Intramuscular vaccination with Strangvac is safe and induces protection against equine strangles caused by Streptococcus equi

Untangling the stranglehold through mathematical modelling of Streptococcus equi subspecies equi transmission

Strangles, a disease caused by infection with Streptococccus equi subspecies equi (S. equi), is endemic worldwide and one of the most frequently diagnosed infectious diseases of horses. Recent work has improved our knowledge of key parameters of transmission dynamics, but important knowledge gaps remain. Our aim was to apply mathematical modelling of S. equi transmission dynamics to prioritise future research areas, and add precision to estimates of transmission parameters thereby improving understanding of S. equi epidemiology and quantifying the control effort required. A compartmental deterministic model was constructed. Parameter values were estimated from current literature wherever possible. We assessed the sensitivity of estimates for the basic reproduction number on the population scale to varying assumptions for the unknown or uncertain parameters of: (mean) duration of carriership (1∕γC), relative infectiousness of carriers (f), proportion of infections that result in carriership (p), and (mean) duration of immunity after natural infection (1∕γR). Available incidence and (sero-)prevalence data were compared to model outputs to improve point estimates and ranges for these currently unknown or uncertain transmission-related parameters. The required vaccination coverage of an ideal vaccine to prevent major outbreaks under a range of control scenarios was estimated, and compared available data on existing vaccines. The relative infectiousness of carriers (as compared to acutely ill horses) and the duration of carriership were identified as key knowledge gaps. Deterministic compartmental simulations, combined with seroprevalence data, suggest that 0.05 Collapse

Key Words

• Epidemiology
• Infectious disease modelling
• Streptococcus equi

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MESH Headings

• Animals
• Horses
• Streptococcal Infections/veterinary
• Streptococcal Infections/epidemiology
• Streptococcal Infections/transmission
• Horse Diseases/transmission
• Horse Diseases/epidemiology
• Horse Diseases/microbiology
• Models, Theoretical
• Prevalence
• Incidence
• Streptococcus equi
• Models, Biological
• Streptococcus

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Affiliation(s)

R M A C Houben Department of Clinical Sciences, faculty of Veterinary medicine, Utrecht University, the Netherlands.
J R Newton Equine Infectious Disease Surveillance (EIDS), Department of Veterinary Medicine, Cambridge, UK
C van Maanen Royal GD, Deventer, the Netherlands
A S Waller Intervacc AB, Stockholm, Sweden; Department of Biomedical Science and Veterinary Public Health, Swedish University of Agricultural Sciences, Uppsala, Sweden
M M Sloet van Oldruitenborgh-Oosterbaan Department of Clinical Sciences, faculty of Veterinary medicine, Utrecht University, the Netherlands
J A P Heesterbeek Department of Population Health Sciences, faculty of Veterinary Medicine, Utrecht University, the Netherlands

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Development of a Real-Time Recombinase-Aided Amplification Method for the Rapid Detection of Streptococcus equi subsp. equi

Streptococcus equi subspecies equi (S. equi) is the causative pathogen of strangles in horses, donkeys, and other equine animals. Strangles has spread globally and causes significant losses to the horse industry. In response to the urgent need for effective disease control, this study introduces a novel nucleic acid diagnostic method known as a real-time recombinase-assisted amplification (RAA) assay, developed based on the eqbE gene, for the rapid detection of S. equi nucleic acid. The real-time RAA method employs specifically designed probes and primers targeting the eqbE gene, enhancing the overall specificity and sensitivity of the detection. After efficiency optimization, this real-time RAA method can detect 10 or more copies of nucleic acid within 20 min. The method demonstrates high specificity for S. equi and does not cross-react with other clinically relevant pathogens. Real-time RAA diagnostic performance was evaluated using 98 nasal swab samples collected from horses and compared with the real-time PCR detection method. Results revealed that 64 and 65 samples tested positive for S. equi using real-time RAA and real-time PCR, respectively. The overall agreement between the two assays was 96.94% (95/98), with a kappa value of 0.931 (p < 0.001). Further linear regression analysis indicated a significant correlation in the detection results between the two methods (R2 = 0.9012, p < 0.0001), suggesting that the real-time RAA assay exhibits a detection performance comparable to that of real-time PCR. In conclusion, the real-time RAA assay developed here serves as a highly specific and reliable diagnostic tool for the detection of S. equi in equine samples, offering a potential alternative to real-time PCR methods. In conclusion, the real-time RAA nucleic acid diagnostic method, based on the eqbE gene, offers rapid and accurate diagnosis of S. equi, with the added advantage of minimal equipment requirements, thus contributing to the efficient detection of strangles in horses.

Survival of Streptococcus equi subsp. zooepidemicus on environmental samples is affected by material type and incubation temperature

Streptococcus equi subsp. zooepidemicus is an opportunistic pathogen associated with disease in a range of domestic and wild animals. Despite its importance, very limited data is available on its survival and persistence on the environment. The goal of this study was to evaluate survival of S. zooepidemicus under ideal culture conditions and farm-like setting, in various surface types. Rubber, plastic, wood, and concrete samples were sterilized and inoculated with 109 CFU/mL of S. zooepidemicus with or without feces, and cultured under ideal conditions (37 °C, 5% CO2) or farm-like settings (20oC on air) for a maximum period of 25 days (n = 3/material/environment/feces-group/time-point). Under ideal conditions without feces, the bacterium survived for up to 17 days on plastic and rubber surfaces, 4 days on wood and less than 1 day on concrete (P < 0.05 between materials). Samples under ideal conditions with feces and farm-like settings without feces were negative by day 1 post-inoculation, regardless of the surface material used. Wood and concrete allowed S. zooepidemicus persistence for up to 3 days under farm-like settings when feces were present. This data suggests that environmental persistence of S. zooepidemicus is affected by surface type and incubation temperature.

Streptococcus equi Subspecies equi

Strangles, caused by the bacteria Streptococcus equi subsp equi, is a highly contagious disease of equids classically characterized by a high fever and enlarged lymph nodes of the head. Diagnostic sampling depends on the stage of the disease. The goal of treating strangles is to control transmission and to eliminate infection while providing future host immunity. Daily temperature checking and isolation of febrile horses is the key to controlling outbreaks. Eradication of this disease will not be possible until S equi carriers are eliminated from the equine population.

Voluntary Biosurveillance of Streptococcus equi Subsp. equi in Nasal Secretions of 9409 Equids with Upper Airway Infection in the USA

This study aimed to describe selected epidemiological aspects of horses with acute onset of fever and respiratory signs testing qPCR-positive for S. equi and to determine the effect of vaccination against S. equi on qPCR status. Horses with acute onset of fever and respiratory signs from all regions of the United States were included in a voluntary biosurveillance program from 2008 to 2020 and nasal secretions were tested via qPCR for S. equi and common respiratory viruses. A total of 715/9409 equids (7.6%) tested qPCR-positive for S. equi, with 226 horses showing coinfections with EIV, EHV-1, EHV-4, and ERBV. The median age for the S. equi qPCR-positive horses was 8 ± 4 years and there was significant difference when compared to the median age of the S. equi qPCR-negative horses (6 ± 2 years; p = 0.004). Quarter Horse, Warmblood, and Thoroughbred were the more frequent breed in this horse population, and these breeds were more likely to test qPCR-positive for S. equi compared to other breeds. There was not statistical difference for sex between S. equi qPCR-positive and qPCR-negative horses. Horses used for competition and ranch/farm use were more likely to test qPCR-positive for S. equi (p = 0.006). Horses that tested S. equi qPCR-positive were more likely to display nasal discharge, fever, lethargy, anorexia, and ocular discharge compared to horses that tested S. equi qPCR-negative (p = 0.001). Vaccination against S. equi was associated with a lower frequency of S. equi qPCR-positive status.

Molecular Features and Antimicrobial Susceptibilities of Streptococcus equi ssp. equi Isolates from Strangles Cases in Indonesia

Strangles, caused by Streptococcus equi ssp. equi (S. equi equi), is a highly infectious and frequent disease of equines worldwide. No data are available regarding the molecular epidemiology of strangles in Indonesia. This study aimed to characterize S. equi equi isolates obtained from suspected strangles cases in Indonesia in 2018. Isolates originated from seven diseased horses on four different farms located in three provinces of Indonesia. Whole genome sequences of these isolates were determined and used for seM typing, multilocus sequence typing (MLST), and core genome MLS typing (cgMLST). Genomes were also screened for known antimicrobial resistance genes and genes encoding for the recombinant antigens used in the commercial Strangvac^® subunit vaccine. All seven S. equi equi isolates from Indonesia belonged to ST179 and carried seM allele 166. Isolates differed from each other by only 2 to 14 cgSNPs and built an exclusive sub-cluster within the Bayesian Analysis of Population Structure (BAPS) cluster 2 (BAPS-2) of the S. equi equi cgMLST scheme. All isolates revealed predicted amino acid sequence identity to seven and high similarity to one of the eight antigen fragments contained in Strangvac^®. Furthermore, all isolates were susceptible to beta-lactam antibiotics penicillin G, ampicillin, and ceftiofur. Our data suggest that the horses from this study were affected by strains of the same novel sublineage within globally distributed BAPS-2 of S. equi equi. Nevertheless, penicillin G can be used as a first-choice antibiotic against these strains and Strangvac^® may also be protective against Indonesian strains.

Conservation of vaccine antigen sequences encoded by sequenced strains of Streptococcus equi subsp. equi

BACKGROUND

Streptococcus equi subspecies equi (S equi) is the cause of Strangles, one of the most prevalent diseases of horses worldwide. Variation within the immunodominant SeM protein has been documented, but a new eight-component fusion protein vaccine, Strangvac, does not contain live S equi or SeM and conservation of the antigens it contains have not been reported.

OBJECTIVE

To define the diversity of the eight Strangvac antigens across a diverse S equi population.

STUDY DESIGN

Genomic description.

METHODS

Antigen sequences from the genomes of 759 S equi isolates from 19 countries, recovered between 1955 and 2018, were analysed. Predicted amino acid sequences in the antigen fragments of SEQ0256(Eq5), SEQ0402(Eq8), SEQ0721(EAG), SEQ0855(SclF), SEQ0935(CNE), SEQ0999(IdeE), SEQ1817(SclI) and SEQ2101(SclC) in Strangvac and SeM were extracted from the 759 assembled genomes and compared.

RESULTS

The predicted amino acid sequences of SclC, SclI and IdeE were identical across all 759 genomes. CNE was truncated in the genome of five (0.7%) isolates. SclF was absent from one genome and another encoded a single amino acid substitution. EAG was truncated in two genomes. Eq5 was truncated in four genomes and 123 genomes encoded a single amino acid substitution. Eq8 was truncated in three genomes, one genome encoded four amino acid substitutions and 398 genomes encoded a single amino acid substitution at the final amino acid of the Eq8 antigen fragment. Therefore, at least 1579 (99.9%) of 1580 amino acids in Strangvac were identical in 743 (97.9%) genomes, and all genomes encoded identical amino acid sequences for at least six of the eight Strangvac antigens.

MAIN LIMITATIONS

Three hundred and seven (40.4%) isolates in this study were recovered from horses in the UK.

CONCLUSIONS

The predicted amino acid sequences of antigens in Strangvac were highly conserved across this collection of S equi.

Estimation of the basic reproduction number for Streptococcus equi spp equi outbreaks by meta-analysis of strangles outbreak reports

BACKGROUND

Streptococcus equi spp equi (S. equi), the cause of strangles in horses, is considered a highly contagious pathogen affecting equines and the equine industry worldwide. Fundamental epidemiological characteristics of outbreaks, such as the basic reproduction number (R₀ ), are not well described.

OBJECTIVES

Estimate R₀ for S. equi in equine populations from outbreak data.

STUDY DESIGN

Systematic review and meta-analysis of published and unpublished data.

METHODS

A literature search for outbreak reports was carried out. Depending on data available in the reports, the early epidemic growth rate or final attack rate (AR) approach was used to estimate the basic reproduction number for that outbreak. Other recorded outbreak characteristics were the type of housing (group vs individual). An overall estimate for R₀ was computed by meta-analysis.

RESULTS

Data from 8 outbreaks were extracted from peer-reviewed publications. Data from two additional, non-published outbreaks was also included in the meta-analysis. A conservative estimate for R₀ was 2.2 (95% CI 1.9- 2.5). A less conservative estimate, including outbreaks with a 100% AR for which a lower limit R₀ was estimated, was 2.7 (95% CI 2.1- 3.3).

MAIN LIMITATIONS

Few papers describing longitudinal incidence data were found so most estimates were based on the outbreaks' final size. Several outbreaks had a 100% attack rate and could therefore only be included as a lower limit estimate in the meta-analysis. The reported result therefore may be an underestimation.

CONCLUSIONS

This estimate for R₀ for S. equi informs parameters for future mathematical modelling, quantifies desired preventive vaccine coverage and helps evaluate the effect of prevention strategies through future modelling studies.

[Management and hygiene measures during an outbreak of herpes, influenza, strangles or infections with multidrug resistant bacteria]

The aim of this review is to describe general guidelines of hygiene measures in the horse stable as well as to provide current recommendations for an outbreak of a common infectious disease. General cleanliness, hand hygiene, avoidance of stress, regular deworming, and vaccinations belong to the basic hygiene measures in a horse herd. All new or returning equids should be submitted to a quarantine period as an important prevention measure. Repeated washing and disinfection of hands may prevent spreading of infectious agents to people and horses.The conception of a hygiene plan, including general biosecurity procedures and standard operating procedures in a case of an outbreak of an infectious disease, zoonosis, or colonization with multi-resistant bacteria is strongly recommended. As soon as the disease is suspected, extended hygiene measures including protective clothing, cleaning, disinfection, and isolation of potentially infected animals should be implemented. Prompt confirmation of the causative agent by examination of appropriate samples is crucial. It is important to adjust all safety measures based on the contagious nature of the respective pathogen and its major transmission routes. Apart from a lock-down of the stable, clinic or show grounds, the segregation of horses plays an important role. Implementation of the "traffic light system" is recommended. In this, the red group ("infected") include animals with clinical signs of the disease or that have been tested positive. All horses with possible pathogen contact should be allocated to a yellow group ("suspected") and regularly controlled for the signs of infection and fever. Clinically normal horses without contact to the infected animals belong to the green group ("healthy"). A change of protective clothing and an extensive disinfection should be performed when moving between the groups.The extended hygiene measures are to be maintained until all animals have been tested negative or fail to exhibit clinical signs of the disease for a certain time period.

Enhanced immune effects and protection conferred by simultaneously targeting GAPDH, SeM, and EAG of S. equi via TLR4

Strangles, which is caused by Streptococcus equi subspecies equi, is one of the most prevalent equine infectious diseases and poses heavy economic losses worldwide. Although various vaccines have been used for decades, they seemed to be sub-optimal to demonstrate effective protection, and the antigen component of vaccines against S. equi remains to be optimized. In the present study, three target antigens (M-like protein, α2-macroglobulin and IgG-binding protein, and glyceraldehyde-3-phosphate dehydrogenase) were selected and expressed. Mice were immunized and challenged, and their immune response and efficacy were evaluated. The results revealed that this optimized multi-antigen treatment elicited a high expression level of T-cell receptor, major histocompatibility complex I, toll-like receptor TLR-4, and increased specific antibody. In addition, the challenge experiment showed an evidently improved protection efficacy. The present work demonstrated that these three proteins might be used as a promising multicomponent subunit vaccine candidate against S. equi infection.

Nasopharyngeal Microbiomes in Donkeys Shedding Streptococcus equi Subspecies equi in Comparison to Healthy Donkeys

Streptococcus equi subsp. equi (S. equi) is the pathogen causing strangles, a highly infectious disease that can affect equids including donkeys of all ages. It can persistently colonize the upper respiratory tract of animals asymptomatically for years, which serves as a source of infection. Several strangles outbreaks have been reported in the donkey industry in China in the last few years and pose a great threat to health, production, and the welfare of donkeys. Nasopharyngeal swab samples for culture and PCR are used widely in strangles diagnosis. Additionally, microbiomes within and on the body are essential to host homoeostasis and health. Therefore, the microbiome of the equid nasopharynx may provide insights into the health of the upper respiratory tract in animals. There has been no study investigating the nasopharyngeal microbiome in healthy donkeys, nor in donkeys shedding S. equi. This study aimed to compare nasopharyngeal microbiomes in healthy and carrier donkeys using 16S rRNA gene sequencing. Nasopharyngeal samples were obtained from 16 donkeys recovered from strangles (group S) and 14 healthy donkeys with no history of strangles exposure (group H). Of those sampled, 7 donkeys were determined to be carriers with positive PCR and culture results in group S. In group H, all 14 donkeys were considered free of strangles based on the history of negative exposure, negative results of PCR and culture. Samples from these 21 donkeys were used for microbial analysis. The nasopharyngeal microbiome composition was compared between the two groups. At the phylum level, relative abundance of Proteobacteria was predominantly higher in the S. equi carrier donkeys than in healthy donkeys (P < 0.01), while Firmicutes and Actinobacteria were significantly less abundant in the S. equi carrier donkeys than in healthy donkeys (P < 0.05). At the genus level, Nicoletella was detected in the upper respiratory tract of donkeys for the first time and dominated in carrier donkeys. It is suspected to suppress other normal flora of URT microbiota including Streptococcus spp., Staphylococcus spp., and Corynebacterium spp. We concluded that the nasopharyngeal microbiome in S. equi carrier donkeys still exhibited microbial dysbiosis, which might predispose them to other airway diseases.

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.

Optimized GAPDH-truncated immunogen of Streptococcus equi elicits an enhanced immune response and provides effective protection in a mouse model

Strangles is an acute and frequently diagnosed infectious disease caused by Streptococcus equi subsp. equi. Infection with this pathogen can cause grave losses to the equine industry. The present work investigates glyceraldehyde-3-phosphate dehydrogenase (GAPDH), an important surface-localized virulence factor of S. equi, to determine whether it could be developed into an efficacious and suitable subunit vaccine against strangles. Two different recombinant fragments of S. equi GAPDH, namely, GAPDH-L and GAPDH-S, were constructed and expressed. Further, the antigenicity and immunogenicity of these two recombinant proteins were compared and evaluated in a mouse model. Our results revealed that immune responses were efficiently induced by the proteins in immunized mice. Remarkably, higher survival rates and significantly lower bacterial loads in the lung, liver, kidney, and spleen were observed in the GAPDH-S group compared with the GAPDH-L group after challenge with S. equi. High levels of specific antibodies, elevated antibody titers, and increased proportions of CD8 + T cells further indicated that GAPDH-S elicited better humoral and cellular immune responses than GAPDH-L. Furthermore, the induction of TCR, TLR-2, TLR-3, and TLR-4 significantly increased in the GAPDH-S group compared with those in the GAPDH-L and negative control groups. In summary, our results indicate that the optimized recombinant protein GAPDH-S is a promising candidate construct that may be further developed into a multivalent subunit vaccine for strangles.

Vaccination of yearling horses against poly-N-acetyl glucosamine fails to protect against infection with Streptococcus equi subspecies equi

Strangles is a common disease of horses with worldwide distribution caused by the bacterium Streptococcus equi subspecies equi (SEE). Although vaccines against strangles are available commercially, these products have limitations in safety and efficacy. The microbial surface antigen β 1→6 poly-N-acetylglucosamine (PNAG) is expressed by SEE. Here we show that intramuscular (IM) injection alone or a combination of IM plus intranasal (IN) immunization generated antibodies to PNAG that functioned to deposit complement and mediate opsonophagocytic killing of SEE ex vivo. However, immunization strategies targeting PNAG either by either IM only injection or a combination of IM and IN immunizations failed to protect yearling horses against infection following contact with infected horses in an experimental setting. We speculate that a protective vaccine against strangles will require additional components, such as those targeting SEE enzymes that degrade or inactivate equine IgG.