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.

The new confirmations push the national total of H5 cases this year to 20. All but one are related to contact with sick cows and poultry. Health officials are still investigating the source of a human H5 infection in Missouri and whether illness symptoms in seven contacts, one household member and six healthcare workers.

The human avian flu has yet to pass from human to human, but state gene detectives are on the lookout for mutations.

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.