Pathogenic fungi arefungi that cause disease in humans or otherorganisms. Although fungi areeukaryotic, many pathogenic fungi aremicroorganisms.[1] Approximately 300 fungi are known to be pathogenic to humans;[2] their study is called "medical mycology". Fungal infections are estimated to kill more people than eithertuberculosis ormalaria—about two million people per year.[3]
In 2022 the World Health Organization (WHO) published a list of fungal pathogens which should be a priority for public health action.[4]
Markedly more fungi are known to be pathogenic toplant life than those of theanimal kingdom.[5] The study of fungi and other organisms pathogenic to plants is calledplant pathology.
According to theWorld Health Organization (WHO) in 2022 pathogens of particular concern are:[4]
Candida species cause infections in individuals with deficient immune systems. Candida species tend to be the culprit of most fungal infections and can cause both systemic and superficial infection.[6]Th1-typecell-mediated immunity (CMI) is required for clearance of a fungal infection.Candida albicans is a kind of diploidyeast that commonly occurs among the humangut microflora.C. albicans is anopportunistic pathogen in humans. Abnormal over-growth of this fungus can occur, particularly inimmunocompromised individuals.[7]C. albicans has aparasexual cycle that appears to be stimulated by environmental stress.[8]
C. auris, first described in 2009, is resistant to many frontline antifungal drugs, disinfectants, and heat, which makes it extremely difficult to eradicate. Like many fungal pathogens it mostly affects immunocompromised people; if in the blood or other organs and tissues, mortality is about 50%.[3]
Other species ofCandida may be pathogenic as well, includingCandida stellatoidea,C. tropicalis,C. pseudotropicalis,C. krusei,C. parapsilosis, andC. guilliermondii.[9]
The most common pathogenic species areAspergillus fumigatus andAspergillus flavus. Aspergillus flavus producesaflatoxin which is both a toxin and acarcinogen and which can potentially contaminate foods such as nuts.Aspergillus fumigatus andAspergillus clavatus can causeallergic disease. SomeAspergillus species cause disease on grain crops, especiallymaize, and synthesizemycotoxins includingaflatoxin.Aspergillosis is the group of diseases caused byAspergillus. The symptoms include fever, cough, chest pain or breathlessness. Usually, only patients with weakenedimmune systems or with otherlung conditions are susceptible.[1]
The spores ofAspergillus fumigatus are ubiquitous in the atmosphere.A. fumigatus is an opportunistic pathogen. It can cause potentially lethal invasive infection in immunocompromised individuals.[10]A. fumigatus has a fully functional sexual cycle that produces cleistothecia andascospores.[citation needed]
Cryptococcus neoformans can cause a severe form ofmeningitis and meningo-encephalitis in patients withHIV infection andAIDS. The majority ofCryptococcus species live in the soil and do not cause disease in humans.Cryptococcus neoformans is the major human and animal pathogen.Papiliotrema laurentii andNaganishia albida, both formerly referred toCryptococcus, have been known to occasionally cause moderate-to-severe disease in human patients with compromised immunity.Cryptococcus gattii is endemic to tropical parts of the continent of Africa and Australia and can cause disease in non-immunocompromised people.[1]
InfectingC. neoformans cells are usually phagocytosed byalveolar macrophages in the lung.[11] The invadingC. neoformans cells may be killed by the release of oxidative and nitrosative molecules by these macrophages.[12] However someC. neoformans cells may survive within the macrophages.[11] The ability of the pathogen to survive within the macrophages probably determines latency of the disease, dissemination and resistance toantifungal agents. In order to survive in the hostile intracellular environment of the macrophage, one of the responses ofC. neoformans is toupregulategenes employed in responses tooxidative stress.[11]
The haploid nuclei ofC. neoformans can undergo nuclear fusion (karyogamy) to become diploid. These diploid nuclei may then undergomeiosis, includingrecombination, resulting in the formation of haploidbasidiospores that are able to disperse.[13] Meiosis may facilitaterepair ofC. neoformans DNA in response to macrophage challenge.[13][14]
Histoplasma capsulatum can causehistoplasmosis in humans, dogs and cats. The fungus is most prevalent in the Americas, India and southeastern Asia. It is endemic in certain areas of theUnited States. Infection is usually due to inhaling contaminated air.
Pneumocystis jirovecii (or Pneumocystis carinii) can cause a form ofpneumonia in people with weakenedimmune systems, such as premature children, patients on immunosuppressive treatment, the elderly andAIDS patients.[15]
Stachybotrys chartarum or "black mold" can cause respiratory damage and severe headaches. It frequently occurs in houses and in regions that are chronically damp.[16]
Mammalianendothermy and homeothermy are potent nonspecific defenses against most fungi.[17] A comparative genomic study found that in opportunistic fungi there are few if any specialised virulence traits consistently linked to opportunistic pathogenicity of fungi in humans apart from the ability to grow at 37 °C.[18]
Theskin,respiratory tract,gastrointestinal tract, and thegenital-urinary tract induced inflammation[vague] are common bodily regions of fungal infection.
Studies have shown that hosts with higher levels ofimmune response cells such asmonocytes/macrophages,dendritic cells, andinvariant natural killer (iNK) T-cells exhibited greater control of fungal growth and protection against systemic infection.Pattern recognition receptors (PRRs) play an important role in inducing an immune response by recognizing specific fungal pathogens and initiating an immune response.In the case of mucosalcandidiasis, the cells that producecytokine IL-17 are extremely important in maintaininginnate immunity.[19]
A comprehensive comparison of distribution ofopportunistic pathogens and stress-tolerant fungi in the fungal tree of life showed thatpolyextremotolerance and opportunistic pathogenicity consistently appear in the same fungal orders and that the co-occurrence of opportunism andextremotolerance (e.g.osmotolerance andpsychrotolerance) is statistically significant. This suggests that some adaptations to stressful environments may also promote fungal survival during the infection.[18]
