Streptococcus suis: An Emerging Human Threat

Abstract

In the world of Streptococcus, S. suis has been somewhat neglected. Indeed, at this year’s Lancefield International Symposium on Streptococci and Streptococcal Diseases, only one oral presentation was devoted to this pathogen. However, this perception should change, because S. suis is emerging as an important threat to human health. S. suis is an encapsulated Grampositive coccus possessing cell-wall antigens related to Lancefield group D. There are 35 serotypes that have been described on the basis of the composition of the polysaccharide capsule [1]. S. suis infections are considered to be a major problem worldwide in the swine industry. The pathogen’s natural habitat is the upper respiratory tract of pigs, as well as the genital and alimentary tracts [1]. The pig carrier rate is near 100%, and, in the absence of treatment, mortality reaches 20% [2]. The most important clinical feature associated with S. suis infection in pigs is meningitis. In severe cases, pigs are found dead with no premonitory signs. Other manifestations are arthritis, endocarditis, pneumonia, rhinitis, abortion, and vaginitis [1]. Until recently, S. suis disease in humans has been rare [3, 4]. Since the first human case in Denmark in 1968 [5] and before 2005, only 250 S. suis infections were reported in Europe and Asia. However, S. suis recently was identified as the third most common cause of communityacquired bacterial meningitis in Hong Kong, behind S. pneumoniae and Mycobacterium tuberculosis [6], and as the leading cause of adult meningitis in Vietnam [7]. Mysteriously, only 2 cases of S. suis meningitis have been reported in Canada [8, 9], and only 1, very recently, in the United States [10]. Although the epidemiology of S. suis infections in humans remains largely undefined, nearly all cases can be ascribed to either handling or consumption of unprocessed pork meat or to close contact with pigs. Therefore, most infected people are pig farmers, abattoir workers, meat inspectors, butchers, and veterinarian practitioners [4]. In humans, S. suis usually produces a purulent or nonpurulent meningitis. Additionally, endocarditis, cellulitis, peritonitis, rhabdomyolysis, arthritis, spondylodiscitis, pneumonia, uveitis, and endopthalmitis may occur [4, 11, 12]. Both severe infections with shock and a high mortality also have been described [13, 14]. One striking feature of S. suis–related meningitis is the complication of deafness and/or vestibular dysfunction, which occurs at a rate consistently higher than that reported for other meningitis-causing bacteria and which reaches 50% and 65% in Europe and Asia, respectively [11, 12]. Most cases of S. suis infection have been attributed to serotype 2 strains; however, cases due to serotypes 4, 14, and 16 also have been reported [4]. Strains isolated from humans are phenotypically and genotypically similar to those recovered from swine within the same geographical region [15].