Coccidioidomycosis, also known as valley fever, has increased significantly in its "endemic area," which includes Arizona.

A report released Thursday by the U.S. Centers for Disease Control and Prevention says the infection, caused by the inhalation of coccidoides spores, increased "substantially" between 1998 and 2011 in the areas where it is most common - New Mexico, Utah, Nevada, California and Arizona.

There were nearly 16,500 cases reported in Arizona in 2011, up from 1,475 in 1998.

Some of this spike may be due to better reporting, improved awareness and improved lab testing, officials say. However, the report's authors say the overall causes of the increase are unclear.

Coccidioides exists in the soil and is sensitive to environmental changes; factors such as drought, rainfall, and temperature might have resulted in increased spore dispersal, and disruption of soil by human activity, such as construction, also might be a contributing factor, the report says.

The CDC analyzed data from the National Notifiable Diseases Surveillance System for the period 1998–2011. During that time the incidence of reported coccidioidomycosis increased from 5.3 per 100,000 population in the endemic area (Arizona, California, Nevada, New Mexico, and Utah) in 1998 to 42.6 per 100,000 in 2011.

Federal officials say healthcare providers should be aware of the increasingly common infection when treating people with influenza-like illness or pneumonia who live in or have traveled to the endemic area.

State and regional annual incidence rates were calculated by dividing the number of cases by U.S. Census Bureau population estimates for each year.

During 1998–2011, a total of 111,717 coccidioidomycosis cases were reported to CDC from 28 states and the District of Columbia —  66 percent from Arizona, 31 percent from California, one percent from other endemic states, and less than one percent from nonendemic states.

Incidence in endemic states increased among all age groups during 1998–2011.

During 1999–2008, most Arizona cases occurred among males, but beginning in 2009, a higher proportion of cases occurred among females, the report says. 

Incidence in 2011 in Arizona was substantially higher among females (286.9 per 100,000) than males (215.7 per 100,000).

In contrast, only 35 percent of California cases occurred among females during 1998–2011.

The report recommends people in endemic areas consider trying to reduce exposure to dusty air, which might contain coccidioides spores.

However, because there are currently no proven preventive measures for coccidioidomycosis, additional research into strategies that reduce the incidence or morbidity of this infection is warranted, the authors state.

The report was co-authored by Clarisse A. Tsang, who is an epidemiologist from the Arizona Department of Health Services.