Zoonotic Streptococci: A Focus on Streptococcus suis

Abstract

Pyogenic zoonotic streptococcal infections are neglected zoonoses which have increasingly been reported, especially in Asia, and can produce a wide variety of invasive diseases that mimic clinical entities produced by strictly human streptococci. The common human pathogen Streptococcus group B, now called Streptococcus agalactiae, may have originated from cows, and it is now recognized to cause invasive disease in fish as well. Sporadic cases of animal-derived streptococci had rarely been reported to cause human infections. These include Streptococcus canis from dogs and cats, Streptococcus equi zooepidemicus from horses and unpasteurized milk, Streptococcus iniae from fish, and more recently Streptococcus suis from pigs. S. suis is a major cause of disease in pigs worldwide with severe infections and high fatality in piglets from septicemia, meningitis, pneumonia, and septic arthritis and costs the global pig industry hundreds of millions of dollars. Moreover, S. suis has emerged as a serious major pathogen in humans throughout Southeast Asia and China in the past two decades. This pathogen has produced two deadly human outbreaks of septicemia and streptococcal toxic shock syndrome in China in 1998 and 2005. Furthermore, S. suis is the leading cause of adult bacterial meningitis in Vietnam, the second in Thailand and the third commonest in Hong Kong. There are 35 serotypes of S. suis recognized in animals, but human infections in Asia are caused predominantly by serotype 2. Animal strains of the bacteria are commonly multidrug resistant especially to tetracycline, clindamycin, macrolides, and the quinolones. Penicillin and beta-lactam agents are the drugs of choice. Numerous studies have been published on the virulence and pathogenic mechanisms of S. suis which are reviewed, and there are some common features shared with the virulent human Streptococcus pyogenes. Potential vaccine candidates for pig use are reviewed.