The Other Pig Plague

The Other Swine Infection Swine Flu has been making headlines all over the world, curtailing travel and trade, and killing a small number of people in several countries. Eventually, it could become a global menace or, like bird flu, continue to percolate in distant pockets of the globe. No one knows what will happen or when, but this spring’s swine flu episode is another reminder that many pathogens can make the leap from bird and animal to people. Another pathogen that has made the leap is Streptococcus suis. It is not a virus, but a bacterium whose natural host is the pig. Among the crowded pens of snorting, slobbering porkers, the bacteria are readily exchanged orally and nasally. In piglets, the bacteria sometimes will cause fatal infections such as sepsis, pneumonia and meningitis. The first recognized case of S. suis in pigs was in 1954; the first case in humans was in 1969. Since then, there have been more than 700 human cases. Most of them have occurred in Southeast Asia where both the pig and the pig-eating populations have exploded. According to Minnesota epidemiologist Mike Osterholm, China had 790 million people and 5.2 million pigs in 1968. Today, they have 1.3 billion people and 508 million pigs. (The population of the U.S. is 300 million.) Perhaps it was no surprise then that China recently had a large outbreak of S. suis infections. That 2005 outbreak sickened 215 people and killed 38. Most of the deaths were due to meningitis. In people, S. suis frequently causes meningitis, with symptoms of headache, fever, and vomiting. Hearing loss, endocarditis and arthritis may occur in some cases, and a severe form of septic or toxic shock also has been seen. In Hong Kong, S. suis is the third most common cause of bacterial meningitis.

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Not surprisingly, most of the patients with S. suis infections tend to be older men who work around pigs: farmers, veterinarians and butchers. That does not mean, however, that many Asian women handling raw pork at home or in fresh meat markets are not at risk for infection. Infections from this bacterium usually are treated with penicillin, ceftriaxone, vancomycin, and other common antibiotics. There is no vaccine for people or pigs, and it is not clear just how useful one would be. S. suis is not transmitted person-to-person so there is little fear of a contagious outbreak among people. (Unlike with swine flu.) Pigs spread the bacteria among themselves and carry it naturally and asymptomatically. A vaccine may not eliminate those carried bacteria or, worse, may eliminate the carried strains and create openings for more pathogenic strains. People who work around pigs also may carry the bacteria. Among 132 studied slaughterhouse workers in Germany, 5.3% were found to be carrying S. suis in their noses or throats. A 1999 study in the Netherlands found 6% of vets and 1% of pig farmers had antibody to S. suis bacteria. They may not have been carriers, but they had enough exposure to the bacteria to provoke antibody immune responses. In the United States, the first human case of S. suis meningitis was reported in February 2007. The patient was a 59-year-old farmer from upstate New York. He may have been the first recognized patient, but he may not be the only patient. For example, a 2008 survey of Iowa hog-farm workers found seven of 73 who tested positive for prior exposure to S. suis bacteria. So the bacteria are present in U.S. pig populations and U.S. workers are being exposed to the bacteria. Because millions of pigs are transported across the U.S. each

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year there are ample opportunities to spread the infection and expose farmers, truckers, and slaughterhouse workers. The bacteria may be especially common on large commercial farms where thousands of pigs and piglets may be crowded together. A world authority on S. suis, Marcelo Gottschalk, recently told Wired magazine, “You could say that between 5 and 10 percent of [pig] workers in the U.S. might be infected. I don’t know if it’s true, but the extrapolation is fair enough.” How Streptococcus suis causes sudden disease—or instead—sits harmlessly in the nasopharynx for years is unknown. But it’s clear that a normal pig pathogen can jump to people, cause infections, or wait patiently for some immunological or evolutionary opportunity to emerge, evolve, and spread. S. suis is a new human pathogen worth keeping a close eye on.

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