Streptococcus equi is an important and highly infectious pathogen in horses. Strict infection control measures are required to prevent or control S. equi. One aspect that is frequently discussed is survival of S. equi on environmental surfaces. Studies performed in laboratories on sterile surfaces have reported prolonged environmental survival of S. equi, however it is unclear whether these results are applicable to the field situation where factors such as sunlight, temperature extremes, variations in temperature and humidity and competing microflora may affect survival. The objective of this study was to evaluate S. equi persistence in an outdoor environment on surfaces found on equine farms.
A field strain of S. equi was grown in pure culture and suspended in either an equal volume of phosphate buffered saline or mucus collected from the upper respiratory tract of euthanized horses that was determined to be S. equi-free. One ml of inoculum was spread onto multiple 15 cm sections of bare wood, painted wood, metal and rubber. Moistened swabs were used to sample the inoculated sites immediately after inoculation (day 0), on day 1, then daily until 2 consecutive negative samples were obtained. Swabs were inoculated onto blood agar for anaerobic and aerobic incubation. Enrichment culture using Todd-Hewitt broth was also performed. Meteorological data were collected daily. Kruskall-Wallis test was used to compare duration of persistence between saline and mucous inoculum groups. Chi-square test was used for categorical comparisons.
Eight replicates were performed between July 5 and Sept 21, 2007. The mean concentration of the S. equi inoculum was 2.1 x 108 colony forming units (CFU)/ml (SD 1.5 x 108, range 1.3 x 107-4.8 x 108). Survival was short term with S. equi identified from only 53/64 (83%) samples after 1 day. The longest duration of persistence was 3 days, which occurred in only 4 (6.3%) samples. There was no difference in persistence on different materials (P=0.95), with greater than 1 day survival in only 2/16 (13%) of wood surfaces and 3/16 (19%) metal, rubber or painted wood surfaces. Persistence was longer in S. equi inoculated in respiratory mucus compared to saline (P=0.02). When surface materials were combined, there was significantly shorter survival on samples inoculated during sunny periods for both mucus and saline groups (both P<0.01).
This study indicates that S. equi persists for only a short time in the outdoor environment, at least under these conditions. It is possible that survival could be longer given different weather conditions, and further study is warranted. The longer survival of S. equi inoculated in mucus is not surprising as mucus could be both physically protective and provide a nutritional source, and indicates that persistence studies should be performed using organic materials such as mucus that would expected to be deposited with S. equi in the natural situation.