PHOENIX  Found tottering alone in the desert with their ribs visible and their heads hung low, horses play a backbreaking, unappreciated role in the multibillion-dollar drug smuggling industry.

Mexican traffickers strap heavy bales of marijuana or other illegal drugs to the horses’ backs and march them north through mountain passes and across rough desert terrain. With little food and water, some collapse under their heavy loads. Others are turned loose when the contraband gets far enough into Arizona to be loaded into vehicles with more horsepower.

“We would pick up 15 to 20 horses a month, and many more of the animals would get past us,” said Brad Cowan, who spent 28 years as a livestock officer for the Arizona Department of Agriculture before retiring a few months back. “They wear poorly fitted equipment. It’s obvious they were not well taken care of. The makeshift saddles rub big sores in their backs.”