Immunologic and genetic comparison of Streptococcus equi isolates from the United States and Europe

Development of novel Streptococcus equi vaccines with an assessment of their immunizing potentials and protective efficacies

Strangles is a highly contagious disease of the equine upper respiratory tract caused by Streptococcus equi subspecies. Streptococcus equi subsp. equi (S. equi) and Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) was isolated, as local, hot, and field strains, from horses clinically suffering from respiratory distress. The isolated Streptococci were identified using bacteriological and molecular techniques. Four formulations of inactivated S. equi vaccines were developed and evaluated. The first formulation was prepared using the S. equi isolates, adjuvanted with MONTANIDE GEL adjuvant, while the second formulation was adjuvanted with MONTANIDE ISA-70 adjuvant. The other 2 formulations were inactivated combined vaccines prepared from both S. equi and S. zooepidemicus isolates. The 3rd formulation was the combined isolates adjuvanted with MONTANIDE GEL while the 4th formulation was the combined isolates adjuvanted with MONTANIDE ISA-70. The developed vaccines' physical properties, purity, sterility, safety, and potency were ensured. The immunizing efficacy was determined in isogenic BALB/c mice and white New Zealand rabbits using the passive hemagglutination test. Also, the antibodies' titer of the combined S. equi and S. zooepidemicus vaccine adjuvanted with MONTANIDE ISA-70 in foals was tracked using an indirect enzyme-linked immunosorbent assay. The protective efficacy of the developed vaccines was determined using a challenge test in both laboratory and field animal models, where a 75% protection rate was achieved. The combined vaccine proved to be more efficacious than the monovalent vaccine. Also, the MONTANIDE ISA-70 adjuvant provided significant protective efficacy than the MONTANIDE GEL. The current work is introducing a very promising mitigative and strategic controlling solution for strangles.

Globetrotting strangles: the unbridled national and international transmission of Streptococcus equi between horses

The equine disease strangles, which is characterized by the formation of abscesses in the lymph nodes of the head and neck, is one of the most frequently diagnosed infectious diseases of horses around the world. The causal agent, Streptococcus equi subspecies equi, establishes a persistent infection in approximately 10 % of animals that recover from the acute disease. Such 'carrier' animals appear healthy and are rarely identified during routine veterinary examinations pre-purchase or transit, but can transmit S. equi to naïve animals initiating new episodes of disease. Here, we report the analysis and visualization of phylogenomic and epidemiological data for 670 isolates of S. equi recovered from 19 different countries using a new core-genome multilocus sequence typing (cgMLST) web bioresource. Genetic relationships among all 670 S. equi isolates were determined at high resolution, revealing national and international transmission events that drive this endemic disease in horse populations throughout the world. Our data argue for the recognition of the international importance of strangles by the Office International des Épizooties to highlight the health, welfare and economic cost of this disease. The Pathogenwatch cgMLST web bioresource described herein is available for tailored genomic analysis of populations of S. equi and its close relative S. equi subspecies zooepidemicus that are recovered from horses and other animals, including humans, throughout the world. This article contains data hosted by Microreact.

Novel seM-types of Streptococcus equi subsp. equi identified in isolates circulating in Argentina

BACKGROUND

Strangles is a worldwide infectious disease caused by Streptococcus equi subsp. equi that affects the upper respiratory tract of horses. Streptococcus equi subsp. equi characterisation by seM-typing is internationally used for epidemiological studies and comparison of isolates.

OBJECTIVES

To identify and to compare the seM-types of Argentinian isolates of Streptococcus equi subsp. equi.

STUDY DESIGN

Investigation of bacterial isolates using molecular and phylogenetic approaches.

METHODS

A total of 59 Argentinian isolates of Streptococcus equi subsp. equi obtained between 2007 and 2019 were studied by seM-typing. The sequence similarity of Argentinian seM-types and the other alleles available on the seM database was determined using BLAST and phylogenetic analysis was performed using the Neighbour-Joining algorithm. The amino acid sequences were predicted and compared with the predicted amino acid sequence of the reference strain 4047 using the MEGA 7 software and PROVEAN tool.

RESULTS

Eight seM-types were found among the isolates. Only one of them (seM-61) has been previously reported and the other seven alleles (seM-129, seM-130, seM-131, seM-132, seM-133, seM-134 and seM-135) were novel seM sequences. High genetic similarity was observed among the Argentinian seM-types, with the exception of seM-130. No functional effects of amino acid differences were predicted.

MAIN LIMITATIONS

The number of related and unrelated isolates per year.

CONCLUSIONS

Seven novel seM-types and seM-61 that were previously reported in Brazil were circulating in Argentina which were identified as circulating in Argentinian horses between 2007 and 2019. The high genetic similarity among the Argentinian and Brazilian seM-types suggests that there is a geographical distribution of strain types. The geographical restriction of strains is likely to reflect the movement of horses between different equine disciplines and neighbouring countries.

New perspectives for the diagnosis, control, treatment, and prevention of strangles in horses

Strangles, characterized by abscessation of the lymph nodes of the head and neck, is the most frequently diagnosed infectious disease of horses worldwide. The persistence of the causative agent, Streptococcus equi, in a proportion of convalescent horses plays a critical role in the recurrence and spread of disease. Recent research has led to the development of effective diagnostic tests that assist the eradication of S equi from local horse populations. This article describes how these advances have been made and provides advice to assist the resolution and prevention of outbreaks. New perspectives on preventative vaccines and therapeutic interventions are discussed.

The Antiphagocytic Activity of SeM of Streptococcus equi Requires Capsule

Resistance to phagocytosis is a crucial virulence property of Streptococcus equi (Streptococcus equi subsp. equi; Se), the cause of equine strangles. The contribution and interdependence of capsule and SeM to killing in equine blood and neutrophils were investigated in naturally occurring strains of Se. Strains CF32, SF463 were capsule and SeM positive, strains Lex90, Lex93 were capsule negative and SeM positive and strains Se19, Se1-8 were capsule positive and SeM deficient. Phagocytosis and killing of Se19, Se1-8, Lex90 and Lex93 in equine blood and by neutrophils suspended in serum were significantly (P ≤ 0.02) greater compared to CF32 and SF463. The results indicate capsule and SeM are both required for resistance to phagocytosis and killing and that the anti-phagocytic property of SeM is greatly reduced in the absence of capsule.

Characterization and protective immunogenicity of the SzM protein of Streptococcus zooepidemicus NC78 from a clonal outbreak of equine respiratory disease

Streptococcus zooepidemicus of Lancefield group C is a highly variable tonsillar and mucosal commensal that usually is associated with opportunistic infections of the respiratory tract of vertebrate hosts. More-virulent clones have caused epizootics of severe respiratory disease in dogs and horses. The virulence factors of these strains are poorly understood. The antiphagocytic protein SeM is a major virulence factor and protective antigen of Streptococcus equi, a clonal biovar of an ancestral S. zooepidemicus strain. Although the genome of S. zooepidemicus strain H70, an equine isolate, contains a partial homolog (szm) of sem, expression of the gene has not been documented. We have identified and characterized SzM from an encapsulated S. zooepidemicus strain from an epizootic of equine respiratory disease in New Caledonia. The SzM protein of strain NC78 (SzM(NC78)) has a predicted predominantly alpha-helical fibrillar structure with an LPSTG cell surface anchor motif and resistance to hot acid. A putative binding site for plasminogen is present in the B repeat region, the sequence of which shares homology with repeats of the plasminogen binding proteins of human group C and G streptococci. Equine plasminogen is activated in a dose-dependent manner by recombinant SzM(NC78). Only 23.20 and 25.46% DNA homology is shared with SeM proteins of S. equi strains CF32 and 4047, respectively, and homology ranges from 19.60 to 54.70% for SzM proteins of other S. zooepidemicus strains. As expected, SzM(NC78) reacted with convalescent-phase sera from horses with respiratory disease associated with strains of S. zooepidemicus. SzM(NC78) resembles SeM in binding equine fibrinogen and eliciting strong protective antibody responses in mice. Sera of vaccinated mice opsonized S. zooepidemicus strains NC78 and W60, the SzM protein of which shared partial amino acid homology with SzM(NC78). We conclude that SzM is a protective antigen of NC78; it was strongly reactive with serum antibodies from horses during recovery from S. zooepidemicus-associated respiratory disease.

Detection of Streptococcus equi subspecies equi using a triplex qPCR assay

Genome sequencing data for Streptococcus equi subspecies equi and zooepidemicus were used to develop a novel diagnostic triplex quantitative PCR (qPCR) assay targeting two genes specific to S. equi (eqbE and SEQ2190) and a unique 100 base pair control DNA sequence (SZIC) inserted into the SZO07770 pseudogene of S. zooepidemicus strain H70. This triplex strangles qPCR assay can provide results within 2 h of sample receipt, has an overall sensitivity of 93.9% and specificity of 96.6% relative to the eqbE singlex assay and detects S. equi at levels below the threshold of the culture assay, even in the presence of contaminating bacteria.

A multiphasic typing approach to subtype Streptococcus equi subspecies equi

The objective of the present investigation was to differentiate between strains of Streptococcus equi subspecies equi implicated in abscess formation in vaccinated horses. Streptococcus equi isolates recovered from clinical specimens associated with equine strangles cases submitted to the University of Illinois Veterinary Diagnostic Laboratory were compared with S. equi isolates representing at least 12 lots of a commercial modified live vaccine (MLV) to determine whether the isolates obtained from the abscesses were vaccine or wild type. Genotyping techniques evaluated included enterobacterial repetitive intergenic consensus polymerase chain reaction (PCR), repetitive extragenic palindrome PCR, BOX element PCR, ribotyping, and pulsed-field gel electrophoresis (PFGE). Phenotypic evaluations were performed using the Biolog GP2 Microplate (hereafter, Biolog). In cases where Biolog and PFGE results did not coincide, a single nucleotide polymorphism located in the upstream regulatory region of szp gene was used to identify the S. equi strains. PFGE and Biolog successfully differentiated wild-type S. equi strains isolated from clinical submissions from isolates of the MLV. PFGE genotyping enabled further subtyping of the wild-type strains, whereas Biolog combined with szp sequencing was useful in differentiating the MLV strain from its wild-type progenitor. Deletion of a single guanine residue located in the upstream regulatory region of the szp gene appears to be conserved among vaccine isolates, and shows a 98.5% correlation to Biolog identification. This multiphasic approach can be used to answer specific diagnostic questions pertaining to the source of infection and/or outbreak, or to address quarantine concerns.

Molecular characterisation of 'strangles' outbreaks in the UK: the use of M-protein typing of Streptococcus equi ssp. equi

REASONS FOR PERFORMING STUDY

Strangles is the most commonly diagnosed and important infectious disease of horses worldwide. Very little is known about the temporo-spatial and molecular epidemiology of strangles. The disease is not notifiable in the UK and there are few published data on the geographical locations of outbreaks.

OBJECTIVE

To investigate whether typing of a surface protein (SeM) of Streptococcus equi ssp. equi (S. equi), the causative agent of strangles, is a useful epidemiological tool.

METHODS

The variable region of the SeM gene was amplified from 145 isolates of S. equi by PCR and sequenced. Different SeM gene alleles were assigned based on the SeM database, grouped into phylogenetic clusters using split decomposition analysis and plotted against the submitting veterinary practices.

RESULTS

In this study 21 S. equi SeM alleles were found, including 9 previously unidentified alleles and representing 4 phylogenetic groups. S. equi containing SeM alleles 9 and 7 were the most commonly isolated and there was a high number of low frequency alleles. The occurrence of an outbreak cluster in the north-west of the UK is also reported.

CONCLUSIONS

Strangles outbreaks can be differentiated on the basis of their SeM allele sequences. The data provide further evidence of SeM mutation leading to the emergence of novel, but related SeM alleles that are geographically linked. Sequencing of the SeM gene is a useful tool for the elucidation of strangles epidemiology at a regional and a national level.

POTENTIAL RELEVANCE

This technique may allow differentiation or linkage of strangles outbreaks and as such may be an effective tool for local as well as national and international disease surveillance.

Use of a novel serological test for exposure to Streptococcus equi subspecies equi in hospitalised horses

Thirty horses with no external signs of strangles were tested for exposure to Streptococcus equi subspecies equi (S equi) using a new, commercially available serological test. The horses were also tested for persistent carriage of S equi by endoscopy of the guttural pouches and PCR analysis of lavage samples. The owners were questioned about the recent medical history of the horses. Serology suggested that four horses had been recently exposed to S equi. None of the horses had a known history of strangles but three of the four seropositive horses had recently shown non-specific signs of respiratory disease. One asymptomatic horse was positive for S equi by PCR, but none had both guttural pouch abnormalities and a positive PCR result. Ten additional horses known to have strangles were all seropositive by the serological test.

Virulence and antigenicity of the szp-gene deleted Streptococcus equi ssp. zooepidemicus mutant in mice

Streptococcus equi ssp. zooepidemicus (SEZ) causes serious disease in pigs. M-like protein (SzP) of SEZ is a virulence factor with opsonin function. To fabricate better vaccine against SEZ, We have generated a szp-knockout strain by homologous recombination. Two fragments, upstream and downstream of the szp gene, were obtained by genome walking. The vector was then used to delete a 1090 bp segment of szp gene from a strain of SEZ (ATCC35246). The mutant strain had 1000-fold decrease in LD50 as compared with wild type, and provided 85% protection against challenge with a virulent strain when administered as a live vaccine. Semi-quantitative RT-PCR analysis showed a marked increased in levels of IL-4 and IFN-gamma mRNA in immunized mice. These data demonstrate that szp-knockout strain is a better candidate for vaccine development.

Streptococcus equi bacteriophage SeP9 binds to group C carbohydrate but is not infective for the closely related S. zooepidemicus

Streptococcus equi (S. equi subsp. equi) is widely believed to have evolved from an ancestral strain of S. zooepidemicus (S. equi subsp. zooepidemicus) based on high sequence homology. A striking difference is the absence of phage sequences from S. zooepidemicus. In this study we show that the receptor for SeP9, a temperate bacteriophage of S. equi, is the Lancefield group C carbohydrate. However, although SeP9 binds to group C carbohydrate from S. zooepidemicus, it appears not to replicate and produce plaques.

IdeE reduces the bactericidal activity of equine neutrophils for Streptococcus equi

Streptococcus equi (S. equi) causes equine strangles, a highly contagious and widespread purulent lymphadenitis of the head and neck. Highly resistant to phagocytosis, it produces long extracellular chains in affected lymph nodes. In a screen of clones reactive with convalescent serum from a gene library of S. equi CF32 we identified IdeE, an IgG-endopeptidase and homologue of the leucocyte receptor Mac-1 (CD11b). IdeE is expressed during S. equi infection eliciting both serum and mucosal antibody responses which persisted at significant levels in serum for over 200 days. Release from S. equi into culture medium was detected during the exponential phase of growth. The closely related Streptococcus zooepidemicus appeared to store the protein but not to release it. Antiphagocytic activity for equine neutrophils was dose-dependent and neutralized by IdeE-specific antiserum. Biotinylated IdeE bound weakly to about 77% of purified equine neutrophils and strongly to the remainder.

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.

Variation in the N-terminal region of an M-like protein of Streptococcus equi and evaluation of its potential as a tool in epidemiologic studies

OBJECTIVE

To develop a method for typing Streptococcus equi on the basis of the DNA sequence of the genes that produce an M-like protein and to compare isolates among the United States, Japan, and other countries.

SAMPLE POPULATION

S equi strains CF32, Hidaka/95/2, and NCTC9682 as well as 82 other isolates from the United States, Japan, and other countries obtained during 1975 to 2001.

PROCEDURE

DNA sequences of the structural genes ( SeM and SzPSe) that produce M-like proteins were determined for 3 representative strains to find a variable region. Variability in this region of SeM was then determined for the other isolates. Amino acid sequences were deduced and analyzed phylogenetically by use of the neighbor-joining method.

RESULTS

Sequence diversity was detected in the N-terminal region of SeM but not in SzPSe of the 3 representative strains. Base substitutions in the variable region of SeM varied in a nonsynonymous manner, resulting in variation in the amino acid sequence. Eighty-five isolates were categorized as 32 types of SeM on the basis of differences in the deduced amino acid sequences.

CONCLUSIONS AND CLINICAL RELEVANCE

This study documented a region in the N-terminal portion of SeM that varies in a nonsynonymous manner. This information should be useful in molecular epidemiologic studies of S equi.

Sequence variation of the SeM gene of Streptococcus equi allows discrimination of the source of strangles outbreaks

Improved understanding of the epidemiology of Streptococcus equi transmission requires sensitive and portable subtyping methods that can rationally discriminate between strains. S. equi is highly homogeneous and cannot be distinguished by multilocus enzyme electrophoretic or multilocus sequence-typing methods that utilize housekeeping genes. However, on sequence analysis of the N-terminal region of the SeM genes of 60 S. equi isolates from 27 strangles outbreaks, we identified 21 DNA codon changes. These resulted in the nonsynonymous substitution of 18 amino acids and allowed the assignment of S. equi strains to 15 distinct subtypes. Our data suggest the presence of multiple epitopes across this region that are subjected to selective immune pressure (nonsynonymous-synonymous substitution rate [d(N)/d(S)] ratio = 3.054), particularly during the establishment of long-term S. equi infection. We further report the application of SeM gene subtyping as a method to investigate potential cases of disease related to administration of a live attenuated S. equi vaccine. SeM gene subtyping successfully differentiated between the vaccine strain and field strains of S. equi responsible for concurrent disease. These results were confirmed by the development and application of a PCR diagnostic test, which identifies the aroA partial gene deletion present in the Equilis StrepE vaccine strain. Although the vaccine strain was found to be responsible for injection site lesions, all seven outbreaks of strangles investigated in recently vaccinated horses were found to be due to concurrent infection with wild-type S. equi and not due to reversion of the vaccine strain.

Characterization and immunogenicity of pyrogenic mitogens SePE-H and SePE-I of Streptococcus equi

Two pyrogenic mitogens, SePE-H and SePE-I, were characterized in Streptococcus equi, the cause of equine strangles. SePE-H and SePE-I have molecular masses of 27.5 and 29.5 kDa, respectively, and each is almost identical to its counterpart in Streptococcus pyogenes M1. Both genes are adjacent to a gene encoding a phage muramidase of 49.7 kDa and are located immediately downstream from a phage genomic sequence almost identical to a similar phage sequence in S. pyogenes M1. Strong mitogenic responses were elicited by both proteins from horse peripheral blood mononuclear cells. However, although both were pyrogenic for rabbits, only SePE-I was pyrogenic in ponies. Convalescent sera contained antibody to each mitogen and horses recovered from strangles or immunized with SePE-I were resistant to the pyrogenic effect of SePE-I. The immunogenicity of SePE-I suggests that it should be included in new generation strangles vaccines. In isolates of S. equi sepe-I and sepe-H were consistently present but they were absent from the closely related Streptococcus zooepidemicus, suggesting that phage mediated transfer was an important event in the formation of the clonal, more virulent, S. equi from its putative S. zooepidemicus ancestor.

Comparison of the fibronectin-binding protein FNE from Streptococcus equi subspecies equi with FNZ from S. equi subspecies zooepidemicus reveals a major and conserved difference

The gene fnz from Streptococcus equi subspecies zooepidemicus encodes a cell surface protein that binds fibronectin (Fn). Fifty tested isolates of S. equi subspecies equi all contain DNA sequences with similarity to fnz. This work describes the cloning and sequencing of a gene, designated fne, with similarity to fnz from two S. equi subspecies equi isolates. The DNA sequences were found to be identical in the two strains, and sequence comparison of the fne and fnz genes revealed only minor differences. However, one base deletion was found in the middle of the fne gene and eight base pairs downstream of the altered reading frame there is a stop codon. An Fn-binding protein was purified from the growth medium of a subspecies equi culture. Determination of the NH(2)-terminal amino acid sequence and molecular mass, as judged by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, revealed that the purified protein is the gene product of the 5'-terminal half of fne. Fn-binding activity has earlier only been found in the COOH-terminal half of FNZ. By the use of a purified recombinant protein containing the NH(2) half of FNZ, we provide here evidence that this half of the protein also harbors an Fn-binding domain.

Investigations towards an efficacious and safe strangles vaccine: submucosal vaccination with a live attenuated Streptococcus equi

As part of a search for a safe and efficacious strangles vaccine, several different vaccines and different vaccination routes were tested in foals. The degree of protection was evaluated after an intranasal challenge with virulent Streptococcus equi by clinical, postmortem and bacteriological examinations. Inactivated vaccines containing either native purified M-protein (500 microg per dose) or whole S equi cells (10(10) cells per dose) administered at least twice intramuscularly at intervals of four weeks, did not protect against challenge. Different live attenuated S equi mutants administered at least twice at intervals of four weeks by the intranasal route were either safe but not protective or caused strangles. In contrast, a live attenuated deletion mutant administered intramuscularly, induced complete protection but also induced unacceptable local reactions at the site of vaccination. Submucosal vaccination in the inner side of the upper lip with the live attenuated mutant at > or =10(8) colony-forming units per dose, appeared to be safe and efficacious in foals as young as four months of age. The submucosal vaccinations caused small transient swellings that resolved completely within two weeks, and postmortem no vaccine remnants or other abnormalities were found at the site of vaccination.

Use of repetitive sequence-based polymerase chain reaction for molecular epidemiologic analysis of Streptococcus equi subspecies equi

OBJECTIVE

To determine whether repetitive sequence-based polymerase chain reaction (rep-PCR) could be used to differentiate Streptococcus equi isolates, to examine S equi isolates from throughout the world, and to determine whether a horse had > 1 subtype of S equi during an outbreak of disease.

SAMPLE POPULATION

An initial group of 32 S equi isolates, 63 S equi isolates from various geographic areas, and 17 S equi isolates obtained during outbreaks of disease.

PROCEDURE

An aliquot of S equi genomic DNA was amplified, using enterobacterial repetitive intergenic consensus primers. Gel electrophoresis was performed on 1.5% agarose gels, and a computed-assisted program was used to compare rep-PCR results.

RESULTS

Use of these primers to analyze 100 ng of S equi genomic DNA resulted in patterns of 6 to 14 bands. The 32 initial isolates were separated into 7 rep-PCR subtypes. There were 30 rep-PCR subtypes found among 29 S equi isolates obtained from Minnesota, Michigan, Canada, and Australia and 34 S equi isolates obtained from Kentucky and other sources. Furthermore, the same clone was identified in several horses during an outbreak of disease. Infected horses on the same farm all had a single clone of S equi.

CONCLUSION AND CLINICAL RELEVANCE

Analysis of these results suggests that rep-PCR is useful for delineating S equi into rep-PCR subtypes. Results revealed that isolates with the same geographic source or similar date of collection did not always have the same rep-PCR subtype. A single clone of S equi usually predominated during an outbreak of disease.

Streptococcus equi with truncated M-proteins isolated from outwardly healthy horses

The M-protein genes of Streptococcus equi isolated from 17 outwardly healthy horses after 4 strangles outbreaks had ended, including a quarantined animal, were compared with those of S. equi isolates from 167 active cases of strangles across 4 countries. The healthy horses included 16 persistent S. equi carriers, at least one from each of the four outbreaks. These carriers, despite being outwardly healthy, had empyema of the guttural pouch(es), an enlargement of the equine Eustachian tube. A persistent carrier from two of these outbreaks, the quarantined animal and a healthy animal with normal guttural pouches, from which S. equi was isolated only once, were colonized by variant S. equi with truncated M-protein genes (24% of outwardly healthy animals with S. equi). The truncated M-protein genes had in-frame deletions in slightly different positions between the signal sequence and the central repeat region, equivalent to approximately 20% of the mature expressed protein. Immunoblotting with antibody to recombinant M-protein confirmed that the variants expressed a truncated form of the M-protein. In contrast to the outwardly healthy S. equi carriers, only 1/167 of S. equi isolates from strangles cases possessed a truncated M-protein gene (<1%; Fisher's exact test, P=0.0002). Compared with isolates from healthy horses with a truncated M-protein, much more of the N terminus of the truncated M-protein was retained in the variant S. equi from a strangles case. Variant S. equi from outwardly healthy animals were more susceptible to phagocytosis by neutrophils in vitro than typical isolates. This is the first report of detection of S. equi with a truncated M-protein. The distribution of the variants between strangles cases and carriers suggests that the 80% of the M-protein retained in the variants may contribute to colonization whilst the deleted portion of the gene may be needed for full virulence.

Detection of DNA restriction fragment polymorphisms in Streptococcus equi

Large-restriction-fragment (LRF) polymorphisms in Streptococcus equi (S equi subspecies equi) were studied by pulsed-field gel electrophoresis. Five or six chromosomal fragments of between 194 and 915 kb were separated by digestion with the restriction endonuclease Notl. All 20 isolates of S equi, including 12 from independent Japanese outbreaks, four from independent American outbreaks, two from a single Irish outbreak, us vaccine strain F43, and type strain NCTC 9682 were successfully typed. Seven distinctive, reproducible and stable types were identified. The 12 Japanese isolates collected between 1992 and 1998 were of LRF type II suggesting that they were derived from the same source. The remaining eight isolates were of six types. The results indicate that LRF typing should be a useful technique for investigating the source and transmission of S equi.

SFS, a novel fibronectin-binding protein from Streptococcus equi, inhibits the binding between fibronectin and collagen

The obligate parasitic bacterium Streptococcus equi subsp. equi is the causative agent of strangles, a serious disease of the upper respiratory tract in horses. In this study we have, using shotgun phage display, cloned from S. equi subsp. equi and characterized a gene, called sfs, encoding a protein termed SFS, representing a new type of fibronectin (Fn)-binding protein. The sfs gene was found to be present in all 50 isolates of S. equi subsp. equi tested and in 41 of 48 S. equi subsp. zooepidemicus isolates tested. The sfs gene is down-regulated during growth in vitro compared to fnz, a previously characterized gene encoding an Fn-binding protein from S. equi subsp. zooepidemicus. Sequence comparisons revealed no similarities to previously characterized Fn-binding proteins, but high scores were obtained against collagen. Besides similarity due to the high content of glycine, serine, and proline residues present in both proteins, there was a nine-residue motif present both in collagen and in the Fn-binding domain of SFS. By searching the Oklahoma S. pyogenes database, we found that this motif is also present in a potential cell surface protein from S. pyogenes. Protein SFS was found to inhibit the binding between Fn and collagen in a concentration-dependent way.

Pulsed-field gel electrophoresis and distribution of the genes zag and fnz in isolates of Streptococcus equi

Streptococcus equi subsp. equi and subsp. zooepidemicus are important pathogens of the equine respiratory tract. Isolates of both subspecies were examined by pulsed-field gel electrophoresis (PFGE). With the exception of eight isolates, a unique band pattern was displayed for each of the 48 subsp. zooepidemicus isolates tested. A method to distinguish isolates of the genetically very homogeneous subsp. equi has hitherto not been available, although several methods have been tested. By the use of PFGE, 50 isolates of subsp. equi could be divided into eleven groups, each with a unique pulsotype. In addition, the recently characterised genes encoding the cell-wall proteins ZAG and FNZ of S. equi subsp. zooepidemicus strain ZV were shown by Southern blots to be present in all 98 tested isolates, including the type strains of the two subspecies. Binding assays showed that the expression of the two genes clearly differentiate between the two subspecies.

Streptococcus equi but not Streptococcus zooepidemicus produces potent mitogenic responses from equine peripheral blood mononuclear cells

Streptococcus equi causes equine strangles. The acute disease has many of the hallmarks of an acute response including high fever, elevated plasma fibrinogen and neutrophilia, affects known to be mediated by proinflammatory cytokines. The objective of this study was to screen-culture supernatants from equine clinical isolates of S. equi and S. zooepidemicus for stimulation of mitogenic responses by horse peripheral blood mononuclear cells. Mitogenicity comparable to that of concanavalin A was detected in culture supernatants of S. equi strains but not in those of S. zooepidemicus. Mitogenicity was neutralised by Proteinase K and a post-strangles convalescent serum, and evidence for the presence of both thermo-resistant and thermo-labile mitogenic factors was obtained. Release of proteinaceous immunogenic mitogens in combination with the antiphagocytic protein SeM unique to S. equi may therefore contribute to some of the severe clinical manifestations of acute strangles in the horse.

Affinity purification and characterization of a fibrinogen-binding protein complex which protects mice against lethal challenge with Streptococcus equi subsp. equi

Cell-wall-associated proteins from Streptococcus equi subsp. equi, the causative agent of strangles, were analysed with a view to identifying a potential protective antigen. Preparations of these proteins, isolated from mutanolysin extracts of cell walls, were shown to contain one major high-M(r) protein species (apparent M(r) 220,000 and 550,000 when analysed by SDS-PAGE and gel-filtration chromatography, respectively). The high-M(r) protein bound horse fibrinogen and was purified under non-denaturing conditions using fibrinogen affinity chromatography. The fibrinogen-binding protein (FgBP) reacted with serum taken from horses recovering from strangles and protected mice against lethal challenge from S. equi subsp. equi. The sequence of the corresponding gene (fbp) was determined and shown to encode a mature protein (M(r) 54,597) with predicted coiled-coil structure. An FgBP truncate, lacking the C-terminal cell wall/membrane anchor domain, was overexpressed in and purified from Escherichia coli and was shown to behave in an analogous fashion to the wild-type product in terms of M(r) estimation, fibrinogen binding and seroreactivity.

Serum and mucosal antibody isotype responses to M-like protein (SeM) of Streptococcus equi in convalescent and vaccinated horses

Equine strangles, caused by the clonal pathogen Streptococcus equi, is a source of serious economic loss despite the widespread use of commercial vaccines. The anti-phagocytic 58 kDa M-like protein (SeM) is an important protective antigen. The objective of this study was to define differences, if any, between SeM-specific convalescent serum and mucosal IgA and IgG subisotypes and those induced by vaccination with commercial strangles vaccine. SeM-specific opsonophagocytic IgGb was the predominant serum antibody in horses intramuscularly vaccinated or recently recovered from infection. Infection also induced high levels of specific opsonophagocytic serum IgGa during and shortly after S. equi infection whereas vaccination stimulated only low levels of serum IgGa. Specific serum IgGc and opsonophagocytic IgA were present at very low levels following infection or vaccination. A strong specific mucosal antibody response occurred during the acute and convalescent phases of infection whereas vaccinated horses made no mucosal response. Specific IgGb was generally predominant in nasopharyngeal washings during the acute phase but was replaced by specific IgA during convalescence. SeM-specific mucosal IgGa and IgG(T) but not IgGc were detected only during the acute and early convalescent phase. The results therefore indicate that vaccination, although inducing SeM-specific serum isotype responses qualitatively and quantitatively similar to those seen in convalescence, did not induce mucosal responses. This suggests that mucosal immunity may be important in acquired resistance to strangles.

Isolation of Streptococcus equi subsp. equi from thoroughbred horses in a racehorse-breeding area of Japan

For determination whether strangles has invaded the Hidaka district of Hokkaido, the main racehorse-breeding area of Japan, a epizootiological survey with bacterial isolation was carried out during the breeding season in 1995. Streptococcus equi subsp. equi, which is the causative agent of strangles, was isolated from two Thoroughbred horses with submandibular lymphadenitis. Isolates were identified by serological grouping, biochemical tests and analysis of cell surface proteins by Western immunoblotting. Through this survey, it revealed that S. equi subsp. equi has invaded the Hidaka district and that strangles has become prevalent in racehorse-breeding farms in this area.

Comparison of the sequences and functions of Streptococcus equi M-like proteins SeM and SzPSe

Streptococcus equi (Streptococcus equi subsp. equi), a Lancefield group C streptococcus, causes strangles, a highly contagious purulent lymphadenitis and pharyngitis of members of the family Equidae. The antiphagocytic 58-kDa M-like protein SeM is a major virulence factor and protective antigen. The amino acid sequence and structure of SeM has been determined and compared to that of a second, 40-kDa M-like protein (SzPSe) of S. equi and to those of other streptococcal proteins. Both SeM and SzPSe are mainly alpha-helical fibrillar molecules with no homology other than that between their signal and membrane anchor sequences and are only distantly related to other streptococcal M and M-like proteins. The sequence of SzPSe indicates that it is an allele of SzP that encodes the variable protective M-like and typing antigens of S. zooepidemicus (S. equi subsp. zooepidemicus). SeM is opsonogenic for S. equi but not for the closely related S. zooepidemicus, whereas SzPSe is strongly opsonogenic for S. zooepidemicus but not for S. equi. Both proteins bind equine fibrinogen. SeM and SzPSe proteins from temporally and geographically separated isolates of S. equi are identical in size. The results taken together support previous evidence that S. equi is a clonal pathogen originating from an ancestral strain of S. zooepidemicus. We postulate that acquisition of SeM synthesis was a key element in the success of the clone because of its effect in enhancing resistance to phagocytosis and because protective immunity entails a requirement for SeM-specific antibody.

M proteins of group C streptococci isolated from patients with acute pharyngitis

We studied 15 strains of group C (Streptococcus equi subsp. equisimilis) [corrected] isolated from the throats of college students with acute pharyngitis and 5 strains isolated from patients with noninfectious problems. Nineteen of the 20 strains resisted phagocytic killing during incubation in normal human blood, suggesting that they might express M proteins. Genomic DNA from all 20 strains hybridized with a probe corresponding to the carboxyterminal one-third of the group A M-protein gene emm24, a region that is highly conserved among M proteins of group A and group G streptococci. The DNA sequences of the N-terminal (variable) regions of the M-protein-encoding genes from two disease-associated group C isolates and one control isolate were determined. The predicted amino acid sequences of the two pharyngitis strains were identical and were 88% homologous to the amino acid sequence of a group G M-protein gene. The predicted terminal amino acid sequence of the control strain does not correspond to any such sequences in the GenBank database. All three strains studied possess the conserved region domain common to class I group A M-protein types epidemiologically associated with rheumatic fever. These studies demonstrate the presence of M proteins in strains of S. equi subsp. equisimilis [corrected] isolated in cases of endemically occurring acute pharyngitis. Certain of these proteins are similar to those of group G streptococci, while others may represent new M types. The similarity in structure and function between M proteins of nonrheumatogenic serogroups and those of rheumatogenic group A streptococci suggests that factors other than or in addition to M protein per se are likely involved in the pathogenesis of rheumatic fever.

An assessment of mucosal immunisation in protection against Streptococcus equi ('Strangles') infections in horses

The ability of mucosally administered antigen to provide protection against Streptococcus equi ('Strangles') infections in horses was examined. First, an enzyme linked immunosorbent assay (ELISA) was developed to detect the immune status of horses to S. equi. This assay was used to select Strangles-naive horses for the study and also to monitor their response to immunisation. Potential vaccine candidates were: (a) orally administered paraformaldehyde killed S. equi; (b) intraperitoneally (IP) administered paraformaldehyde killed S. equi in a non-inflammatory adjuvant; (c) orally administered live avirulent S. equi; (d) orally administered microencapsulated streptococcal M protein. The latter three preparations were first assessed in a rat model, using rate of lung bacterial clearance following intratracheal inoculation of live virulent bacteria as an indication of efficacy. Candidates (a) and (b) were then assessed in an equine model. IP immunisation of horses was shown to effectively induce production of specific antibody in mucosal and systemic sites. Four weeks after initial immunisation, horses were challenged intranasally with live virulent S. equi. Both groups of immunised horses demonstrated partial protection following vaccination. Of the IP immunised horses, only two out of four developed clinical signs of Strangles following live challenge. The orally immunised horses all developed submandibular abscesses containing S. equi. However, none of the immunised horses became as ill as the control horses in terms of fever, anorexia, loss of condition and general malaise.

Cloning and sequence analysis of a protective M-like protein gene from Streptococcus equi subsp. zooepidemicus

Streptococcus equi subsp. zooepidemicus, a Lancefield group C streptococcus, is a frequently isolated opportunist pathogen from a variety of animal hosts, including the horse. Previous studies have indicated that equine strains carry antigens with characteristics of the antiphagocytic M proteins on the Lancefield groups A and G streptococci. We have cloned a protective M-like protein gene (SzPW60) of an equine strain of S. equi subsp. zooepidemicus W60 and determined its sequence. This gene encodes a protein with a molecular weight of 40,123 which protects mice against subsp. zooepidemicus but not subsp. equi, stimulates antibodies which opsonize subsp. zooepidemicus but not equi, and reacts with antiserum to the protein of the parent strain. The predicted amino acid structure shows significant homology with the carboxy termini of groups A and G M proteins but no other homology. The M-like protein, although showing an extensive region of alpha helix, lacks the A, B, and C repeats found in group A M proteins and has a shorter signal sequence. A proline-rich region upstream from the LPSTGE motif contains 20 repeats of the tetrapeptide PEPK. The presence of this repeat region may account for the slow migration of the M-like protein in sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Genetic structure of populations of beta-haemolytic Lancefield group C streptococci from horses and their association with disease

The genetic structure of beta-haemolytic Lancefield group C streptococci isolated from horses in Australia was examined by multilocus enzyme electrophoresis. The 249 isolates comprised 70 classified phenotypically as Streptococcus equi subspecies equi, 177 classified as S equi subspecies zooepidemicus and two which were unclassifiable. Forty-one electrophoretic types were identified which could be classified into three major clusters, A, B and C. Of the isolates, 178 fell into cluster B (types 4 to 22) and lay within a genetic distance of 0.36. Sixty-nine of the 70 S equi subspecies equi isolates fell into type 12, which suggests that they were members of a single clone, and the isolates from abscesses were significantly more likely to belong to type 12 than those from horses with no clinical signs (P < 0.001). There were no other significant associations between electrophoretic types or clusters and the isolation of the organism from particular sites. These data suggested that S zooepidemicus may be the archetypal species from which the clone designated subspecies equi has been derived. If isolates of the subspecies equi from other geographical regions also prove to be members of electrophoretic type 12, this hypothesis would be strengthened.

Inhibition of C3 deposition on Streptococcus equi subsp. equi by M protein: a mechanism for survival in equine blood

The effect of the M protein of Streptococcus equi subsp. equi on complement deposition, complement degradation, and bacterial survival in equine whole blood was examined in vitro. Preincubation of bacteria with rabbit M protein-specific immunoglobulin G (IgG) inhibited the survival of the M+ strain in whole blood by 20-fold (P < 0.01). In addition, preincubation of bacteria with M protein-specific F(ab')2 fragments inhibited the survival of M+ cells in whole blood by 3.8-fold (P < 0.01). In the absence of specific antibody, an M+ strain (CF32) of S. equi subsp. equi survived 100-fold better in whole blood than an M- isolate (strain 19) (P < 0.01). Complement inactivation by cobra venom factor significantly enhanced the ability of the M- and M+ strains of S. equi subsp. equi to survive in whole blood, the latter in the presence or absence of M protein-specific IgG. The major opsonic forms of C3, C3b and iC3b, were present on both M- and M+ cells after opsonization in nonimmune plasma. However, colloidal gold staining indicated that the M- strain bound four times as much C3 as the M+ strain (P < 0.02) and that preincubation of the M+ strain with M protein-specific IgG or F(ab')2 fragments also enhanced the amount of C3 deposited by a factor of 4 (P < 0.02). Therefore, at least part of the M protein's ability to enhance bacterial survival in equine whole blood may be related to its ability to interfere with the deposition of equine complement on the bacterial surface.

The protective M proteins of the equine group C streptococci

The group C streptococci are the most commonly isolated bacteria from disease states in the horse. Important virulence factors of S. equi and S. zooepidemicus are the hyaluronic acid capsule and the antiphagocytic fibrillar M protein located on the surface of the cell wall and extending into and through the capsule. The hyaluronic acid capsule is non-antigenic and so is not involved in protective immunity. The M protein, a superantigen, elicits very strong B and T cell responses that may result in protective immunity mediated by opsonic antibodies in plasma and by locally synthesized IgG and IgA on the pharyngeal mucosa. However, vaccines based on acid or mutanolysin extracted M protein do not confer a high level of protection against field exposure. Protective antibodies to S. equi or S. zooepidemicus can in part be assayed by the bactericidal test that measures opsonization for equine neutrophils. A mouse-challenge model has also been used to test immunizing potency of streptococcal extracts and in a passive protection test for protective antibody. There is as yet no means of distinguishing protective opsonic or mucosal antibodies from other antibodies produced against the many epitopes on the M molecule.

Strangles

The etiology, epizootiology, pathogenesis, and clinical presentation of strangles are described. Streptococcus equi, the causative organism, is highly host-adapted to Equidae and shows no antigenic variation. Protective immunity apparently is mediated by a combination of serum opsonic and nasopharyngeal mucosal humoral responses. Vaccines based on M protein or inactivated bacterial suspensions may reduce the clinical attack rate by 50%, a level of protection much lower than that produced during recovery from strangles.