The College of Environmental Science and Engineering at Tongii University, Shanghai (Zhao et al, 2022) have completed an important review and analysis of published literature on the presence of fungi in drinking water (Aspergillus, Fusarium, Candida, Cryptococcus andothers). The paucity of easy and reliable analysis methods, lack of regulation and limited control strategies has led to opportunistic fungal pathogens in drinking waters to be overlooked compared to the relatively spotlighted waterborne Gram-negative bacteria. As with many waterborne pathogens, research and understanding on managing drinking water associated fungi is in its infancy.
Drinking Water Regulation & Risk Assessing
Fungi are so widespread in our environments it is not surprising that drinking water systems in most countries, including the UK, have reported their presence. The authors found reports of over 70 genera of fungi in drinking water with the Top 5 genera being Aspergillus, Penicillium, Cladosporuim, Fusarium, Trichoderma, and within hospitals attention/management should also include Candida and Cryptococcus. Only Sweden’s water regulations have specifications on fungi with a general population value not to exceed 100 CFU/100 mL. The Spanish Society of Infectious Disease and Clinical Microbiology (SEIMC) have guidance that patients at risk from invasive fungal infections are not recommended to take a shower or bath in hospital. Little guidance or regulation is available.
Considering the hazards and risks for exposed individuals, the source of fungi could be via air, water and/or dust. Waterborne transmission routes are via drinking, inhalation of aerosols, or via direct contact with skin and mucous membranes. Topical infections of the skin, hair, nails and mucous membranes are usually successfully treatable, allergies which can trigger asthma, fungal rhinitis, hypersensitivity pneumonitis and extrinsic allergic alveolitis are more difficult to manage but the more seriously invasive infections from lung and blood stream infections have high mortality rates (> 60%).
Vulnerable Patient Groups
Prevalence of invasive fungal disease has increased in recent decades with new and extended cancer treatments, organ transplantations and long term immunodeficiency treatments. The authors noted that intensive care patients have an incidence of fungal disease between 8-15%, organ transplantation patients 20-40% and those with haematological tumours approximately 31%. More recently patients with COVID-19 appear to have a higher risk of fungal disease (Song et al., 2020).
Aspergillus spp. (e.g. Aspergillus fumigatus, A. niger, A. flavus) and Fusarium spp. were noted as the most frequently present pathogenic families, and in concentrations of 16-64 CFU/100 mL within in-premise drinking water systems. Fungal spores are highly resistant to disinfection by biocides and can regrow in drinking water distribution systems following treatment within water plants. They are a health threat to end-users, particularly the immunocompromised. Fungal contamination is also a source of taste and odour complaints from consumers, often associated with an earthy and musty odour.
Are we positively selecting for these fungi with sub-lethal levels of biocides at water treatment plants and where secondary treatment is undertaken within buildings?
Free chlorine and monochloramine are the top two most frequently used drinking water disinfectants, and their typically used dosage concentrations is insufficient to inactivate or impact fungal pathogens, indeed levels can significantly increase through the water treatment plant process (Ma et al., 2017). Fungal spores rather than the hyphae or vegetative spore are the predominant form that thrive and constitute drinking water biofilms.
Disinfection effectiveness is typically measured as “Ct” – which is the biocide concentration x contact time required to inactivate a stated % of microorganism. The indicator organism for biocide activity is frequently the highly sensitive Escherichia coli bacterium which has a 0.04-0.6 mg min/L chlorine exposure Ct value for a 2 log (99%) inactivation. Exposure to such low levels has little effect on many waterborne pathogens, and fungi (like non-tuberculous mycobacteria and Pseudomonas spp.) are highly tolerant of chlorine requiring significantly higher quantities for a meaningful effect: up to 1400 mg min/L Chlorine (Pereira et al., 2013) and up to 530 mg min/L monochloramine (Ma & Bibby, 2017). High concentrations such as these are not tolerated well in drinking water systems for neither taste nor plumbing material compatibility.
Multibarrier Approach to Control
Protected by their complex and strong cell walls, fungal spores often aggregate to allow even better tolerance of extreme environments including oxidising biocides, UV light, temperature, osmosis and both high and low pH. As with many tenacious waterborne pathogens a multibarrier approach to their mitigation and control is needed. Chlorine dioxide at concentrations of 2 mg / L has been shown to be more effective than monochloramine or free chlorine. Often a combination of filtration, UV light plus biocide is needed to control risks where there are vulnerable users. Whilst this text is focused on drinking water systems fungi and mycotoxins produced by them can be a problem within bottled and flasks of water and should be considered for vulnerable users, and sterile water may be the only suitable alternative in some cases. Drinking water dispensers, dish washers, washing machines and laundry may also be key areas to consider when assessing for fungal risk.
Drinking water is complex and variable from area to area geographically, and inconsistent locally within buildings. The options for environmental control of fungi are limited, and better understanding is needed for good hazard and risk evaluation. Yet how often does “Waterborne Fungi” get raised at Water Safety Group Meetings?
Be alert to this hazard and consider if your user populations could be at risk, and the additional processes which may be needed to protect vulnerable patients.
Zhao HX, Zhang TY, Wang H, Hu CY, Tang YL, Xu B. Occurrence of fungal spores in drinking water: A review of pathogenicity, odor, chlorine resistance and control strategies. Sci Total Environ. 2022 Sep 8;853:158626. doi: 10.1016/j.scitotenv.2022.158626. Epub ahead of print. PMID: 36087680.
Song G, Liang G, Liu W. Fungal Co-infections Associated with Global COVID-19 Pandemic: A Clinical and Diagnostic Perspective from China. Mycopathologia. 2020 Aug;185(4):599-606. doi: 10.1007/s11046-020-00462-9. Epub 2020 Jul 31. PMID: 32737747; PMCID: PMC7394275.
Ma X, Vikram A, Casson L, Bibby K. Centralized Drinking Water Treatment Operations Shape Bacterial and Fungal Community Structure. Environ Sci Technol. 2017 Jul 5;51(13):7648-7657. doi: 10.1021/acs.est.7b00768. Epub 2017 Jun 21. PMID: 28562026.
Pereira VJ, Marques R, Marques M, Benoliel MJ, Barreto Crespo MT. Free chlorine inactivation of fungi in drinking water sources. Water Res. 2013 Feb 1;47(2):517-23. doi: 10.1016/j.watres.2012.09.052. Epub 2012 Oct 9. PMID: 23164218.
Ma X, Bibby K. Free chlorine and monochloramine inactivation kinetics of Aspergillus and Penicillium in drinking water. Water Res. 2017 Sep 1;120:265-271. doi: 10.1016/j.watres.2017.04.064. Epub 2017 Apr 29. PMID: 28501787.