A grass awn is the barbed seed head of certain grasses, shaped so that it moves in only one direction. Once embedded, the barbs prevent it backing out, so it travels steadily inward through tissue.
A dog quartering a field with its nose down inhales these directly. Some lodge in the nasal passages, producing violent sneezing fits that start abruptly during or immediately after a session in cover, often with discharge from one nostril only. That one-sided pattern is highly suggestive.
Others are inhaled further, reaching the lower airways, where they migrate into lung tissue and set up abscessation. This version can present weeks after the original outing as a persistent cough, lethargy, fever or weight loss, by which time nobody connects it to a walk in August.
Awns also enter through the skin, particularly between the toes and in the armpit, and can then migrate internally along tissue planes to reach the chest.
This is a genuinely breed-relevant hazard rather than a theoretical one, and the sudden onset of violent sneezing in a field dog during summer should always raise it. A grass awn found early is a straightforward retrieval; one that has migrated is a considerably larger problem.