Pig and human strains isolated from clinical cases of infection
For clinical cases in pigs, isolation and identification of strains is relatively
easy since successful identification can be achieved by a minimum
of biochemical tests and confirmed by serotyping (see below).17
However, the use of rapid multi-test biochemical kits may be misleading
as some strains of S. suis can be misidentified.18 Although S. suis
field isolates readily grow on media used for culturing meningitiscausing
bacteria and veterinary diagnostic laboratories easily identify
this pathogen, many human diagnostic laboratories are less aware of
this pathogen and may misidentify it as enterococci, Streptococcus
pneumoniae, Streptococcus bovis, viridans group streptococci (e.g.,
Streptococcus anginosus and Streptococcus vestibularis) or even Listeria
monocytogenes.14,18–22 In many cases, the initial gramstain presumptive
diagnosis of the cerebrospinal fluid specimen is pneumococcal meningitis.
This confusion may have led to the misdiagnosis of S. suis infections
in the past. Many cases were diagnosed retrospectively after the
isolates were initially misidentified.23 More recently, polymerase chain
reaction (PCR) tests have been used to directly detect S. suis DNA from
cerebrospinal fluid samples with a sensitivity considerably higher than
direct culture, especially if antibiotics have been used.2 However, most
PCR tests used for humans detect serotype 2 strains only and will not
detect infections caused by other S. suis serotypes (see below). Instead
of the serotype 2 PCR test, it is advisable to use other validated PCR
tests that allow the detection of the S. suis species (see below).