Ask a Doc: The origins of potentially deadly Valley Fever

• A TGen study sets the stage for tracking future outbreaks of Valley Fever
• Valley Fever is an infection caused by the microscopic fungus Coccidioides
• The geographic range of the fungus is expanding

Question:

Where did Valley Fever originate, and what is being done to monitor the spread of this potentially deadly pathogen?

Answer: Using the latest in genomic analysis technologies, scientists at the Phoenix-based Translational Genomics Research Institute, or TGen, have tracked the likely origins and dispersal of the fungus that causes Valley Fever.

In a story that spans 2 million years and includes the effects of glaciation and the pre-historic movements of animal hosts, the study sets the stage for tracking future outbreaks of this potentially deadly disease as it emerges in arid regions of North and South America.

Valley Fever is an infection caused by the microscopic fungus Coccidioides, which lives in desert soils and typically enters the body through the lungs. An estimated 150,000 Americans are infected annually by Valley Fever, and as many as 500 die each year.

Nearly 60 percent of infected people develop no significant symptoms from exposure to Valley Fever.

However some infected patients — and other mammals, especially dogs — develop highly debilitating symptoms, such as cough, fever and fatigue. Very severe Valley Fever can require lifelong treatment with antifungal drugs and may lead to death.

Distinct species

There are two distinct species of the fungus Coccidioides that cause Valley Fever:  C. posadasii, the oldest species, which originated in the Sonoran Desert of southern Arizona and northern Mexico; and C. immitis, which is the species found in California’s Central Valley, as well as parts of southern California and Baja Mexico.

Using the genetic data derived from 86 Coccidioides genomes, a TGen study published recently in the scientific journal mBio estimates that C. posadasii diverged, forming Arizona and non-Arizona subpopulations, between 820,000 and 2 million years ago.

The Arizona subtype today is found throughout central and southern Arizona. Within Arizona, TGen researchers identified multiple distinct genetic groups likely originating in the Tucson region, one of which also includes all the Phoenix isolates, where the largest concentrations of Valley Fever cases occur.

The fungus' spread

The study’s authors also estimate that the fungus spread to Mexico and Texas as many as 675,000 years ago, to South America about 527,000 years ago, and to Guatemala in Central America less than 190,000 years ago.

C. immitis appears to have diverged from the Arizona-based C. posadasii species between 365,000 and 920,000 years ago in central California’s San Joaquin Valley.

This was an inland sea during periods of the Pleistocene epoch, up to 2.6 million years ago. It was submerged by glacial runoff as long as 700,000 years ago, and the emergence of C. immitis likely followed its drainage.

The barrier formed by the Sierra Nevada mountain range also may have played a role in the formation of C. immitis as a separate species.

Expanding range

The geographic range of the fungus is expanding. New clusters of C. immitis have been identified in eastern Washington state, which likely emerged from California.

Valley Fever also is also known to occur in Argentina, Paraguay, Brazil and Honduras. The previous dispersal of this pathogen is attributed in large part to major animal migrations during the last several hundred thousand years.

Highly susceptible mammals, besides humans, includes dogs and other canids, rodents, and in South America llamas, armadillos and bats. The cause of the more recent expansion to Washington is under active investigation.

Dr. David Engelthaler is director of programs and operations for the Pathogen Genomics Division (TGen North in Flagstaff) of TGen, a non-profit biomedical research institute based in Phoenix. He can be reached atdengelthaler@tgen.org.

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