This week, a horse show in Halifax (UK) was cancelled because of some local cases of strangles (Streptococcus equi subsp equi infection). We can debate whether that was prudent or overkill (or whether measures could have been put in place to reduce the risk and make it manageable), but it highlights the concerns that accompany infections caused by this bacterium.
Strangles outbreaks are, in some ways, easy to control, and in others, quite difficult. Realistically, in my experience, when people can and will do what I recommend during an outbreak, it’s readily contained. Where things go off the rails is when people are unwilling to listen, only do a half-hearted job of infection control measures, are unable to control horse movement, or start well but lapse back into old habits too early.
Anyway, that’s not the point of this post. When dealing with cases on farms, the question of what to do about the outdoor environment often comes up. Horses can pick up the bacterium from contaminated objects or surfaces (even the ground), so if an infected horse deposits S. equi in a paddock or pasture (i.e. in nasal secretions or pus), what’s the risk to other horses and what can be done?
On a recent ProMED-mail posting about this horse show, a widely circulated but outdated line of “Under ideal environmental circumstances, the organism can survive 7-9 weeks outside the host. Paddocks and barn facilities used by infected horses should be regarded as contaminated for about 2 months after resolution of an outbreak.”
Firstly, the reference that was cited (the Merck Veterinary Manual) doesn’t actually say this (anymore). Rather, it now says “Survival of the organism in the environment depends on temperature and humidity; it is susceptible to desiccation, extreme heat, and exposure to sunlight and must be protected within mucoid secretions to survive. Under ideal environmental circumstances, the organism can survive ~4 wk outside the host. Under field conditions, most organisms do not survive 96 hr.” (updated January 2014)
That’s a more reasonable approach, and it’s presumably based on some research we did a few years ago (Weese et al, Can Vet J 2009) that showed the bug dies very quickly (often within a day) when outside, exposed to sunlight and other stressors.
That doesn’t mean someone with a strangles case on a farm should immediately open up potentially contaminated areas to unexposed horses. But, 2 months is quite excessive. Realistically, each scenario is different and a risk assessment based on the type of exposure, surface materials, ability to keep pastures/paddocks closed and local weather (including temperature and exposure to sunlight especially) are used to make a reasonable estimate of how long to quarantine outdoor environments. It’s never perfect but almost always it can be well under 2 months with little risk.