Three years ago we discussed (we were still on blogspot then) a spreading outbreak of a mysterious disease in Sichuan (Szechuan), China. Any mysterious disease outbreak in China always raises warning flags since southern China is the incubator for influenza. The Chinese discounted the possibility of bird flu, saying it was instead a massive outbreak caused by the bacterium Streptococcus suis, Type 2, an important cause of disease in pigs. S. suis sometimes causes human menigitis or septic shock, but the final total of 215 cases and 38 deaths seemed atypical. But it turned out Chinese authorities were right. Sporadic cases of S. suis infections have occurred elsewhere (the UK, Vietnam, Hong Kong) but they are uncommon. The US reported its first case in 2006 in a meningitis patient who reported contact with piglets he had bought at a farm in New York. How come we don't see more cases or Chinese-like outbreaks in the US?
Fellow scienceblogger Tara Smith (Aetiology) and her colleagues at the University of Iowa's Center for Emerging Infectious Diseases set out to shed some light on this by examining serum samples from 73 swine-exposed workers living in Iowa and comparing the frequency of antibodies to S. suis in their serum with the serum of 67 non-swine exposed adults. The antibodies were specifically directed against S. suis, Type 2, the same serotype as the Chinese outbreak (paper is in CDC's Emerging Infectious Diseases, Smith TC, Capuano AW, Boese B, Myers KP, Gray GC. Occupational exposure to Streptococcus suis among US swine workers. E Emerg Infect Dis. 2008 Dec; [Epub ahead of print]).
59 of the 73 swine exposed adults (most of them) had contact with both swine in both nursery and finishing work settings and all but 1 of the 73 had been working with pigs for more than 10 years. 7 of this group showed antibodies to S. suis Type 2. None of the non-swine exposed adults had antibodies. The odds ratio estimate for swine exposure was 8.8, meaning that the odds of working with swine was almost 9 times higher for swine work as for non-swine exposed adults in Iowa.
This work shows that Iowa swine workers indeed have antibodies to S. suis, despite the fact the disease is essentially never reported in the US. Hw come? Tara and her colleagues suggest several possible reasons. One is that S. suis infections do indeed occur but they are underdiagnosed or misdiagnosed. There are some plausible reasons to think this might be the case but at this point we don't know. A second possibility is that the infections that occur are mild or asymptomatic because the US version of S. suis Type 2 is not as virulent as the one that caused the Chinese outbreak. There is some molecular evidence to show that the US version is of a different Sequence Type than the Chinese bug that supports this. Maybe both are true. Finally, it might be that the antibodies are not caused by infection at all but exposure to some S. suis antigen in pig manure or other exposures related to swine work.
S. suis infection is a serious occupational disease. Tara's work is the first to try to figure out how important it is, might be or might become in the very large US pork industry. Occupational health was languishing in the Bush administration. Maybe things will change during the Obama years. Let's hope so.
Meanwhile, kudos to Tara and her colleagues.
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Sorry, but I can't make sense out of this sentence. Will you explain?
Susan: Yes, it is not completely clear to a non epidemiologist what this means. Probably the easiest way to approximate it is to think of "odds" as "the chances" of having antibodies if you work with swine versus not working with swine (it's not exactly accurate, since odds aren't the same as "chances" but for things that aren't common they are almost the same).
Let me take a crack at this. And I do it while acknowledging that I have just enough statistics to get myself into trouble.When you have a cohort study (i.e. you define the entire population without regard to risk and then do the study) you end up with a Relative Risk (RR) which says "The risk of having the outcome (illness) was X times as great for those who had the exposure (swine work) as for those who did not have the exposure." When you don't/can't know whe entire population up-front and select the subjects based on either exposure or outcome, you end up with an Odds Ratio (OR) which is an estimate of the Relative Risk. In that case, the statement you make is that those who had the outcome (illness) were more likely to have had the exposure (swine work) as those who did not have the illness. It isn't exactly kosher to say they were X times as likely, but we often do.In the first case, you start with exposure and work toward outcome. In the second, you start with outcome and work toward exposure. That means (as is the case here) that those with the outcome (illness) were more likely to have the exposure (pigs) than those without the outcome (not ill).
Thanks revere!
I think the confusion in the text is that it should read:
"The odds ratio estimate for swine exposure was 8.8, meaning that the odds of having antibodies to Strep suis was almost 9 times higher for swine work as for non-swine exposed adults in Iowa."
Not the odds of working with swine was nine times higher among swine workers.
Brian: Yes, of course that's what it means. Too little sleep these days.
Many people have antibodies, yet no symptoms, well that means natural immunity probably knocked it out.
Any evidence that this microbe actually capable of causing any disease? Do they even remotely fulfill Kochs postulates?
Are they visible in the tissues with they electron microscope? Do they induce disease in lab animals? If so many people have it without any symptoms can you dare propose another hypothesis, toxins etc?
Typical responses are that Kochs rules are outdated, but then all you're basically saying is that partial poor correlations with antibody, not actual virus, prove causality. Any microbe could be said to cause disease with these low standards.
cooler: The post references plenty of human disease from this organism (meningitis, sepsis). That's not in doubt.
There are also a number of intriguing viral diseases of swine in the news such as PRRS in China and Vietnam, African Swine Fever in Geogia and Russia, and Classical Swine Fever in the Philipines. Interestingly there has been a scarcity of data on any influenza infections of swine in these areas. It could be very interesting if the Iowa labs were let loose to do extended testing on some of these specimens.
I have to disagree with Brian
Actually, if I interpret it correctly, the odds of working with swine were higher for those who had antibodies than for those who did did not have antibodies.This is my understanding of the difference between RR and OR. What Brian describes would actually be a Risk Ratio, rather than an Odds Ratio which, I gather, was precluded by the study design. If I've botched this, somebody please straighten me out.