Dust storm clouds moving over hills in the Arizona desert at sunset

Scientists have yet to establish a clear link between dust storms — such as this one in Arizona — and the risk of Valley Fever.Credit: Getty

What qualifies as a dust storm? That is one of the questions at the centre of a debate over the phenomenon’s role in the spread of Valley Fever, an infectious disease caused by inhaling the soil-dwelling fungi Coccidioides, in the Western United States.

A study published in GeoHealth on 17 July1 calls into question research from 2021 suggesting that no consistent link exists between dust storms and Valley Fever2. The authors of the new paper say the data set used in the 2021 analysis — the Storm Events Database, maintained by the US National Oceanic and Atmospheric Administration (NOAA) — is known to contain errors, lacks certain information and uses a definition of ‘dust storm’ that is inconsistent with that used by most meteorological organizations. As a result, they say the jury is still out on the dust storm–Valley Fever connection.

“Based on our knowledge of the fungus, it’s transported in dust, and there is no reason to believe that dust storms couldn’t carry [it],” says co-author Morgan Gorris, an Earth-systems scientist at Los Alamos National Laboratory in New Mexico.

Andrew Comrie, a climate and health scientist at the University of Arizona in Tucson, who wrote the 2021 paper, acknowledges that the database could be more complete, but still thinks it is a robust enough representation of big dust storms that his analysis would have picked up subsequent spikes in Valley Fever cases. “If there was a reliable signal, it should show up,” he says.

Determining whether — and how — dust storms increase the risk of the disease is important, researchers say, because cases are on the rise, and it would help to prioritize effective disease-mitigation strategies.

Fungal fever

Of the roughly 20,000 cases of Valley Fever reported to the US Centers for Disease Control and Prevention in 2019, the vast majority occurred in California and Arizona. Symptoms of the disease, which include fatigue, fever and cough, can last for a few weeks to months. In severe cases, infection can lead to scarring on the lungs or death.

Gorris and colleagues analysed nearly all of the 76 dust events in Comrie’s paper and found that 47% of them did not constitute a dust storm, as defined by the World Meteorological Organization (WMO), the United Nations body dealing with atmospheric sciences. The WMO defines a dust storm as an event in which dust in the air is so thick that visibility reduces to one kilometre or less. The team found a further 30 events in the Phoenix area that do meet the definition and are missing from the NOAA database.

The NOAA Storm Events Database currently defines a dust storm as an event in which visibility is reduced to about 0.4 kilometres, a limit that some researchers think is too narrow. Although the vast majority of reports come from trained storm spotters, other sources include automatic weather stations, law enforcement and the public. “We are working on ways that we can potentially make improvements to the database and data reporting going forward,” says Gordon Strassberg, storm-data programme manager at the National Weather Service, run by NOAA. “As we get more storm events, and as any database continues to gets larger, it can be challenging,” he says, “but we will continue to provide the best data we can.”

Although NOAA’s database contains the best available information on dust storms, the authors of the new study think that it is unsuitable for assessing the phenomenon’s relationship with Coccidioides, partly because of the inconsistent categorization of dust events. “Entries are very subjective; the same dust event could be categorized as a dust storm, blowing dust or simply haze,” says study co-author Karin Ardon-Dryer, a dust researcher at Texas Tech University in Lubbock.

“The devil is in the details,” says co-author Daniel Tong, an atmospheric scientist at George Mason University in Fairfax, Virginia. “We need to exercise caution when using this data — and be careful about the conclusions we draw from it,” he adds.

Soil studies

Comrie maintains that his work is solid. Both he and the authors of the new research agree that a better understanding of the environmental conditions that favour the fungus that causes Valley Fever is key. “We need to get to the bottom of how Coccidioides works,” he says.

Further efforts to determine which soils carry the greatest risk of transmitting Valley Fever are needed, he adds. A 2020 study found that soils home to burrowing animals have a high probability of harbouring the fungus3. Comrie suggests that small burrowing mammals favour desert habitats that typically have vegetative cover or a biocrust, which makes the surface less likely to emit dust, whereas big dust storms often begin in disturbed areas, such as abandoned farmland, during high-wind events.

The authors of the new paper identified several actions needed to better understand the relationship between dust storms and Valley Fever risk, including a consistent definition of dust storm, a quality-controlled database for dust storms and mechanistic models of Coccidioides transport. Comrie adds another: measurements of viable spores of Coccidioides, to identify which dust events carry the greatest risk. To date, only one study has taken this approach, and it found more spores on non-dust-storm days than on dust storm days4.

Progress has been slow, Comrie says, because Valley Fever is a regional disease and those studying it do not receive a lot of funding. “We are just chipping at the edges of understanding.”

Although scientists are eager to understand how, when and where Valley Fever transmission and risk is greatest, physicians say there are plenty of other health reasons to avoid dust, including its links to chronic lung and heart diseases. “The bottom line is that you don’t want to be breathing those tiny particles anyway,” says George Thompson, co-director of the Center for Valley Fever at the University of California, Davis.