Freedom from Disease

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Shedding of S. equi usually ends rapidly after complete recovery but may continue intermittently for several weeks after clinical signs have resolved in some carrier horses. Therefore, no convalescent horse or in-contact horses can be considered free from infection until either three negative nasopharyngeal swabs have been obtained or the horse has been tested negative on bilateral guttural pouch samples. Negative results for S. equi by qPCR alone or by culture and qPCR indicate freedom from infection and the carrier state in the large majority of cases, but not all, so vigilance must be maintained. In deciding on the best time to commence testing to confirm freedom from infection after an outbreak of strangles it should be noted that this is likely to be a trade-off between starting sooner and finding a proportion of convalescing horses that continue to harbour S. equi that would if left longer have cleared the infection naturally and starting later and identifying fewer true subclinical S. equi carriers that require treatment and re-testing. Experience suggests that the best compromise is for clearance testing to commence at least four weeks after the last clinical signs of strangles have been observed.