Repeated nasopharyngeal lavage predicts freedom from silent carriage of Streptococcus equi after a strangles outbreak

Background The value of repeated nasopharyngeal lavage (NPL) to detect silent carriers of Streptococcus equi has not been investigated. Hypothesis/Objectives Determine if results of serial testing for S. equi by NPL predicts subsequent true carrier status as determined by both NPL and guttural pouch lavage. Animals An outbreak of strangles with 100% morbidity in 41 mature Icelandic horses was followed prospectively to investigate development of silent carriers. All were initially positive to S. equi on NPL. The farm was closed to horse movement during the entire study. Methods Prospective observational study. Testing for S. equi was performed by NPL at weeks 18, 28, 29, and 30 postindex case and subsequently at week 45 by both NPL and guttural pouch lavage. Carrier status at week 45 was compared to results obtained at weeks 18, 28, 29, and 30. Descriptive statistics were calculated. Comparisons were made using Fisher's exact test or the Freeman-Halton extension with a P < .05 level of significance. Results Of 24 noncarriers at week 45, only 4 horses were negative on all 3 consecutive weekly NPL samples at weeks 28 to 30. However, 10 of the 11 horses with at least 3 negative NPL obtained from weeks 18, 28, 29, and 30 were S. equi-free at week 45 (P = .03). Conclusions and Clinical Importance Repeated NPL on at least 3 separate occasions can assist in predicting S. equi carrier-free status in horses after recovery from a strangles outbreak.