Novel Streptococcus equi strains causing strangles outbreaks in Arabian horses in Egypt

Abstract

Strangles displays a major challenge to veterinary medicine worldwide. However, no data on Streptococcus equi subsp. equi (S. equi) M protein alleles have been reported so far from Arabian horses. We report here for the first time the S. equi SeM alleles causing strangles in Arabian horses, and the associated risk factors for the disease. Duplicate samples from one hundred Arabian horses with acute strangles in confirmed outbreaks and sporadic cases were analysed by phenotypic methods and multiplex polymerase chain reaction (PCR) targeting streptokinase precursor, seeI and sodA genes. PCR and sequencing of S. equi SeM gene were employed for strains typing, and the four superantigens were determined among the allelic variants. Directsample PCR confirmed and highly positively correlated (r = .85) with the phenotypic results, and detected S. equi in five samples more than the conventional culture. A combination of multiplex PCR from samples and culture could successfully identify S. equi (92%), S. zooepidemicus (5%) and S. equisimilis (3%). SeM typing demonstrated five SeM alleles, including four previously unidentified alleles that were deposited in the PubMLST-SeM database. SeM-139 and SeM-141 are related to some strains that were recently recovered from donkeys in China. SeM-140 and SeM-199 are related to a group of alleles from horses in Europe. Variation in the presence of seeM, seeH and seeL superantigens was found across the four novel alleles without interference with the severity of strangles and clinical presentation seen in different outbreaks. Horse age was the most important factor in developing strangles, followed by seasonality and the diagnosis of strangles in the previous year. These new findings comprise a significant contribution to the horse industry through the identification of novel S. equi SeM types that may bolster measures for strangles control as the identified SeM alleles will certainly help in the development of SeM-containing vaccine.

An Outbreak of Strangles in Arabian Horses in Saudi Arabia: Epidemiology, Clinical Signs and Treatment Outcomes

“…Strangles is a highly infectious upper respiratory disease affecting horses of all ages, but most commonly in horses less than five years of age, especially in weanling foals or yearlings (Khoo et al 2011;Tartor et al 2020). The morbidity and mortality rates in this outbreak were significantly higher (P≤0.05) in young horses less than 5 years than those over 5 years.…”

Conservation of vaccine antigen sequences encoded by sequenced strains ofStreptococcus equisubsp.equi

“…Despite the majority of the isolates originating from a small number of countries, the diversity identified within the encoded SeM protein was very high. The SeM protein is an important immunodominant component of S equi and the selective pressure exerted on this protein is evident in the variation in amino acid sequences encoded by different strains 17,19,29–31,46 . This selective pressure indicates the potential for SeM‐containing vaccines to exert a protective effect 47,48 .…”

“…The SeM protein is an important immunodominant component of S equi and the selective pressure exerted on this protein is evident in the variation in amino acid sequences encoded by different strains. 17 , 19 , 29 , 30 , 31 , 46 This selective pressure indicates the potential for SeM‐containing vaccines to exert a protective effect. 47 , 48 However, sequencing of the SeM gene has shown that variants emerge over time within outbreaks and individual animals, suggesting that variation may permit evasion, at least in part, of an immune response directed only at this protein.…”

“…However, genes encoding antigenic proteins may be subject to much greater selective pressure, leading to much more rapid genetic change. For example, genetic variation in the gene encoding the immunodominant SeM protein has been exploited in strain typing schemes, with 242 alleles described to date 11,17,19,29–31 . The SeM protein is used within several cell extract and live vaccines that target S equi 32–36 .…”

“…For example, genetic variation in the gene encoding the immunodominant SeM protein has been exploited in strain typing schemes, with 242 alleles described to date. 11 , 17 , 19 , 29 , 30 , 31 The SeM protein is used within several cell extract and live vaccines that target S equi . 32 , 33 , 34 , 35 , 36 However, the multicomponent fusion protein vaccine Strangvac does not contain SeM and instead uses eight different S equi proteins, the diversity of which has not been described previously.…”