Give your horse a kiss on the nose and you may catch a whiff of sweet grain, fresh hay and perhaps the peppermint candy you just gave him. But if the smell of your horse’s breath causes you to recoil, something in his mouth, sinus passages or respiratory system may be amiss.

Bad breath—usually a sickly sweet odor not unlike trash left in the sun—can signal an abscessed tooth, a tumor or an advanced lung infection, such as pneumonia. The odor comes from bacterial overgrowth in decomposing tissues. Foul breath can also signal that a horse has not eaten in an extended period of time: Saliva helps to flush organisms from a horse’s mouth, and when he hasn’t been chewing regularly, the bacterial population can flourish.

Click here to learn the facts about heaves in horses.

If your horse’s breath becomes malodorous, ask your veterinarian to investigate. In addition to a full physical and dental exam, she may want to use an endoscope to search for the source of the problem in your horse’s sinuses and airways.

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