Listeria and the public health infrastructure

Food poisoning can be a miserable experience and sometimes you feel like you want to die rather than endure another minute of the agony, but people rarely die from food poisoning. At least for most causes of food poisoning. The exceptions are the exceedingly rare cases of botulism and the much more common occurrence of listeriosis, infection with the organism Listeria monocytogenes. Listeria is a really nasty bug and serious cases are often fatal. The most at risk are the elderly, the immunocompromised, and pregnant women, biologically immune altered, and their fetuses and neonates. Listeria outbreaks can involve many food vehicles. Canada recently had an outbreak of more than 50 cases and 10 deaths. A wide variety of Ready To Eat (RTE) meat products from one Canadian food processing plant were implicated, probably by contaminating pre-cooked foods via a meat slicing machine. Listeria has the unusual ability to grow at refrigerator temperatures. RTE food is therefore risky for pregnant women and other vulnerable groups. When my daughter was pregnant she didn't touch RTE foods or deli foods, like cold cuts. However heating these foods to a reasonable temperature for even short periods will kill Listeria. A hot dog heated in a microwave to the point where it steams when cut open is fine. But many people eat hot dogs straight from the refrigerator, and that's not good if you are elderly or pregnant. Unpasteurized dairy products are also problematic and almost all listeriosis outbreaks in the US involving dairy products like soft Mexican cheeses were in unpasteurized items. Pasteurization kills Listeria. But recently the Massachusetts Department of Public Health reported on a five case common source listeria outbreak involving pasteurized dairy products, only the third such report in the US involving pasteurized fluid milk:

On November 27, 2007, a local health officer in central Massachusetts contacted the Massachusetts Department of Public Health (MDPH) to report listeriosis in a man aged 87 years. Pulsed-field gel electrophoresis (PFGE) performed on the patient's Listeria monocytogenes isolate produced a pattern indistinguishable from that of isolates from three other cases identified in residents of central Massachusetts in June, October, and early November 2007. MDPH, in collaboration with local public health officials, conducted an investigation, which implicated pasteurized, flavored and nonflavored, fluid milk produced by a local dairy (dairy A) as the source of the outbreak. (CDC, Morbidity and Mortality Weekly Reports [MMWR])

Listeria infections usually produce only mild illness, but in high risk groups the results can be serious. Of the five cases, all hospitalized, three were elderly males and all died. The two females were pregnant women. One had a premature but otherwise healthy infant after suffering a listeria infection of the amniotic sac. The other woman suffered a stillbirth at 37 weeks. Thus of the detected cases, there was a 60% mortality and a fetal death.

The other thing of note is the role laboratory investigation and surveillance played in detecting this outbreak. The five cases were scattered in space and time, and if the health department had not noticed that the genetic fingerprint of the five cases was identical, they never would have been linked. Listeria symptoms appear anywhere from days to a month after consuming contaminated food, and in some unusual cases, can be more than two months afterward. By that time the food has vanished from the refrigerator. Moreover the bulk of listeriosis cases are not so severe and don't come to the attention of the medical system and no clinical isolates are obtained. The serious cases are the tip of the iceberg. The laboratory based foodborne illness surveillance system was able to work backward from the serious cases and find a common source, the family owned dairy. An investigation of the bottling facility suggested that the milk was contaminated with listeria after the pasteurization process. The onset dates of the five cases show that the contamination had been going on for an extended period and wasn't over at the time of the investigation.

This is a good example of the value of public health infrastructure. Public health surveillance systems aren't glamorous and remain essentially invisible to the public and elected officials. Without the surveillance system the dairy would likely have gone on supplying contaminated product with the likely result of more deaths and tragically ended pregnancies.

Starving the public health system, which is what we are doing now in the name of cutting taxes, is not just stupid and short sighted. It is deadly.

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