California and Arizona "attract elderly persons to migrate and settle down, thereby introducing new, unexposed populations" to the fungus, they wrote.
People spending their vacations or entire winters in those states before going back to their hometowns may also be bringing the disease with them. Researchers noted that people died from valley fever in every state, "which probably reflects population mobility and movement in and out of coccidioidomycosis-endemic areas after exposure."
The study found that more than one-third of the deaths associated with valley fever were people between ages 65 and 84.
Researchers also found that there were certain health conditions more often associated with the disease. The most common was diabetes, with 384 deaths that listed both diabetes and coccidioidomycosis on the death certificate. That was followed by chronic obstructive pulmonary disease, HIV infection and non-Hodgkin lymphoma.
"The conditions that were associated with coccidioidomycosis were all inherently associated with immunosuppression: HIV, tuberculosis, diabetes mellitus, autoimmune diseases, organ transplant and cancers of lymphatic cells," Huang and colleagues wrote.
The connection between valley fever and people whose immune systems are already weakened by aging or other diseases is no surprise, said Dr. Navin Amin, an infectious disease specialist and chairman of family medicine and pediatrics at Kern Medical Center.
"We used to have a lot of HIV people who would get cocci and if it wasn't picked up early, they would die," Amin said. "Every HIV patient we had, when they developed cocci, it spread like wildfire."
Amin said he suspects the correlation between valley fever and lupus and rheumatoid arthritis found in the study could be caused by the medications patients take for those two conditions that weakened their immune system.
Deaths expected to rise
Researchers also looked at valley fever-related deaths across different race and ethnic groups. While the majority of deaths were among white people, the study found that people of other ethnicities, particularly Native Americans and Hispanics, had the highest mortality rate.
Native Americans were about six times more likely to have valley fever appear on their death record than white people, and Hispanics were about four times more likely to have the disease listed on their death record, the study said.
Researchers wrote that the high rates of valley fever-related deaths among Native Americans could be tied to cultural practices and exposure to dust. Health care could play a part as well, they wrote.
"Poor access to health care services might delay diagnosis, resulting in more severe disease," they wrote.
But researchers also cautioned that Native Americans' high rates should be viewed carefully because they accounted for a small number -- just 79 -- of the total deaths.
"I think we have a duty to really put our efforts toward those that are most impacted by illness," Bristow said, adding that those people often have less access to other resources such as health care and housing.
The study's researchers concluded that valley fever-related deaths will probably rise.
"One can probably expect to see more valley fever in the future because we're seeing more people with problematic immune systems," Bristow said.
Bristow said that it is now up to physicians, public health officials and policymakers to look at the burden from the disease and design an appropriate response.
"Understanding the problem is the first step, and that's what this paper helps us do," Bristow said. "This gives valley fever a voice. This says, 'Valley fever is real, it is present, it is killing Americans and it deserves attention.' "