Lack of hard information on the levels and diversity of microfungi in drinking water could be concealing significant pathogenic and other hazards
I've started to worry about fungi in drinking water. It is not a major worry, like that engendered by the unfolding of a worst-case scenario for H1N1 influenza. But it is significant and has become cumulative over recent years. The question is: do these neglected organisms pose pathogenic and other threats to all of us, and to certain vulnerable individuals in particular?
When I first attended lectures on water microbiology (half a century ago, alas), fungi did not merit a single sentence. Even today, they get comparatively little attention alongside bacteria (agents of the classical and best-known waterborne infections) and viruses (now recognized as the most frequent causes of gastrointestinal maladies worldwide). Many textbooks simply distinguish between microfungi that can occur naturally in water and those that may find their way there from the soil and air. They say nothing of possible dangers associated with either variety. Nevertheless, filamentous fungi in particular (moulds) seem to be causing growing concern. They are recognized as being much commoner in drinking water than we realised in the past, while concrete evidence has also been emerging of the infections and allergenic and other problems they may cause.
Two of the first papers on the subject that I read and filed away a decade ago were based on unexpected findings in water used in hospitals. One of them reported a high level of recovery of fungi from dialysate in artificial kidney units (M. Arvanitidou et al., J. Hosp. Infect. 45:225, 2000). The second described the isolation of filamentous fungi from water in a pediatric bone marrow transplantation unit (A. Warris et al., J. Hosp. Infect. 47:143, 2001). In this instance, the investigators recovered Aspergillus fumigatus from 49% of taps which they examined at Rikshospitalet University Hospital in Oslo, Norway.
The health implications of these discoveries were augmented by two further examples. In one project, researchers described fusariosis associated with pathogenic Fusarium species that had colonised a hospital water system (E. J. Anaissie et al, Clin. Infect. Dis. 33:1871, 2001). The same group then carried out a three-year prospective search and found pathogenic Aspergillus species in several samples of hospital water (E. J. Anaissie et al, Clin. Infect. Dis. 34:780, 2002).
Even before suggestions of this sort emerged, there was evidence that drinking water might also be a significant source of fungi capable of triggering allergic reactions. As a result of one investigation in Finland, researchers reported that raised levels of allergenic Aureobasidium pullulans in water used in a home sauna were responsible for hypersensitivity pneumonitis. They labelled the condition "sauna-taker's disease" (W.J. Metzger et al, JAMA 236:2209, 1976). And investigations into an outbreak of extrinsic allergic alveolotis, in over 100 people taking saunas, showers and baths, identified water contaminated with Aspergillus fumigatus and other organisms as the cause (A. Muittari et al, Clin. Allergy 10:77, 1980).
More recent surveys of drinking water in Norway, based on DNA barcoding, have incriminated Trichoderma viride as a dominant organism (G. Hageskal et al, Mol. Ecol. Resources 8:1178, 2008). The same fungus has been pinpointed as a cause of asthma among children living in water-damaged homes (S. J. Vesper, J. Occupational Environ. Med. 48: 852, 2006).
A third type of risk linked with fungi in water-in this instance a largely hypothetical one-is their capacity to generate mycotoxins. There have been brief reports of the detection of afllatoxins formed by A. fumigatus in water, including that from a cold-water tank. Although the concentrations of mycotoxins produced in this situation are likely to be vanishingly small, it is possible that their ingestion over long periods of time might constitute a health hazard.
Despite an accumulation of reports on pathogenic, allergenic and other consequences, it would be misleading to argue that there is yet convincing evidence that microfungi in drinking water pose serious or widespread threats to human health. In particular, a literature survey suggests that cases of acute illness arising from this source must be rare. At the same time, the combination of well-validated disease incidents and at-best sporadic surveillance indicates that a more searching analysis would be timely.
In a recent review in Mycological Research (113:165, 2009), Gunhild Hageskal and other workers at the National Veterinary Institute in Oslo point out that the proportion of drinking water samples found to be carrying fungi in recent studies ranged from 7.5 to 89%. They emphasize that the levels of fungi in the positive samples also varied considerably, probably at least in part as a result of differences in methodology, These, in turn, jeopardize meaningful comparisons between the results of one survey and another.
"The main disadvantage in culturing fungi is that it requires the intermediate and often timeconsuming step of obtaining each organism in isolation on appropriate media," Hageskal and coauthors write. "There is no international standard method for analyses of fungi in drinking water, hence several isolation procedures are used in the various studies performed." While membrane filter techniques are the most popular, others include direct plate-spread.
It is hardly surprising that standards for the occurrence of fungi in drinking water are virtually nonexistent. It seems that only the Swedish water regulation authority includes specifications on fungi.
One country that has addressed the specific issue of hospital-acquired aspergillosis is the USA. In 2000, the Centers for Disease Control and Prevention, Atlanta, published recommendations to minimize exposure of high-risk patients to potential sources of Aspergillus species, to minimize activities that may aerosolize this and other fungi, and to eliminate sources of aspergilli. The Oslo mycologists argue, however, that "no specific practical measures with respect to water were suggested, leaving the hospitals with as good as no recommendation."
Hageskal and his coworkers have another concern, arising from their demonstration of the capacity of several species of filamentous fungi to become established in biofilms in water distribution systems. In these situations, the organisms are relatively protected. They can colonize filters in treatment plants and are more resistant to water treatment procedures. The behavior and ecology of such biofilms has not yet been properly investigated.
Probably the most pressing reason why we require greater understanding of the prevalence of microfungi in tap water, and of any associated hazards, is the danger to hospital patients whose immune systems are impaired either by their illness or by drugs given to treat cancer or to prevent the rejection of transplanted organs. A. fumigatus is one of the most important fungal pathogens that can cause infections in immunocompromised individuals, and the rate of such infections is rising in many countries.
Despite these and other real and putative problems, the true significance of the threat posed by drinking water remains uncertain. "Some researchers state that nosocomial aspergillosis is waterborne, and recommend that hospitalised patients with high risk of fungal infection should avoid exposure to hospital water and use sterile water instead," The Oslo mycologists write. "Others state that the problem with fungi in water is substantially overstated, and requires further evidence before acting."
While the nature and size of the risk to immunocompromised hospital patients remain unresolved, they have hardly been evaluated at all as regards the population at large. For a start, we surely need more data. These might include epidemiological information on the contribution made by water supplies to the burden of fungal disease. Augmented by findings from other types of study, such data could then be used to determine defensible standards for acceptable levels of filamentous fungi (and yeasts) in drinking water. Not asking a lot, is it?