Markers of long term silent carriers of Streptococcus equi ssp. equi in horses.
Authors: Pringle John, Venner Monica, Tscheschlok Lisa, Waller Andrew S, Riihimäki Miia
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Identifying Silent Carriers of Strangles Streptococcus equi continues to spread through horse populations largely because silent carriers—animals harbouring the bacterium without clinical signs—are virtually impossible to identify through routine assessment. This prospective study tracked 235 horses across three separate strangles outbreaks (6 months to 2 years post-infection) to determine whether physical examination findings, inflammatory markers, or serological responses could distinguish carriers from non-infected herd-mates. Nasopharyngeal lavage and guttural pouch endoscopy cultures identified carriers, whilst serology was performed using enhanced ELISA against S. equi antigens A and C; critically, 3 of 12 culture-positive carriers were seronegative, demonstrating incomplete antibody responses. Across all outbreaks except 6-month-old weanlings in one group, no significant differences emerged between carriers and non-carriers in clinical signs, white blood cell counts, serum amyloid A, or antibody titres (P = 0.06–1.0). For equine practitioners, this underscores that neither clinical examination nor single-point serology can reliably identify silent carriers within mixed populations, making carrier identification dependent on direct culture or molecular testing of nasopharyngeal and guttural pouch samples—a critical consideration when designing biosecurity protocols and deciding which horses require isolation following strangles exposure.
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Practical Takeaways
- •Silent carriers of strangles are indistinguishable from non-infected herd-mates on clinical grounds—culture or qPCR from nasopharyngeal lavage/guttural pouch lavage is essential for identification
- •Antibody testing alone will miss infected carriers and should not be relied upon as a sole screening tool when managing post-outbreak populations
- •Breeding farms and multi-horse facilities must implement direct pathogen detection protocols rather than relying on clinical signs or serology to prevent silent transmission to naive groups
Key Findings
- •Silent carriers of S. equi showed no significant clinical or inflammatory markers distinguishing them from non-carrier herd-mates (P = 0.06-1.0) across three separate outbreaks
- •Serology alone cannot reliably identify carriers in comingled horses, with 3 of 12 culture-positive carriers being seronegative
- •Clinical examination, white blood cell counts, and serum amyloid A levels failed to differentiate carriers from non-carriers except in weanlings at 6 months post-outbreak