Review of the speculative role of co-infections in Streptococcus suis-associated diseases in pigs

Abstract

Streptococcus suis is one of the most important bacterial swine pathogens affecting post-weaned piglets, causing mainly meningitis, arthritis and sudden death. It not only results in severe economic losses but also raises concerns over animal welfare and antimicrobial resistance and remains an important zoonotic agent in some countries. The definition and diagnosis of S. suis-associated diseases can be complex. Should S. suis be considered a primary or secondary pathogen? The situation is further complicated when referring to respiratory disease, since the pathogen has historically been considered as a secondary pathogen within the porcine respiratory disease complex (PRDC). Is S. suis a respiratory or strictly systemic pathogen? S. suis is a normal inhabitant of the upper respiratory tract, and the presence of potentially virulent strains alone does not guarantee the appearance of clinical signs. Within this unclear context, it has been largely proposed that co-infection with some viral and bacterial pathogens can significantly influence the severity of S. suis-associated diseases and may be the key to understanding how the infection behaves in the field. In this review, we critically addressed studies reporting an epidemiological link (mixed infections or presence of more than one pathogen at the same time), as well as in vitro and in vivo studies of co-infection of S. suis with other pathogens and discussed their limitations and possibilities for improvement and proposed recommendations for future studies.

Genomic and pathogenic investigations of Streptococcus suis serotype 7 population derived from a human patient and pigs

“…Therefore, we proposed that the public health threat of S. suis serotype 7, especially those virulent ST373 strains, should not be ignored. Coincidentally, healthy pigs were reported to be a reservoir of strains with high virulence potential in humans [ 39 , 42 ]. Moreover, four strains isolated from diseased pigs were classified as lowly virulent.…”

“…A similar result was also reported in our previous study [ 43 ]. It should be noted that the presence of clinical signs in pigs may also depend on co-infection with some viral and bacterial pathogens [ 42 ]. Thus, the public health significance of strains may not be accurately evaluated only based on the clinical information of their host.…”

Experimental evaluation of protection and immunogenicity of Streptococcus suis bacterin-based vaccines formulated with different commercial adjuvants in weaned piglets

“…Experimental infections using virulent S. suis strains are not easy to reproduce [ 20 ]. In fact, presence of serious clinical signs in the field are frequently associated to co-factors, which are usually absent in research animal facilities [ 10 ]. Indeed, when using the more natural intranasal route of infection with conventional animals, the success rate on reproducing the disease is usually very low (unpublished observations).…”

“…These antibodies might have been generated in adult animals by S. suis strains that are part of the normal microbiota or even by other S. suis -like microorganisms that are present in the upper respiratory tract [ 3 , 24 ]. Adult animals usually present high levels of antibodies tested for any serotype of S. suis [ 13 , 25 ] (unpublished observations), which may explain why S. suis -associated diseases are almost never observed in older animals [ 10 ]. Finally, in addition to the presence of some residual maternal antibody interference, other features that might also explain the lack of immunological response after one vaccine dose include an immature immune system, diet change, and other stressing factors [ 26 , 27 ].…”

“…Prevention of clinical disease caused by S. suis is mainly based on the control of predisposal factors and the use of vaccines [ 10 ]. Since there are no universal efficacious commercial vaccines against S. suis infection, the use of autogenous vaccines (in general, bacterins) is widespread [ 11 ].…”