Bird flu has been reported in 94 dairy herds across 12 states since late March, according to the U.S. Department of Agriculture.

The risk to public health is currently low, though additional spread of the virus could increase that risk, said Nirav Shah, principal deputy director for the U.S. Centers for Disease Control and Prevention, on a briefing call with reporters.

"The more infections there are among cows, the more risk there is for infections to occur among humans," Shah said.

The CDC has monitored more than 500 people and tested at least 45 during the current outbreak, Shah said.

Three dairy farm workers have tested positive since the virus began circulating among dairy cows. The U.S. has advised farm workers to wear protective gear to try to reduce their risk of exposure to the virus. The U.S. and Europe are both preparing vaccines that could be used to help protect farm workers this year.

A genetic analysis of the third human case found no signs of mutations in the virus that would make it easier to transmit among people, Shah said.

Asked whether the CDC should be testing people who have been exposed to the virus but are not symptomatic, Shah said the agency is confident in its current testing strategy.

Testing asymptomatic people could result in identifying people who are carrying virus in their nose that would not actually cause an infection, he said.

The spread among dairy farms is likely occurring from animal movement, shared personnel, and vehicles and equipment that travel between farms, Kammy Johnson, a veterinary epidemiologist with the USDA said on the call.

Wild birds, an early vector of the virus, do not appear to be spreading the virus from herd to herd or to poultry farms, a USDA report released on Thursday found.

Biosecurity practices like cleaning equipment and limiting movement of sick animals are important to containing the spread of the virus, Johnson said.

Asked how many farms with infected cows the USDA may be missing, Johnson said: "That's the $64,000 question. We don't know what we don't know."

The USDA is encouraging farmers to test their herds if they suspect symptoms of the virus and is providing financial support for milk and animal testing. Farmers from 11 of the 94 infected dairy herds have signed up for those programs, said Mark Lyons, a USDA official.

Two dozen companies are currently in various stages of development on an avian flu vaccine for cattle, Agriculture Secretary Tom Vilsack told Reuters on Wednesday.