− | RAO usually becomes apparent in middle-aged and older horses, and persists for life<sup>allen</sup>. There is some evidence that there may be a genetic component to the disease, as many horses do not suffer RAO when they are housed in environments that can provoke clinical disease in others<sup>marti</sup>. | + | RAO usually becomes apparent in middle-aged and older horses, and persists for life<sup>6</sup>. There is some evidence that there may be a genetic component to the disease, as many horses do not suffer RAO when they are housed in environments that can provoke clinical disease in others<sup>7</sup>. |