Fungal infections are major killers and claim more lives every year than HIV or malaria. There are even some that infect insects and turn them into zombies. A new TV show bringing this horror story into the spotlight has got a lot of people wondering – are humans next?
Meet the famous fungal villains
Bingeworthy HBO series The Last of Us based on the computer game of the same name presents a scenario of a zombie apocalypse resulting from the infection of humanity by a fungus. The fungal group Cordyceps is blamed for these infections in the show, turning victims into crazed zombies that attack and bite the uninfected turning them into more crazed zombies. As with all the best horror stories there is a tiny element of truth in the basic concept and scenario.
The taxonomy of the fungi that have been traditionally placed in the genus Cordyceps is very complex. We now know that they are in two big families, the Cordycipitaceae and the Ophiocordycipitaceae in the Order Hypocreales.
Each of these families include a number of different genera, many of which are entomopathogenic – meaning they attack insects and other arthropods – and which have the capacity to kill the insect they infect. In many of these cases the infection is relatively straightforward – the insect gets infected and slowly the fungus colonises the host until the insect dies because it can no longer function effectively. The insects are a good vehicle for reproduction purposes and provide a good food source too.
The hypnotic power of the zombie ant fungus
Things get a little strange with many of the species of Ophiocordyceps. The most celebrated is Ophiocordyceps unilateralis – the zombie ant fungus – which was first discovered by Alfred Russel Wallace in 1859. Wallace was one of the greats of natural history and along with Charles Darwin developed the theory of evolution. He explored the jungles of South America and south-east Asia collecting vast numbers of specimens and in that process discovered these “zombie ants”.
The ant that is infected, Camponotus leonardi, lives high in the canopy of tropical forests, but has to on occasions descend to the ground to then climb into adjacent trees. It is at this point that it can be infected by the fungus, as the spores can attach to the insect and then penetrate through the tough exoskeleton of the ant. The fungus spreads through the ant and it is presumed to produce compounds that change the behaviour of the ant. These behavioural changes are where the term “zombie ant” comes from and are specifically for the benefit of the fungus.
As a result of the infection the ant will climb up a plant, and with huge force and in a zombie- like state, embed its mandibles, or jaws, in the mid rib of a leaf at the specific height of 26cm from the ground. This occurs on the northern side of a plant, which in a tropical forest environment, will provide a temperature between 20C and 30C and relative humidity of around 95 per cent.
The fungus produces a fruiting structure out of the ant (which is definitely dead by this stage) on a long stalk with a club-like head – very similar to some of the structures appearing in the hapless zombies portrayed in the TV series. It has to be said that some of the depictions of the fruiting structures of these fungi in show are more reminiscent of the bracket fungi we often see on tree branches and stems or on fallen logs in the rainforest. But who is to say there could be co-infections of other species of fungi once a person is infected!
It is now recognised that there are many hundreds of species in the Cordyceps and Ophiocordyceps groups – often with each species infecting a specific insect or spider in what is a marvellous example of co-evolution – and bizarre infections can result. Some related species infect caterpillars underground – in Australia Drechmeria gunnii (previously known as Cordyceps gunnii) infects ghost moths – and are sometimes called vegetable caterpillars. These species are often collected as they are believed to have beneficial effects as a health supplement. We have a spectacular specimen in our herbarium spirit collection that was collected in 1899.
Another kind of mind-altering affect caused by fungi in humans comes from an important group of fungi, the Clavicipitaceae, which are closely related to Cordycep group. This group contains the fungi that produce ergots, which contain a number of alkaloids including lysergic acid, a precursor of LSD, that can affect neurotransmission and are known to be mind-altering in humans.
Fungal diseases kill 1.5 million people a year
Of course, it is a long jump from infecting insects and other arthropods and turning them into zombies to being able to infect humans in this way (or even other vertebrates).
Some cause topical lesions and problems such as thrush, Athlete’s foot, eye and nail infections but there are a number that cause much more serious problems. It is estimated that more than 1.5 million people die from fungal diseases globally each year caused by species of yeast like Candida and Cryptococcus species and other moulds such as Aspergillus and Fusarium species.
With the group of fungi I work on, Fusarium (more commonly known as pathogens of plants), we are often seeing more and more infections in people with suppressed immune systems – as a result of organ transplant drugs or infection as a result of pathogens such as HIV. In these cases, a normally benign fungus systemically moves through the body, often causing a fatal disease. Unfortunately, there are very few, and often no drug treatments that work on the fungus that don’t adversely affect the patient. I guess with this scenario in mind perhaps it is not too far-fetched to see other species of fungi become important in humans, and maybe even adapt or evolve to prefer humans as their main host. Could it happen with Cordyceps fungi? That’s highly unlikely and very remote but you can never say never in the fascinating world of mycological science.
Prof. Brett Summerell is Chief Scientist and Director Science, Education and Conservation at the Gardens and a world leading fungi expert. He is one of the world's foremost experts on the deadly Fusarium pathogen, which can wreak havoc on our food crops and native plants.