A blistering hot summer coupled with a never before seen virus in dairy cattle, has caused more cow deaths than anticipated in California, according to statewide dairy expert.

Cows in California are dying at much higher rates from bird flu than in other affected states, industry and veterinary experts said, and some carcasses have been left rotting in the sun as rendering plants struggle to process all the dead animals.

All of the state's cases have been linked to contact with infected dairy cows, officials said. No human-to-human transmission has been documented.

Of the 27 known bird flu cases in humans, about half have been seen in California dairy workers. Most are fighting mild symptoms such as conjunctivitis and fever. None were hospitalized. The Centers for Disease Control and Prevention say the current public health risk of the disease to humans is low.