Be on guard against this tricky fungus
The Parksville-area fungus that has sickened people, pets and wildlife during the past two decades plays clever tricks on our immune systems, say scientists.
Since 1999, a virulent form of Cryptococcus gattii has infected more than 400 people and untold numbers of animals that have spent time in and near the popular seaside community. Of these, a few have died each year.
Humans and other animals become exposed to the fungus when they inhale its airborne spores. Once inhaled, the pathogen might spread through the body to cause pneumonia or meningitis.
Rathtrevor Beach Provincial Park was once fingered as the disease’s hotspot by epidemiologists, who traced the fungus to the park’s soil, trees and air. Since then, however, researchers have found the fungus throughout the southeast Island, the Lower Mainland and down the coast into Oregon, where another virulent variety of the same fungus has emerged. Other forms of the fungus have also shown up in California and the northeastern U.S.
Genetic studies show the region’s dominant strains are descended from tropical strains. International trade and travel likely introduced them here some time ago, and changes in regional weather patterns — warmer, drier summers and milder, wetter winters — might have enabled them to spread and emerge. Researchers have identified several genes in the fungus that might make these strains better able to survive this environment.
The same genes might also make them more virulent and harmful. The Island’s outbreak variety differs from other strains in that it threatens healthy people — those with strong immune systems, rather than those already at risk for infections.
That super-virulence hinges on C. gattii’s ability to turn healthy immune responses to its advantage. For example, a common immune reaction in healthy mammals automatically triggers a division of labour between infecting C. gattii cells.
Some of the invading cells form a protective shield around their neighbouring fungal fellows, sacrificing themselves to buy time for the others to get established.
Infecting C. gattii cells also hijack immune cells that normally protect the lungs. Immune cells called macrophages patrol the lungs, vacuum up microbes and other foreign material found there, then digest them.
The Island’s C. gattii outbreak strain not only kills macrophages, it can hide inside them. A digestion-proof shell keeps the fungus safe and allows it to use macrophages as Trojan horses and avoid detection by antibodies and other immune cells.
More critically for the patient, inside a macrophage, C. gattii may also enjoy a free ride from the lungs into the bloodstream and across the blood–brain fortress-barrier. There, the brain’s ample supply of blood sugars feeds the fungus, permitting it to grow and spread rapidly.
Few animals or people survive such brain infections.
Research shows that C. gattii thrives in the Island’s temperate southeastern coastal region, where winters rarely freeze and summers remain mild.
One study indicates the greatest concentrations and risk of exposure to airborne C. gattii occur on sunny, breezy days in late summer and early fall, coinciding with summer holidays, camping season, hikes and barbecues in the region’s parks.
The greatest concentrations and risk of exposure to soil-borne C. gattii occurs on cool June and July summer days — gardening season.
Which means most of us almost certainly have been exposed to the fungus, perhaps repeatedly. Fortunately, despite its local virulence and prevalence, it succeeds in infecting relatively few people and animals. When illness occurs, treatment exists. Early treatment, of course, is always best.
But diagnosis can be difficult. In the few people who do become infected, symptoms might not appear until up to three years after exposure. Furthermore, the symptoms resemble those of other, more common illnesses, which increases the chance of misdiagnosis and further delayed treatment.
And, even though the region’s physicians are advised to watch for symptoms and to test for the fungus, cases — tragically — slip through.
More education and greater awareness, of course, are key. Even if it were possible, we’re not going to hermetically seal ourselves inside bubbles during the loveliest time of year, every year. The best way we can counter C. gattii’s immune-system tricks is by becoming C. gattii-aware.