I just finished reading an interesting article from The Horse about the disease Strangles.
I had always been told (and had read in at least several places) that the strangles bacteria, Streptococcus equi, stays in the ground for years and years and years. This is why many people are quite afraid of Strangles, not only does your horse suffer, but once it gets in the soil, you never know when the next outbreak will be. And indeed, it does appear this way. If the disease goes through a whole herd or barn eventually all the horses will recover (or die), then, a year or two later, another outbreak will occur.
But is this really what’s happening? Actually subsequent outbreaks are not the fault of the barn, pasture, farm, etc. After an outbreak of strangles, “a number of horses (can be as high as 10%) cannot clear the bacteria and become persistently infected. This bacteria can survive in the guttural pouches, which are located in the pharyngeal region, for years.” Then, later on, if the carrier horse undergoes stress, they can shed some of the bacteria, causing a new outbreak. Transmission can occur both through direct social contact with infected horses and indirectly through interaction with environments that have recently been contaminated, including water sources. (However, besides for water sources, the bacteria cannot live for long without a host). Once horses have had strangles, they can be tested to see whether they have become carriers.
Since strangles is transmitted by horses and not by contaminated pasture, it’s important to be careful what other horses you expose your horse to, whether you are introducing new horses to your herd or interacting with other horses at shows or on other outings. Also, if you frequently travel with your horse or have horses constantly coming and going from your facility, vaccinating for strangles could be a wise move. Although the vaccine does not provide full protection from the infection, it does reduce the severity of strangles if your horse becomes infected.