Cases of infant botulism linked to ByHeart baby formula have doubled to at least 31 across 15 states since Aug. 9, according to the FDA, though no deaths have been reported.

Symptoms include poor feeding, muscle weakness, and breathing difficulties.

Despite an expanded recall, the formula remains on store shelves and online.

Parents of affected infants have filed lawsuits alleging the product was defective and the company was negligent, CBS News has reported.

The FDA is testing leftover and unopened formula, but cautions that negative results do not guarantee the product is safe.