Los Angeles - A mysterious illness afflicting California brown pelicans continues to take its toll.

The Northern California-based International Bird Rescue Research Center said Thursday it had counted 460 sick or dead birds so far, up from 265 last week. The birds were found along the West Coast from Baja California in Mexico to Oregon.

Wildlife experts remain baffled by the sickness, but birds so far have tested negative for avian influenza and for high levels of domoic acid, a toxic chemical produced by algae.

The center says some pelicans were found suffering from frostbite after record-breaking cold occurred on the Oregon coast in mid-December, but a center veterinarian says it's unlikely the problems were caused by weather alone.