There are thousands of types (species) of moulds, but only very few can cause infection in humans, partly because most cannot grow at human body temperature. They live by feeding on dead organic matter in the main which is why they are often seen spoiling our food, some becoming very specialised e.g. they can feed on antibacterial soap, survive in the freezing conditions of Antarctica and even live on aviation fuel! Fungi grow happily without any exposure to sunlight but cannot grow without sufficient water.
Moulds (also known as filamentous fungi) outside the home can often be seen as mushrooms which are structures entirely made up of millions of hyphae. Many tend to live in soil and on dead plant material so will be found in leaf litter in autumn and winter. However, where leaf litter piles up to form compost heaps few moulds can survive as the temperature in the heap can reach 60C. These are the growing conditions that some Aspergillus species have adapted to, to take advantage of the lack of selective competition from other moulds.
Aspergillus is a large family of moulds found all over the world and is common both in the outside air and in the home. Only a few species of Aspergillus that have adapted to grow at high temperatures can cause infection in humans and animals – these include A. fumigatus, A. niger and A. nidulans. Of these A. fumigatus is by far the most commonly found causing infection.
Aspergillus has more than one adaption that helps it infect humans. Chief among these is due to how it reproduces. Filamentous fungi reproduce and spread by producing millions of ‘seeds’ known as spores, each capable of growing into a new fungus. Aspergillus spores are amongst the smallest and that enables them to float easily on the slightest wind, consequently, they can be blown a very long way away from their parent – spores produced in the Sahara Desert have been traced to the Caribbean! Unfortunately for us, the small size (2-3 microns) of these spores means that we can easily breathe them in and they can readily penetrate deep into our lungs and sinuses. If they happen to alight somewhere where they can avoid our immune system eg scarred tissue from an earlier infection (Tb), other lung damage eg COPD, cystic fibrosis or happen to find a host who has a weakened immune system eg genetic mutations, patients on immune-suppressing medication, they can start to grow. This can cause severe allergy, tissue erosion or tissue invasion depending on the health status of the patient.
The vast majority of us are completely naturally resistant to Aspergillus infection and do not develop disease caused by Aspergillus despite breathing in their spores every day. However, when the conditions are right disease does occur, and it takes several forms.
The types of diseases caused by Aspergillus are varied, ranging from an allergy-type illness to life-threatening invasive infections (1).
Diseases caused by Aspergillus are called aspergillosis. The severity of aspergillosis is determined by various factors, but one of the most important is the state of the immune system of the person.
- If a patient has a severely limited immune system such as a bone marrow transplant, then the fungus can grow quite quickly and invade surrounding tissues. This is an acute invasive infection that is rare outside of transplant clinics. The infection is treated at those clinics.
- The National Aspergillosis Centre sees patients who are much better at fighting fungal infections. They often cannot eliminate the infection but they slow its growth down to such an extent that patients can live for years without it being identified. The commonest part of the body that gets infected is the lungs and here it can closely resemble other more common infections or other lung tissue disorders. There are no diagnostic techniques that can accurately identify chronic pulmonary aspergillosis by themselves, diagnosis usually takes multiple tests and the experience of an aspergillosis specialist to finalise.
Types of chronic aspergillosis infection:
- CPA – chronic pulmonary aspergillosis
- FRS – fungal rhinosinusitis
- Ear, eye and nail infections caused by Aspergillus
- Aspergillus bronchitis
Types of acute aspergillosis:
Types of Aspergillus allergy:
- ABPA – allergic bronchopulmonary aspergillosis
- SAFS – severe asthma with fungal sensitivity
- Fungal sensitisation from a damp home
- George R. Thompson, III, M.D., and Jo-Anne H. Young, M.D. Aspergillus Infections., October 14, 2021, N Engl J Med 2021; 385:1496-1509 DOI: 10.1056/NEJMra2027424. Article