Introduction

Wind is commonly the mechanism for the dispersal of fungi, and as a consequence, the air we breathe contains spores of many different fungi. LINK Because of high irradiance and low water availability, air is a hostile environment for fungi. Spores of most fungi do not survive significant periods in air because of the absence of energy. Those that survive have quite specific mechanisms to prevent damage from desiccation and irradiation.

The Spores

The spores of common air-borne fungi have thick melanised walls. The walls usually contain complex carbohydrates which are hydrophobic, and waxy. LINK Hydrophobicity enables control of water loss, but the wall must also enable uptake of water prior to germination. Thus spore survival in air is a fine balance between water loss and retention of metabolic activity. The cytoplasm of air-borne spores may be constituted of greater concentrations of compatible solutes such as glycerol and other polyols, which enable continued metabolism in conditions of reduced water availability. LINK, LINK Use of glycerol in the cytoplasm enables loss of water from the cell without catastrophic consequences for water uptake in moist conditions.

Many molecules, including melanin reduce the penetration of radiation especially in the UV range. Melanin is contained in walls of spores, and so is the first barrier to UV. However, a range of other UV absorbing molecules are found in spores. The consequence is that energy is transformed usually to heat, which can be readily radiated from the spore in air.

INHALATION OF FUNGI

When spores are inhaled, in most cases, inhalation has no effect on humans. Most spores lodge on the surface of the lining of the airways, and are then expelled in mucus. The rest are usually neutralised because the immune response of the body is immediate and effective. The following segment is about those fungi that lodge deep in the respiratory system, evade the immune system, and cause respiratory and other disease.

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Asthma

Asthma is an allergic response in the lungs to allergens (sinusitis is a similar response of the sinuses). The cells lining bronchi and alveoli become inflamed, severely reducing the capacity to inhale. Among common allergens are house dust mite, cat dander, rye grass pollen and the spores of some fungi including Alternaria, Cladosporium and Aspergillus.

The fungus Alternaria is a common saprotroph and endophyte on many plants. It is commonly isolated from stems, and young and old leaves. It is particularly common on old leaves, and sporulates profusely especially as the leaf senesces. 

Spores of the fungus are usually dispersed only short distances in high densities. However, mechanical manipulation of the plant debris during harvesting and handling of crops may result in the simultaneous release to the moving air of very high densities of spores. Once airborne, spores may disperse long distances in the air stream. LINK In NSW Australia, the period for harvesting various crops coincides with the “spore season”. Crop processing and transport also contribute to the spore load in air. Cases of asthma induced by Alternaria increase especially in children during the harvesting period.

The fungi Alternaria and Cladosporium are similar in many ways. Cladosporium is a common saprotroph on plants, and can be isolated from soil. Spores are released to the air and dispersed significant distances in wind. Cladosporium is more common in the cooler areas of southern Australia; Alternaria is more common further north in the drier inland areas. Cladosporium requires moist conditions to germinate and cool conditions to grow. Alternaria, on the other hand, can germinate at lower water potential than Cladosporium, and the fungus can grow in drier and hotter habitats.LINK Cladosporium causes a similar syndrome, but more commonly in southern Australia.

Asthma in children of western NSW is primarily caused by the fungus Alternaria, especially in cropping areas. While the condition is associated with harvesting of crops, the fungus grows in dry conditions on all plants and can thus be expected across agricultural and non-agricultural areas. The fungus is uncommon on the coast, where many other fungi grow readily, thus competitively reducing populations of Alternaria in plants. Because plant growth will remain common in inland NSW, asthma caused by Alternaria will continue to be a major health problem.

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Farmers Lung

This is a disease commonly found in people who handle hay and compost. In the strict sense, Farmers lung is caused by actinomycetes, not fungi, though the fungus Aspergillus fumigatus is often blamed. The fungus, however, can lodge and grow in the lungs where it may cause Aspergillosis, a serious condition when the person is immunocompromised (eg. the young and very old, pregnant, when using drugs of recreation, immune suppressed by drugs). Most people with a functional immune system remain unaffected by Aspergillus.

A. fumigatus is a thermotolerant and thermostable saprotrophic fungus commonly isolated from compost especially during the high temperature stage of composting. LINK, LINK The fungus is found world wide. It is more commonly associated with farming because of the readily available plant materials, but is also easily isolated from gardens and parks, bark, potting mixtures and mulches. Spore densities are normally low, maybe around 5 colony forming units per cubic metre air. In hay sheds, silos and other farm buildings, up to 20 million colony forming units per cubic metre have been collected. Thus Farmers lung is a syndrome associated with composting and plant materials, though the role of fungi in causing the disease is less than commonly indicated.

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Other Medically Important Fungi

A number of fungi are becoming increasingly important because of the increased numbers of people whose immune system is compromised in some way. Traditional agents of fungal disease include Cryptococcus neoformans, Coccidioides immitis and Histoplasma capsulatum LINK.

C. neoformans

is a yeast-like fungus, found naturally in bird droppings and the reproductive parts of some Eucalyptus trees. The spores are distributed in wind. When spores lodge in the lungs of susceptible individuals, development of pneumonia-like symptoms is followed by systemic infection, possibly involving the central nervous system. Normally, the immune system will remove the fungus and the symptoms will be transient.

C. immitis

is a dimorphic fungus LINK found in soil of the arid regions of the Americas. Spores become air-borne in dust and may become lodged in the lungs. Infection is established and the fungus spreads through the body. The fungus may lodge in bones, skin and meninges. The disease is most common in people who, never having been exposed to the fungus, suddenly become exposed to very high doses, such as army personnel, archaeologists and farm workers. The fungus is highly infectious, so can be readily transmitted from culture or tissue to laboratory workers. Immunisation is an effective preventative measure.

H. capsulatum

is a dimorphic fungus LINK found in soil. Spores become airborne in high winds. When spores lodge in the lungs of sensitive individuals, they may cause a mild, chronic or severe infection. Generally the bodies natural immune response clears the fungus from the lungs and the person shows antibody response to applied antigens.

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Conclusion

With the spread of HIV, which suppresses the immune system, and use of immune-suppressant drugs in patients with transplant tissues, the range of fungi able to cause significant disease has increased LINK. These airborne fungi, have in the past, been unimportant because the human body has an effective immune response to the presence of the fungi. The response has been removed by agents of disease or medical intervention. With the increased medicalisation of human life, it is likely that diseases caused by fungi will increase.

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