Many studies have been performed to evaluate the persistence of Strep equi in the environment. Strangles can be debilitating to infected horses and is highly infectious to other horses on a property. It is known that S. equi can persist for months or years in the guttural pouch despite clinical recovery, making those horses carriers of infection.
A study looked at the duration of viability of this bacterium in conditions outside the nasopharyngeal tissues of the horse [Durham, A.E.; Hall, Y.S.; Kulp, L.; and Underwood, C. A study of the environmental survival of Streptococcus equi subspecies equi. Equine Veterinary Journal 2018 DOI: 10.1111/evj.12840].
While the highest risk of infection is associated with direct contact with a horse infected with S. equi, other potential sources of infection can be found in the environment. One study looked at survival on bare wood, painted wood, metal and rubber under various weather conditions and protection of the bacteria with mucus. Most of the inoculated samples did not survive longer than four days.
In this recent study, S. equi was able to survive up to 34 days in a wet bucket in the winter. Another important source for potential contamination is the inside of a moist nasogastric tube—S. equi survived up to 21 days in winter. The report noted, “Maintenance of a wet environment will prevent bacterial death from desiccation as well as affording some degree of temperature moderation.”
Ultraviolet from bright sunlight typically kills S. equi within 24 hours. The Strep bacteria in this study were purposely shielded from direct sunlight by inoculating surfaces only in indoor or outdoor shaded areas. This allowed for results based on temperature and humidity during two distinct seasons.
“Time of year had a large and significant effect on both survival times and growth scores of S. equi, with cold and humid winter conditions associated with significantly longer survival times and greater three-day growth scores.”
Examples of potential fomites included:
- Rubber sole of a shoe – growth persisted for 3 weeks
- Plain indoor wood surface – growth maintained for 2 weeks
- Wet bucket in winter – similar 3-day growth scores as the two above
- Dry wood or clothing – better growth in winter
Despite a wet environment such as in a stomach tube or bucket, in warm summer conditions S. equi lasts about nine days.
The study summarized, “Although survival in warm and dry locations was detected for only up to 2 days, in wet and cold indoor conditions, S. equi may remain viable for more than 30 days.”
This does not necessarily imply that a horse will become sick since many factors determine infectivity, including the amount of infectious material the horse receives and the horse’s native immune system. However, the authors suggested that knowing that S. equi can survive in the environment for extended periods of time under specific environmental conditions, especially with wet and cold, allows formulation and implementation of appropriate control strategies.