The death of an 8-month-old boy at a Coon Rapids home day care last month was caused by a rare heart disease, according to a medical examiner's report released Friday.

The Midwest Medical Examiner's Office listed "active lymphocytic myocarditis" as the cause of death for David Keith Brigman, of Andover.

The boy was in the care of licensed providers Janelle Rutzen and her daughter Kari Rutzen at their home Feb. 15 when they put him down for a nap at 1:15 p.m.

When they went to wake him at 3 p.m., he wasn't breathing. They called 911 and tried to revive him.

The state temporarily closed the day care after it was discovered that the Rutzens had not followed safe sleeping practices by not placing the infant on his back. But that was not listed as a contributing factor in the death, which was ruled by the medical examiner to be natural.

Myocarditis is a rare disease in children that can result in inflammation or weakness of the heart, according the National Institutes of Health MedlinePlus website. Symptoms can occur suddenly in infants and the result can be heart failure, according to the site.

It was not immediately clear whether or when the day care would reopen. Efforts to reach the Rutzens have been unsuccessful.

Fifty-one children have died in Minnesota's licensed child-care facilities for the five-year period ending in 2011. Almost all have been at in-home care facilities, not child-care centers. The increase has raised concern among state health officials, who say they are trying to determine any common links or reasons for the spike.