Decreased Clinical Severity of Strangles in Weanlings Associated with Restricted Seroconversion to OptimizedStreptococcus equisspequiAssays

Abstract

Background Streptococcus equi ssp. equi causes characteristic clinical signs that are most severe in young horses, including fever, purulent nasal discharge, and lymph node abscessation in the head region.Hypothesis/ObjectivesClinical, serologic, and microbiologic factors related to unexpectedly mild disease severity in a natural outbreak of strangles in immunologically naïve weanlings were investigated.AnimalsOne‐hundred and twelve warmblood weanlings.MethodsProspective longitudinal observational study of a natural outbreak of strangles. The entire cohort was examined at the peak of the outbreak by deep nasal swabs for culture and quantitative PCR (qPCR) for the presence of S. equi and clinically and serologically in a sequential manner by an optimized ELISA from the index case throughout the outbreak until resolution. Descriptive statistics were calculated and comparisons made using a nondirectional Wilcoxon signed‐rank test.ResultsOutbreak morbidity was 53%, with 9 of 14 horses culture positive and 26 of 53 horses qPCR positive for S. equi lacking clinical signs characteristic of strangles. By resolution, 91 of 112 had seroconverted to Antigen A by ELISA but seroconversion to antigen C (part of the SeM protein) was minimal. Sequencing of the isolates detected no alterations in the SeM protein, but identified a 61 bp deletion in the gene SEQ_0402.Conclusions and Clinical ImportanceAbsence of clinical signs alone in naïve horses may be an insufficient criterion to release horses from strangles quarantine measures. Restricted seroconversion to antigen C may have been associated with decreased clinical severity. The role of a minor gene deletion in SEQ_0402 in the virulence of S. equi warrants further investigation.

Influence of penicillin treatment of horses with strangles on seropositivity toStreptococcus equissp.equi‐specific antibodies

“…Samplings T27 and T46 took place during acute clinical illness, approximately 4 and 7 weeks after the index case, and samplings T123‐T313 were conducted after clinical signs had resolved (approximately 4, 6, and 10 months after the index case; Figure ). On each sampling occasion, all horses had complete physical examinations with clinical scoring assigned according to a modification of a previous scoring scheme (Table ). Nasopharyngeal lavages were performed on all horses on T27, and T123‐T313, and on 2 horses on T46 (the 2/41 horses that were S. equi negative on the first sampling occasion).…”

“…Penicillin is the drug of choice for treatment of streptococcal infections in horses . However, there is controversy about whether to treat acutely infected horses with antibiotics or not, because most cases of strangles resolve without specific treatment . Nonetheless, recent reports suggests that, in some practice settings, the majority (78%) of horses with strangles receive some form of antibiotic treatment .…”

Markers of long term silent carriers of Streptococcus equi ssp. equi in horses

“…In the autumn of 2014, strangles occurred in the foals shortly after weaning. 8 Isolation of clinically affected animals was not instituted and the outbreak was allowed to resolve over time with only supportive care. None of the foals (outbreak A, weanlings) were treated with antibiotics during clinical strangles and no resident brood mares (outbreak A, broodmares) were affected.…”

‘Subtle strangles’ – the more elusive signs of the disease

“…This test is on blood and it looks to see if the body has had an immune response to the strangles young animals are usually naïve (have not been exposed to the bacteria and subsequently have not developed any immunity) and therefore normally display more severe disease. Reasons for this were hypothesised as being due to a less virulent strain (Tscheschlok et al, 2018). More research is currently being conducted to compare the different strain types with the severity of clinical signs.…”