Aspergillus fumigatus and other species capable of growing at 37°C are opportunistic pathogens. About 40 of the 250 species of Aspergillus have been reported as human pathogens but the majority of cases are associated with just one species: A. fumigatus. The respiratory system is the normal portal of entry. The animal diseases caused by Aspergillus infection are all lumped together under the term 'aspergillosis.' Aspergillosis is a 'big umbrella' term, with an accompanying enormous literature. It refers to all infections caused by Aspergillus, including both veterinary and human disease, and ranging from localized and minor maladies to those that are systemic and life threatening. Although not contagious, human aspergillosis is of growing importance in modern medical care. Aspergillosis has been reported from all species of domestic animals and many wild species. Birds show a particularly high susceptibility; historically, aspergillosis was first recognized as an avian disease.

Different forms of aspergillosis include severe asthma with fungal sensitization; 'fungus ball,' (aspergilloma) whereby the mould colonizes usually without spreading, causing a gramulomatous disease of the lung; and systemic (invasive) aspergillosis, in which the fungus disseminates throughout the body. Aspergilloma and invasive aspergillosis both involve direct growth of the fungus inside of host tissues; the former is often found associated with tuberculosis and is 'semi-invasive.' Invasive aspergillosis, also called systemic aspergillosis, is the most life threatening form of Aspergillus infection. The clinical categorization of these conditions defies neat classification but provide a good introductory framework for understanding aspergillosis. Despite advances in antifungal drug therapy, the invasive forms of aspergillosis have extremely high mortality rates.

Clinical manifestations and the severity of aspergillosis reflect the immunological status of the patient. The best protection is a robust immune system. Dissemination of Aspergillus in the body indicates a break of, or deficiency in, host defences. Immunosuppressive agents and other medical developments have created a new ecological niche for aspergilli to grow on people with impaired immune systems, where they can cause serious and often fatal infections. Invasive aspergillosis, with an attendant high mortality rate, has become increasingly common as the number of susceptible hosts is increased. Bone marrow recipients constitute particularly vulnerable populations. Unfortunately, drug resistance to known antifungal drugs is becoming more common. As the disease has become more prevalent, there is a great need for expanding the number of safe and more reliable anti fungal drugs.

The early diagnosis of invasive Aspergillus infections is still difficult. It is usually based on the isolation and subsequent identification of species from appropriate clinical specimens and/or the detection of characteristic septate hyphae in sections of tissue following biopsies. Unfortunately, disseminated aspergillosis is frequently not diagnosed until necropsy. Another growing problem is the increasing number species causing invasive aspergillosis in organ transplant patients. Many of these 'non-fumigatus' aspergilli are resistant to the drugs commonly usually used to treat aspergillosis. A. flavus, the second most common species involved in invasive aspergillosis, is the most common cause of superficial infection.

A good definition of human pathogen is 'a microbial or parasitic species that can infect and is capable of causing disease in humans under natural transmission conditions'. The best-known bacterial pathogens have co-evolved with their hosts to subvert host functions and they possess special virulence factors that have indispensable functions in mediating host-pathogen interactions. Aspergillus infections do not resemble classical bacterial diseases. A. fumigatus has no obvious need for passage in humans or other animals. The fact that Aspergillus can become a deadly pathogen may be a biological accident associated with it extreme opportunism. With invasive aspergillosis, the immunological status of the host - not the virulence of the fungus - is the single most significant element. Despite intensive study, the molecular basis of the pathogenic potential of A. fumigatus remains elusive. Suggested reading: Medical Mycology Publications

In summary, as disease categories go, aspergillosis is an extremely capacious rubric. Occupational mycoses, allergies, localized mycoses, and invasive mycoses are all assembled together under the same name. 'Aspergillosis' encompasses any and all medical and veterinary diseases caused by any and all members of the genus Aspergillus. The word 'aspergillosis' describes so many different clinical entities that, without qualifications, the term is almost meaningless. It would be useful if we did a better job of stipulating definitions for the various specific diseases grouped together under the aspergillosis rubric. Hypersensitivity reactions are not the same as fungus ball. Invasive aspergillosis is profoundly different from both. Further, there is growing evidence that 'non-fumigatus' Aspergillus infections have different manifestations other than their patterns of differential drug resistance. One of our greatest challenges in studying aspergillosis may be to break conventional mindsets about the nature of pathogenicity. As the numbers of immune suppressed individuals increase within the human population, cases of invasive aspergillosis are bound to increase too and they will become a mounting burden on our health care systems. Suggested reading: Medical Mycology Publications

Adapted from An Overview of the Genus Aspergillus by Joan W. Bennett writing in Aspergillus: Molecular Biology and Genomics

Further reading

• Aspergillus: Molecular Biology and Genomics
• An Overview of the Genus Aspergillus
• Fungal Publications
• Microbial Toxins: Current Research and Future Trends
• Microbial Biodegradation: Genomics and Molecular Biology
• Microbial Production of Biopolymers
• Probiotics Publications
• Foodborne Pathogens: Microbiology and Molecular Biology

Labels: Allergies, Aspergillosis, Invasive mycoses, Localized mycoses, Mycoses