James Estrin/The New York Times

The image of cows as placid, gentle creatures is a city slicker’s fantasy, judging from an article published on Friday by the Centers for Disease Control and Prevention, which reports that about 20 people a year are killed by cows in the United States. In some cases, the cows actually attack humans—ramming them, knocking them down, goring them, trampling them and kicking them in the head—resulting in fatal injuries to the head and chest.

Mother cows, like other animals, can be fiercely protective of their young, and dairy bulls, the report notes, are “especially possessive of their herd and occasionally disrupt feeding, cleaning, and milking routines.”

The article, in the Morbidity and Mortality Weekly Report, discusses 21 cases in which people were killed by cattle from 2003 to 2007 in Iowa, Kansas, Missouri and Nebraska.

In 16 cases, “the animal was deemed to have purposefully struck the victim,” the report states. In 5 other cases, people were crushed against walls or by gates shoved by the cattle. Ten of the attacks were by bulls, 6 by cows and 5 by “multiple cattle.” A third of the deaths were caused by animals that had been aggressive in the past.

All but one of the victims died from head or chest injuries; the last died after a cow knocked him down and a syringe in his pocket injected him with an antibiotic meant for the cow. In at least one case the animal attacked from behind, when the person wasn’t looking. Older men with arthritis and hearing aids have the highest risk of being injured by livestock, the report says, probably because they don’t hear the animals charging and can’t move fast enough to get out of the way.