listeria in adults. Non-reheated frankfurters and undercooked chicken have been prime culprits. In one study, cases were more likely to have eaten food bought at a deli or have eaten soft cheeses. In about two thirds of the cases it was possible to isolate Listeria from at least one food item in their refrigerators, especially RTE meats, leftovers, cheeses and raw vegetables. Since this is a lot of what is in almost any refrigerator this isn't a lot of help. Most cases are isolated or "sporadic" cases. Only a small fraction occur in large outbreaks, but outbreaks can be large. The mean size is 40 cases. In one large outbreak Mexican-style soft cheese was responsible for 142 cases, with a mortality of 34%.

Listeria is difficult to control because it grows at refrigerator temps on RTE food. It is also ubitquitous. If you don't kill the organism by cooking (e.g., heating the hot dog in the microwave to the point where you see steam coming from it) you might be at risk. So pregnant women shouldn't eat deli food or other RTE items.

I think we said that already, didn't we. But some things bear repeating.