Getting a grip on strangles: recent progress towards improved diagnostics and vaccines

'Strangles', caused by infection with the bacterium Streptococcus equi, remains one of the most commonly diagnosed and important infectious diseases of horses world-wide. This review discusses the diagnosis and pathogenesis of strangles with particular attention to the significance of persistent infections in disease transmission and the rapid progress now being made towards the development of effective preventative vaccines. It is now possible combine recent sequence data from the N-terminal region of the SeM protein and reassign the SeM alleles using the on-line database http://pubmlst.org/szooepidemicus/seM/. Hypotheses concerning the origin of this variation and the potential for its exploitation for the epidemiological analysis of outbreaks are proposed. Advances in understanding of the molecular evolution of S. equi highlight the role played by phage-mediated acquisition of virulence factors and suggest new avenues for prophylactic intervention.     

Streptococcus equi in Equine: Diagnostic and Healthy Performance Impacts

Although the strangles disease of Streptococcus equi was discovered many decades ago in 1,251 by Jordanus Ruffus, it has still remained a major frequently diagnosed infection in horses all over the world. The S. equi subspecies pathogen is known to be often resistant to antibiotic treatment, and it makes the antibiotics inefficient; hence, this review was conducted to study how the disease can be managed. The age-long sign of this infection is the oozing of pus through the mucous and skin membranes. Affected horses lose appetite, develop fever, and become depressed, which result in them losing weight and becoming lethargic, reducing their physical activity especially when they are being raised for athletic and sport purposes. This article reviews various solutions proffered by several researchers about the healthy performance impacts of S. equi in equine.     

First Report of Molecular Characterization of Argentine Isolates of Streptococcus equi subsp. equi by Pulsed-Field Gel Electrophoresis

Strangles is one of the most frequently diagnosed equine respiratory infectious diseases in the world. It is caused by Streptococcus equi subsp. equi (S. equi), and it is an acute infection characterized by pyrexia, nasal discharge, pharyngitis, and abscessation of lymph nodes. Frequently, healthy horses might continue to harbor S. equi after clinical recovery. Although the genetic distance between S. equi isolates is short, strains can be differentiated by pulsed-field gel electrophoresis (PFGE) and single locus sequence typing for epidemiological studies. The aim of this study was to characterize by PFGE Argentine isolates of S. equi obtained from horses with acute strangles and those that had recovered. Bacterial isolation and identification of 80 S. equi isolates by phenotypic and genotypic tests were performed using samples from 29 horses with acute strangles and 95 from healthy animals. Also, the isolates were characterized by PFGE using Bsp120I and SmaI. Visual comparison of macrorestriction patterns generated with both enzymes revealed three different DNA fragment profiles with variations of one or two bands. Interestingly, an identical profile was found in isolates from the same horse and from horses that were infected at the same time, and the horses recovered from strangles continue to carry the same strain. Some vaccinated horses have been mild infected for a different strain from that of carriers suggesting other source of infection. This is the first molecular characterization of Argentine isolates of S. equi, which shows the presence of three strains between 2010 and 2013 in Buenos Aires.     

Identification of a Streptococcus equi Strain Responsible for Four Outbreaks of Strangles in Colorado

The objective of this study was to use a new subtyping technique to determine the identity of five Streptococcus equi isolates from four outbreaks of strangles in Colorado during 2005−2006. All five of the isolates from the four strangles outbreaks in Colorado contained SeM allele 28. This SeM allele is typical of American isolates of S. equi and has previously been linked with strangles outbreaks in Minnesota in 1994 and Kentucky in 1995. Use of the new S. equi strain subtyping method allowed for characterization of recent S. equi isolates associated with outbreaks of strangles in Colorado. To our knowledge, this is the first report of typing of more recent isolates from North America. This approach, however, has been used in situations in the United Kingdom to differentiate the vaccine strain of S. equi from that of the wild type of the bacteria.     

Lineages of Streptococcus equi ssp. equi in the Irish equine industry

Background

Streptococcus equi ssp. equi is the causative agent of ‘Strangles’ in horses. This is a debilitating condition leading to economic loss, yard closures and cancellation of equestrian events. There are multiple genotypes of S. equi ssp. equi which can cause disease, but to date there has been no systematic study of strains which are prevalent in Ireland. This study identified and classified Streptococcus equi ssp. equi strains isolated from within the Irish equine industry.

Results

Two hundred veterinary isolates were subjected to SLST (single locus sequence typing) based on an internal sequence from the seM gene of Streptococcus equi ssp equi. Of the 171 samples which successfully gave an amplicon, 162 samples (137 Irish and 24 UK strains) gave robust DNA sequence information. Analysis of the sequences allowed division of the isolates into 19 groups, 13 of which contain at least 2 isolates and 6 groups containing single isolates. There were 19 positions where a DNA SNP (single nucleotide polymorphism) occurs, and one 3 bp insertion. All groups had multiple (2–8) SNPs. Of the SNPs 17 would result in an amino acid change in the encoded protein. Interestingly, the single isolate EI8, which has 6 SNPs, has the three base pair insertion which is not seen in any other isolate, this would result in the insertion of an Ile residue at position 62 in that protein sequence. Comparison of the relevant region in the determined sequences with the UK Streptococcus equi seM MLST database showed that Group B (15 isolates) and Group I (2 isolates), as well as the individual isolates EI3 and EI8, are unique to Ireland, and some groups are most likely of UK origin (Groups F and M), but many more probably passed back and forth between the two countries.

Conclusions

The strains occurring in Ireland are not clonal and there is a considerable degree of sequence variation seen in the seM gene. There are two major clades causing infection in Ireland and these strains are also common in the UK.     

Characterization of a Streptococcus equi ssp. equi Isolate From a Strangles Outbreak in Thailand

Strangles, which is caused by Streptococcus equi ssp. equi, is one of the major infectious respiratory diseases in horses. Knowledge of isolates from different areas of the world is important for investigating the different strains of the disease. In contrast to many other countries, currently little is known about S. equi ssp. equi isolates in Thailand. In 2014, a farm in Thailand imported 20 horses from Europe. Approximately 1 month after arrival, 50% of the horses had developed pyrexia, mucopurulent nasal discharge, and abscesses of the mandibular lymph nodes. Nasal swabs of mucopurulent discharge were sent to a diagnostic laboratory, and two isolates of S. equi ssp. equi were identified. One of the isolates was further characterized using seM gene polymerase chain reaction and sequence analysis. The seM sequence was then compared to the database of PubMLST-seM. It was found to contain SeM allele 48, an allele isolated from horses in the United Kingdom in 2006 and 2010. This result demonstrates the usefulness of SeM allele identification as a tool for investigating the source of related strains and for the epidemiologic study of strangles. To the best of the authors' knowledge, this is the first report of the identification of an SeM allele of the S. equi ssp. equi isolate in Thailand.