Climate Change, Ag Practices Expand Histoplasmosis Range

Largely confined to the Ohio and Mississippi River valleys just a half century ago, the soil-dwelling fungus that causes the lung infection histoplasmosis has migrated west into the upper Missouri River basin due to a combination of climate change and changing land use patterns.

The infection, caused by exposure to airborne spores from the soil-dwelling fungus Histoplasma capsulatum, is generally asymptomatic in healthy people, but can cause significant disease and even death in people who are immunocompromised due to illness or drug treatments.

Recent outbreaks of the infection in Montana and Nebraska led the U.S. Centers for Disease Control and Prevention (CDC) to suggest that histoplasmosis is now endemic in these regions.

In an effort to better understand the current geographic distribution of H. capsulatum, Amelia Maiga, MD, of Vanderbilt University Medical Center in Nashville, and colleagues, used satellite imagery integrating land cover use, distance to water, and soil pH to create a site suitability model for the fungus.

Via logistic regression modeling, the researchers compared their map with state-level histoplasmosis incidence data from a 5% sample from the Centers for Medicare & Medicaid Services (CMS).

"Our histoplasmosis exposure suitability score predicted state-level histoplasmosis incidence rates from recent CMS data with an AUC [area under the receiver operator curves] ≥ 0.72," the researchers wrote online in the CDC journal Emerging Infectious Diseases. "Our model confirms previous reports that preferred soil environments for Histoplasma have expanded into the upper Missouri River basin."

Study co-author Stephen Deppen, PhD, also of Vanderbilt, told MedPage Today that the Missouri River basin region -- which includes Montana, North Dakota, South Dakota, and parts of Nebraska, Iowa, Kansas, and Missouri -- is hotter and wetter than it was a half century ago. There is also more farming in this region than previously.

"There has been a radical change over the last 50 or 60 years in how we use the land," he said.

Deppen added that the recognition that H. capsulatum is now endemic west of its traditionally recognized location is especially relevant to clinicians caring for immunocompromised patients who have not previously had histoplasmosis on their radar.

"Clinicians in the heartland who may not have seen this infection before may now see it in their immunocompromised patients. They need to understand that."

A recent analysis of histoplasmosis outbreaks between 2011 and 2014 by CDC researchers identified 3,409 cases of symptomatic infection occurring in 12 states: Alabama, Arkansas, Delaware, Illinois, Indiana, Kentucky, Michigan, Minnesota, Mississippi, Nebraska, Pennsylvania, and Wisconsin. The results showed a very high prevalence of hospitalization (57%) and death (7%) among the cases.

Given that most infected people remain asymptomatic, the high hospitalization and death rate in the study suggests both underreporting and underdiagnosis of infection, Paige A. Armstrong, MD, MPH, and colleagues wrote. "Increased awareness about histoplasmosis among the public, the public health community, and healthcare providers could improve diagnosis, leading to appropriate treatment and better patient outcomes and reducing harm from administering multiple courses of antibacterial drugs ineffective against fungi, as commonly occurs for coccidioidomycosis."

"To identify populations at highest risk and opportunities for prevention, additional study of the incidence and epidemiologic, clinical, and laboratory features of histoplasmosis cases nationwide is needed."