{"id":59011,"date":"2020-12-03T09:09:45","date_gmt":"2020-12-03T08:09:45","guid":{"rendered":"https:\/\/ew-nutrition.com\/understanding-and-managing-strep-suis-in-swine-the-essentials\/"},"modified":"2020-12-04T17:03:42","modified_gmt":"2020-12-04T16:03:42","slug":"understanding-and-managing-strep-suis-in-swine-the-essentials","status":"publish","type":"post","link":"https:\/\/ew-nutrition.com\/en-uk\/understanding-and-managing-strep-suis-in-swine-the-essentials\/","title":{"rendered":"Understanding and managing Strep suis in swine: The essentials"},"content":{"rendered":"
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Strep suis<\/em> causes vast losses in pig production and threatens human health, too. We still rely on antibiotics to control it \u2013 but we will have to change tactics to contain antimicrobial resistance.<\/strong><\/p>\n<\/div>\n

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Streptococcus suis<\/em>\u00a0is one of the most economically harmful pathogens for the global swine industry.\u00a0When I started working in pig production 25 years ago, S. suis<\/em> was already a problem on practically all the farms that I visited. Back then, our understanding of the pathogen and hence our control strategies were rudimentary: in farrowing rooms, we cut piglets’ teeth, used gentian violet spray on their navels, and sometimes applied penicillin lyophilized with iron. For the nursery phase, we only had penicillin or phenoxymethylpenicillin at our disposal \u2013 until the first amoxicillin-based premixes arrived, which turned out to be highly effective.<\/p>\n

To this day, we control S. suis<\/em> mainly through oral beta-lactam antibiotics (in feed or water) or injectable solutions, administered to piglets at an early age. However, pig production has evolved dramatically over the past decades, and so has the scientific research on this complex pathogen. Crucially, we now know that the excessive use of antibiotics contributes to the development of antimicrobial resistance.<\/p>\n

Recent Australian research<\/a> has discovered S. suis<\/em> strains (both in humans and pigs) with a high degree of resistance to macrolides or tetracyclines, strains with intermediate sensitivity to Florfenicol, and others that are developing resistance to penicillin G. Additionally, we now know that S. suis<\/em> is a zoonotic bacteria that affects not only at-risk farm or slaughterhouse personnel: S. suis<\/em> is among the leading causes of death from meningitis in countries such as Thailand, China or Vietnam<\/a>. In light of these threats to human health, we in the swine industry more than ever have a duty to help control this pathogen.<\/p>\n

This article first reviews our current state of knowledge about the epidemiology and pathogenesis of Strep suis<\/em>; it then lays out virulence factors and the role of coinfections. The second part considers the dimensions of a holistic approach to S. suis<\/em> prevention and control and highlights the central role of intestinal health management.<\/p>\n<\/div>\n

What we know about S. suis<\/em> epidemiology and pathogenesis<\/h1>\n

Practically all farms worldwide have carrier animals, but the percentage of animals colonized “intra-farm” varies between 40 and 80%, depending on several factors such as environmental conditions, hygiene measures, and the virulence of the S. suis<\/em> strains involved.<\/p>\n

How S. suis<\/em> strains are classified<\/h2>\n

S. suis<\/em> strains were once classified into 35 serotypes, according to their different capsular polysaccharides<\/em>(CPS<\/em>),\u00a0the<\/em>outermost layer of\u00a0the<\/em>\u00a0bacterial cell. Due to phylogenic and genomic sequencing, some of the old serotypes (20, 22, 26, 32, 33, and 34) are now reclassified, either in other bacterial genera or in other Streptococcus<\/em> species. This has reduced the total to 29 S. suis<\/em> serotypes.<\/p>\n

Globally, the prevalence of the disease varies between 3% and 30%. The main serotypes affecting pig population are type 2 (28%), 9 (20%), and 3 (16%); differences in the geographical distribution are shown in Figure 1.<\/p>\n

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Figure 1: Global distribution of S. suis serotypes
\n<\/span>Based on different sources, incl. Goyette-Desjardins et al. (2014)<\/a>, Zimmermann et al. (2019)<\/a>, and Gebhart (2019)<\/a><\/span><\/em><\/h4>\n

In addition to the serotype classification based on CPS antigens, S. suis<\/em> has also been genetically differentiated into “sequence types” using the MLST (Multi Locus Sequence Typing) technique. The distribution of both porcine and human sequence types is detailed in Figure 2.<\/p>\n

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Figure 2: S. suis sequence types and their worldwide distribution<\/span><\/em><\/h4>\n

How S. suis<\/em> is transmitted in swine<\/h2>\n

The main transmission routes are, firstly, the vertical sow-piglet route; the mucosa of the vagina is the first point of contamination. In the farrowing room, respiratory transmission from the sow to the piglets takes place. Horizontal transmission between piglets has also been proven to occur, especially during outbreaks in the post-weaning phase. This form of transmission happens through aerosols, feces, and saliva<\/a>.<\/p>\n

While in humans, the possibility of infection via the digestive tract has been confirmed, there are discussions about this route for swine. De Greeff et al. (2020)<\/a> argue, based on in vitro<\/em> and in vivo<\/em> data, that infection through the digestive tract is associated with specific serotypes. Serotype 9, for example, would have a greater capacity for colonizing the gastrointestinal tract, and from there, the bacteria’s translocation takes place. The same authors point out that, in Western Europe, S. suis<\/em> serotype 9 has become one of the most prevalent serotypes in recent years.<\/p>\n

How S. suis<\/em> colonization occurs<\/h2>\n

Although there are still unknown mechanisms in the pathogenesis of the disease, it can be schematically summarized how colonization occurs (Figure 3). From the different infection routes, the pathogen always passes through the mucosa. When S. suis<\/em> enter the bloodstream, it can lead to a systemic infection, ending in septicemia, meningitis, endocarditis, or pneumonia, or a local infection at the joints level, causing arthritis.<\/p>\n

According to Haas and Grenier (2018), different pathogenicity factors<\/a> intervene in each of the processes. The CPS, for example, are relevant during colonization and the initial progression (indicated by black arrows). Microvesicles released by S. suis<\/em> cell membranes are more involved in the passage to the bloodstream or, for example, the progression towards local or systemic infection (indicated by white arrows).<\/p>\n

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Figure 3: Pathogenesis of S. suis infection<\/em>
\nSource: based on Haas and Grenier (2018)<\/a><\/em>
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Depending on the host and the immune response, the well-known clinical signs of the disease will occur. Although they may begin in the lactation phase, the highest prevalence of meningitis (the main clinical symptom) usually occurs between the 5th and the 10th week of life, that is, between two and three weeks after weaning.<\/p>\n

How to diagnose S. suis<\/em> infection<\/h2>\n

Diagnosing S. suis<\/em> is relatively simple at a clinical level; however, we need to know how to differentiate it from G. parasuis<\/em> in the case of animals with nervous symptoms. We also need to distinguish S. suis <\/em>from other pathogens responsible for producing arthritis, such as M. hyosynoviae <\/em>or the fibrin-producing agent M. hyorhinis<\/em>.<\/p>\n

Laboratory techniques are developing on two fronts. Among molecular techniques, multilocus sequence typing (MLST) is considered the gold standard for serotyping. It is still costly and not yet practicable for large samples at the farm level. In contrast, several types of polymerase chain reaction (PCR) show greater practical applicability. Quantitative PCRs (qPCR) are used for the evaluation of bacterial load, and some PCRs are based on the identification of specific virulence genes.<\/p>\n

Due to the relevance of S. suis<\/em> for human health, more complex techniques are also available, such as the complete sequencing of the bacterial genome. This type of method aims to develop epidemiological analyzes together with the differentiation between virulent and non-virulent S. suis<\/em> strains. Research is also underway in serology, particularly on evaluating maternal immunity and its interference with the piglet, as well as autogenous vaccines monitoring.<\/p>\n

Why S. suis<\/em> sometimes causes disease: Virulence factors and coinfections<\/h2>\n

Streptococcus suis<\/em> is a pathobiont, i.e., a microorganism that belongs to the commensal flora of animals but generates disease under certain conditions. In their daily work on farms, clinical veterinarians, for instance, find that S. suis<\/em> often colonizes the upper respiratory tract, nasal cavity, and tonsils without causing disease. S. suis<\/em> pathogenicity is associated with an astounding range of different circumstances or triggering factors; some sources list more than 100 virulence factors. Several factors are considered essential in the development of pathogenesis; others, however, are the subject of ongoing research (cf. Xia et al., 2019<\/a>, and Segura et al., 2017<\/a>).<\/p>\n

Critical virulence factors<\/h3>\n