It all starts with a plume of smoke on the horizon. Wispy and ethereal, the twisting, blooming columns of white and grey belie the danger and devastation to come. Within moments, the plume begins to darken, an orange glow appears and a faint crackling might be heard above the rustling trees and the consternation of birds taking flight. Though it may not always be immediately apparent that a wildfire has taken hold, it will waste little time in making its presence felt. Driven by soaring temperatures and dry brush, flames race across a landscape leaving ash and devastation in their wake. Fauna, flora and even metal are no match for the ferocity of the flames. Temperatures within the inferno rise to 800 C, eviscerating organic life enough to eviscerate organic life, melt the plastics and metals that make up homes and businesses and cremate a human body. Their power is unequivocal yet they are still underestimated: a wildfire that seems to be burning in the distance one moment can be racing towards you in another, whipped up by the ferocious winds they themselves generate, with new fires created all the time by the unique weather systems born from flame.
This destructive force leaves a transformed world in its wake. Blackened remnants of trees sit adrift in a sea of ash. The fuel for the inferno – plants, animals, structures – has been transformed into thick, acrid smoke, which coats the landscape and spreads for thousands of miles – a messenger for the death and destruction which has occurred. Smoke is a demonstration of destruction and devastation, of life being snuffed out.
The wildfire season of 2020 in California had been one of the most brutal on record. 10,000 fires had burned over 4 million hectares across the state, with five of its six largest ever fires coming in that summer alone. The fires were driven by ever-increasing temperatures and a worsening draught, leaving forests and grasslands parched and ready to ignite with a single spark.
As the wildfire season was coming off its peak in the late summer of 2020, UC Davis Medical Center, located state capital, Sacramento, was treating 10 burn victims who were unable to escape the fires. There were also three additional patients on the burn ward who had received their injuries from a plane crash and a housefire. Infection is one of the single biggest risks for these patients, who often have wounds that expose a great deal of their tissue to potential pathogens.
Despite the best efforts of the treatments teams at UC Davis, the burn ward was experiencing a series outbreak of fungal infections: 7 of the 13 patients had developed infections, far above the 10% rate of infection that would typically be seen on a ward. Attempts to maintain sanitised conditions in hospitals are not always successful. Infectious agents can escape sanitation practices and patients can develop hospital-borne – or nosocomial – infections. Though sometimes severe, in the case of Methycillin-resistant Staphylococcus aureus (MRSA), these outbreaks can often be contained relatively quickly, as a single infectious agent if often the culprit. The UC Davis outbreak was different. The 7 infected patients were all experiencing different fungal infections.
Panicked, the team worked tirelessly to both identify the fungi responsible and establish how such a severe breakdown in sanitation practices could have happened. Doctors and nurses combed over the entire ward, pouring over patient charts, tracking visitors and checking cleaning rotas. Nothing seemed out of place and nobody seemed negligent. However, when Dr Naomi Hauser, desperate to find the cause began to investigate the air conditioning systems in the patients’ rooms, she discovered that the filters were pitch black and caked in soot and ash. She, like many other residents of Northern California, knew how bad the fires had been that summer – even if you avoided the news, you couldn’t avoid the orange haze in the sky and the acrid smell in the air. While on the one hand she was glad that the air filtration system was clearly doing something to avoid coating the room in the small particles that made up the smoke released by the fires, she couldn’t help but wonder what might have made it through, and weather these particularly aggressive fires where at the heart of the outbreak.
On the face of it, her hunch simply didn’t make sense. Sure, smoke can cause serious medical complications: inhalation can damage the tissue in our respiratory system and it is particularly hazardous for people with existing conditions that impact breathing. Some of the particles carried in smoke are thought to be inherently toxic and potentially carcinogenic. But one thing that smoke does not do is transmit the kinds of pathogens that were threatening the lives of her patients. Surely nothing lives in smoke?
Two years later, another set of fires was burning in the picturesque Fishlake National Forest in central Utah. These were not wildfires, but rather controlled burns, designed to regenerate woodland that was littered with decaying trees hit hard by a woodworm outbreak.
As the smoke billowed high over the pristine lakes and cragged peaks adorned with pines and firs, an object noiselessly swept through the haze. An unmanned aircraft laden with sensors and probes flitted from plume to plume, completing thousands of recordings for a team assembled by Dr Leda Kobziar, Professor of Wildland Fire Science at the University of Idaho and her team. Their analysis of the Fishlake Forest burn was designed to determine the composition of the smoke, but they weren’t interested in the proportion of gases or determining the toxicity of small particles whipped up by the inferno. Rather, they were searching for anything living in the smoke – microorganisms, such as bacteria and, yes, the kinds of fungi that had swept through the UC Davis burn ward two summers previously.
They had run the same tests on ‘clean’ air above the same stretches of the forest before the controlled burn was lit. When the team compared the two sets of samples, they were shocked to discover four times as many microorganisms present in the wildfire smoke as in the pre-fire air. Some samples had up to ten times as many. What’s more, there was a similar order of magnitude increase in the number of viable cells, which are those that have avoided serious damage by the harsh conditions of the fire and could continue to thrive, grow and reproduce in the right conditions.
The findings were truly perplexing, given that the conditions deep within a wildfire should snuff out all life. Though it isn’t clear exactly how microorganisms are ejected from fires, researchers believe that the burning plant life itself is the source of the microbes. Like any organism, trees and other plants are home to countless forms of microorganisms, including bacteria, fungi, single-celled eukaryotes and protists. As a fire sweeps through a forest such as Fishlake, outer layers of the plants and soils are incinerated, but some remaining particles are launched into the building column of smoke by powerful updrafts created by the power of the burning vegetation. It is thought that bacteria and fungi found deeper in plant structures might be protected from the harshest conditions and survive, clinging on to particles that eventually combine with ash and soot to form the thick columns of smoke spreading hundreds of miles from the fire.
Further analysis of the microorganisms in the Fishlake Forest fire identified a highly diverse range of potential human pathogens, including Bacillus anthracis-cereus, Pseudomonas syringae, Streptococcus species, Escherichia-Shigella coli, Corynebacterium jeikeium, Acinetobacter ursingii, Haemophilus haemolyticus-influenzae, and some Staphylococcus species.
The devastating impact of a wildfire is clear to see – swathes of incinerated animal and plant life, melted and contorted buildings. People caught in the fire’s path suffer burns and the terrible results of smoke inhalation. Studies like these, however, suggest that fires could have an ongoing impact stretching hundreds, if not thousands, of miles from where the fire sparked into life and wreaked havoc. With wildfires becoming ever more common and powerful with the acceleration of the climate emergency, the pathogens being swept into the air and rained down on vulnerable people may be yet another deadly impact of these devastating disasters.
Inspiration for this piece was from an episode of Radiolab, entitled ‘Up in Smoke‘ and information was sourced from the following articles:
- Fungal infections in burn patients during Northern California wildfires: https://academic.oup.com/ofid/article/9/Supplement_2/ofac492.1207/6902972?login=false
- Wildland fire smoke alters the composition, diversity, and potential atmospheric function of microbial life in the aerobiome: https://www.nature.com/articles/s43705-022-00089-5
