Direct contact between infected horses is the most obvious means of transmitting the infection but the hands and equipment of staff, farriers or veterinary surgeons can spread it indirectly. The bacterium is discharged (shed) from draining abscesses and the nose, and it may survive in the environment, particularly in water troughs. Good hygiene is therefore essential in controlling the disease. The incubation period (the time between infection occurring and clinical signs developing) is usually about one week but may be longer. Horses incubating the disease may shed S. equi before the onset of obvious clinical signs and so may spread the infection to in-contacts before the first case becomes apparent.
A small but important proportion of horses that have recovered from strangles become persistently infected (most commonly in their guttural pouches) with S. equi for months or even years. These ‘carriers’ are less susceptible to reinfection, and they may have no obvious clinical signs of disease but can intermittently shed S. equi, which can then infect naive horses. These subclinical carriers are probably the most important factor in persistence of infection on premises between outbreaks and can initiate new outbreaks following their inadvertent movement to new premises.